Colloids Lecture Notes - PDF

DISPERSIONS Ms. Colette Gouveia Lecture 8  Dispersed Systems Dispersed systems consist of two phases: particulate matter (dispersed phase) continuous phase (dispersion medium/solvent). Based on the size of the dispersed phase, three types of dispersed systems are...

DISPERSIONS Ms. Colette Gouveia Lecture 8  Dispersed Systems Dispersed systems consist of two phases: particulate matter (dispersed phase) continuous phase (dispersion medium/solvent). Based on the size of the dispersed phase, three types of dispersed systems are generally considered: (a) molecular dispersions, (b) colloidal dispersions, and (c) coarse dispersions.  Introduction to Colloidal Dispersions The word colloid comes from a Greek word ‘kolla’ which means glue and ‘eidos’ which means like, thus colloidal particles are glue - like substances. The colloidal system consist of two phases: - A dispersed phase - discontinuous phase - A dispersion medium - continuous phase A colloid may be defined as a heterogenous (two phase system consisting of minute particulate of (1nm – 1000nm) substance microscopically dispersed into a continuous phase or dispersion medium. Examples of natural colloids: fogs, moist, smoke, Ferric hydrosol. COLLOIDS Classification of Dispersed Systems Properties of Solutions, Colloids, and Suspensions Property Solution Colloid Suspension Particle Size 0.1-1.0 nm 1-1000 nm >1000 nm Settles on No No Yes Standing? Filter with No No Yes Paper? Separate by No Yes Yes Dialysis? Homogeneou s? Yes Borderline No Size and Shape of Colloidal Particles The colloidal particles may have different shapes and sizes. The more extended the particle shape, the greater its specific surface and the attractive forces are greater between the particles of the dispersed phase and the dispersion medium.. Particles of colloidal size have a comparatively large surface area when compared with the surface area of an equal volume of larger particles. Specific surface is the surface area per unit weight or volume of material The shape of colloidal particles has a direct effect on the flow, sedimentation and osmotic pressure of the colloidal system The size of colloidal particles causes a change in colour, for e.g.an ↑ in size of particle causes red gold sol to change to blue. Size and Shape of Colloidal Particles The following forces play an important role in the interaction of colloid particles: EXCLUDED VOLUME REPULSION IN LIQUID THEORY : In liquid state theory, the 'excluded volume' of a molecule is the volume that is inaccessible to other molecules in the system as a result of the presence of the first molecule. The excluded volume of a hard sphere is eight times its volume VAN DER WAALS FORCE : It is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds, the hydrogen bonds, or the electrostatic interaction of ions with one another or with neutral molecules or charged molecules. ELECTROSTATIC INTERACTION : Colloidal particles often carry an electrical charge and therefore attract or repel each other. The charges of both the continuous and the dispersed phase, as well as the mobility of the phases are factors affecting colloids STERIC FORCES : Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds (Pauli or Born repulsion), and this may affect the molecule's preferred shape (conformation) and reactivity.  CLASSIFICATION OF COLLOIDS Colloids are classified based on:- 1. Physical state of dispersed phase and dispersion medium. 2. Molecular size of the dispersed phase. 3. Nature of interaction between dispersed phase and dispersion medium. 4. Appearance of colloids. 5. Electric charge on dispersion phase  Physical State of Dispersed Phase and Dispersion Medium Dispersed Phase Dispersion Name Example Medium Solid Solid Solid-sol Coloured glass, gemstones Solid Liquid Sol Ink, blood Solid Gas Aerosol Smoke Liquid Solid Gel Jelly Liquid Liquid Emulsion Milk , cream Liquid Gas Liquid Aerosol Fog Gas Solid Solid Form Pumice stone Gas Liquid Foam Shaving cream Gas Gas None All gases are miscible  Molecular Size of the Dispersed Phase MULTIMOLECULAR COLLOIDS - Individual particles of the dispersed phase consists of aggregates of atoms or small molecules having diameter less than 10-7cm. The particles are held by weak vander waal’s forces. Example; gold sol, sulphur sol MACROMOLECULAR COLLOIDS - The particles of dispersed phase are sufficiently large in size enough to be of colloidal solution (100 nm). Cellulose, starch, proteins are examples of naturally occurring macromolecular molecules  Appearance of Colloids SOLS When a colloidal solution appears as fluid i.e. dispersion of solids in liquids, but also for dispersions in solid or gas. The sols are generally named for the dispersion medium. When the dispersion medium is water, the sol is known as hydrosol or aquosol. When the dispersion medium is alcohol or benzene it is called alcosol and benzosol respectively. When the dispersion medium is air, it is an aerosol GELS A colloidal system which under a set of conditions of concentration and temperature, "sets" into a solid or semisolid The rigidity of a gel is due to an intertwining network which traps the dispersion medium. It varies from substance to substance. Examples : jelly, butter, cheese, curd.  Nature of Interaction between Dispersed Phase And Dispersion Medium Based on the interaction or attraction of the particle molecules, colloid are of three types : - Lyophilic colloids (solvent loving) Lyophobic colloids (solvent hating) Association colloids (amphiphilic) N.B. SOL. = COLLOIDAL SOLUTION. DISPERSION MEDIUM = SOLVENT. DISPERSED PHASE = MATERIAL = COLLOIDAL PARTICLES  Lyophilic Colloids The term lyophilic means ‘solvent loving’, where there is a considerable attraction between the disperse phase and disperse medium. Hence colloidal solutions are those where the dispersed phase has a great affinity for the dispersion medium. They are also termed as intrinsic colloids since such substances have tendency to pass into colloidal solution when brought in contact with dispersion medium.  Lyophilic Colloids The lyophilic colloids are more stable than lyophobic colloids since the dispersed phase does not precipitate easily. The sols are quite stable as the solute particle is surrounded by two stability factors: a) negative or positive charge b) Layer of solvent If the dispersion medium is separated from the dispersed phase, the sol can be reconstituted by simply remixing with the dispersion medium. Hence, these sols are reversible in nature and are heavily hydrated Lyophilic colloids are very common in biological systems and in foods.  Preparation of Lyophilic Colloids The affinity of lyophilic particles for the dispersion medium leads to the spontaneous formation of colloidal dispersions. For example, acacia, tragacanth, methylcellulose and certain other cellulose derivatives readily disperse in water. This simple method of dispersion is a general one for the formation of lyophilic colloids.  Lyophobic Colloids Lyophobic means solvent hating, where there is a little attraction between the dispersed phase and dispersion medium i.e colloidal solutions in which the dispersed phase has no affinity to the dispersion medium. These are also referred as extrinsic colloids. Such substances have no tendency to pass into colloidal solution when brought in contact with dispersion medium. Lyophobic colloids are all inherently unstable; they will eventually coagulate and they are irreversible by nature. They are poorly hydrated. If the dispersion medium is water, the lyophobic colloids are known as hydrophobic or suspenoids. Examples: sols of metals like Au, Ag, sols of metal hydroxides and sols of metal sulphides. N.B "eventually" can be a very long time (the settling time for some clay colloids in the ocean is years!).Example: Gold sol  Preparation of Lyophobic Colloids The preparative methods for lyophobic colloids may be divided into those methods that involve the breakdown of larger particles into particles of colloidal dimensions (dispersion methods) and those in which the colloidal particles are formed by the aggregation of smaller particles such as molecules (condensation methods) Large particles of the substances can be broken, into particles of colloidal dimensions in presence of a dispersion medium. Since the sols formed are highly unstable. They are stabilized by adding some suitable stabilizer. Some of the methods employed for carrying out the dispersion are as follows: A – DISPERSION METHODS 1. Mechanical disintegration - Ball mill - Colloid mill 2. Ultrasonic Treatment 3. Peptization 4. Electrical dispersion or Bredig’s Arc Method  Mechanical Dispersion Mechanical disintegration Ball mill: It is based on breaking. It contains a rotating vessel with metal balls within the vessel and arranged systemically. The balls continuously move up and down and thus strike the material taken into the vessel. Ultimately the material is crushed to coarse particles. The material finally dispersed in a suitable medium to obtain the formulation (suspension). Colloid mill: It contains two plates arranged at a particular distance from each other. It may be of two types- a) One is fixed and other rotates or b) Both are rotating in opposite direction at very high speed (7000 revolution per minute. ). The solid along with dispersion medium is passed through the mill. So, the particles become disintegrated and dispersed in the medium. The space between the discs of the mill is so adjusted that coarse  Peptization and Ultrasonic Wave 1.By peptization: The process of converting a freshly prepared precipitate into colloidal form by the addition of suitable electrolyte is called peptization. Cause of peptization is the adsorption of the ions of the electrolyte by the particles of the precipitate. The electrolyte used for this purpose is called peptizing agent or stabilizing agent. Important peptizing agents are sugar, gum, gelatin and electrolytes Some freshly precipitated solids are dispersed into colloidal solution in water by the addition of small quantities of electrolytes. During peptization, the precipitate adsorbs one of the ion of the electrolyte on its surface. The adsorbed ion is generally common with those of the precipitate. For example: When freshly precipitated Fe(OH)3 is shaken with aqueous solution of FeCl3 (Peptizing agent) it adsorbs Fe3+ ions and thereby breaks up into small sized particles of type Fe(OH)3 / Fe3+.  Bredig’s Arc Method Metal electrodes are dipped in the dispersion medium, and an electric arc is struck between these electrodes. By doing so, intense heat is generated that helps in the vaporization of metal. Then, condensation of metal takes place and results in the formation of particles having colloidal size.  Preparation of Lyophobic Colloids B – CONDENSATION METHODS (Association Methods) Materials of sub colloidal dimensions are caused to aggregate into particles of colloidal size range by; 1) Chemical reactions. 2) Change in solvent. Chemicals Reactions 1) Oxidation: Addition of oxygen and removal of hydrogen is called oxidation. For example: Colloidal solution of sulphur can be prepared by oxidizing an aqueous solution of H2S with a suitable oxidizing agent such as bromine water. H2S + Br2 → 2HBr + S 2H2S + SO2 → 2H2O + 3S 2) Reduction: Addition of hydrogen and removal of oxygen is called reduction. For example: Gold sol can be obtained by reducing a dilute aqueous solution of gold with stannous chloride. 2AuCl 3 + 3SnCl2 → 3SnCl4 + 2Au 3 3) Hydrolysis: It is the breakdown of water. Sols of ferric hydroxide and aluminum hydroxide can be prepared by boiling the aqueous solution of the corresponding chlorides. For example. FeCl3 + 3H2S → Fe(OH)3 + 3HCl 4) Double Decomposition: The sols of inorganic insoluble salts such as arsenous sulphide, silver halide etc may be prepared by using double decomposition reaction. For example: Arsenous sulphide sol can be prepared by passing H 2S gas through a dilute aqueous solution of arsenous oxide. As 2O3 + 3H2S → As2S3(OH)3 + 3H2O Change of solvent In the condensation method, the smaller particles of the dispersed phase are aggregated to form larger particles of colloidal dimensions. Some important condensation methods are described below: a) Thermal condensation: This process involves the passing of hot vapor through water, which condenses releasing heat to form precipitate. Example: Solutions of substances like mercury and sulphur are prepared by passing their vapours through a cold water containing a suitable stabilizer such as ammonium salt or citrate. b) By excessive cooling: A colloidal solution of ice in an organic solvent like ether or chloroform can be prepared by freezing a solution of water in solvent. The molecules of water which can no longer be held in solution, separately combine to form particles of colloidal size. The colloidal solution of ice in an organic solvent such as CHCl3 or ether can be obtained by freezing a solution of water in the solvent. The molecules of water which can no longer be held in solution separately combines to form particles of colloidal size. c) By exchange of solvent: When a true solution is mixed with an excess of another solvent in which the solute is insoluble, but solvent is soluble, a colloidal solution is obtained. For example, when a solution of sulphur which is soluble in alcohol (ethanol) (but insoluble in water) is added to an excess of water, a milky colloidal solution of sulphur is obtained due to decrease in solubility. d) By reducing solubility: If a concentrated solution of a substance is poured in another liquid in which the substance is insoluble, it undergoes precipitation due to super saturation.  Differences between Lyophilic & Lyophobic Colloids Lyophilic colloids Lyophobic colloids Prepared by direct mixing with dispersion Not prepared by direct mixing with the medium medium Little or no charge on particles Particles carry positive or negative charge Particles generally solvated No solvation of particles Viscosity higher than dispersion medium; Viscosity almost the same as of medium; do set to a gel not set to a gel Precipitated by high concentration of Precipitated by low concentration of electrolytes electrolytes Reversible Irreversible Do not exhibit Tyndall effect Exhibit Tyndall effect Particles migrate to anode or cathode or Particles migrate to either anode or cathode. not at all  Association Colloids Association colloids are thermodynamically stable. Certain molecules or ions are termed amphiphiles Surface active agents (SAA) are characterized by two distinct regions of opposing solution affinities within the same molecules or ions. Organic compounds which contain large hydrophobic moieties together with strongly hydrophilic groups in the same molecule are said to be amphiphilic.  Association Colloids At low concentrations, these colloids behave as normal electrolytes i.e. amphiphiles exist separately (sub colloidal size). At higher concentrations they behave as colloids i.e. they form aggregates or micelles (50 or more monomers) (colloidal size). These associated colloids are also referred to as micelles. Sodium stearate (C18H35NaO2) behave as electrolyte in dilute solution but colloidal in higher concentrations. The individual molecules are generally too small to bring their solution into colloidal size range; they tend to associate in aqueous or oil solution into micelles. Such preparations are called Association Colloids. Surface active molecules such as soaps and synthetic detergents form associated colloids in water.  Association Colloids As with lyophilic sols, formation of association colloids is spontaneous, provided that the concentration of the amphiphile in solution exceeds the CMC. Amphiphiles may be : 1. Anionic (e.g., Na. lauryl sulfate) 2. Cationic (e.g., cetyl triethylammonium bromide) 3. Nonionic (e.g., polyoxyethylene lauryl ether) 4. Ampholytic (zwitterionic) e.g., dimethyl dodecyl ammino propane sulfonate. Comparison of properties of Colloidal Sol  Electrical Charge of Dispersion Phase Colloidal particles has a surface charge on it. The interior of these particles is electronically neutral however the surface gets charged. Generally, the charge on the surface is originated to : a) The surface which adsorbs the ions present within the medium b) The ionization of functional groups present on the surface. POSITIVE COLLOIDS - When dispersed phase in a colloidal solution carries a positive charge. Examples : Metal hydroxides like Fe(OH)3, Al(OH)2, methylene blue sol etc. NEGATIVE COLLOIDS- When dispersed phase in a colloidal solution carries a negative charge. Examples : Ag sol, Cu sol Purification of Colloidal Solutions  Purification of Colloidal Solutions When a colloidal solution is prepared is often contains certain electrolytes which tends to destabilize the sol. The common methods used for purification of colloidal solutions: 1.Dialysis 2.Electro dialysis. 3.Ultra filtration. 4.Ultra- centrifugation  Dialysis The process of separating the colloidal particles from those of molecular particles, by means of diffusion through a suitable membrane. Its principle disallows colloidal particles from passing through a semi permeable membrane (e.g. collodion (nitrocellulose), cellophane) while the ions of the electrolyte can pass through it. Based on the pore size, the small molecules and ions (impurities) such as urea, glucose, and sodium chloride, slowly diffuse out of the bag leaving behind pure colloidal solution. The distilled water is changed frequently to avoid accumulation of the crystalloids otherwise they may start diffusing back into the bag. Dialysis can be used for removing HCl from the ferric hydroxide sol.  Dialysis At equilibrium, the colloidal material is retained in compartment A, while the sub colloidal material is distributed equally on both sides of the membrane. By continually removing the liquid in compartment B, it is possible to obtain colloidal material in A that is free from sub colloidal contaminants. In dialysis, molecules and ions always diffuse from areas of higher concentration to areas of lower concentration Electro dialysis To increase the process of purification, the ordinary dialysis is carried out by applying an electrical potential. This process is called electrodialysis. Application of electrical potential causes cations to migrate to the negative electrode compartment and anions to move to the positive electrode compartment, in both of which running water ultimately removes the electrolyte. Electrodialysis is carried out in a three- compartment vessel with electrodes in the outer compartments containing water and the sol in the center compartment.. Ultra filtration It is a process of high-pressure filtration through a semi permeable membrane in which colloidal particles are retained while the small sized solutes and the solvent are forced to move across the membrane by hydrostatic pressure forces. The pores of the ordinary filter paper are made smaller by soaking the filter paper in a solution of gelatin and subsequently hardened by soaking in formaldehyde. The treated filter paper may retain colloidal particles and allow the true solution particles to escape. Such filter paper is known as ultra - filter and the process of separating colloids by using ultra – filters is known as ultra – filtration. The membrane must be supported on a sintered glass plate to prevent rupture due to high pressure. Pore size of the membrane can be increased by soaking in a solvent that cause swelling e.g. cellophane swell in zinc chloride solution. e.g.nd collodion (nitrocellulose) swell in alcohol. Application of ultra filtration: Ultra filtration is a vital process that takes place in the kidneys. Ultra – centrifugation Ultracentrifugation involves the separation of colloidal particles from the impurities by centrifugal force. The impure sol is taken into a tube which is placed in an ultra-centrifuge. The tube is rotated at high speeds. Because of this, the colloidal particles settle at the bottom of the tube and the impurities remain in the solution. This solution is termed as centrifugate. The settled colloidal particles are removed from the tube and are mixed with an appropriate dispersing medium. Thus, the pure sol is obtained.by using high speed centrifugal machines having 15,000 or more revolutions per minute. Such machines are known as ultra– centrifuges. PROPERTIES OF COLLOIDS - PHYSICAL PROPERTIES - OPTICAL PROPERTIES - MECHANICAL PROPERTIES - ELECTRICAL PROPERTIES  Physical Properties Of Colloids (i) Heterogeneity: Colloidal solutions consist of two phases-dispersed phase and dispersion medium. (ii) Visibility of dispersed particles: Colloidal solutions are heterogeneous in nature, yet the dispersed particles present in them are not visible to the naked eye and they appear homogenous. This is because colloidal particles are too small to be visible to the naked eye. (iii) Stability: Lyophilic sols in general and lyophobic sols in the absence of substantial concentrations of electrolytes are quite stable. Their particles are in a state of motion and do not settle down at the bottom of the container. (iv) Filterability: Colloidal particles are readily passed through the ordinary filter papers. However, they can be retained by animal membranes, cellophane membrane and ultrafilters. (v) Particle size: The particle size of colloids generally varies from 1 nm to 1 um. (vi)Color: The color of a hydrophobic sol depends on the wavelength of the light scattered by the dispersed particles. The wavelength of the scattered light again depends on the size and the nature of the particles. Larger particles absorb the light of longer wavelength and therefore transmit light of shorter wavelength.  Optical Properties of Colloids When an intense converging beam of light is passed through a colloidal solution kept in dark and viewed at right angles,, the path of the beam gets illuminated as a hazy beam or cone (bluish light ). This is because sol particles absorb light energy and then emit it in all directions in space When a strong beam of light is passed through a sol and viewed at right angles, the path of light scatter light due to the colloidal particles. This phenomenon is called Tyndall effect, and the illuminated path is known as Tyndall cone. Tyndall effect is not exhibited by true solutions. This is because the particles present in a true solution are too small to scatter light.  Importance of light scattering measurements 1. Estimate particle size. 2. Estimate particle shape. 3. Estimate particles interactions.  Instruments of Detection ULTRAMICROSCOPE ELECTRON MICROSCOPE Zsigmondy (1903) used the Tyndall In an electron microscope, beam of phenomenon to set up an apparatus electrons is focused by electric and named as the Ultramicroscope. magnetic fields. An intense beam of light is focused on This focused beam is allowed to pass a sol contained in a glass vessel. through a film of sol particles on to a photographic plate. The focus of light is then observed This allows a picture of the individual with a microscope at right angles to particles showing a magnification of the the beam. order of 10,000. Individual sol particles appear as This instrument shows, the size and bright specks of light against a dark shape of several sol particles background (dispersion medium). including paint pigments, viruses, and bacteria become known. An ultramicroscope does not give These particles have been found to be any information regarding the spheroid, rod-like, disc-like, or long shape and size of the sol filaments. particles.  Light scattering measurements are of useful for estimating particle size, shape and determination of molecular weight of colloids.. Turbidity (Ʈ): Used to estimate concentration of dispersed particles and molecular weight of solute. Turbidity: the fractional decrease in intensity due to scattering as the incident light passes through 1 cm of solution. - Turbidity is proportional to the molecular weight of lyophilic colloid Mathematically expressed as Hc / T = 1/M + 2Bc Where T: turbidity C: concentration of solute in gm / cc of solution M: molecular weight B: interaction constant H: constant for a particular system These measurements are particularly suitable for finding the size of the micelles of surface-active agents (association colloids) and for the study of proteins and natural and synthetic polymers. The ultra microscope and the electron microscope are used for the detection of the particles of colloidal dimensions. Equipment used to measure turbidity 1. Spectrophotometer 2. Nephelometer.  Mechanical Properties of Colloids Kinetic properties which relate to the motion of the particles within the dispersion medium are as following: a. Brownian motion b. Diffusion c. Sedimentation d. Osmotic pressure e. The Donnan membrane effect. f. Viscosity.  BROWNIAN MOVEMENT The rapid and continuous zigzag movement of the colloidal particles in the dispersion medium of a colloidal solution is called Brownian movement. Brownian movement is due to the unequal bombardments of the moving molecules of dispersion medium on colloidal particles. – particles are generally small enough to be influenced by the collision with molecules of the dispersion medium – when particles are observed, they are seen to move in a random, erratic manner The Brownian movement decreases with an increase in the size of colloidal particle and viscosity. Suspensions do not exhibit this type of movement. Increasing the viscosity of dispersion medium (by glycerin)decrease then stop Brownian motion.  Consequences of Brownian movement Stable colloids are systems in which the dispersed particles do not settle, because the force of gravity is counteracted by Brownian movement Colloidal sols will diffuse from a region of high concentration to a region of low concentration Colloidal sols show colligative properties  Colloidal Diffusion Diffusion results in mass transport i.e sol particles diffuse from higher concentration to lower concentration region without requiring bulk motion. Rate of Diffusion is expressed by Fick’s first law where the diffusion flux is proportional to the minus gradient of concentrations. It goes from regions of higher concentration to regions of lower concentration. J = -DA dC dx where J is the flux (flow of particles per unit time) i.e dm/dt where dm is the mass of substance diffusing in time dt across an area A under the influence of a concentration gradient dC/dx. - D is the diffusion coefficient and has the dimensions of area per unit time. (the minus sign denotes that diffusion takes place in the direction of decreasing concentration).  Sedimentation Stoke’s Law –- The velocity of Stoke’s law; V = 2r2( p-po) sedimentation is given by Stokes‘ Law: g/9η At small particle size (less than 0.5 um) where Brownian motion is significant and tend to prevent sedimentation due to gravity and v: velocity of sedimentation promote mixing instead. of spherical particles. The colloidal particles settle down under p: density of the spherical the influence of gravity at a very slow rate. particles. This phenomenon is used for determining po: density of the medium. the molecular mass of the η: viscosity of the medium. macromolecules. In the limit of g: acceleration due to - Slow particle motion gravity. - Dilute colloidal suspensions - Solvent is considered as a continuum of viscosity  Sedimentation Sedimentation: is influenced by gravitational force, applicable for particle size > 0.5 µm. Stokes law equation – velocity of sedimentation. - Colloidal particles have Brownian motion - No sedimentation - Forced sedimentation – ultra centrifuge. Applications: 1. Molecular weight estimation 2. Study micellar properties of drug. Under gravity Balance method – cumulative mass of settling particles is obtained as a function of time Practical lower limit is about 1 micron Under centrifugal force High Field – up to 4 x 105g is applied. Displacement of boundary is monitored with time Under low field ERYTHROCYTE SEDIMENTATION RATE Sedimentation rate is the rate at which red blood cells sediment in a period of one hour. To perform the test, anticoagulated blood is placed in an upright tube, known as a Westergren tube. The rate at which the red blood cells fall is measured and reported in mm/h. The ESR is governed by the balance between pro- sedimentation factors, mainly fibrinogen, and those factors resisting sedimentation, namely the negative charge of the erythrocytes (zeta potential). When an inflammatory process is present, the high proportion of fibrinogen in the blood causes red blood cells to stick to each other. The red cells form stacks called 'rouleaux,' which settle faster. Rouleaux formation can also occur in association with some lymphoproliferative disorders in which one or more immunoglobulin are secreted in high amounts. The ESR is increased by any cause or focus of inflammation. The ESR is increased in pregnancy, inflammation, anemia or rheumatoid arthritis, and decreased in sickle cell anemia and congestive heart failure. The basal ESR is slightly higher in females. Osmotic pressure The method is based on Van's Hoff's law; P = RTC / M From the equation; a) The osmotic pressure (P)depends on molar conc. of the solute (C) and on absolute temperature (T). b) The osmotic pressure is inversely proportional to molecular weight (M). R= molar gas constant The equation is valid for very dilute solutions in which the molecules do not interact mutually.  Osmotic pressure INTERNAL SOLUTION - It is a solution of an electrolyte containing non penetrating ions of about colloidal dimension and ions small enough to be penetrable. EXTERNALSOLUTION - It is a solution of electrolyte containing both penetrable ions. SEMIPERMEABLEMEMBRANE - It separates the two solutions and is permeable to all Crystalloid solutions are solutions of sugar or salt or mixture of salt and sugar in water. Crystalloids remain intravascular for short period and metabolize to carbon dioxide, water, ions and yields energy. In simple terms Osmotic pressure is exerted by salts to push fluid out Oncotic pressure is exerted by colloids to get fluid into the blood vessel  Osmotic Pressure Osmotic pressure of a colloidal solution is a colligative property useful in determination of molecular weight of dispersed phase. The Van’t Hoff equation is used to determine the molecular weight of colloids in dilute solution especially polymers. Van’t Hoff equation □ = cRT Where , c= concentration of solute particles in g/L □ = osmotic pressure exerted by colloidal particles. R= molar gas constant T= Absolute temperature Replacing c with C/M in equation. □ = C g RT/ M □ /Cg = RT/M Where, Cg is grams of solute per liter of solution. M= molecular weight □ = osmotic pressure R= Molar gas constant. Substituting values of experimental observations in this gives molecular weight of the colloidal particle  Viscosity Viscosity: the resistance to flow of a system under an applied pressure. Viscosity of colloids allow 1. Calculation of the molecular weight. 2. Provide useful information about the shape of the colloidal particles. N.B. Sphero colloidal dispersions are of relatively low viscosity. While linear colloidal dispersions are of high viscosity. If linear colloidal particles coil up into spheres ,the viscosity of the system falls due to a change in the shape. Viscosity is used to obtain molecular weight of material comprising disperse phase and the shape of particles in solution.  Viscosity Einstein developed an equation of flow applicable to colloidal dispersion of spherical particles: η= ηo ( 1+2.5φ) where ηo= viscosity of dispersion medium η= viscosity of dispersion when volume fraction is φ η can be measured by using viscometer. Relative viscosity= ηrel = η/ ηo= 1+2.5 φ Specific viscosity= ηsp = (η/ ηo)-1= 2.5 φ ηsp / φ = 2.5  A comparison Viscosity Osmotic Pressure The viscosity of colloids It depends on the number depends upon the shape of particles in dispersion. of colloidal particle. In associated colloids Spherical colloid particles each aggregate acts as show dispersion of low one particle and osmotic viscosity whereas linear pressure is small. particles shows more This can be used to viscous dispersion. calculate the MW of Viscosity increases are colloidal material (π=CRT) due to the degree of solvation.  Electrical Properties of Sols The sol particles carry an electric charge : The most important property of colloidal dispersions is that all the suspended particles posses either a positive or a negative charge. The mutual forces of repulsion between similarly charged particles prevent them from aggregating and settling under the action of gravity. This gives stability to the sol.  Electrical Properties a) Electrical properties of interfaces. b) The electrical double layer.  Electrical Properties of Interfaces Most surfaces acquire a surface electric charge when brought into contact with an aqueous medium, the principal charging mechanisms being as follows; 1) Ion dissolution. 2) Ionization. 3) Ion adsorption.  Ionization Here the surface charge of colloidal particles is controlled by the ionization of surface groupings. Examples; a) polystyrene latex has carboxylic acid group at the surface, ionize to give negatively charged particles. b) acidic drugs as ibuprofen and nalidixic acid acquire surface negative charged particles. c) Amino acids and proteins have carboxyl and amino groups which ionize to give –COO- and NH3 + ions. d) Their ionization and hence the net molecular charge depends on the pH as follow; NH2-R-COO- ↔NH3+-R-COO- ↔ NH3+-R-COOH  NH2-R-COO- ↔NH3+-R-COO- ↔ NH3+-R-COOH NH2-R-COO- ↔NH3+-R-COO- ↔ NH3+-R-COOH Represented as follows : R – NH2 – COO- Alkaline Solution ↓↑ R– NH3+ – COO- Isoelectric point (zwitterion) ↓↑ R – NH3+ – COOH Acidic solution At high PH ,Alkaline medium ,Negatively charged COOH →COO- Zwitter ion,Iso electric point, Zero charge At low PH ,Acidic medium ,Positively charged NH2 →  Suit ionization At a certain definite pH, specific for each individual protein, the total number of positive charges will equal the total number of negative charges, and the net charge will be zero. This pH is termed the isoelectric point of the protein, and the protein exists as its zwitterion. A protein is least soluble (the colloidal sol is least stable) at its isoelectric point Iso electric point: a) pH at which +ve charges = -ve charges, b) i.e. net charge of the amino acid = zero. c) It is a definite pH specific for each protein. d) At this pH protein is least soluble and precipitated. Q; How can you precipitate insulin??? BY ADJUSTING THE pH OF the SOLUTION TO THE ISO ELECTRIC POINT OF INSULIN (PH 5.2).  Ion adsorption A net surface charge can be acquired by the unequal adsorption of oppositely charged ions. Surfaces in water are more often negatively charged than positively charged, because cations are generally more hydrated than anions. Hence cations have the greater tendency to reside in the bulk aqueous medium, whereas the smaller, less hydrated and more polarizing anions have a greater tendency to reside at the particle surface. Surface-active agents are strongly adsorbed and have a pronounced influence on the surface charge, imparting either a positive or negative charge depending on their ionic character.  Ion dissolution Ionic substances can acquire a surface charge by virtue of unequal dissolution of the oppositely charged ions of which they are composed. Surface charge of colloidal particle is controlled by the charge of ion present in excess in the medium. Examples; AgNo3 + NaI → AgI +NaNo3 a) The particles of silver iodide in a solution with excess [I -] will carry a negative charge, but the charge will be positive if excess [Ag + ] is present. b) Aluminum hydroxide in a solution with excess hydroxide particles will acquire a –ve charge and vice versa. Since, the concentrations of Ag + and I- determine the electric potential at the particle surface, they are termed potential determining ions. Similarly, H+ and OH- are potential determining ions for metal oxides and hydroxides such as magnesium and aluminum hydroxides.  Silver iodide sols can be prepared by the reaction, AgNo3 + NaI → AgI +NaNo3 In the bulk of AgI particles 1 : 1 ratio of Ag+ and I – If the reaction is carried out with an excess silver nitrate, there will be more Ag+ than l- ions in the surface of the particles The particles will thus be positively charged and the counterions surrounding them will be N03-. The combination of the positively charged surface and the atmosphere of counter ions surrounding it is called the electric double layer. If the reaction is carried out with an excess NaI, there will be more l-than Ag+ ions in the surface of the particles The particles will thus be negatively charged and the counter ions surrounding them will be Na+  Electrical Double Layer The surface of colloidal particle acquires a positive charge by selective adsorption of a layer of positive ions around it. This layer attracts counterions from the medium which form a second layer of negative charges. More recent considerations have shown that the double layer is made of - A Compact layer of positive and negative charges which are fixed firmly on the particle surface. - A Diffuse layer of counterions (negative ions) diffused into the medium containing positive ions. The combination of the compact and diffuse layer is referred to as the Stern Double layer previously known as the Helmholtz Double layer.  Electrical Double Layer Because of the distribution of the charge around the particle, there is a difference in potential between the compact layer and the bulk of solution across the diffuse layer. This is known as Electrokinetic or Zeta potential. Development of a net charge at the particle surface affects the distribution of ions in the surrounding interfacial region, As a result: concentration of counter ions increase at the surface, Thus, an electrical double layer exists around each particle.  Electrical Properties of Colloids The movement of a charged surface with respect to the adjacent liquid phase is the basic principle underlying four electrokinetic phenomena : 1. Electrophoresis 2. Electroosmosis 3. Sedimentation Potential 4. Streaming Potential  ELECTROPHORESIS ELECTROPHORESIS : The movement of sol particles towards a particular electrode under the influence of an applied electric potential is called electrophoresis or cataphoresis. Thus, by noting the direction of movement of the sol particles, it can be determined whether they carry a positive or negative charge. If the colloidal particles carry positive charge, they move towards cathode when subjected to an electric field and vice versa.  ELECTROSMOSIS A sol is electrically neutral. Therefore, the dispersion medium carries an equal but opposite charge to that of the dispersed particles, whether they carry a positive or negative charge. The movement of dispersion medium under the influence of an applied electric field in the situation when the movement of colloidal particles is prevented with the help of a suitable membrane is known as electroosmosis. During electrosmosis, colloidal particles are checked, and it is the dispersion medium that moves towards the oppositely charged electrode.  Streaming Potential Streaming Potential: Differs from electro-osmosis in that the potential is created by forcing a liquid to flow through a bed or plug of particles.  Sedimentation Potential The sedimentation potential also called the (Donnan effect). It is the potential induced by the fall of a charged particle under an external force field. If a colloidal suspension has a gradient of concentration (such as is produced in sedimentation or centrifugation), then a macroscopic electric field is generated by the charge imbalance appearing at the top and bottom of the sample column.  GIBBS– DONNAN MEMBRANE EQULIBRIUM The Gibbs–Donnan effect (also known as the Donnan's effect,) is a name for the behavior of charged particles near a semi-permeable membrane that sometimes fail to distribute evenly across the two sides of the membrane. Some ionic species can pass through the barrier while others cannot. E.g gels or colloids as well as solutions of electrolytes, and as such the phase boundary between a gel and a liquid, can also act as a selective barrier. The electric potential arising between two such solutions is called the Donnan potential. The usual cause is the presence of a different charged substance that is unable to pass through the membrane and thus creates an uneven electrical charge. For example, the large anionic proteins in blood plasma are not permeable to capillary walls. Because small cations are attracted, but are not bound to the proteins, small anions will cross capillary walls away from the anionic proteins more readily than small cations. GIBBS– DONNAN MEMBRANE EQULIBRIUM PHYSICAL STABILITY OF COLLOIDAL SYSTEMS  Stability of Colloids (SOLS) Stabilization serves to prevent colloids from aggregation. The presence and magnitude, or absence of a charge on a colloidal particle is an important factor in the stability of colloids. Two main mechanisms for colloid stabilization: 1. Steric stabilization i.e. surrounding each particle with a protective solvent sheath which prevent adherence due to Brownian movement. 2. Electrostatic stabilization i.e. providing the particles with electric charge  Stability of Colloidal (SOLS) A- Lyophobic sols: - Unstable. - The particles are stabilized only by the presence of electrical charges on their surfaces through the addition of small amounts of electrolytes. - The like charges produce repulsion which prevent coagulation of the particles and subsequent settling. Coagulation also result from mixing of oppositely charged colloids. B- Lyophilic sols and association colloids: - Stable - Present as true solution - Addition of moderate amounts of electrolytes do not cause coagulation (opposite lyophobic)  Stability of Colloidal (SOLS) Salting out Definition: agglomeration and precipitation of lyophilic colloids. This is obtained by: 1. Addition of large amounts of electrolytes - Anions arranged in a decreasing order of precipitating power: citrate > tartrate > sulfate > acetate > chloride> nitrate > bromide > iodide - The precipitation power is directly related to the hydration of the ion and its ability to separate water molecules from colloidal particles 2. addition of less polar solvent - e.g. alcohol, acetone  DLVO Theory In considering the interaction between two colloidal particles, the stability of hydrophobic sols can be determined according to the DLVO Theory of colloid stability. This theory was developed to describe stability of lyophobic colloids by Derjaguin and Landau and later by Verwey and Overbeek. Based on two principles: 1. Electrostatic repulsive force Vr - Repulsion between particles arises because of the osmotic effect produced by the increase in the number of charged species on overlap of the diffuse parts of the electrical double layer 2.Van der waal force of attraction Va - The energy of attraction, FA, arises from van der Waals universal forces of attraction. According to this theory, the distance between two dispersed particle influences particle- particle interaction. In a colloidal dispersion ,the Brownian motion result in frequent collision between particle. Interactions like attraction and repulsion are responsible for stability of colloids.  Hardy-Schulze rule By addition of Electrolytes. Addition of electrolytes beyond necessary for maximum stability results in accumulation of opposite ions and decrease zeta potential coagulation precipitation of colloids. The electrolyte contains both positive and negative ions in the medium ,the sol particles adsorb the oppositely charged ions and get discharged. The electrically neutral particles then aggregate and settle down as precipitate. From a study of the precipitating action of various electrolytes on particular sol, Hardy and Schulze gave a general rule. Lyophilic colloids → Addition of large amount of electrolyte solution → Forms precipitate. Lyophobic colloids → Addition of small amount of electrolyte solution → Forms precipitate. Hardy-Schulze Rule states that the precipitating effect of an ion on  COAGULATION OR FLOCCULATION The stability of a lyophobic sol is due to the adsorption of positive or negative ions by the dispersed particles. The repulsive forces between the charged particles do not allow them to settle. If, some how, the charge is removed, there is nothing to keep the particle apart from each other. They aggregate (or flocculate) and settle down under the action of gravity. 1. Aggregation is a general term signifying the collection of particles into groups. 2. Coagulation is the destabilization of colloids by addition of an electrolyte which causes precipitation because of rapid mixing. 3. Flocculation is the agglomeration of destabilized particles into a large size particles known as flocs by slow mixing which can be effectively removed by sedimentation or flotation In flocculation the aggregates have an open structure in which the particles remain a small distance apart from one another.  Coagulation and Precipitation The coagulation of colloidal solution can also be achieved by any of the following methods. - By electrophoresis - By mixing two oppositely sols - By persistent dialysis - By boiling (a) By Electrophoresis. In electrophoresis the charged sol particles migrate to the electrode of opposite sign. As they come in contact with the electrode, the particles are discharged and precipitated. (b) By mixing two oppositely charged sols. The mutual coagulation of two sols of opposite charge can be effected by mixing them. The positive particles of one sol are attracted by the negative particles of the second sol. This is followed by mutual adsorption and precipitation of both the sols. Ferric hydroxide (+ve sol) and arsenious sulphide (–ve sol) form such a pair. ( c) By addition of Electrolytes. When excess of an electrolyte is added to a sol, the dispersed particles are precipitated. The electrolyte furnishes both positive and negative ions in the medium. The sol particles adsorb the oppositely charged ions and get discharged. The electrically neutral particles then aggregate and settle down as precipitate. From a study of the precipitating action of various electrolytes on particular sol, Hardy and Schulze gave a general rule. Hardy-Schulze Rule states that the precipitating effect of an ion on dispersed phase of opposite charge increases with the valence of the ion. The higher the valency of the effective ion, the greater is its precipitating power. (d) By boiling. Sols such as sulphur and silver halides dispersed in water, may be coagulated by boiling. Increased collisions between the sol particles and water molecules remove the adsorbed electrolyte. This takes away the charge from the particles which settle down Potential Energy Curves  Coacervation and Microencapsulation Coacervation is the separation of a colloid-rich layer from a lyophilic sol on the addition of another substance. It is the process of mixing negatively and positively charged hydrophilic colloids, and hence the particles separate from the dispersion to form a layer rich in the colloidal aggregates (coacervate) Complex coacervation occurs when two oppositely charged lyophilic colloids are mixed, e.g. gelatin and acacia.. Any large ions of opposite charge, for example cationic surface-active agents (positively charged) and dyes used for colouring aqueous mixtures (negatively charged), may react in a similar way. If the coacervate is formed in a stirred suspension of an insoluble solid the macromolecular material will surround the solid particles. The coated particles can be separated and dried, and this technique forms the basis of one method of microencapsulation. Several drugs, including aspirin, have been coated in this manner. The coating protects the drug from chemical attack, and microcapsules may be given orally to prolong the action of the medicament.  Sensitization and Protective Colloidal Action : Sensitization: the addition of small amount of hydrophilic or hydrophobic colloid to a hydrophobic colloid of opposite charge tend to sensitize(coagulate) the particles. Polymer flocculants can bridge individual colloidal particles by attractive electrostatic interactions. For example, negatively-charged colloidal silica particles can be flocculated by the addition of a positively-charged polymer.  Sensitization and Protective Colloidal Action Protection /Protective colloids by addition of hydrophilic colloid A larger concentration of hydrophilic colloids increase the stability of hydrophobic colloids towards precipitation of electrolytes. This may be due to the hydrophobic particles. The hydrophile being adsorbed as a monomolecular layer on the surface of hydrophobic colloids particles and forming a protective layer, Thus preventing them from precipitation on addition of an electrolyte, this phenomenon is called protection which increases stability of lyophobic colloids. The hydrophilic solution used for the purpose of protecting the hydrophobic colloid is known as the protective colloid.  ADVANTAGES & DISADVANTAGES OF COLLOIDAL PREPARATIONS Advantages Disadvantages 1.Higher degree of catalytic activity: Due 1. As colloids are small in particle to increased surface area in colloidal size so it is easily absorbed and preparation, the activity of a catalyst is gives extensive bioavailability generally accelerated. which may support toxicity. 2.Color: Colloidal preparations generally 2. Preparation of lyophobic colloid is possess attractive color. difficult. Stabilization of colloids is 3.Taste: Colloidal preparation may also be often difficult as it may be used to pronounce the taste of a destabilized by a lot of factors pharmaceutical preparation. (radiation, heat, drying etc.) 4.Better solubility, absorption and 3. There is a great restriction on the bioavailability particle size of the particles. 5.Compatibility with biological system: Ionic silver salt may itself produce toxicity-argyria and less bioavailability due to the formation of silver chloride which is insoluble and rapidly excreted from body. But it does not occur when colloidal preparations are used.  Additional Advantages of Colloids Colloids allow the dispersion of normally insoluble materials, such as metallic gold or fats. These can then be used more easily or absorbed more easily. Colloidal gold, for example, can be used in medicine to carry drugs and antibiotics, because it is highly non- reactive and non-toxic. Pharmaceutical industry makes use of colloidal solution preparation in many medicines. A wide variety of medicines are emulsions. An example is Cod Liver Oil. Paint industry also uses colloids in the preparation of paints. In milk, the colloidal suspension of the fats prevents the milk from being thick and allows for easy absorption of the nutrients. Sewage water contains particles of dirt, mud etc. which are colloidal in nature and carry some electrical charge. These particles may be removed by using the phenomenon of electrophoresis. The sky is the empty space around earth and as such has no colour. It appears blue due to the scattering of light by the colloidal dust particles present in air (Tyndall effect). Asphalt emulsified in water and is used for building roads. The sugar present in milk produces lactic acid on fermentation. Ions produced by acid, destroy the charge on the colloidal particles present in milk, which then coagulate and separate as curd. Soap solution is colloidal in nature. It removes the dirt particles either by adsorption or by emulsifying the greasy matter sticking to the cloth. Large numbers of food particles which we use in our daily life are colloidal in nature. Example: Milk, butter, & ice cream.  Application of Colloids Area Example Paints, Adhesives, Floor polishes, Print inks, Carpet backing, Industrial Paper sizing, Water/sewage treatment, Secondary oil recovery, Rubberized concrete Size standard for electron microscopy Research High resolution chromatography column packing Magnetic particles targeted drug Medical delivery Immunodiagnostics Controlled released drugs Catalysts, Colloids & surface chemistry, Coagulation kinetics Chemical Model liquid crystals, Rheology, Dielectric spectroscopy Particle interaction *Dispersion forces *Electrostatics  Pharmaceutical Applications Colloidal silver preparation are effective germicides and do not cause GI irritation that is the characteristic of ionic silver state. Colloidal preparations are used in treatment and diagnosis of diseases. Example , Colloidal Hg (for syphilis),Colloidal Cu (In the treatment of cancer) Protein is a colloidal preparation. Plasma protein binds with certain drugs in our body, which affects the pharmacological activity of the drug. Colloidal hydroxyethyl starch (HES) are used as plasma substitutes. Colloidal macromolecules are used for coating purpose of the pharmaceutical products. Colloidal electrolytes are sometimes used to increase the solubility, stability and taste of certain products. Colloidal Al(OH)3 shows better rate of neutralization of stomach acid. Dextran injection is a colloidal dispersion which acts plasma substitute.