CML-528:Interfacial Science and Engineering Dr.C.Ravikumar Assistant Professor Department of Chemical Engineering VNIT Course Description • The course aims to introduce the basic concepts and tools for the analysis of colloidal and interfacial properties of the materials which depend on their size, inter-particle distances and forces acting on them. • The course will be very useful in understanding the basic concepts of adhesion, particle-aggregation, wetting, detergency, oil recovery, flotation, nucleation, bio-surfaces, chromatography, paints, and composite materials. • This course is interdisciplinary will be of very useful for the undergraduate students, post-graduate students and researchers in the field of chemical, mechanical, civil, materials and electrical engineering; chemistry and physics; and materials science. Outcomes • Students will be able to understand the basic concepts of colloids, surface and interfacial energies and tensions, intermolecular and surface forces, stability of colloids, preparation of emulsions, and micro emulsions. • The knowledge acquired from this course will enable the students to solve the advance research problems effectively in the field of chemical engineering, analytical and physical chemistry, biochemistry and environmental science, materials science, petroleum engineering and nanotechnology. Syllabus • Introduction to Colloids: Colloids- Classification, thermodynamic and kinetic stabilities of colloids, Sedimentation of colloids, Brownian motion, Osmotic pressure, Optical properties and electrical properties of colloids, Hofmeister series, Electroviscous effect, Gelation, Imbibition, Syneresis, Gold number, Rheological properties of colloidal dispersions, characterization of colloids. SurfactantsProperties and Classification, micelles, reverse micelles, Bilayers, liposomes and vesicles, Hydrophilic–lipophilic balance. • Surface and Interfacial Tension: Surface tension of liquids, Calculation and measurements of surface tension. Interfacial tension- Girifalco–Good and Fowkes correlation, Measurement of interfacial tension. Concepts of surface and interfacial energies and tensions; van der Waals and acid-base components of interfacial tensions, Young-Laplace equation of capillarity, Stability of equilibrium solutions; Contact angle and Young's equation, Free energies of adhesion, kinetics of capillary flows. • Intermolecular and Surface Forces: van der Waals force, Mie and Lennard-Jones potentials, Retardation effect, van der Waals force between macroscopic bodies, Derjaguin approximation, Lifshitz theory of van der Waals interaction, van der Waals force in electrolyte solutions. Electrostatic Interaction- Electric Interaction in Colloidal Particles, electrostatic interaction (Double Layer), Gouy Chapman Theory, Debye Huckel approximation, charge Density, force between two particles, potential Energy, Zeta potential. Syllabus • • • • • • • ✓ ✓ ✓ Stability of Colloids: DLVO theory- Effect of concentration, calculations for critical coagulation concentration, relation between surface charge and electrostatic potential, Extensions to DLVO theory, kinetics of coagulation, slow Coagulation, Non-DLVO forces. Emulsions, and Microemulsions: Preparation, stability, Flocculation and coalescence of drops, Mechanism of coalescence of drops, Stochastic theory, Phase inversion, applications of emulsions. Microemulsionsclassification and Stability. Textbooks P.C. Hiemenz, and R. Rajagopalan, Principles of Colloid and Surface Chemistry, 3rd Edition, Marcel Dekker, N.Y., 1997. M.J. Rosen, Surfactants and Interfacial Phenomena, Wiley-Interscience Publication, New York, 2004. ✓ ✓ ✓ Reference Books: A.W. Adamson, A. P. Gast, Physical Chemistry of Surfaces, Wiley-Interscience, New York, 1997. J. Israelachvili, Intermolecular and Surface Forces, Academic Press, New York, 1992. R. J. Hunter, Foundations of Colloid Science, Clarendon, Oxford, Volumes 1 & 2, 1989.W. B. Russel, D. A. Saville and W. R. Schowalter, Colloidal Dispersions, Cambridge University Press, 1989. D.J. Shaw, Colloid & Surface Chemistry, Butterworth Heinemann, Oxford, 1991. H. Butt, K. Graf, M. Kappl, Physics and Chemistry of Interfaces, WILEY-VCH, 2003. D. Myers, Interfaces, and Colloids: Principles and Applications, Wiley, Second Edition, 2002. • • Internet notes: NPTEL lecture notes-Interfacial science and Engineerng by Dr.Pallab Ghosh (IIT Guwahati) Introduction to Colloids We will discuss about • Definition of colloids • Various types of colloid dispersions with examples • Classification of colloids • Coagulation and flocculation • Critical coagulation concentration • Thermodynamic and kinetic stabilities of colloids • Sedimentation in gravitational field • Sedimentation in centrifugal field Colloids-Definition • A colloid is defined as a particle that has some linear dimension between 1 nm -1000 nm (1µm). • A dispersion of such particles is called a colloid dispersion. Note: Emulsion having size > 1µm are also considered as colloids. Colloidal Dispersion Colloids solutions are heterogeneous in nature and always consist of at least two phases : the dispersed phase and the continuous phase. Dispersed Phase : It is the substance present in small proportion and consists of particles of colloids size (1 to 100 nm). Continuous phase : It is the medium in which the colloids particles are dispersed. States of Colloidal Dispersion Applications of colloidal solutions: 1- Therapy--- Colloidal system are used as therapeutic agents in different areas. e.g- Silver colloid-germicidal Copper colloid-anticancer Mercury colloid-Antisyphilis 2- Stability---e.g. lyophobic colloids prevent flocculation in suspensions. e.g- Colloidal dispersion of gelatin is used in coating over tablets and granules which upon drying leaves a uniform dry film over them and protect them from adverse conditions of the atmosphere. Applications of colloidal solutions: 4- Absorption--- As colloidal dimensions are small enough, they have a huge surface area. Hence, the drug constituted colloidal form is released in large amount. e.g- sulphur colloid gives a large quantity of sulphur and this often leads to sulphur toxicity 5-Targeted Drug Delivery--- Liposomes are of colloidal dimensions and are preferentially taken up by the liver and spleen. Applications of colloidal solutions: 6- Photography: A colloidal solution of silver bromide in gelatine is applied on glass plates or celluloid films to form sensitive plates in photography. 7- Clotting of blood: - Blood is a colloidal solution and is negatively charged. On applying a solution of Fecl3 bleeding stops and blood clotting occurs as Fe+3 ions neutralize the ion charges on the colloidal particles. How colloids differ from solution? Colloid particles scatter light-true solution does not! Only continuous medium passes through the filter/membrane-not the colloidal particles Important properties of true solutions, colloids and suspensions Types of Colloidal Dispersions Sol (Solid in Liquid type), gels (liquid in solid type), emulsions (liquid in liquid type) Hydrosol: Dispersion medium is water Alcosol. dispersion medium is alcohol Classification of Colloids (upon interaction) • (a) Lyophilic (or solvent loving) colloids : Dispersed phase have a great affinity (or love) for the dispersion medium. • Substances like gum, gelatine, starch etc when mixed with suitable dispersion medium, they form colloidal dispersion easily • Reversible in nature- once lyophilic colloidal solution has been formed then dispersed phase and dispersion medium can be separated easily. • Once separated these can again be formed by remixing the two phases. These sols are quite stable. • If water is used as dispersion medium then it is termed as hydrophilic colloid. Classification of Colloids (upon interaction) (b) Lyophobic (or solvent hating) colloids: • Dispersed phase has no affinity for the dispersion medium. • Metals like Au, Ag and their hydroxides or sulphides etc., when simply mixed with dispersion medium do not pass directly into colloidal state. • Sols can be readily precipitated and once precipitated they have little tendency to go back into the colloidal state. • Sols are irreversible in nature. • Not very stable and require a stabilizing agent to remain in the colloidal form. • In case water is used as dispersion medium it is called as hydrophobic sol. Classification of Colloids (upon interaction) Classification of Colloids (Based on Molecular Size) 1.Macromolecular colloids – Size of the particles of the dispersed phase are big enough to fall in the colloidal dimension (i.e.,100 nm). Examples of naturally occurring macromolecular colloids are starch, cellulose, proteins etc. 2. Multi molecular colloids – Individual atoms are not of colloidal size but they aggregate to join together forming a molecule of colloidal dimension. E.g.,sulphur sol contains aggregates of S molecules which fall in colloidal dimension. 3. Associated colloids – These are substances which behave as normal electrolyte at low concentration but get associated at higher concentration to form micelle and behave as colloidal solution. Soap is an example. Soap is sodium salt of long chain fatty acid R COONa. When put in water, soap forms RCOO— and Na+. These RCOO— ions associate themselves around dirt particles as shown below forming a micelle. Association colloids Colloids-surface charge • Colloidal particles have a large specific surface area, and the surface generally carries electrical charge. • Because of these properties, soil colloidal particles are able to adsorb a large amount of fertilizer, agricultural chemicals, and contaminants. • As long as colloidal particles do not migrate, they play a role as the storing warehouse of substances in the environment. Coagulation and flocculation • Coagulation and flocculation are widely used in colloid chemistry to mean aggregation of the particles. Coagulation to implies the formation of compact aggregates leading to the macroscopic separation of particles. Flocculation is used implying the formation of a loose or open network which may or may not separate macroscopically. • Coagulation and flocculation are widely used in colloid chemistry to mean aggregation of the particles. Coagulation to implies the formation of compact aggregates leading to the macroscopic separation of particles. Flocculation is used implying the formation of a loose or open network which may or may not separate macroscopically. Critical coagulation concentration (CCC) • When the colloid particles are charge stabilized, the process is known as peptization. • • Destabilization of colloids is coagulation. Certain ions are necessary to cause peptization or coagulation. Critical coagulation concentration (CCC) • The amount of electrolyte required to induce coagulation depends upon the valence of the counterion in the salt. This concentration of electrolyte is known as critical coagulation concentration (CCC) or flocculation value Critical coagulation concentrations of some salts for the negatively charged silver iodide (AgI) sol The valence of the counterion is a very important parameter for the coagulation of a sol. The amount of divalent counterion required to coagulate the AgI colloid is much smaller than the amount of the monovalent counterion. The amount of trivalent counterion required is even less. Critical coagulation concentration (CCC) • • Schulze Hardy rule: Critical coagulation concentration varies with the inverse sixthpower of the valence of the counterion. The specific nature of these ions is less important. Also, the effect of the valence of the coions (i.e., the ions of the electrolyte having the same charge as the colloid particles) is less significant on coagulation. Thermodynamic and kinetic stabilities of colloids Sedimentation of colloids in gravitational field • Consider the movement of an uncharged spherical particle in a liquid under the gravitational force. • For small colloid particles, the flow occurs at very low velocities relative to the sphere. This is known as creeping flow. Two other forces act on the particle: buoyant force and drag force, as shown in the following figure. Sedimentation of colloids in gravitational field • If the particle is falling in the viscous fluid under its own weight, then a terminal velocity, or settling velocity, is reached when this frictional force combined with the buoyant force exactly balances the gravitational force. This is the expression for terminal velocity based on Stokes’ law. It is evident that when ρp > ρ , the particle settles or sediments to the bottom (e.g., paint pigments settle to the bottom of a paint container). When ρp < ρ , the converse is true and the particle rises, which is known as creaming (e.g., cream rises to the top of a bottle of milk). Assumptions considered: (i) Settling is not affected by the presence of other particles in the fluid. This condition is known as free settling. When the interference of other particles is appreciable, the process is termed hindered settling. (ii) The walls of the container do not exert an appreciable retarding effect. The particle is large compared with the mean free path of the molecules of the fluid. Otherwise, the particles may slip between the molecules and thus attain a velocity that is different from that calculated. Sedimentation of colloids in Centrifugal field • The equation of motion for the particles in a centrifugal field is similar to the equation for gravitational field, except that the acceleration due to gravity needs to be replaced by the centrifugal acceleration rω2 , where r is the radius of rotation and ω is angular velocity. • For a spherical particle under the influence of centrifugal for ce, the Stokes’ law the equation of motion is given as • As the particle moves outwards, the accelerating force increases. Therefore, it never acquires an equilibrium velocity in the fluid. The above equation can be simplified to give, Sedimentation of colloids in Centrifugal field • The instantaneous sedimentation velocity dr/ dt can be considered as the terminal velocity in the gravity field that is increased by a factor equal to the acceleration ratio, rω2/ g . Let us define a sedimentation coefficient as, • Integrating we get, or Ans: 585.6 rad/sec or 5600 revolution/min Exercise Problems • 1. A spherical particle of 2 cm diameter is broken uniformly into a large number of tiny spheres such that the diameter of each of the new particles is 1 x10-7 m. What is the total surface area of these particles? • 2. 1 dm3 of water is atomized to form droplets of 1 μm diameter. Calculate the total surface area of the droplets. What would be the surface area if the diameter of the droplets is 10 nm? • 3. Calculate the terminal velocity of a 10 micrometer diameter carbon tetrachloride drop in water. Given: density of carbon tetrachloride = 1600 kg/m3, viscosity of water = 1 mPa s. What are the assumptions made in your calculation? • 4. A 1 micrometer diameter quartz particle (density = 2650 kg/m3) suspended in water is placed in a centrifuge. The centrifuge rotates at a speed of 1000 rad/s. Calculate the sedimentation coefficient. What will be radial position of the particle with respect to its initial position after operating the centrifuge for 60 s? Exercise Problems • 5. The critical coagulation concentrations for NaCl, MgCl2 and AlCl3 for negatively charged As2S3 colloids are 60 mol/m3, 0.7 mol/m3 and 0.09 mol/m3, respectively. Verify whether these values are consistent with the Schulze-Hardy rule or not.
