COLLOID SYSTEMS Colloidal chemistry is the science, which is a branch of chemistry that researches the dispersions, colloids and surface phenomena that form at the interface. Colloidal chemistry is inextricably linked to physical chemistry (forming physcolloidal chemistry), general chemistry (organic and inorganic), physics and biology.
COLLOID SYSTEMS Colloidal have been defined classically as systems involving characteristic length scales ranging from a few thousand nanometers. Until a few decades ago, this range of dimensions has attracted relatively little scientific interest. The two far ends of our perception of space, the subatomic elementary particles on one side and the universe with its galaxies at the other, have been much more appealing to scientists.
COLLOID SYSTEMS Fig. 1.1 shows the colloidal domain on a logarithmic length scale. It is situated between the nanoworld of atoms and molecules and the macroworld of biological and technological systems involving organisms and products. Colloidal dimensions may therefore be classified as mesoscopic. Fig.1. Length scales relevant for various scientific disciplines.
COLLOID SYSTEMS colloidal solutions show strongly reduced colligative properties, such as osmotic pressure and freeze point suppression, as compared to solutions of regular-sized molecules of the same mass concentration. The upper limit for the particle size in a colloidal system is marked by the point where thermal (Brownian) motion, which tends to keep the particles in a dispersed state, is superseded by the gravitational force, which tends to segregate and settle the particles from solution.
COLLOID SYSTEMS Modern colloid chemistry, the science of colloids [1] and surfaces, deals with the highly dispersed state of a substance. The fundamental characteristic of colloid chemistry is the dispersity, i.e. scattering, of a substance. Naturally, in the broad meaning of the word, dispersity at the molecular, atomic, or nuclear level is a feature of any substance and is the granularity of matter. In colloid chemistry, the concept of dispersity covers a broad range of particle sizes, namely, from sizes larger than simple molecules to ones visible to the naked eye, i.e. from to cm. [1] [1] From the Greek (kolla) glue and (eidos) form, shape.
COLLOID SYSTEMS Disperse systems are heterogeneous and consist of two (or more) phases: an internal or dispersed phase and an external, continuous, or dispersing phase (medium). Hence, modern colloid chemistry studies both coarsely dispersed systems (for example, suspensions, emulsions, powders) with a particle size exceeding 1 μm ( cm) and highly dispersed or colloidal systems proper with a size below 1 μm (<10 -6 m = cm), namely, from 1 μm to 1 nm ( cm). Strictly speaking, they display a dialectic unity of heterogeneity and homogeneity.
Principles of disperse system classification Classification by dispersity Classification by state of aggregation Classification by structure Classification by interphase interaction Сlassification by phase distinguish ability
Classification by dispersity The size of particles or pores makes it possible to divide disperse systems into coarsely and highly dispersed ones. Particles under cm in size do not relate to colloidal ones and form molecular or ionic solutions. Coarsely dispersed systems such as settling dispersions (suspensions) in natural water, suspensions, and emulsions differ in practice from highly dispersed systems in that the particles of the dispersed phase settle (or rise to the surface) in a gravitational field, do not pass through paper filters, and are visible under an ordinary microscope. Particles of a highly dispersed system pass through ordinary filters, hut are retained by ultrafilters (for example, cellophane and parchment), do not practically settle (do not float up), and are not visible under an optical microscope.
Classification by state of aggregation Types of disperse systems Dispersed phase Dispersing medium Symbol of system Type of systemExamples 1SolidLiquidS/LSols, suspensions Dispersions (suspensions) in natural water, sols of metals in water, bacteria 2Liquid L/LEmulsions Milk, lubricants, crude petroleum 3GaseousLiquidG/LGaseous emulsions, foamsSoapy foam 4Solid S/SSolid colloidal solutions Minerals, some alloys (semi-precious stones,steel, iron) 5LiquidSolidL/S Porous bodies, capillary systems, gels, jellies Adsorbents, moist soils, and some minerals (opal, pearls) 6 GaseousSolidG/S Porous and capillary systems, xerogels Pumice, silica gel, aluminogel, activated carbons 7SolidGaseousS/GAerosols (dusts, smokes) Tobacco smoke, coal and cosmic dusts, powders 8LiquidGaseousL/GAerosols (fogs) Fog, cumulus clouds, storm clouds 9Gaseous G/GSystems with density fluctuations The Earth's atmosphere
Classification by state of aggregation Soils:Size: sand> 50 μm powdery1-50 μm Red blood cells7 μm Coli bacillus3 μm Influenza virus0.1 μm -100 nm Sol of Au (blue)50 nm Sludge in natural water nm Smoke (charcoal)30-40 nm Sol of Au (red)20 nm Virus of foot-and-mouth disease 10 nm Molecule of glycogen10 nm Sol of Au (seed)3 nm Thin pores of coal1-10 nm Table 1.1 acquaints us with the main types of disperse systems. Some examples are given below with indication of the maximum size of the dispersed particles: The surface of Venus is covered with particles μm in size. The classification by the state of aggregation is limited because when the size of particles approaches molecular values, not only the concept of an interface, but also that of the state of aggregation of the dispersed phase lose their meaning.
Classification by structure All disperse systems can be divided into two classes, namely, freely dispersed, in which the dispersed particles are not bound to one another and can freely move (suspensions, emulsions, sols, including aerosols), and coherently or bond- dispersed systems in which one of the phases does not move freely because it is fixed structurally. They include capillary- porous bodies frequently called diaphragms or capillary systems, membranes – thin films, generally polymeric that are permeable for liquids and gases, gels, jellies, foams – liquid networks with air cells, and solid solutions. Multimolecular colloids : Consists of aggregates of a large number of atoms or smaller molecules whose diameter is less than 1 nm Macromolecular colloids: In these colloids, the molecules have sizes and dimensions comparable to colloidal particles. For example, proteins, starch, cellulose.
Classification by phase distinguish ability High-polymer and high-molecular compounds (HMC) and their solutions occupy a special place in the classification of colloid chemistry. Solutions of HMC, being in essence true molecular solutions, also have many features of the colloidal state. In spontaneous dissolving, HMC disperse up to individual macromolecules, forming homogeneous, single-phase, stable and reversible systems (for example, solutions of a protein in water, of rubber in benzene) that do not differ in principle from ordinary molecular solutions. But the size of these macromolecules is gigantic in comparison with that of ordinary molecules and is commensurate with the size of colloidal particles. The above data show that the size of macromolecules (glycogen) can he even larger than that of ordinary colloidal particles (sol of Au) and thin pores.
Classification by interphase interaction Lyophobic colloids (solvent hating colloids ) When metals and their sulphides simply mixed with dispersion medium, they dont form colloids. need stabilizing to preserve them. irreversible. For example, colloidal solutions of gold,silver, Fe(OH) 3, As 2 S 3, etc. Lyophilic colloids ( solvent loving) Directly formed by substances like gum, gelatine rubber etc. on mixing with a suitable liquid(the dispersion medium). self-stabilizing reversible sols For example, gums, gelatin, starch, albumin in water.
Colloidal particles in natural water
Association Colloids
Types of colloids: C- Association / amphiphilic colloids - Certain molecules termed amphiphiles or surface active agents, characterized by two regions of opposing solution affinities within the same molecule.
Types of colloids: -At low concentration: amphiphiles exist separately (subcolloidal size) -At high concentration: form aggregates or micelles (50 or more monomers) (colloidal size)
Association colloids
Properties of colloids Optical properties: Tyndall effect When a beam of light falls at right angles to the line of view through a solution, the solution appears to be luminescent and due to scattering of light the path becomes visible. Quite strong in lyophobic colloids while in lyophilic colloids it is quite weak.
Properties of colloids Brownian movement: Zig- zag movement of colloidal particles in a colloidal sol
Properties of colloids Electrophoresis Movement of colloidal particles under influence of electric field
Properties of colloids Electro-osmosis: molecules of dispersion medium are allowed to move under influence of electric field Coagulation or flocculation:Process which involves coming together of colloidal particles so as to change into large sized particles which ultimately settle as a precipitate or float on surface.It is generally brought about by addition of electrolytes. The minimum amount of an electrolyte that must be added to one litre of a colloidal solution so as to bring about complete coagulation or flocculation is called coagulation or flocculation value.Smaller is the flocculation value of an electrolyte,greater is the coagulating or precipitating power.
Properties of colloids For positively charged, then the coagulating power of electrolytes follow the following order: Hardy schulze law : Coagulating power of an electrolyte increases rapidly with the increase in the valency of cation or anion. For negatively charged sol, the coagulating power of electrolytes are AlCl 3 > BaCl 2 > NaCl or Al 3+ > Ba 2+ > Na +
Gold Number Covering up of lyophobic particles by lyophilic particles is known as its protective action and such colloids are called protective colloids. Gold number is defined as amount of protective sol that will prevent the coagulation of 10 ml of a gold solution on the addition of 1 ml of 10% NaCl solution. Smaller the gold number,higher is protective power
Emulsion A colloidal dispersion of one liquid in another immiscible liquid is known as an emulsion, e.g. milk, Na-soaps, vanishing cream, etc. 1.Oil in water, where oil is the dispersed phase and water is the dispersion medium, e.g. milk. 2.Water in oil where water is the dispersed phase and oil is the dispersed medium, e.g. butter, cream. Types of emulsions
Stability of colloids
Alternative Explanation of Surface Tension Suppose we have a thin liquid film suspended on a wire loop as follows liquid film expanded liquid film dx dA f = force needed to move wire dw = dG = dA l = length of wire
Measurement of Surface Tension Early measurements – even pure liquids has been described as a comedy of errors Today – possible to routinely measure the surface tension of liquids and solutions to an accuracy of mN/m
Capillary Action The tendency of liquids to rise up in narrow tubes - capillary action. Due to the phenomenon of surface tension.
The Complication of Contact Angles The balance of forces that results in a contact angle, c. The contact angle gives information on the wettability of a surface.
Capillary Rise The pressure exerted by a column of liquid is balanced by the hydrostatic pressure. This gives us one of the best ways to measure the surface tension of pure liquids and solutions.
The Wilhelmy Plate Method a) detachment b) static
The Du Nüoy Ring Method Measure the force required to pull the ring from the surface of the liquid or an interface by suspending the ring from one arm of a sensitive balance Water F R
The Correction Factor The correction factor takes into account of the small droplets that are pulled up by the ring when it detaches from the surface
Drop Weight/Drop Volume Method A stream of liquid (e.g., H 2 O) falls slowly from the tip of a glass tube as drops
Drop Weight Method The drop weight is found by – Counting the number of drops for a specified liquid volume passing through the tip; – Weighing a counted number of drops V g= mg = 2 r g A correction factor - F r/v 1/3
Sessile Drop Method The surface tension of a liquid may be obtained from the shape and size of a sessile drop resting on a horizontal surface e Surface Sessile Drop h
Sessile Drop Method (Contd) Three techniques for obtaining the surface tension from the image of the sessile drop – Measure the height of the top of a large sessile drop above its maximum diameter. – Estimate the shape factor of the drop from the coordinates of the drop profile. – Fit the drop profile to ones that are generated theoretically.
Drop Profiles The sessile drop method may also be used to obtain the value of the equilibrium contact angle. Contact angle, e < 90° e
The Maximum Bubble Pressure Method The maximum pressure required to force a bubble through a tube is related to the surface tension of the liquid. gas stream b l
The Bubble Pressure Technique The maximum bubble pressure is related to the surface tension of the liquid as follows P = g l + 2 / b – = the density difference between the liquid and the vapour – b = radius of curvature at the apex of the bubble – l = hydrostatic height to the bottom of the bubble – g = m / s 2
The Differential Maximum Bubble Pressure Method Two probes of different diameters. A differential pressure is generated, P. b2b2 gas stream b1b1 z2z2 z1z1 t
The Differential Bubble Pressure Equations The maximum bubble pressure is related to the surface tension of the liquid as follows P = g z / b 1 - g z / b 2 – = the density difference between the liquid and the vapour of the first bubble – = the density difference between the liquid and the vapour of the second bubble – z 1 = the distance from the tip to the bottom, of the first bubble – z 2 = the distance from the tip to the bottom, of the second bubble
Methods of Measuring Surface Tension
Molecular Contributions to an Oil- water Interfacial Tension = Oil= water Oil Phase Water Phase oil water oil x d water ) 1/2
The Work of Adhesion Energy required to reversibly pull apart to form unit surface areas of each of the two substances
1 1 The Work of Cohesion Defined in terms of the energy required to reversibly separate a column of a pure liquid to form two (2) new unit surface areas of the liquid.
The Definition of the Surface Excess To obtain a clearer meaning of the surface excess, lets consider the following system. z CiCi zozo - + C J (1) C J (2)
The Spreading Coefficient Substance (usually liquid) already in contact with another liquid (or solid) spreads – increases the interfacial contact between the first and second liquid (or the liquid and the solid) – decreases the liquid-vapour interfacial area
Three Cases of Spreading Place a drop of oil on a clean water surface Define the spreading coefficient
The spreading coefficient (to be defined later) is indicative of the difference in the adhesive forces between liquid 1 and liquid 2 (or the solid), and the cohesive forces that exist in liquid 1
S > 0, spreading occurs spontaneously S < 0, formation of oil lenses on surface Water Air wa ow oa Oil e
A third possibility is the a monolayer spreads until spreading is not favourable; excess oil is left in equilibrium with the spread monolayer
Wetting Ability and Contact Angles Wetting - the displacement of a fluid (e.G., A gas or a liquid) from one surface by another fluid Wetting agent - a surfactant which promotes wetting Three types of wetting nSpreading wetting nImmersional wetting nAdhesional wetting
Spreading Wetting Liquid already in contact with another liquid (or solid) wets the surface of the second component (liquid or solid) by spreading across the surface of the second component Using the spreading coefficient defined earlier, we find that the liquid spreads spontaneously over the surface when S > 0
Solid Surfaces Consider the case of a liquid drop placed on a solid surface (non-spreading) For a liquid drop making a contact angle with the solid surface
A spreading drop e < 90° e Solid Surfaces/Different Contact Angles Examine the following two surfaces.
A drop with a contact angle << 90 e
The Derivation of Youngs Equation la sa ls e change in the liquid-solid interfacial area = dA dA e change in the solid-air interfacial area = - dA change in the liquid-air interfacial area = dA Cos e
Youngs Equation For a liquid (as a drop or at at the surface of a capillary) making a contact angle c with the solid surface
Adhesional Wetting The ability of the liquid to wet the solid will be dependent on its ability to stick to the solid liquid droplets Solid Surface la droplets adhering to solid surface sl
from the Young Equation Note: the solid is completely wetted if e = 0; it is partially wetted for finite values of e.
Immersional Wetting Immerse a solid substance in a pure liquid or solution – area of the solid-air interface decreases – interfacial contact between solid and liquid is increased solid particle Water sa immersed solid particle sl
Work required to immerse the solid in the liquid – Examine the difference ion the solid-airsurface tension and the solid-liquid interfacial tension
Applying youngs equation If sa > sl, spontaneous wetting while if sa < sl, work must be done to wet the surface
Degrees of Liquid-solid Interaction
Surfactants What is a surfactant? Surface active agent Headgroup Tail
Heads or Tails? Headgroup – hydrophilic functional group(s) Tail – hydrocarbon or fluorocarbon chain Typical headgroups (charged or uncharged) – Sulfate – Sulfonate – Trimethylammonium – Ethylene oxide – carboxybetaine
Properties of Surfactant Molecules Aggregate at various interfaces due to the hydrophobic effect – Air-water interface – Oil-water interface Form aggregates in solution called micelles at a specific concentration of surfactant called the critical micelle concentration (the cmc) – Micellar aggregates are known as association colloids
Applications of Surfactants Surfactants are an integral part of everyday life; they are formulated into a wide variety of consumer products – Shampoos – Dish detergents – Laundry detergents – Conditioners – Fabric softeners – Diapers – Contact lens cleaners
Applications of Surfactants (Contd) Surfactants are also widely used in industry due to their ability to lower surface and interfacial tensions and act as wetting agents and detergents – Heavy and tertiary oil recovery – Ore flotation – Dry cleaning – Pesticide and herbicide applications – Water repellency
Interfacial Properties of Surfactant Molecules Surfactants – used in a large number of applications due to their ability to lower the surface and interfacial tension Gibbs energy change to create a surface of area dA dG = dA
Using the Gibbs adsorption equation for a 1:1 ionic surfactant Where surf = n surf / A
Surfactants H2OH2O Hydrophobic / Lipophilic core Surfactant Concentration Unimers Micelles Critical Micelle Concentration (CMC)
Surfactants Types - Anionic - Cationic - Amphoterics - Non-ionics
Emulsion Polymerization Recipe
Emulsion Polymerizations Polymz Rate Surfactant Concentration Critical Micelle Concentration
Kinetics of Emulsion Polymerization Percent Conversion Time I II III
Kinetics of Emulsion Polymerization Rate % Conversion IIIIII
Surfactant – polymer interaction and compatibility When surfactant and polymer are injected in the same slug (SP flooding), their compatibility is an issue. Sometimes, polymer is injected before surfactant as a sacrificial agent for adsorption or for conformance improvement. Sometimes polymer is injected behind surfactant to avoid chase water fingering in the surfactant slug. Even though polymer is not injected with surfactant in the same slug, they will be mixed at their interface because of dispersion and diffusion. Polymer may also mix with surfactant owing to the inaccessible pore volume phenomenon when it is injected behind surfactant.
Surfactant – polymer interaction and compatibility Factors affecting surfactant – polimer interaction Electrolyte concentration Alcohol Oil Polymer concentration Competitive Adsorption Phase trapping
Phase separation: surfactants