MINISTRY OF SCIENCE AND EDUCATION OF THE REPUBLIC OF KAZAKHSTAN
STATE UNIVERSITY of SEMEY named after SHAKARIM
Document of SQM
EMCD
of 3rd level
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EMCD
The of discipline of «Physical and
Edition № 1from
Colloidal Chemistry»
«_11_»___09___2014
EDUCATIONAL-METHODOLOGICAL COMPLEX OF DISCIPLINE
on
«Physical and Colloidal Chemistry»
for speciality
5B011200– «Chemistry»
EDUCATIONAL METHODOLOGICAL MATERIALS
Semey
2014
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Preface
1 WORKED OUT
Compiled by: ______________ B.B. Bayakhmetova, Ph.D. in Chemistry, senior teacher of the
Chemistry and Geography Department
2 DISCUSSED
2.1 At the meeting of Chemistry Department
Protocol _1__, September _2__, 2014
Head of the Department ____________ D.R. Ontagarova
2.2 At the meeting of Educational-Methodological Bureau of the Natural Science Department
Protocol _1__, September _3__, 2014
Head of the Bureau____________ Z.V. Abdisheva
3 APPROVED
Approved and recommended for publishing at the meeting of the Teaching-Methodological Council
of the University
Protocol _1__, September _11__, 2014
Head of the TMC, Vice-principal__________________ G.К. Iskakova
4 INTRODUCED instead of edition №1 from 18.09.2013
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Content
1
A briefsynopsis of the lectures
2
Laboratory works
3
Independentworkofstudents
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The glossary on discipline " Physical and Colloidal Chemistry"
Absorption is a phenomenon that occurs when matter crosses from one phase to another passing
through the border surface and in the other phase more or less monotonously distributes itself in a
concentration higher than the one within the first phase.
Adsorption is a process in which molecules of gas, of dissolved substances in liquids, or of liquids
adheres in an extremely thin layer to surfaces of solid bodies with which they are in contact.
Aerosols are colloidal dispersions of liquid or solid particles in a gas, as in a mist or smoke. The
commonly used aerosol sprays contain an inert propellant liquefied under pressure. The pressure of
the gas causes the mixture to be released as a fine spray (aerosol) or foam (aerogel) when a valve is
opened.
Absorption coefficient (a) is the relative decrease in the intensity of a collimated beam of
electromagnetic radiation, as a result of absorption by a medium, during traversal of an infinitesimal
layer of the medium, divided by the length traversed.
Adsorbent is a substance on the surface of which a substance is adsorbed.
Binary solution is a mixture of two liquids that are completely miscible one with another.
Colloids are systems in which there are two or more phases, with one (the dispersed phase)
distributed in the other (the continuous phase). Moreover, at least one of the phases has small
dimensions, in the range between 1 nm and 1 μm (10-9 m- 10-6 m). Dimension, rather than the
nature of the material, is characteristic. In this size range, the surface area of the particle is large
with respect to its volume so that unusual phenomena occur, e.g., the particles do not settle out of
the suspension by gravity and are small enough to pass through filter membranes. Macromolecules
(proteins and other high polymers) are at the lower limit of this range; the upper limit is usually
taken to be the point at which the particles can be resolved in an optical microscope.
Colloidal particles may be gaseous, liquid, or solid, and occur in various types of suspensions:
Dialysis a very slow process, where the aim is to remove a large part ot any ionic material that may
have accompanied their formation.
Sols - dispersions of small solid particles in a liquid.
Emulsions are colloidal systems in which the dispersed and continuous phases are both liquids.
Gels are colloids in which both dispersed and continuous phases have a three-dimensional network
throughout the material.
Foams are dispersions of gases in liquids or solids.
Colloid ions emerge when colloid particles adsorb certain type of ion from solution and thus
become charged with the same charge. The charge can also originate form a chemical reaction of
colloid particle’s surface. Colloid ions formed by absorption of silver chloride particle can be show
as follows:
(nAgCl)Cl- i (nAgCl)Ag+
Adsorbed layer is monomolecular (one molecule thick) and which type of ion will be formed
depends upon which ions are present in a greater number in the solution in. Because of this colloid
particles are charged with the same charge, mutual repelling occurs, and the colloid solution
becomes stable. Colloid charge can be determined by electrophoresis.
Colloid mills are machines used to grind aggregates into very fine particles or to apply very high
shearing within a fluid to produce colloid suspensions or emulsions in which the particle sizes are
less than 1 micrometer. One type of colloid mill is called a disc mill, in which a mixture of a solid
and liquid (or two liquids) is passed between two discs a small distance apart, which rotate very
rapidly relative to each other. Applications of colloid mills occur in food processing, in paint
manufacture, and in the pharmaceutical industry.
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Colloid silver is a bright blue-green powder which dissolved in water gives colloid solution of red
colour.
Colorimeter is an instrument used to measure the strength of colorification in a solution.
Diffusion is the spontaneous mixing of one substance with another when in contact or separated by
a permeable membrane. Diffusion is a result of the random motions of their component atoms,
molecules, ions, or other particles. Diffusion occurs most readily in gases, less so in liquids, and
least in solids. The rate of diffusion is proportional to the concentration of the substance and
increases with temperature. The theoretical principles are stated in Fick’s laws.
Filtration is a procedure in which liquids are separated from the precipitate by passing a suspension
through the filter. The precipitate remains on the filter and through it the filtrate passes. Gaseous
heterogeneous mixtures can also be filtrated.
Flotation is a procedure in which hydrophobic solid substances are separated from hydrophilic one
using bubbles of air. If air is blow through a suspension, in which substances promoting easier
creation of foam are added, bubbles of air are created which stick to the hydrophobic matter and
carry it out to the surface.
Molar absorption coefficient (ε) is the absorption coefficient divided by amount-of-substance
concentration of the absorbing material in the sample solution (ε = a/c). The SI unit is m2mol-1. Also
called extinction coefficient, but usually in units of dm3cm-1mol-1.
Nonpolar solvent is a liquid with nonpolar molecules. It dissolves covalent compounds, nonwater
solvent.
Osmotic pressure (Π) is the excess pressure necessary to maintain osmotic equilibrium between a
solution and a pure solvent separated by a membrane permeable only to the solvent. In an ideal
dilute solution
Π = cB RT
where cB is the amount-of-substance concentration of the solute, R is the molar gas constant, and T
the temperature.
Ostwald’s viscometer is a simple appliance used for determining relative viscosity.
Phase is a portion of a physical system (solid, liquid, gas) that is homogeneous throughout, has
definable boundaries, and can be separated physically from other phases.
Saturated solution is a solution that holds the maximum possible amount of dissolved material.
When saturated, the rate of dissolving solid and that of recrystallisation solid are the same, and a
condition of equilibrium is reached. The amount of material in solution varies with temperature;
cold solutions can hold less dissolved solid material than hot solutions. Gases are more soluble in
cold liquids than in hot liquids.
Spectrophotometry is a determination of the concentration of a material in a sample by
measurement of the amount of light the sample absorbs.
Thermostat is a device which controls the heating or cooling of a substance, by turning the
machinery on or off, in order to maintain a constant temperature.
Tyndall’s effect occurs when light disperses on colloid particles. This phenomenon can be seen
when a ray of light enters in dark room through a small hole. In the beam some dust particles of
colloid dimensions can be seen sparkling.
Viscosity. (η) (coefficient of viscosity) is the resistance a liquid exhibits to flow. Experimentally,
the frictional force between two liquid layers moving past each other is proportional to the area of
the layers and the difference in flow speed between them.
Condensation, in colloid systems, is a process where smaller particle join in one colloid size
particle
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Electrostatic stabilization is based on the mutual repulsion of like electrical charges. In general,
different phases have different charge affinities, so that an electrical double layer forms at any
interface.
Steric stabilization consists in covering the particles in polymers which prevents the particle to get
close in the range of attractive forces.
Spectroscopy: The study of the interaction of matter and electromagnetic radiation, usually as a
function of the radiation wavelength.
Calibration Calibration is a process in which the operation of the mass spectrometer in a specified
manner is adjusted and certified to produce the accurate and known ion masses in the spectrum of a
standard compound.
Ionization energy The ionization energy is the minimum energy required to remove an electron
from an atom or molecule in order to produce a positive ion.
1 LECTURES
A brief synopsis of the lectures
Lecture #1,2. Introduction to physical and colloid chemistry.
Purpose: To familiarize with the basic group of dispersion systems
Key questions:
1. The nature and classification of colloid system.
2. The basic types of dispersion systems
Summary:
Colloidal system is a highly dispersed system in which dispersed particles are not molecules
but aggregates of many molecules. The size range consume the nanometer (10–9m) to micrometer
(10–6m) range. There is no sharp distinction between colloidal and non-colloidal systems. Usually
colloidal system is found due to the nature of the substances dissolved in the media and does not
depend on aggregation, chemical nature and origin. Colloid science is a part of surface science. The
surface interfacial phenomena associated with colloidal systems such as emulsions and forms are
often studied by means of experiments on artificially flat surfaces rather than on the colloidal
systems themselves.
A mixture in which one substance is divided into minute particles (called colloidal particles)
and dispersed throughout a second substance. The substances are present as larger particles than
those found in solution, but are too small to be seen with a microscope. There are no strict
boundaries on the size of colloidal particles, but they tend to vary between 10-9 m to 10-6 m in size.
The mixture is also called a colloidal solution, colloidal system, or colloidal dispersion. The
three forms in which all matter exists are solid, liquid or gas. Colloidal systems can be any
combination of these states.
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A colloidal system is not a true solution but it is not a suspension either because it does not
settle out like a suspension will over time.
Colloids are larger than most inorganic molecules and remain suspended indefinitely. They are
large molecules, such as proteins, or groups of molecules. They have many properties, depending
on their large specific surface. Colloid formation can be classified in two systems, namely
reversible and irreversible. In an irreversible system, the products are so stable or removed so well
that the original reactants cannot be reproduced. A reversible system is one in which the products
can be made to react to reproduce the original reactants.
The word "Colloid" was derived from the Greek, "kolla" for glue, as some of the original
organic colloidal solutions were glues. This term was first coined in 1862 to distinguish colloids
from crystalloids such as sugar and salt.
Colloids have been studied by scientists since the early 1800's. The early part of the 20 th
century saw a number of major developments in both chemistry and physics, some of which had
direct influences on the study of colloids. A number of methods for studying colloidal particles
were developed, including diffusion, electrophoresis, and scattering of visible light and X-rays.
Due to colloidal particles being so small, their individual motion changes continually as a result of
random collision with the molecules of the dispersion medium. This random, zig-zagging
movement is called Brownian motion after the man who discovered it. This motion helps keep the
partilces in suspension. In the early 19th century, Michael Faraday showed that when you pass a
strong beam of light through a colloidal solution, it is scattered. This method to study colloids was
further developed by John Tyndall and became known as the "Tyndall effect".
Types of colloids:
Colloids are usually classified according to the original states of their constituent parts:classified
according to the original states of their constituent parts:
Dispersing
medium
Dispersed
phase
Name
Solid
Solid
Solid sol
Solid
Liquid
Gel
Solid
Gas
Solid foam
Liquid
Solid
Sol
Liquid
Liquid
Emulsion
Liquid
Gas
Foam
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

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Gas
Solid
Solid
aerosol
Gas
Liquid
Aerosol
Dispersing medium (external phase) - the constituent found in the greater extent in the
colloid.
Dispersed phase (internal phase) - the constituent found in the lesser extent.
A further classification is as lyophilic (solvent attracting), lyophobic (solvent repelling) or
association colloids (a mixture of the two).
If water is the dispersing medium, it is often known as a hydrosol.
Colloidal dispersions may be lyophobic (water hating) or lyophilic (water loving). Lyophilic sols
are formed spontaneously when the dry coherent material (e.g. gelatin, rubber, soap) is brought in
contact with the dispersion medium, hence they are thermodynamically more stable than in the
initial state of dry colloid material plus dispersion medium. Lyophobic sols (e.g. gold sol) cannot be
formed by spontaneous dispersion in the medium. They are thermodynamically unstable with
respect to separation into macroscopic phases, but they may remain for long times in a metastable
state.
Preparation
There are two main ways of forming a colloid; reduction of larger particles to colloidal size
or condensation of smaller particles, e.g. molecules, into colloidal particles. This latter generally
makes use of chemical reactions such as hydrolysis or displacement. Laboratory and industrial
methods make use of several techniques. This page gives a brief overview of some of these
techniques, but it should be noted that there are a broad range used in practice.
A method of forming an aerosol is to tear away a liquid spray with a gas jet. The process can
be helped by separating the liquid into droplets with electrostatic repulsions, done by applying a
charge to the liquid.
Emulsions are usually prepared by vigorously shaking the two constituents together, often
with the addition of an emulsifying agent, e.g. a surfactant such as soap, in order to stabilise the
product formed.
Semi-solid colloids, known as gels, may be formed from the cooling of lyophilic sols that
contain large linear molecules and have a much greater viscosity than the solvent.
Colloids are often purified by dialysis, a very slow process, where the aim is to remove a
large part ot any ionic material that may have accompanied their formation. A membrane is selected
that will not allow colloid particles through but will let the solvent and ions permeate through. The
method relies on diffusion, osmosis and ultrafiltration.
Properties
Colloidal particles are generally aggregates of numerous atoms or molecules. They pass
through most filter papers, but can be detected by light-scattering, sedimentation and osmosis. A
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characteristic of colloids is absorption, as the finely divided colloidal particles have a large exposed
surface area.
The chemical and physical properties of inorganic colloids can be changed dramaticaly
when their size is reduced to a number of nanometers. This effect is due to the increasing
importance of the colloid surface.
The presence of colloidal particles in a solution has only minor effects on its colligative properties
(boiling, freezing point, etc.)
Thixotropy is a property exhibited by certain gels. This is where a gel appears solid and
maintains its own shape until it is subjected to some force or disturbance, such as shaking. It then
tends to act as a sol, flowing freely. This behavior is reversible, and the sol will return to a gel if left
undisturbed. Examples of thixotropic gels include certain paints, printing inks and clays.
The particles of a colloid selectively absorb ions and acquire an electric charge. The
existence of an electric charge on the surfaces of the colloidal particles is a source of kinetic
stability for colloids. All of the particles of a given colloid are repelled by one another as they all
take on the same charge. The movement of colloidal particles through a fluid under the influence of
an electric field is known as electrophoresis.
Examples of colloids
These are just a few of the many examples of colloids, both man-made and naturally occuring.
Aerosols: Man-made: Aerosol sprays, insecticide spray, smog. Natural: Fog, clouds.
Solid aerosol: Natural: Smoke, dust.
Foam: Man-made: Shaving lather, whipped cream.
Emulsions: Man-made: Mayonnaise, cosmetic lotion, lubricants. Natural: Milk.
Sols: Man-made: Paint, ink, detergents, rubber (a latex - also occur naturally).
Solid foams: Man-made: Marshmallow, styrofoam, insulation, cushioning.
Gels: Man-made: Butter, jelly.
Solid sols: Man-made: Certain alloys. Natural: Pearl, opal.
Biological macromolecules and cells may be considered to be biocolloids and many foods are also
colloidal in nature. Colloids are also an important feature of the natural environment.
Questions for self-control:
1. What does the colloidal chemistry study?
2. What did types of dispersion systems classify?
3. Give examples of colloids.
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References
1. Е.Д. Щукин, А.В. Пернов, Е.А. Амелина. Коллоидная химия, 3-е изд., М.: Высш.школа,
2004г., 435с.
2. Д.А. Фридрихсберг. Курс коллоидной химии, Л.: Химия, 1984, 352 с.
3. Ю.Г. Фролов. Курс коллоидной химии: Поверхностные явления и дисперсные системы.
М.: Химия, 1988, 462 с.
Lecture #3, 4. Molecular – kinetic and rheological properties of colloid systems
Purpose: learn the molecular – kinetic and rheological properties of colloid systems
Key questions:
1. Thermal molecular motion and Brownian motion.
2. Diffusion in solutions, Osmotic pressure
3. Dispersions, sedimentation stability.
Summary:
Kinetic properties of colloidal systems relate to the motion of particles with respect to the
dispersion medium. They are;
Brownian Motion
Brownian motion is seen in particular of sizes 6 mm, as a result of the bombardment of the
particles by the molecules of the dispersion medium. It is not observable due to small size. The
velocity of the particle increases with the decrease in particle size. Brownian movement can be
stopped by increasing the viscosity of medium by the addition of glycerine or similar agents.
Brownian movement: Robert Brown (1927) an English Botanist, observed that the pollen grains in
aqueous suspensions were in constant motion. Similar phenomenon was, later on, found in case of
colloidal solution, when observed ultra-microscopically.
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Brownian movement. This continuous and rapid zig-zag motion of the colloidal particles is called
Brownian movement. This motion is independent of the nature of the colloidal particles. It is more
rapid when the size of the particles is small and the solution is less viscous.
Diffusion. Diffusion is another kinetic property of colloid which occur as a result of brownian
movement. Here particles get diffused spontaneously from a region of higher concentration to one
of lower concentration until the concentration of system is uniform throughout.
According to fick's 1st law, the amount dm of a substance diffusing in time dt across an area S is
directly proportional to charge of concentration dc with distance dx:
dm   D
dc
sd
dx
D- is the diffusion coefficient
Diffusion coefficient may be obtained in colloidal chemistry by diffusion experiments in which
the material is allowed to pass through a porous disc and samples are removed and analysed
periodically.
If the colloidal particles are assumed to be spherical, the following equation is used to obtain radius
of the particle and molecular weight.
RT 1
kT
D
 
N A B 6r
В = 6πηr for spherical particles
R = Molar gas constant
T =Absolute temperature
η= Visocity of the solvent
NA = Avagadros number
r =radius of the particles
Osmotic pressure
If two solutions of different concentration are separated by a semi-permeable membrane
which is permeable to to the smaller solvent molecules but not to the larger solute molecules, then
the solvent will tend to diffuse across the membrane from the less concentrated to the more
concentrated solution. This process is called osmosis. The energy which drives the process is
usually discussed in terms of osmotic pressure.
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The osmotic pressure π, is given by van't Hoff formula, which is identical to the pressure
formula of an ideal gas:
π = cRT
where c is the molar concentration of the solute, R = 0.082 (liter∙bar) / (deg∙mol), is the gas
constant, and T is the temperature on the absolute temperature scale (Kelvin).
Van't Hoff received the first Nobel prize in chemistry, in 1901, for his interpretation of osmosis.
Sedimentation
The velocity U of sedimentation of spherical particles having density ρ in a medium of density ρ0
and η viscosity ho is given by Stoke's law.
2r 2
   0 g
u
9
r- radius of the particles
g- velocity of acceleration
if the particles are subjected only to force of gravity then the lower size limit of particles obeying
stoke's equation is about 0.5 mm. this is because Brownian movement become significant and tends
to offset sedimentation due to gravity and promotes mixing. 50 a stronger force must be applied to
bring about the sedimentation of colloidal particles. This is accomplished by ultra centrifuge,
developed by Svedberg. The instantaneous velocity U = dx/dt of a particle in a unit centrifugal field
is expressed in terms of the Svedberg sedimentation coefficient, S
m
S
B
2
2 r    0 
For of spherical particles S 
9
Molecular weight can be determined by two method,
1) Sedimentation velocity technique
2) Sedimentation equilibrium method
VISCOSITY
Viscosity is an expression of the resistance to flow of a system under an applied stress. The more
viscous a liquid, the greater the applied force required to make it flow at particular rate. Viscosity
studies also provide information regarding the shape of the particle in solution.
Einstein developed an equation of flow applicable to dilute colloidal dispersion of spherical
particles namely,
h = h0 (1 + 2.5 q)
h0 = viscosity of the dispersion medium
h = viscosity of the dispersion when the volume fraction of colloidal particles present is q. The
volume fraction is defined the volume of particles divided by the total volume of dispersion, it is
therefore equivalent to a concentration term.
Several viscosity coefficients may be defined with respect to this equation. These include relative
viscosity, specific viscosity, intrinsic viscosity.
Several viscosity coefficients may be defined with respect to this equation. These include relative
viscosity, specific viscosity, intrinsic viscosity.
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Since volume fraction is directly related to concentration.
in which C is expressed in grams of colloidal particles per 100 ml of total dispersion. For highly
polymeric materials dispersed in the medium at moderate concentration.
If is plotted against C, the line which is polated to infinite dilution, the intercept is K1 known as
intrinsic viscosity (h) is used to calculate the approximate molecular weight of polymers.
According to the Mark-Houwink equation,
[h] = KMa
K & a are constants, characteristics of the particular polymer-solvent system.
M=molecular weight
The characteristics of polymers used as substitutes for blood plasma depend in part on molecular
weight of the material. These characteristics include the size and shape of macromolecules and the
ability of the polymers to impart the proper viscosity and osmotic pressure to the blood. These
methods are used to determine the average molecular weight of hydroxyl ethyl starch and gelatine
preparation used as plasma extenders.
Questions for self-control:
1. What is thermal molecular motion and Brownian motion?
2. What is diffusion in solutions and Osmotic pressure?
3. What is determined sedimentation velocity?
References
1. Е.Д. Щукин, А.В. Пернов, Е.А. Амелина. Коллоидная химия, 3-е изд., М.: Высш.школа,
2004г., 435с.
2. Д.А. Фридрихсберг. Курс коллоидной химии, Л.: Химия, 1984, 352 с.
3. Ю.Г. Фролов. Курс коллоидной химии: Поверхностные явления и дисперсные системы.
М.: Химия, 1988, 462 с.
Lecture #5,6. Optical properties of colloid systems
Goal: to learn the basics of optical phenomena
Key questions:
1. Introduction to optical phenomena.
2. The Rayleigh’s and Bouguer-Lambert - Beer’s equations
Summary:
The light scattering methods for studying colloidal systems can be classified in two wide
groups: SLS and DLS. The latter is often called quasi-elastic light scattering (QELS) or photon
correlation spectroscopy (PCS). In SLS methods, the averagedover-time intensity of the scattered
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light is measured as a function of the particle concentration and scattering angle. In DLS methods,
the time fluctuations of the scattered light are measured. The light scattering methods possess a
number of advantages, which make them particularly suitable for investigation of colloid systems.
In general, these methods are noninvasive; applicable to very small and unstable (when dried)
particles, such as surfactant micelles and lipid vesicles; suitable for characterization of particle size
and shape, as well as of interparticle interactions; and relatively fast, and not requiring very
expensive equipment.
The law states that there is a logarithmic dependence between the transmission (or
transmissivity), T, of light through a substance and the product of the absorption coefficient of the
substance, α, and the distance the light travels through the material (i.e., the path length), ℓ. The
absorption coefficient can, in turn, be written as a product of either a molar absorptivity (extinction
coefficient) of the absorber, ε, and the molar concentration c of absorbing species in the material, or
an absorption cross section, σ, and the (number) density N' of absorbers.
For liquids,these relations are usually written as:
whereas for gases, and in particular among physicists and for spectroscopy and spectrophotometry,
they are normally written
where 0 and are the intensity (or power) of the incident light and the transmitted light,
respectively; σ is cross section of light absorption by a single particle and N is the density (number
per unit volume) of absorbing particles.
The base 10 and base e conventions must not be confused because they give different values for the
absorption coefficient:
. However, it is easy to convert one to the other, using
The transmission (or transmissivity) is expressed in terms of an absorbance which, for liquids, is
defined as
whereas, for gases, it is usually defined as
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This implies that the absorbance becomes linear with the concentration (or number density of
absorbers) according to
and
for the two cases, respectively.
Thus, if the path length and the molar absorptivity (or the absorption cross section) are known and
the absorbance is measured, the concentration of the substance (or the number density of absorbers)
can be deduced.
Although several of the expressions above often are used as Beer–Lambert law, the name should
strictly speaking only be associated with the latter two. The reason is that historically, the Lambert
law states that absorption is proportional to the light path length, whereas the Beer law states that
absorption is proportional to the concentration of absorbing species in the material.[1]
If the concentration is expressed as a mole fraction i.e., a dimensionless fraction, the molar
absorptivity (ε) takes the same dimension as the absorption coefficient, i.e., reciprocal length (e.g.,
m−1). However, if the concentration is expressed in moles per unit volume, the molar absorptivity
(ε) is used in L·mol−1·cm−1, or sometimes in converted SI units of m2·mol−1.
The absorption coefficient α' is one of many ways to describe the absorption of electromagnetic
waves. For the others, and their interrelationships, see the article: Mathematical descriptions of
opacity. For example, α' can be expressed in terms of the imaginary part of the refractive index, κ,
and the wavelength of the light (in free space), λ0, according to
In molecular absorption spectrometry, the absorption cross section σ is expressed in terms of a
linestrength, S, and an (area-normalized) lineshape function, Φ. The frequency scale in molecular
spectroscopy is often in cm−1, wherefore the lineshape function is expressed in units of 1/cm−1,
which can look funny but is strictly correct. Since N is given as a number density in units of 1/cm3,
the linestrength is often given in units of cm2cm−1/molecule. A typical linestrength in one of the
vibrational overtone bands of smaller molecules, e.g., around 1.5 μm in CO or CO2, is around 10−23
cm2cm−1, although it can be larger for species with strong transitions, e.g., C2H2. The linestrengths
of various transitions can be found in large databases, e.g., HITRAN. The lineshape function often
takes a value around a few 1/cm−1, up to around 10/cm−1 under low pressure conditions, when the
transition is Doppler broadened, and below this under atmospheric pressure conditions, when the
transition is collision broadened. It has also become commonplace to express the linestrength in
units of cm−2/atm since then the concentration is given in terms of a pressure in units of atm. A
typical linestrength is then often in the order of 10−3 cm−2/atm. Under these conditions, the
detectability of a given technique is often quoted in terms of ppm•m.
The fact that there are two commensurate definitions of absorbance (in base 10 or e) implies that the
absorbance and the absorption coefficient for the cases with gases, A' and α', are ln 10
(approximately 2.3) times as large as the corresponding values for liquids, i.e., A and α,
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respectively. Therefore, care must be taken when interpreting data that the correct form of the law is
used.
The law tends to break down at very high concentrations, especially if the material is highly
scattering. If the light is especially intense, nonlinear optical processes can also cause variances.
The main reason, however, is the following. At high concentrations, the molecules are closer to
each other and begin to interact with each other. This interaction will change several properties of
the molecule, and thus will change the molar aborbtivity. If the absorbtivity is different at higher
concentrations than at lower ones, then the plot of the absorbance will not be linear, as is suggested
by the equation, so you can only use it when all the concentrations you are working with are low
enough that the absorbtivity is the same for all of them.
Derivation
Classically, the Beer-Lambert law was first devised independently where Lambert's law stated that
absorbance is directly proportional to the thickness of the sample, and Beer's law stated that
absorbance is proportional to the concentration of the sample. The modern derivation of the BeerLambert law combines the two laws and correlate the absorbance to both, the concentration as well
as the thickness (path length) of the sample.
In concept, the derivation of the Beer–Lambert law is straightforward. Divide the absorbing sample
into thin slices that are perpendicular to the beam of light. The light that emerges from a slice is
slightly less intense than the light that entered because some of the photons have run into molecules
in the sample and did not make it to the other side. For most cases where measurements of
absorption are needed, a vast majority of the light entering the slice leaves without being absorbed.
Because the physical description of the problem is in terms of differences—intensity before and
after light passes through the slice—we can easily write an ordinary differential equation model for
absorption. The difference in intensity due to the slice of absorbing material
is reduced; leaving
the slice, it is a fraction of the light entering the slice . The thickness of the slice is
, which
scales the amount of absorption (thin slice does not absorb much light but a thick slice absorbs a
lot). In symbols,
, or
. This conceptual overview uses to describe
how much light is absorbed. All we can say about the value of this constant is that it will be
different for each material. Also, its values should be constrained between −1 and 0. The following
paragraphs cover the meaning of this constant and the whole derivation in much greater detail.
Assume that particles may be described as having an absorption cross section (i.e., area), σ,
perpendicular to the path of light through a solution, such that a photon of light is absorbed if it
strikes the particle, and is transmitted if it does not.
Define z as an axis parallel to the direction that photons of light are moving, and A and dz as the
area and thickness (along the z axis) of a 3-dimensional slab of space through which light is passing.
We assume that dz is sufficiently small that one particle in the slab cannot obscure another particle
in the slab when viewed along the z direction. The concentration of particles in the slab is
represented by N.
It follows that the fraction of photons absorbed when passing through this slab is equal to the total
opaque area of the particles in the slab, σAN dz, divided by the area of the slab A, which yields
σN dz. Expressing the number of photons absorbed by the slab as dIz, and the total number of
photons incident on the slab as Iz, the number of photons absorbed by the slab is given by
Note that because there are fewer photons which pass through the slab than are incident on it, dIz is
actually negative (It is proportional in magnitude to the number of photons absorbed).
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The solution to this simple differential equation is obtained by integrating both sides to obtain Iz as
a function of z
The difference of intensity for a slab of real thickness ℓ is I0 at z = 0, and Il at z = ℓ. Using the
previous equation, the difference in intensity can be written as,
rearranging and exponentiating yields,
This implies that
and
The derivation assumes that every absorbing particle behaves independently with respect to the
light and is not affected by other particles. Error is introduced when particles are lying along the
same optical path such that some particles are in the shadow of others. This occurs in highly
concentrated solutions. In practice, when large absorption values are measured, dilution is required
to achieve accurate results. Measurements of absorption in the range of to 1 are less affected by
shadowing than other sources of random error. In this range, the ODE model developed above is a
good approximation; measurements of absorption in this range are linearly related to concentration.
At higher absorbances, concentrations will be underestimated due to this shadow effect unless one
employs a more sophisticated model that describes the non-linear relationship between absorption
and concentration
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Questions for self-control:
1. give a classification of optical phenomena
2. what is the The Rayleigh’s equation based?
3. what is the Bouguer-Lambert - Beer’s based?
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References
1. Е.Д. Щукин, А.В. Пернов, Е.А. Амелина. Коллоидная химия, 3-е изд., М.: Высш.школа,
2004г., 435с.
2. Д.А. Фридрихсберг. Курс коллоидной химии, Л.: Химия, 1984, 352 с.
3. Ю.Г. Фролов. Курс коллоидной химии: Поверхностные явления и дисперсные системы.
М.: Химия, 1988, 462 с.
Lecture # 7-9. Electrical properties of dispersions
Purpose: To familiarize with the main types of electrokinetic phenomens
Key questions:
1. Introduction to electrokinetic phenomena.
2. Structure of the electric double layer (Theories of Helmholz - Perrin, Gouy-Chapman and
Stern).
3. Effects of electrolytes on zeta-potential, Electrophoresis and Electroosmosis, Measurement of
zeta-potential, Isoelectric point.
Summary
The term ‘‘electrokinetic phenomena’’ refers to several processes which appear when a charged
surface (or colloidal particle) is set in a relative motion with respect to the adjacent liquid phase.
Classically, four types of electrokinetic phenomena are distinguished: electroosmosis, streaming
potential, electrophoresis, and sedimentation potential
Electrophoresis – a suspended, charged particle moves as a result of an applied electrical field
• Sedimentation potential – an electrical potential created by the movement of charged particles
through a liquid by gravity
• Electrosmosis – a liquid flows along a charged surface when an electric field is applied parallel to
the surface
• Streaming potential – an electric potential created when a liquid is forced to move along a
charged surface
Electrophoresis - Movement of particle in a stationary fluid by an applied electric field.
Electroosmosis - Movement of liquid past a surface by an applied electric field
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Streaming potential - Creation of an electric field as a liquid moves past a stationary charged
surface
Sedimentation potential - Creation of an electric field when a charged particle moves relative to
stationary fluid
Electrophoresis the movement of a charged particle through a liquid under the influence of an
applied potential difference (electric current)
Voltage source
Electroosmosis. The movement of a liquid relative to an immobile charged surface of a capillary
tube under the influence of an electric field
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Sedimentation potential (“Dorn-effect) creation of an electric field when a charged particle moves
relative to stationary fluid
ΔE = sedimentation potential
Streaming potential creation of an electric field as a liquid moves along a stationary charged
surface
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”steady state” conditions:
ΔE = streaming potential
i = flow current
A solid surface in contact with a solution of an electrolyte usually carries an electric charge,
σo. This gives rise to an electric potential, ψo, at the surface, and a decreasing potential, ψ, as we
move through the bulk solution away from the surface, and in turn this effect the distribution of ions
in the liquid. Two regions are of primary importance: the Stern layer immediately adjacent to the
surface where ion size is important; and outside this region there is a diffuse layer. Because of
difference in charge between the diffuse layer and the solid suface,
movement of one relative to the other will cause charge separation and hence generate a potential
difference, or alternatively, application of an electrical potential will cause movement of one
relative to the other. The relative movement of the solid surface and the liquid occurs at a surface of
shear. The potential at the shear plane is known as the zeta (ζ) potential and its value can be
determined by measurement of electrokinetic phenomena. Zeta potential is almost identical with
the Stern potential thus gives a measure of the potential at the beginning of the diffuse layer.
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Electrical Double Layer Theory
1. Helmholtz model (1879)
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2. Guoy Chapman Model (1910-1913)
Stern Model (1924)
Takes into account specifically adsorbed ions, Stern layer.
Defined a slip plane between specifically adsorbed ions and diffuse layer of the
electrolyte solution. Potential at this plane is called
.
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Questions for self-control:
1.
2.
3.
4.
5.
6.
What types of electrokinetic phenomens do you know?
What is Zeta potential?
What causes the surface charge on oxides?
What is the significance of and 1/ ?
What does mean by "compression of electrical double layer" ?
How do the three models for the electrical double layer differ from each other?
References
1. Е.Д. Щукин, А.В. Пернов, Е.А. Амелина. Коллоидная химия, 3-е изд., М.: Высш.школа,
2004г., 435с.
2. Д.А. Фридрихсберг. Курс коллоидной химии, Л.: Химия, 1984, 352 с.
3. Ю.Г. Фролов. Курс коллоидной химии: Поверхностные явления и дисперсные системы.
М.: Химия, 1988, 462 с.
Lecture # 10- 13. Surface chemistry and phenomens
Goal: to learn the basics of Surface chemistry and phenomens
Key questions:
1. Structure of the surface and interface, surface tension, energy of the surface.
2. Surface active and inactive substances.
3. The types of adsorption, chemical and physical adsorption.
4. The kinetic of adsorption, influence of the properties of adsorbent and adsorptive on
adsorption, adsorption equation of Gibbs.
5. Nature of adsorption forces, Langmuir monomolecular adsorption theory.
6. The polymolecular theory of Polany and BET-theory.
Summary
Measurement of surface tension of solutions by Du Nouy tensiometer
Theory
The molecules at the surface of a liquid are subjected to an unbalanced force of molecular
attraction as the molecules of the liquid tend to pull those at the surface inward while the vapor does
not have as strong an attraction. This unbalance causes liquids to tend to maintain the smallest
surface possible. The magnitude of this force is called the surface tension.When this lowest possible
energetic state is achieved the surface tension acts to hold the surface together where the force is
parallel to the surface. The symbol for surface tension is "gamma". Conventionally the tension
between the liquid and the atmosphere is called surface tension while the tension between one
liquid and another is called interfacial tension.
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The specific surface free energy or surface tension of surface γ is equal to the expenditure of
work required to increase the net area of surface isothermally and reversibly way by unit of area, in
2
J m (Joule per meter); if the increase in the surface area is accomplished by moving a unit length of
line segment in a direction perpendicular to itself, γ is equal to the force, or “tension”, opposing the
-1
moving of the line segment. Accordingly, it is usually expressed in units of N m (Newton per
meter). Surface may be a free surface (exposed to air or vapor or vacuum) or an interface with
another liquid or solid. In the event that the surface is an interface, this quantity is called interface
tension. The value of surface tension is dependent on the nature of the liquid and also on the
temperature (see Eötvös and Ramsay empirical rule). The temperature of measurement of surface
tension must be kept constant. It is found that the surface tensions of solutions are in general
different from those of the corresponding pure solvents. It has also been found that solutes whose
addition results in a decrease in surface tension tend to concentrate slightly in the neighborhood of
the surface (positive surface concentration). The migration of solute either toward or away from the
surface is always such as to make the surface tension of the solution (and thus the free energy of the
system) lower than it would be if the concentration of solute were uniform throughout (surface
concentration equal to zero). Equilibrium is reached when the tendency for free-energy decrease
due to lowering surface tension is balanced by the opposing tendency for free-energy increase due
to increasing nonuniformity of solute concentration near the surface.
A surface-active molecule, also called a surface active agent or surfactant, possesses
approximately an equal ratio between the polar and nonpolar portions of the molecule. When such a
molecule is placed in an oil-water system, the polar group(s) are attracted to or oriented toward the
water, and the nonpolar group(s) are oriented toward the oil. This orientation of amphiphilic
molecules is described by Hardy-Harkins principle of continuity. The surfactant is adsorbed or
oriented in this manner, consequently lowering interfacial tension between the oil and water phase.
When a surfactant is placed in a water system it adsorbs at the surface and lowers the surface
tension between the water and air. When it is place in a mixture of solid and liquid it adsorbs on the
surface of the solid and lowers the interfacial tension between the solid and the water. Since the
surfactant is adsorbed at the surface it is logical that the concentration of surfactant at the surface
would be greater than the concentration in the bulk solution. Mathematically such a relationship has
been derived by Willard Gibbs. It relates lowering of surface tension to excess concentration of
surfactant at the surface.
The Gibbs equation can write for a dilute solution in two forms:
Г 
C d

RT dC
-3
where c is the concentration (in mol m ) in the solution, T (K) the absolute temperature, R the gas
-1
-1
-1
2
constant (8.314 JK mol ), γ (Nm ) the surface tension and Γc (mol m ) is the surface excess
concentration. It follows from Equation 1 that Γc is positive if dγ/dc is negative, that is the surface
tension decreases with increasing solute concentration. On the basis of experimental surface tension
vs. solute concentration function the dγ/dc can be determined and the Γc =f (c) adsorption isotherm
(Eq. 2) can be calculated.
Г  Г
bC
1  bC
(2a)
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The area (φm) occupied per molecule is determined as: φm=1/Γ∞ NA where NA is the Avogadro’s
23
-1
2
number (6×10 mol ). The φm for alcohols are about 0.22 nm and for carboxylic acids about 0.25
2
2
nm extrapolated from the liquid condensed regions and φm are about 0.20 nm extrapolated from
the solid regions of two-dimensional isotherm of monolayer, idependent of both the length of the
hydrocarbon chain and the nature of the head.
Adhesion and Cohesion
To understand the origin of surface tension at a molecular level one needs to look at adhesion and
cohesion.
Adhesion is the attraction between two different phases.
Cohesion is the attraction between molecules of the same phase.
We can define two quantities
Work of adhesion per unit area, WAB, which is the work required to pull two liquid phases apart and
measures the attraction between the two phases.
A
WAB
A
B
= final -initial
= A+B -AB
B
Work of cohesion per unit area, WAA, which is the work required to pull a liquid apart. It measures
the attraction between the molecules of the liquid
WAA
A
=2 A
A
A
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The difference between the work of adhesion and the work of cohesion of two substances defines a
quantity known as the spreading coefficient of A on B, SA/B:
SA/B
Physical:
physisorption
Chemical:
chemisorption












= WAB - WAA=
B
-
A
-
AB=
B-
(
A+
AB)
Physisorption
Van der Waals forces between molecules
multilayer adsorption
predominates at low temperatures
occurs rapidly
reversible
heat of adsorption less than 40 kJ mol-1
Chemisorption
Chemical bond formed that is stronger than Van der Waals
Heat of adsorption> physisorption ( > 80 kJ mol-1)
irreversible
activation energy involved
monolayer
chemical adsorption decreases at low temperatures.
Adsorption from solution
Theory
Adsorption is the enrichment (positive adsorption, or briefly, adsorption) or depletion
(negative adsorption) of one or more components in an interfacial layer. The material in the
adsorbed state is called the adsorbate, while that present in one or other (or both) of the bulk phases
and capable of being adsorbed may be distinguished as the adsorptive. When adsorption occurs (or
may occur) at the interface between a fluid phase and a solid, the solid is usually called the
adsorbent. Sorption is also used as a general term to cover both adsorption and absorption.
Adsorption from liquid mixtures is said to have occurred only when there is a difference between
the relative composition of the liquid in the interfacial layer and that in the adjoining bulk phase(s)
and observable phenomena result from this difference. For liquids, accumulation (positive
adsorption) of one or several components is generally accompanied by depletion of the other(s) in
the interfacial layer; such depletion, i.e. when the equilibrium concentration of a component in the
interfacial layer is smaller than the adjoining bulk liquid, is termed negative adsorption and should
not be designated as desorption. Equilibrium between a bulk fluid and an interfacial layer may be
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established with respect to neutral species or to ionic species. If the adsorption of one or several
ionic species is accompanied by the simultaneous desorption (displacement) of an equivalent
amount of one or more other ionic species this process is called ion exchange.
It is often useful to consider the adsorbent/fluid interface as comprising two regions. The
region of the liquid phase forming part of the adsorbent/liquid interface may be called the
adsorption space while the portion of the adsorbent included in the interface is called the surface
layer of the adsorbent. With respect to porous solids, the surface associated with pores
communicating with the outside space may be called the internal surface. Because the accessibility
of pores may depend on the size of the fluid molecules, the extent of the internal surface may
depend on the size of the molecules comprising the fluid, and may be different for the various
components of a fluid mixture (molecular sieve effect). In monolayer adsorption all the adsorbed
molecules are in contact with the surface layer of the adsorbent. In multilayer adsorption the
adsorption space accommodates more than one layer of molecules and not all adsorbed molecules
are in contact with the surface layer of the adsorbent.
The surface coverage (θ) for both monolayer and multilayer adsorption is defined as the ratio of a
the amount of adsorbed substance to am the monolayer capacity (the area occupied by a molecule in
a complete monolayer); a/am=θ. Micropore filling is the process in which molecules are adsorbed
in the adsorption space within micropores. The micropore volume is conventionally measured by
the volume of the adsorbed material, which completely fills the micropores, expressed in terms of
bulk liquid at atmospheric pressure and at the temperature of measurement. Capillary condensation
is said to occur when, in porous solids, multilayer adsorption from a vapour proceeds to the point at
which pore spaces are filled with liquid separated from the gas phase by menisci. The concept of
capillary condensation loses its sense when the dimensions of the pores are so small that the term
meniscus ceases to have a physical significance. Capillary condensation is often accompanied by
hysteresis.
Adsorption from solution is important in many practical situations, such as those in which
modification of the solid surface is of primary concern (e.g. the use of hydrophilic or lipophilic
materials to realize stable dispersions in aqueous or organic medium, respectively) and those which
involve the removal of unwanted material from the solution (e.g. the clarification of sugar solutions
with activated charcoal). Adsorption processes are very important in chromatography, too.
The theoretical treatment of adsorption from solution is general, complicated since this
adsorption always involves competition between solute(s) and solvent. The degree of adsorption at
a given temperature and concentration of solution depends on the nature of adsorbent, adsorbate and
solvent. Adsorption from solution behaviour can often be predicted qualitatively in terms of the
polar/ nonpolar nature of the solid and of the solution components. A polar adsorbent will tend to
adsorb polar adsorbates strongly and non-polar adsorbates weakly, and vice versa. In addition, polar
solutes will tend to be adsorbed strongly from non-polar solvents (low solubility) and weakly from
polar solvents (high solubility), and vice versa.
Experimentally, the investigation of adsorption from solution is comparatively simple. A known
mass of adsorbent solid is shaken with a known volume of solution at a given temperature until
there is no further change in the concentration of supernatant solution. This concentration can be
determined by a variety of methods involving colorimetry, spectrophotometry, refractometry,
surface tension, also chemical and radio-chemical methods where is appropriate. The apparent
amount of solute adsorbed per mass unit of adsorbent can be determined from the change of solute
concentration in the case of diluted solution. The specific adsorbed amount can be calculated as
follows:
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V С  С о 
(1)
m
where or a is the specific adsorbed amount, mg/g , at constant volume , V is the volume of solution,
а
3
dm , m is the mass of adsorbent, g in volume V, c0 and c are the initial and equilibrium
3
concentrations , mg/dm of dissolved substance, respectively. Subscript 1 denotes the dissolved
substance (the adsorbate). In many practical cases it is found that the adsorption obeys an equation
known as the Langmuir isotherm. This equation was derived for adsorption of gases on solids and
assumes that:
1. the adsorption is limited to monolayer
2. and occurs on a uniform surface; i.e. all “sites” for adsorption are equivalent,
3. adsorbed molecules are localized,
4. no interaction between molecules in a given layer; independent stacks of molecules built up on
the surface sites.
The Langmuir equation may be written as follows:
a C
(2)
а m
1/ b  C
s
where a m is the monolayer capacity or the amount adsorbed at saturation, c is the equilibrium
concentration of solute and b is a constant. The monolayer capacity can be estimated either directly
from the actual isotherm or indirectly by applying the linear form of the Langmuir equation which
is given by:
C C
1


(3)
a am b am
A plot of c/a against c must be a linear line with a slope of 1/am The surface area of the solid
(usually expressed as square meters per gram) may be obtained from the derived value of am
provided that the area occupied by the adsorbed molecule on the surface is known with reasonable
certainty.
Asurface=amNA y (4)
where NA is Avogadro’s number and y is the cross-sectional are of an adsorbed molecule. Recall
that the surface area determined by this method is the total area accessible to the solute molecules.
If these are large (e.g. dyestuffs, long chain molecules) they may not penetrate the pores and cracks,
and the area obtained may be only a fraction of the true surface area of the solid.
Adsorption Isotherms
Type 1
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


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Rapid rise, limiting value
Monolayer
Langmuir type
Example of
1. Chemisorption isotherm
2. Physisorption onto a solid with very fine pore structure eg nitrogen on microporous
carbon at 77K
Type 2



increasing positive slope
multilayer
physisorption
Langmuir Isotherm
Assumptions

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all adsorption sites equivalent
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



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ability of adsorbate to bind is independent of whether the adjacent sites are occupied or not.
adsorbate behaves as an ideal gas in gas phase
only monomolecular adsorption takes place
adsorbed molecules occupy fixed sites
heat of adsorption is independent of surface coverage.
BET Adsorption Isotherms
(Brunauer, Emmett, Teller)
Assumptions:
1. Multiple layers form and langmuir model applies to each layer.
2. Heat of adsorption, Hads for first layer has a value determined by properties of surface and
adsorbate, but for second and all subsequent layers, it is equal to heat of vapourization H vap.
3. Evaporation (or desorption) only occurs from exposed surfaces.
4. Rate of evaporation is equal to rate of condensation on preceding layer.
BET Equation
v
cx

Vm 1  x  1  c  1 x 
x=p/po
p is gas pressure
po is vapour pressure
c = constant related to heat of adsorption and desorption
Gas-solid interfaces, adsorption
Two types of adsorption- physisorption, chemisorption
Adsorption Isotherms Langmuir type for monolayer adsorption
BET type for multilayer adsorption

Langmuir Equation
α=bpa/ ( 1 + bpa)
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More direct form for experimental data is
pa/v = pa/Vm + 1/bV
BET Equation
v
cx

Vm 1  x 1  c  1x 
For plotting purposes the equation arranged as
c  1x
x
1


, where x  p / p o
V 1  x  cVm
cVm

Gas-solid adsorption important in heterogenous catalysis
To measure wetting tendency - measure contact angle
If contact angle less than 90°C then liquid wets surface
 If contact angle greater than 90°C then liquid doesn't wet surface.
Wetting - displacement from a surface of one fluid by another. Water wets a surface by displacing air
Young's Equation
Contact angle related to
of various components.
Consider a liquid making an equilibrium contact angle, , to spread an infinitesimal amount so as
to cover an extra area, dA, of a solid surface. The increase in the free energy of the system is given
by
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dG =
SLdA
+
LG
dAcos
-
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SGdA
If the system is at equilibrium dG = 0 and
SL
SG
+
LG
cos
-
SG
= 0 (Young's Equation)
surface tension of solid in equilibrium with vapour of wetting liquid.
Many surfaces are modified in order to change there wetting properties
Examples







Fabrics treated to become water repellent
Soils treated to become wetting
Mineral ores and ink particles treated to make them hydrophobic so they can be separated by
flotation
Detergents made to wet a surface to remove oil or dirt
Dips for sheep and cattle
Insecticide and horticultural sprays
Dry cleaning
Figure 4.2 Detachment of oily dirt from a solid surface. The sequences (left to right) show: (a) the
substrate/dirt system in contact with pure water, (b) the lowering of contact angle caused by
detergent [ (1) < 90°C, (2) > 90°C], and (c) and (d) mechanical (hydraulic) detachment of oil
droplets.
Contact angle of a liquid on a solid- measure of wetting tendency

Young’s Equation - relationship between contact angle and surface tensions.
SG = 0
LS
+
LG
cos
-
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Adsorption of liquids onto solids similar to gases onto solids. More complicated. Adsorption
from solution involves competition between solute and solvent for adsorption sites.
Physisorption. Generally follows Langmuir Isotherm. Depends on properties of solid, solute,
concentration of solute, temperature.
Polymers adsorb as trains, loops and tails. Conformation of polymer on surface changes with
time. Gradual displacement of lower molecular weight fractions by higher molecular weight
fractions.Adsorption dependent on pH, ionic strength and solvent.
Polymers used for flocculating solid particles. Too much polymer adsorbed. Steric stabilisation
occurs.
Questions for self-control:
1.
2.
3.
4.
5.
What types of Surface phenomens do you know?
What is surface tension?
What is the significance of Adhesion and Cohesion?
What does mean by chemical and physical adsorption?
How do the Langmuir monomolecular adsorption theory differ from polymolecular
theory of Polany and BET-theory?
Lecture # 14, 15. Stability and Coagulation of dispersions
Goal: to learn the basics of Stability and Coagulation of dispersions
Key questions:
1. Stability ratio and overall flocculation rate.
2. Kinetic of coagulation, interparticle energy potential, solvation, structural-mechanical and
entropy effects, coagulation through electrolytes.
3. Adsorption phenomena and coagulation, Micelle.
The stability of a colloidal disperse system is strongly dependent on the attractive pair
potential, VA, between the dispersed particles. VA is determined by the geometric arrangement, G,
of the particles (e.g. lamella-lamella; sphere-sphere; sphere-lamella; etc. interactions, independently
of the chemical composition), and the Hamaker constant, A, of the overall system (which depends
on the chemical composition of the constituting species, but is independent of the geometrc
arrangement). Formally:
VA = A × G
The Hamaker constant A of the overall system originates from a combination of the individual
Hamaker constants, Ai, of the dispersion medium and that of the dispersed particles. It can be
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derived from the summation of the der Waals dispersion forces between the constituting species
(dispersion medium; dispersed particles).
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A micelle (pronounced /maɪˈsɛl/ or /maɪˈsiːl/, plural micelles, micella, or micellae) is an
aggregate of surfactant molecules dispersed in a liquid colloid. A typical micelle in aqueous
solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding
solvent, sequestering the hydrophobic single-tail regions in the micelle centre. This phase is caused
by the packing behavior of single-tailed lipids in a bilayer. The difficulty filling all the volume of
the interior of a bilayer, while accommodating the area per head group forced on the molecule by
the hydration of the lipid head group, leads to the formation of the micelle. This type of micelle is
known as a normal phase micelle (oil-in-water micelle). Inverse micelles have the head groups at
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the centre with the tails extending out (water-in-oil micelle). Micelles are approximately spherical
in shape. Other phases, including shapes such as ellipsoids, cylinders, and bilayers, are also
possible. The shape and size of a micelle is a function of the molecular geometry of its surfactant
molecules and solution conditions such as surfactant concentration, temperature, pH, and ionic
strength. The process of forming micelles is known as micellisation and forms part of the Phase
behaviour of many lipids according to their polymorphism.
Individual surfactant molecules that are in the system but are not part of a micelle are called
"monomers". Lipid micelles represent a molecular assembly, in which the individual components
are thermodynamically in equilibrium with monomers of the same species in the surrounding
medium. In water, the hydrophilic "heads" of surfactant molecules are always in contact with the
solvent, regardless of whether the surfactants exist as monomers or as part of a micelle. However,
the lipophilic "tails" of surfactant molecules have less contact with water when they are part of a
micelle—this being the basis for the energetic drive for micelle formation. In a micelle, the
hydrophobic tails of several surfactant molecules assemble into an oil-like core the most stable form
of which has no contact with water. By contrast, surfactant monomers are surrounded by water
molecules that create a "cage" of molecules connected by hydrogen bonds. This water cage is
similar to a clathrate and has an ice-like crystal structure and can be characterized according to the
hydrophobic effect. The extent of lipid solubility is determined by the unfavorable entropy
contribution due to the ordering of the water structure according to the hydrophobic effect.
Micelles composed of ionic surfactants have an electrostatic attraction to the ions that
surround them in solution, the latter known as counterions. Although the closest counterions
partially mask a charged micelle (by up to 90%), the effects of micelle charge affect the structure of
the surrounding solvent at appreciable distances from the micelle. Ionic micelles influence many
properties of the mixture, including its electrical conductivity. Adding salts to a colloid containing
micelles can decrease the strength of electrostatic interactions and lead to the formation of larger
ionic micelles. This is more accurately seen from the point of view of an effective charge in
hydration of the system.
When surfactants are present above the CMC (Critical micelle concentration), they can act
as emulsifiers that will allow a compound that is normally insoluble (in the solvent being used) to
dissolve. This occurs because the insoluble species can be incorporated into the micelle core, which
is itself solubilized in the bulk solvent by virtue of the head groups' favorable interactions with
solvent species. The most common example of this phenomenon is detergents, which clean poorly
soluble lipophilic material (such as oils and waxes) that cannot be removed by water alone.
Detergents also clean by lowering the surface tension of water, making it easier to remove material
from a surface. The emulsifying property of surfactants is also the basis for emulsion
polymerization.
Micelle formation is essential for the absorption of fat-soluble vitamins and complicated
lipids within the human body. Bile salts formed in the liver and secreted by the gall bladder allow
micelles of fatty acids to form. This allows the absorption of complicated lipids (e.g., lecithin) and
lipid soluble vitamins (A, D, E and K) within the micelle by the small intestine.
Micelles are used for targeted drug delivery.
2 LABORATORY WORKS
3 INDEPENDENT WORK OF STUDENTS
List of topics for independent work of students
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1. The first law of thermodynamics.
2. Thermochemistry.
3. The second law of thermodynamics.
4. Entropy and its change in various processes.
5. Thermodynamic functions of the state.
6. Helmholtz energy, Gibbs energy, Gibbs-Helmholtz equations, chemical potential.
7. Thermodynamicsofchemicalequilibrium.
8. Isotherm equation, isobars and isochores of chemical reactions.
9. Rule of Gibbs phase.
10. Clapeyron- Clausius equation.
11. State diagrams of one-component systems.
12. State diagrams of two-component systems.
13. State diagrams of three-component systems.
14.Conductivity of electrolyte solutions.
15. Thermodynamics of electrode processes.
16. Effect of temperature on the rate of chemical reactions.
17. Theory of active collisions.
18. Basic concepts of the theory of the activated complex.
19. Gibbs surface energy.
20. Surface tension.
21. Adsorption at the boundary of liquid-liquid, liquid-gas.
22. Gibbs adsorption isotherm equation.
23. Adsorption at the interface between gas-solid, liquid- solid.
24. Langmuir adsorption isotherm equation.
25. Osmosis. Sedimentation. Optical properties of disperse systems.
26. The structure and electrical charge of the colloidal particle.
27. Stability and coagulation of dispersed systems.
28. Types of resistance. Sustainability factors. Coagulation of sols by electrolytes.
29. Coagulation kinetics of disperse systems. Gelation.
30. Colloidal protection.
31. Coagulation theory.
32. Classes of disperse systems.
33. Sprays, suspensions, emulsions, their properties.
34. Lyophilic disperse systems formed micelle-forming surface - active substances.