Surface Chemistry Toolkit
Making sense of the role played by colloid science in cosmetics &
personal care
Distance Learning Course in Cosmetic Science
Society of Cosmetic Scientists
Dr Kevan Hatchman
Introduction
• The toolkit brings together elements of the Colloid Science &
Surfactant modules:
• What is surface chemistry?
• The colloidal state & the role of the interface
• Practical applications: ‘Looking for clues’
• Product instability, appearance (homogeneous), sensory
(‘feel’)….
• Providing sensible solutions - application of surface chemistry
• Surfactants (micelles & phase behaviour)
• Polymers (steric stabilization, rheology, interactions)
What is surface chemistry?
• Colloid & Interface Science
• Size is important (surface area 
interfacial area) – dispersed phases
• Reduce particle size, the total surface area
to volume ratio of the ‘system’ increases –
affects performance
• ‘The world of neglected dimensions’ Wolfgang Ostwald (1915)
• Welcome to the ‘twilight zone’…..
What is a colloid ?
• ‘Colloid’ : term introduced by Thomas Graham
(1861)
Dispersed phase
Continuous phase
•
•
It is comprised of one phase dispersed in another
May be comprised of several different types – liquid, gas or solid Multiple
combinations, e.g. w/o/w
What is a colloid ?
•
Describing colloidal systems is not easy, but it is possible to characterise them according to
the following behaviours:
•
Lyophilic or solvent ‘loving’, i.e. the dispersed phase appears to be miscible with the
continuous phase
• Polymer colloids (‘swell’ in the solvent)
• Spontaneously form and are stable
• With respect to thermodynamics and kinetics (time)
•
•
•
•
•
Lyophobic or solvent ‘hating’, i.e. dispersed phase is immiscible in the continuous
phase
Majority of personal care and cosmetic products fall into this category
Requires energy to make them
− Not stable: Thermodynamics and kinetics
− Composition will change with time
How can we differentiate a colloid from a dispersion?
It is purely down to the dimensions of the dispersed phase!
How Are Colloids Made?
•
It usually involves an energy change
• 2nd law of thermodynamics
•
Creation of ‘new’ interface
•
Achieved by….
•
Communition
– High Shear mixing
– Two immiscible liquids
– Dispersing particles in a liquid
– Breaking up large particles in a ball mill
•
A phase change
– Nucleation & growth
• Sols
• Polymer matrices
The colloidal state – properties of the
dispersed phase
Cube (abrasive)
Flat plate (clay)
Colloidal dimension (1 – 1000 nm)
Dealing with systems
comprised of phases
with dimensions of the
order of 10-9 m to 10-6 m
Sphere (oil droplet)
Cylinder (fibre)
Putting size into perspective!
© BASF
•
Particulate size of the dispersed phase is important  interfacial area
•
Affects appearance and performance of the product, e.g. opacity, rheology (phase volume)
•
Product trends towards ‘nanotechnology’ – properties of the interface become very relevant
•
Is Nanotechnology really new?
• Nature has being doing it for millions of years!
Characteristic features of colloids
• Surface-to-volume ratio is high
• Potentially, colloidal systems may have interfacial areas comparable in
size to a football pitch!
• 6 cm diameter jar containing 25 cm3 oil and 25 cm3 water respectively
• Form emulsion droplets with a
diameter of 0.0001 cm
• New interfacial area created
• 150,1681 cm2 (~150 m2)
• S/V ratio: ~ 60,000
• 50,000 times increase in interfacial area!
Surface area/volume ratio
Volume = 25 cm3
S/ V ratio: variation with particle size
60000
d
50000
S/V Ratio
40000
30000
Oil
20000
10000
Water
0
0.0001
0.001
0.01
0.1
1
Particle diameter (cm)
Area of oil/water interface:
Area = p (d2/4)
Add emulsifier and shake to form particles with a diameter of x cm:
Pvol = (4/3) p (x3/8)
Number of particles (N) = V/Pvol
S/V Ratio = S/V
Total surface area (S) = 4 p (d2/4) N
V = volume of the continuous phase
Properties of colloidal dispersions
Increase in
surface area
leads to better
absorption
properties,
e.g.
sunscreens
© BASF
Characteristic features of colloids
• The dispersed phase has an affect on the properties of the formulation,
e.g. rheology or the phase volume (emulsions)
Monodisperse system (uniform
droplets) : phase volume ~ 0.75
max
Polydisperse system (non-uniform
droplets): phase volume > 0.75
Characteristic features of colloids
• Size matters!
• Large oil droplets (macroemulsions)
forms occlusive layer on surface of the Oil droplets
substrate (e.g. skin) – delivery triggered
by rubbing
• Small oil droplets (microemulsions)
Stratum
penetrate surface of skin
corneum
• Improve deposition of silicones on hair, e.g. polydimethylsiloxane (PDMS)
• Increase molecular weight (viscosity) or use cationic emulsifiers
• Tailor particle size distribution
• Increase particle size to improve deposition
• Deposition is poor for very small particulate sizes (microemulsions)
though can be improved by presence of cationic polyelectrolytes and
anionic surfactants (coacervates)
The interface
n
What is an interface?
n It is the transition region separating two or more immiscible phases
The following interfaces are involved in cosmetic science:
Gas/liquid – foams, aerosols
liquid/liquid – emulsions
solid/liquid – pastes, slurries, suspension emulsion systems
Gas/solid – aerosols, foams
n
n
Properties of the interface affect the performance of the product:
Surface tension (gas/liquid or gas/solid)
Interfacial tension (liquid/liquid or solid/liquid), e.g. wetting
and spreading
n
Related to physical characteristics of the interface:
Composition (polarity – hydrophilic or hydrophobic)
Surface roughness (solids)
n
Governed by intermolecular interactions (Van der Waals forces)
The interface
Liquid (
)
Liquid (
)
A broad diffuse boundary
region separates the two
immiscible liquids
Liquid (
The composition of the boundary
region is not the same as the
liquid/liquid or gas/solid interface.
There is an abrupt transition from one
phase to another at the point
separating them
)
Solid (
)
Formulating cosmetic and personal care products




What happens when we put a formulation together?
Assess the properties/identify the colloidal system
What types of interfaces are we dealing with…
Interfacial area increases during preparation  particle size
distribution (dispersed phase)
The processes required to make it….
Do we need an input of energy?
What happens when it goes wrong….
Storage…
Performance…
What steps are needed take to rectify any problems….
Formulating cosmetic and personal care
products

Raw materials
Compatibility
Choice - are they really up to the job?

Quality - what you put in is what you get out!

Understand the problem
Stability – manner of phase separation
Performance (foaming, conditioning or cleansing)

Look for clues, colloid science can help to find the solution
Looking for clues….
• We know most personal care and cosmetic
formulations are ‘lyophobic’ colloids
• The dispersed and continuous phases are not
compatible with each other – i.e. immiscible
• Not stable - will separate very quickly into two
or more phases to reduce interfacial area
(thermodynamics)
• Overcome Van der Waals attractive forces
(‘balancing act’)
Colloid Stability
•
Colloidal systems are quite energetic
•
The particles in the continuous phase are always moving
•
We call it Brownian Motion….
© Nanosight
Colloid Stability
• Notice anything about the way the particles were moving?
• Particles are moving in a random manner
• Rate is determined by a number of factors
• Temperature
• The viscosity of the medium
• Collisions between particles will happen….
• They can bounce off each other….
• Or stick together….
• But that’s another story!
Feel the force….
• The stability of cosmetic and personal care
formulations (lyophobic colloids) are
influenced by the following intermolecular
interactions:
• Van der Waals attractive forces
• Leads to product instability
• Electrostatic and steric interactions
• Stabilise the dispersion
‘Do not underestimate the power of
the force….’
– Darth Vader
Van der Waals attractive forces
• Forces with the greatest effect are :
• London Dispersion Forces or Universal
Attractive Forces.
• Keesom or Orientation Forces (Dipole-Dipole
Interactions), e.g. hydrogen bonding
• Debye Forces (Dipole Induced Dipole Interactions).
• Magnitude of the interactions affect properties such as
surface/interfacial tension
Interfacial forces – surface tension
Limited interaction at surface
Surface (gas)
Net Force
Liquid
Molecules in bulk interact equally in all directions with each other
•
The properties of the surface/interface are dictated by the Van der Waals forces operating at
the surface and in the bulk material
•
The surface tension of a liquid is a product of the attractive interactions between the gas and
liquid molecules at the surface (weak) and within the interior (stronger)….
Looking for clues….
•
Lyophobic colloids require mechanical energy - mixing
• High shear mixer (Silverson)
•
Stability of systems governed by thermodynamics and
defined by kinetics (time – reference point)
•
Possible for ‘unstable’ formulations (thermodynamics)
to be ‘stable’ for several years
•
Performance of the product will be determined by the
properties of the dispersion, i.e. phase separation
•
Instability arises from random particle-particle
collisions (Brownian motion)
•
State where intermolecular forces are in ‘balance’ is
often called ‘metastable’
Thermodynamics – the fly in the ointment
•
Energy changes (DG) during preparation of the
dispersion is described by the 2nd law of
thermodynamics
DG = g A – TDS
g is the interfacial tension (emulsion), A is the ‘new’
interfacial area, T is temperature and DS is the
entropy contribution (mixing)
•
Driving force for instability is determined by the
magnitude of DG.
•
Reason why interfacial area plays an important role
Energy changes : emulsion stability
Free Energy (G)
Add emulsifiers to reduce interfacial tension and
create ‘energy’ barrier (steric and electrostatic
repulsions). Work needs to be done to overcome
interactions (DE)
DE
Preferred
pathway
Rate is determined by
the thinning and
rupturing of
the film separating the
two droplets
Two
Droplets
Film
Rupture
One
Droplet
Time (t)
Anionic emulsifiers - charge stabilisation
• Adding a nonionic surfactant allows closer packing at the
interface and contributes to stabilizing the interface
Nonionic surfactant
- Anionic surfactant (charge repulsions)
Mixed or paired emulsifiers (HLB)
• Use of mixed surfactants allows more surfactant to pack
effectively at the oil - water interface.
• This produces lower interfacial tensions and therefore a more
stable emulsion (steric stabilisation)
High HLB - more water soluble
Low HLB - more oil soluble
Routes to instability – kinetic mechanisms
• ‘Lyophobic’ colloidal systems are not stable
• 2nd law of thermodynamics
• We can, however, stabilise the dispersion by the creation of an energy
barrier
• Adsorption of surface active agents or polymers
• There are a number of pathways through which a colloidal system can
breakdown
• The preferred route however depends on the composition of the
dispersion
• Density and rheological changes due to temperature effects
• Compatibility of ingredients, i.e. solubility
• And the properties of the interface…..
Phase separation
•
Density changes alter the composition of the formulation
• Change in temperature
•
Densities of the continuous and disperse phases do not ‘match’
•
Appearance of the colloid will change over time….
Creaming
•
•
•
Sedimentation
(caking)
‘Stable’ colloidal dispersions can irreversibly separate
Large particles will move much faster than smaller ones
We can slow down the rate of separation by observing a few simple rules….
Interfacial Effects
•
Phase separation is also influenced by the molecular interactions at the interface
•
The inter-particle interactions are affected by the magnitude of the intermolecular forces
•
It is possible for appearance of the colloid to change with time
Flocculation
Coalescence
•
The particles can stick together to form floccs comprised of discrete particles
•
Or fuse together to form larger ones  coalescence
•
It is possible to retard the process but we need to know a little more about the interface….
Stokes’ law - predicting phase separation
For a spherical particle (dilute solution):
Rate = x = 2r2 (rm - rp) g
t 9hm
hm = viscosity of the continuous phase
rm = density of continuous phase
rp = density of dispersed phase
r = radius of spherical particle
t = time taken to move specified distance (x)
g = acceleration due to gravity
Relevance – suspending pearlescent agents or pigments in
cosmetic formulations
Stokes’ Law
Decrease
particle
size
Use
polymers
surfactants
waxes
clays
Match
densities
(Dr ~ 0)
Structure the continuous phase (increase viscosity) to slow movement of the particles
Stokes’ law - problem solving
• Phase separation prevented by determining the
mechanism
• Matching the density of the dispersed and
continuous phase – ensure Dr is small
• ‘Weighting’ the oil phase (changing the
density)
• Increasing the viscosity
• Surfactant system (phase behaviour)
• Polymers
• Inorganics (clays, silicas)
Ion adsorption (electrostatic repulsions)
Ionic surfactants adsorb at the interface and affect the
resultant surface charge
Oil
+ve
Cationic surfactant
Oil
-ve
Anionic surfactant
Electrostatic interactions – the electrical
double layer
Electric Potential (Y)
Surface potential
Stern layer
Cation
-ve
Zeta potential
(z)
Distance (x)
Zeta potential (z)
Boundary of double
layer in contact with
the solution (‘slipping
plane’)
Stern layer
Surface potential
Electrical double layer
described by Guoy
Chapman or Stern models
z – magnitude affected by
pH
DLVO theory – electrostatic stabilisation
Potential energy (VT)
+ve
VR
Repulsive electrostatic
(electrical double layer)
interactions
A
X
B
Resultant interaction
Energy
barrier
Particle Separation (X)
Vv
Van der Waals attractive
interactions
Primary minimum
-ve
VT = Vv + VR
Potential energy (VT)
Potential energy (VT)
+
a) No electrolyte
+
EB
EB
Distance
(x)
Primary minimum
-
+
-
Potential
energy (VT)
b) Electrolyte added
-
Distance
(x)
Secondary minimum
(weak flocculation)
Primary minimum
Energy barrier (EB) decreases as
the electrical double layer is
compressed and is eventually
Distance neutralised
(x)
c) High electrolyte concentration
Making use of electrostatic interactions
Negative charged surface
Clay particle
-- - -- - - - Positive
- + charged
‘House of cards’ structure
++
+ +++ +++ + + surface
----+
+ - -+
--- - ----+ - - - - - + +- - - - - +
Shearing force
- - - - - ++ - - - - + - - - -- - - - -+
+
- - - - - ++ - - - - - +
+ - - - -- - - -Dispersed
+ +
- - - -- +
phase trapped
+ - +
+
+
- within the
Particles slide over each other
- structure
(electrostatic repulsions) – low
- +
- viscosity
Ionic non-associative thickeners
Polyacrylic acid chain untangles as a result of ionised groups repelling each
other
Thickening effect is greatest for high molecular weight polymers and is
sensitive to changes in pH
raise pH
HO
O
HO
O
O
O
O
lower pH
h
-
-
O
-
-
-
-
-
pH
0
2
4
6
8
10
12
14
Steric stabilisation - oil in water (O/W)
emulsion
Polymer chains act as ‘barrier’
to coalescence.
Oil
Oil droplets stabilised by anchored polymer
chains.
Oil
Steric stabilisation – performance engineering
n
Molecular weight and chemical structure are important
‘Comb’ polymer
n
n
Dispersing agents
Anchor to substrate to provide stability
(hydrophobic or ionic interactions with surface)
Conformation is important (loops & tails)
Electrostatic/steric stabilisation
Select dispersant for the application, e.g.
molecular weight
Problems:
Poor adsorption (solvent quality), e.g. depletion
flocculation
Particle size is very small, bridging flocculation
may become an issue – assess particle size
distribution (photon correlation spectroscopy
(PCS)
Pigment
Reduce
particle size
Bridging flocculation
Steric stabilisation – conformation effects
Tail
Loop
Water phase
Oil phase
Train
Hydrophobic
group
Steric stabilisation – conformation effects
Polymer ‘mushroom’
Radius of gyration
Polymer ‘brush’
Polymer chains extend
into solvent owing to
interactions with
neighbouring molecules
at high concentrations
HO
Limited penetration of the polymer
chains occurs during collision
Adsorbed layers of polymer are fully
extended into the solvent
H1
Compression of the
polymer chains prevents
the particles from
coalescing and
flocculating
Solvent concentration gradient
between bulk phase and adsorbed
polymer layer. Polymer prefers
solvent and particles are forced to
part, allowing the chains to be
solvated
Steric stabilisation - solvent effects
‘The Good, The Bad And The Theta!’
‘Good’ solvent
‘Bad’ solvent
• ‘Good’ solvent
• Polymer chain segments extended in solvent producing an open
configuration (polymer is miscible).
• ‘Bad’ solvent
• Polymer chain collapses into a more compact form.
• Transition occurs at the theta (q) temperature
• Polymer separates from solution, e.g. cloud point of PEGs
Stabilisation method – pro’s and cons
Electrostatic
Steric
Need to add stabilising agent (polymer)
n Not reversible
n Sensitive to temperature changes (solvent
quality)
n Operates in aqueous and non-aqueous
systems
n
Easier to control
n Reversible
n Change ionic strength
n Predominantly aqueous
based
n
Dealing with liquid/solid interfaces
• Dispersing solids in a liquid phase
• Cleansing product
• Make-up
• Applying a product to the skin
• Sensory (‘feel’) and penetration
• The properties of the interface dictates how
the formulation will behave
• Wetting and spreading
Wetting and spreading – an historical
perspective
The Ancient Egyptians used oils to
make coloured cosmetics
n
They found it was easier to disperse
coloured pigments
n
Why? – surface tension of the oils
were comparable to the critical surface
tensions of the pigments. It was easier
to ‘wet’ the solid and therefore aided
their dispersion in the oil.
n
The oils also permitted the
formulation to spread easily on the
Egyptian cosmetic jar (2000 - 1000 BC)
skin.
n
Wetting and spreading – an historical perspective
The Romans also understood
spreading. They found oils were good for
cleaning the skin whilst bathing or as a
moisturiser.
n
Why? Surface tensions of oils were
similar to that of skin – easily spreads on
the surface
n
They also used oils and fats as
lubricants – formation of protective layers
on surfaces
n
Pliny wrote about fishermen pouring oil
onto the sea to form lenses to look for
fish
n
Wetting
• Why does a droplet of water refuse to form a film on a greasy
surface?
• What causes a material to absorb a fluid, whilst another repels it?
• We are dealing with the properties of the interface and…
• Balancing the ‘driving’ forces of cohesion and adhesion
• Cohesive forces are result of the Van der Waals interactions
between the molecules in the liquid
• Adhesive forces are the result of Van der Waals interactions
between the molecules residing at the interface, i.e. fluid and
substrate
• Wetting is purely: Adhesion >> Cohesion
Wetting
• Wetting is the displacement from a surface of one fluid by
another
• Involves three phases - at least two must be fluids (liquid or
gas) or a solid
• Wetting must take place before:
• Spreading, dispersing and emulsification, e.g.
detergency (cleansing)
Spreading
What happens when an oil drop is placed on a clean
liquid surface?
Remains as a drop (lens on the surface)
gGL
gOG
Gas
Oil
gOL
Liquid
Or spreads as a thin (duplex) film
Gas
Liquid
Oil layer
Spreading
•
What happens when a liquid droplet (oil) is placed on a surface?
S is -ve
S is + ve
O
q
•
It can reside as a droplet or….
•
Form a thin layer (spreading)
•
The contact angle (q) of the fluid in contact with the surface will change over time
•
We can predict whether the droplet will spread on the surface by considering the
Initial Spreading Coefficient (S)  interfacial tension (g)
S = gGS - (gOG + gOS )
The surface tension of the fluid (gOG) <<< critical surface tension (CFT (gGS)) for the
liquid to spread along the interface (liquid or solid)
What happens when a liquid is in contact
with a solid surface ?
Complete wetting
Incomplete wetting
q
Formation of contact angle (q)
Contact angle
Contact angle (q) decreases as droplet spreads
Substrate can affect contact angle (chemical nature or surface
roughness)
q
Contact angle results from a balance of interfacial surface
tensions acting at the point of contact (Young’s equation)
Wetting – the Young Equation
Spreading and wetting can be explained by the Young
equation (1800’s).
gOL
Liquid (or air)
gSL
At equilibrium:
Oil
q
Substrate
gOS + gOLCOS q - gSL = 0
gOS
q = contact angle
g = surface tension
Relevance of contact angle – Pickering emulsions
Best effect obtained for
hydrophobic particles that form
a contact angle around 90o
(partially embedded) - will
flocculate in either phase
140o
Electrostatic repulsions
help to stabilise the
emulsion
Oil
90o
Oil
30o
Particle size
smaller than oil
droplet
Particle completely wetted by oil
phase (q ~ 0o)
Emulsions
• Classified into two types:
• Oil in water (O/W) and water in oil (W/O)
10-9 – 10-3 m
O/W
W/O
The type formed is determined by the relative proportions of
the components
n Particle size – macroemulsions, nanoemulsions and
microemulsions
n
Emulsions
• An input of energy (work) is required to form the emulsion
Work = g x DA
A = interfacial area
g = interfacial tension
• The lower the interfacial tension, less work is required to form
an emulsion with a specific droplet size/interfacial area
Emulsion Stability
• Emulsifier (surfactant) lowers interfacial tension
• Surfactant adsorbed around droplet and acts as a physical barrier
(can form liquid crystalline phase around oil droplet)
• Electrostatic repulsion (ionic surfactants)
• Steric repulsions (nonionic surfactants)
• Polymers stabilise emulsions by steric interactions
• Surfactant selection is important
• Mixed surfactant systems are beneficial (packing of the surfactant
molecules at the interface)
• Use HLB numbers (Griffin) to select emulsifier
Hitting the target: HLB system (1940’s)
• HLB = Hydrophilic Lipophilic Balance
• Aids selection of nonionic emulsifiers
(surfactants) by characterising their
solubility in oil and water
• Assign number, defines water-liking and
oil-liking properties of a surfactant
• Arbitrary scale 0-20
• 0 - totally oil soluble
• 20 - totally water soluble
HLB system (1940’s)
•
Many oils are assigned required HLB values
•
This allow you to select appropriate emulsifiers for it
•
Paired or mixed emulsifiers desirable
•
Low and high HLB values
•
Closer matching to actual HLB
•
Gives more stable emulsions (packing at the interface)
•
The HLB values assigned to surfactants are related to their structure
•
Determine by calculation or experiment
The HLB of a nonionic surfactant gives an
indication of its role
HLB value
1-5
5-8
8-12
12-15
15-20
Surfactant function
Water in oil emulsifier
Water in oil emulsifier
Oil in water stabiliser
Wetting agent
Oil in water emulsifier
Wetting agent
Oil in water emulsifier
Detergent and solubiliser
Oil in water emulsifier
Detergent and solubiliser
Hydrophile-Lipophile Balance (HLB)
Nonionic Surfactants
HLB
1
4
6
8
10
13
15
18
Emulsifier
O/W
Functions for cleaning formulations
•
Mixture of ‘low’ & ‘high’ HLB surfactants
•
•
•
Emulsion stability (HLB range) can be affected by:
Temperature
Alcohol ethoxylate solubility in water decreases with increasing temperature
− The cloud point
− Electrolytes
− ‘Salting out’ electrolytes, e.g. NaCl, can affect the solubility of surfactants in
water
Emulsifier selection - summary
• Points to consider
• Emulsion type O/W or W/O
• Selection based upon HLB
• Preferential solubility of the emulsifer in the oil or
aqueous phase dictates which type of emulsion
will be formed (Bancroft’s rule)
• Use of paired emulsifiers
• pH range
• Temperature range (nonionic surfactants)
• Compatibility with salts & actives
• Surfactant level, aim for 10% of oil concentration
(macroemulsions)
Emulsion instability
• Emulsions can be stabilized by:
• Using the correct combination of surfactants
(steric stabilisation), e.g. HLB system
• Creating charge repulsions between oil droplets
(ionic surfactants)
• Thickening the continuous phase - e.g. polymer
• Thickening (strengthening the interface) with
waxes - e.g. liquid crystals formed with long
chain alcohols
Dispersion
• Surfactant (dispersant) wets the surface of the solid and
displaces any adsorbed fluids, e.g. gas.
• Solid disperses more readily in liquid.
Solid not wetted by
surfactant
Pigment dispersions
Input of energy – high shear, grinding, milling
Breakdown of
agglomerates
Aggregates of
primary particles
Initial wetting of
agglomerates by
dispersant
Primary pigment
particles
Increase in interfacial area
Detergency
• Detergency is the removal of a soil (matter) by mechanical and chemical
action (pH) under favourable conditions (temperature) in the presence of a
surfactant
• Combination of the following functions
•
•
•
•
Wetting
Dispersing
Solubilisation
Emulsification
• Oily droplet on substrate….
No surfactant
With surfactant
Detergency
Sebum
Hair
Wetting
Emulsification
Dispersion
Foams – gas/liquid interface
• Personal cleansing products formulated to give long
lasting creamy foam
• Consumers will buy products that will produce copious
amounts of foam
• Foams deliver actives to the skin or hair and help to
remove oils and dirt
• Generated with the aid of surfactants
• Formulations that produce the most foam with the
minimum quantity of surfactant are desirable
What is foam ?
n
Dispersion of a gas in a liquid
Trap gas by mechanical action
(agitation)
Can be a problem (industrial
processes)
n
n
Not stable (lyophobic colloid)….
n
Foam is a collection of bubbles
Stabilise using surface active agents –
surfactants, polymers, particulates
n
Life cycle of foams
Time
Gas bubbles
trapped in liquid
Liquid drains from the films
surrounding the gas
bubbles (honeycomb
structure)
Polyhedral structure
is eventually formed
Foam instability
n
Gravitational force - drainage
n
Capillary pressure (squeeze liquid from film
separating bubbles) – liquid flows to regions of low
pressure, i.e. separating cells (Plateau regions)
n
Diffusion of gas across foam lamellae (bubble
disproportionation)
n
Leads to bursting of bubbles and rearrangement of
foam lamellae
Foam persistance
• Prevent drainage and diffusion of gas across
foam lamellae (increase viscosity or retard fluid
drainage by presence of liquid crystals)
• Polyelectrolytes bind to surfactant at interface –
impart mechanical rigidity
• Close packing of surfactants at the interface
• Maintain low interfacial tension
• Ionic surfactants (electrostatics) – can be
screened by electrolytes and affect stability
• Annealing of foam lamellae by surfactant (GibbsMarangoni effect)
• Maintain equililibrium interfacial tension – foams
can be deformed, i.e. stretchy
Film elasticity (e) - Gibbs Marangoni effect
(rubber band)
-
-
-
-
-
-
-
-
-
g1
-
g2
-
-
Gravity thins lamellae
-
f
-
-
• A =Area
• g = Surface
tension
f
g1
-
-
dA
g1
-
-
-
dg
-
-
ε = 2A
-
g1
Gibbs-Marangoni effect (combination of two separate processes) restores equilibrium
(fills holes in the film) - lowers surface tension
Concentration dependent (migration of surfactant to the interface from bulk solution)
Polymer-surfactant interactions – foam stabilisation
Polymer
binds to the
surfactant to
make the
film more
rigid
Liquid
Gas
Gas
Cationic
polymer
Anionic
surfactant
Polymer forms ‘bridge’
between neighbouring films
Foam performance
• Foam performance of cleansing formulations containing
surfactants depends on:
• Surfactant ratio (primary: secondary) and concentration
• Presence of additives, e.g. oils, polymers
• We can assess the foaming ability using a combination of
different techniques which includes:
• Rotary foam measurements (Beh-James) – screen
several formulations at a time
• Beating/pouring (Hart De George)
• Static methods, e.g. Ross-Miles
• Dynamic foam test
• Instrumental (e.g. Foam Scan)
When foam is a problem !!!!
• Presence of foam may not be desirable
• Severe agitation
• Need to use some kind of control
Foam inhibition
• Why is this beer is flat?
• Oil slicks ! (grease….)
• Provides some form of foam control
• Care needed when formulating products
with oils
• Too much can prevent foam from
forming, e.g. oils with ‘low’ surface
tensions can spread along the interface
(antifoam)
Antifoams
• Compounds that inhibit foam formation are called
antifoams or defoamers
• Antifoam compounds include:
• Silicones (e.g. polydimethylsiloxane) laundry/industrial processes
• Branched alcohols (affects packing of the surfactant
molecules at the interface)
• Oils, fats and waxes - may form solid particles
(contact angle)
Foam prevention - antifoams
Air
Liquid
Air
Oil
Oil
Oil spreads on the film and
displaces surfactants
gO/L << gSurface
Film thins and ruptures –
result of change in interfacial
tension between film and oil
Foam collapses
What is a surfactant?
• It is a “surface active agent”
• A chemical compound that combines oil soluble and water
soluble properties
• Surfactants are “active” at a surface or interface
Oil soluble portion
Lipophillic
Water soluble portion
Hydrophillic
Surfactants - Four Types
Anionic
-
- ve charge
Nonionic
No charge
Cationic
+ve charge
+
Amphoteric
+/-
Acidic
-
+
Alkaline
Natural vs synthetic routes
• Feedstocks for the hydrophobe (alkyl chain) obtained from
two main sources.
• ‘Natural’ or renewable sources – animals or plants
• ‘Synthetic’ or non-renewable sources – oil and coal
• Both require processing to obtain either the fatty acids or
triglycerides and olefins.
• It is energy intensive.
• Synthetic routes need more processing steps, e.g. cracking
Natural vs synthetic routes
• Fatty alcohols are one of the most important feedstocks
• Natural – oils and fats are purified before conversion to fatty
acids or methyl esters. The products are then
distilled/fractionated to give the desired cut. Fatty alcohols
are obtained by hydrogenation of fatty acids with a catalyst.
• Synthetic – olefins are converted to the fatty alcohol by
• ‘OXO’ process
• Ziegler process
• The fatty alcohols prepared by the different routes have
different properties
Purification carried out
at high temperatures &
high pressure
Transesterification
Oils & Fats
Purification
Hydrolysis
Esterification
Fatty acid
Methyl ester
Glycerol
Distillation
Fractionation
Hydrogenation
Fatty alcohols
Glycerol
Crude oil & natural gas
Ethylene
n-paraffins
a-Olefins
i-Olefins
Ziegler process
OXO process
n-Alkanols
Oxo-alcohols
Natural vs synthetic routes
• The hydrocarbon feedstocks are then processed further
with hazardous chemicals to produce the surfactants
• Hydrophiles used to make the surfactants fall into two
groups
• Inorganic – H2SO4/SO2, SO3 and P2O5
• Organic – Ethylene oxide/propylene oxide, polyols and
alkanolamines
• Performance of the surfactant is influenced by the relative
‘strengths’ of the hydrophilic and hydrophobic groups
Integrated oleochemical routes
Oils and fats
Fatty acid
methyl ester
Alkanolamides
Ethoxylated
alkanolamides
Alkyl amido betaines
Fatty acids
Glycol + glyceryl
esters
Amphoacetates
Hydroxysultaines
Fatty acid
isethionates
N-acyl derivatives
Alkyl
polyglycoside
Fatty alcohol
Alkyl ether + ester
carboxylates
Sulfo-succinate
Alcohol
ethoxylates
Phosphate
esters
Alkyl
sulphates
Alkyl
dimethylamines
Alkyl ether
sulphates
Alkylamine oxides,
betaines + quats
Micelles – association colloids
• Breaking up and reforming
• Comprised of 100s of molecules
• Surfactant molecule structure - affects micelle shape (sphere, rod….)
• Micelle shape and size can effect the rheology and behaviour of
detergent systems
Sphere
Rod
Disc
Critical micelle concentration (CMC)
Surface
tension (g)
G= Gibbs surface excess
G= - 1 dg
RT dln C
Gradient
Area of adsorbed surfactant molecule = 1
(NA G)
NA is Avogadro’s number
CMC - critical micelle concentration
Surfactant concentration (ln C)
Critical micelle concentration (CMC)
• CH3(CH2)nX
• CMC decreases as n increases (Traube’s rule)
• CMC decreases from being ionic to nonionic
• CMC at minimum where X is at the end of the molecule
• When X=(EO)m , CMC decreases as m decreases
• CMC decreases as the surfactant becomes less soluble
(Krafft point)
Importance of CMC
Concentration
Low
High
Nonionic surfactant
Ionic surfactant
Poor solubility in water
Soluble in water
(Krafft point)
Mild
Irritant
CMC can be reduced by additives, e.g. polymers, amphoteric surfactants
Polymer-surfactant interactions – hydrophobic
polymer
Micelles form a ‘string of
pearls’ arrangement along
the chain
Micelle
Surfactant
molecules bind to
polymer chain
Micelles force polymer chain
to open and expand
(repulsions between
aggregates)
Polymer-surfactant interactions - conditioning
Precipitation region
Micelles form
along the chain
Polymer (polyquaternium)
and anionic surfactant
(negative) below critical
micelle concentration (cmc)
Deposition of coacervate
(complex) from solution
Coacervate structure expands
as micelles form and the
complex is solubilised
Increasing surfactant concentration
Speed is everything - interfacial properties
•
Surfactants readily adsorb at interfaces
•
Rate determined by:
• Diffusion of molecules from bulk solution to surface
• Size of the molecule
• Orientation of the molecule into preferred packing arrangement at
the interface
•
Crucial for:
• Wetting, emulsification and dispersing (detergency)
• Foaming, liquid aerosols (sprays)
The Krafft Point
• The Krafft phenomena is the temperature
dependent solubility of ionic surfactants
• Below the Krafft point the surfactant exists as
hydrated crystals - turbid appearance at low
temperature
• Krafft point increases with increasing chain length
• Addition of salting out electrolytes increases the
Krafft point
The Krafft Point
• Krafft point is lowered by branched chains
• Unsaturation (double bonds)
• Insertion of EO groups between alkyl chain and the head
group - alkyl ether sulphates have lower Krafft points than
alkyl sulphates
• Hydrotropes - enhance solubility of surfactants in water,
e.g aryl sulphonates, short chain (C8/10 phosphate ester,
APG...), amphoteric surfactants
Micelle shape (critical packing parameters)
‘Driving force’ for different micelle structures – head
& tail interactions
P=v
lc a
a
lc
P = critical packing parameter
a = cross sectional area of the head
group
v = volume of hydrocarbon tail
lc = all ‘trans’ length of tail
Micelle shape (critical packing parameters)
Surfactants molecules have different geometries – affects
packing at interfaces
P>1
P~1
1/3 < P < 1/2
Micelle Shape (critical packing parameters)
Critical packing
factor (P)
Packing shape
Structure
P < 1/3
Cone
Spherical micelles
1/3 < P < ½
Truncated cone
Rod micelles
½<P<1
Truncated cone
Vesicles
P~1
Cylinder
Bilayer micelles
P>1
Inverted truncated cone
Inverse micelles
Importance of surfactant molecular structure
Head group size: hydrophilic character
Hydrophobe group: lipophilic character
Micelle shape (critical packing parameters)
Weak head group repulsions
- ‘Salting out’ electrolytes for ionic
surfactants
Small head group, large bulky tail (branching,
unsaturation (kinky), di-alkyl derivatives)
Low curvature structure (disc shaped micelles),
P ~ 1 – favoured for microemulsions
Bulky tails favour ‘reverse’ structures – w/o emulsions or liquid crystalline
phases (bicontinuous cubic (V2 & I2) and hexagonal (H2))
Low or planar curvature – ideal for multi-lamellar vesicles
Micelle shape (critical packing parameters)
Strong head group repulsions (electrostatic or
steric)
Large head group, small tail
Micelle has highly curved structure (spheres
and rods)
‘Gaps’ at interface – o/w emulsions
Strengthen film (low interfacial surface tension) with mixture of different
molecular structures (mixed HLB’s)
Surfactant phase behaviour – rheology
Viscosity
Rod micelles
Viscosity build relies on
entanglement of
rod/cylindrical micelles
‘Salting out’ electrolyte (%)
Surfactant phase behaviour – lyotropic liquid
crystals
Surfactants form micelles in
aqueous/polar media
Cubic phase (I1)
n
Increase concentration micelles form organised structures called liquid crystals
n
Three main types – cubic (I and V), hexagonal and lamellar
n
Exhibit birefringence and have defined crystal lattice spacings (x-ray)
n
Phase diagrams are used to ‘map’ the regions where these structures are found
Surfactant lyotropic liquid crystalline phases
Small Angle X-ray Scattering (SAXS)
Hexagonal phase
(H1)
Lamellar phase
(La)
Importance of lyotropic liquid crystals emulsion stability
Liquid crystal provides a ‘barrier’ to
coalescence
Oil droplet
Droplets appear as ‘maltese’ crosses Oil droplet is coated by layers of
lamellar phase (multi-lamellar vesicle
when viewed with a polarized light
structure)
microscope
Summary
• Use principles of colloid and surface chemistry to solve the
problem
• Identify causes and their effect on the formulation –
evaluate/performance indicators
• Problems can be caused by more than one process
• Need to bear in mind….
‘Nae cannae change the laws
of physics’
Montgomery Scott
Thermodynamics rules ok!
Solutions…
•
More than one solution….
•
Increase the viscosity of the continuous phase
• Polymers, surfactants….
•
Adapt the formulation e.g. Krafft point, tolerant to water
hardness…
•
Reduce level of oils (emollients) if they are suspected of
acting as a defoamer or remove them completely
•
Replace immiscible components, e.g. compatibility issues
•
Evaluate performance (rheology, tests…)
•
Carry out storage tests…
Summary
• Use the INCI listings on back of products as a
guide
• Review patents
• Raw materials - careful selection  what you put
in is what you get out!
• Contact raw material manufacturers!
Further reading
• ‘Basic Principles of Colloid Science’, D H Everett, RSC (1987)
• ‘Introduction to Colloid and Surface Chemistry’, D J Shaw, Butterworth
Heinemann, 4th ed (2000)
• ‘Surfaces, Interfaces & Colloids : Principles & Applications’, D Myers,
Wiley & Sons (1999)
• ‘Interfacial Science’, M W Roberts, Blackwell Science (1997)
• ‘Introduction to Soft Matter: Polymers, Colloids, Amphiphiles and Liquid
Crystals’, I W Hamley, J Wiley & Sons (2000)
Further reading
•
‘Colloid Science, Principles, Methods and Applications’, Ed T Cosgrove,
Blackwell (2005)
•
‘A guide to the Surfactants World’, X Domingo, Proa (1995)
•
‘Surfactants in Cosmetics’, ed. M M Rieger and L D Rhein, 68, Surfactant
Science Series, Marcel Dekker Inc (1997)
• ‘Surfactants and Polymers in Aqueous Solution’, B Jonnson, B Lindman, K
Holmberg and B Kronberg, John Wiley & Sons (1998)
Further reading
•
M Garvey, Chemistry in Britain, 2003, February, 28
•
J Mufti, D Cernasov, R Macchio, HAPPI, 2002, February, 71
•
R Y Lochhead, L R Huisinga, Cosmetics & Toiletries, 2004, 119(2), 37
•
R E Stier, Cosmetics & Toiletries, 2004, 119(12), 75
•
R Y Lochhead, S Jones, HAPPI, 2004, July, 67
•
R Y Lochhead, L R Huisinga, Cosmetics & Toiletries, 2005, 120 (5), 69
‘Size matters……’
Va, Va voom!….’
Thierry Henry
E-mail: kevan.hatchman@solvay.com
www.solvay.com