chapter 16 Colloidal Dispersions.pdf

Physical Pharmacy Colloidal Dispersions chapter 16 chapter 16 | Colloidal Dispersions Contents : Colloidal Dispersions 3 Classification of Dispersed Systems Based on Particle Size 6 Types of Colloidal Dispersions 13 Lyophilic Colloids 18 Lyophobic Colloids 22 Association Colloid 24 Kinetic Propertie...

Physical Pharmacy Colloidal Dispersions chapter 16 chapter 16 | Colloidal Dispersions Contents : Colloidal Dispersions 3 Classification of Dispersed Systems Based on Particle Size 6 Types of Colloidal Dispersions 13 Lyophilic Colloids 18 Lyophobic Colloids 22 Association Colloid 24 Kinetic Properties of Colloids 31 1. Thermally induced 32 2. Gravitationally induced 42 3. Applied externally 45 4. Electrically induced motion 49 chapter 16 | Colloidal Dispersions Colloidal Dispersions : Dispersed Systems Dispersed systems consist of particulate matter, known as the dispersed phase, distributed throughout a continuous or dispersion medium. The dispersed material may range in size from particles of atomic and molecular dimensions to particles whose size is measured in millimeters. Accordingly, a convenient means of classifying dispersed systems is on the basis of the mean particle diameter of the dispersed material. chapter 16 | Colloidal Dispersions 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. The size ranges assigned to these classes, together with some of the associated characteristics, are shown in the accompanying table chapter 16 | Colloidal Dispersions chapter 16 | Colloidal Dispersions Classification of Dispersed Systems Based on Particle Size : Class Molecular dispersion Particle Size* Characteristics of System Less than 1 nm 1. Invisible in electron microscope 2. Pass through ultrafilter and semipermeable membrane 3. Undergo rapid diffusion Examples Oxygen molecules , ordinary ions, glucose chapter 16 | Colloidal Dispersions Classification of Dispersed Systems Based on Particle Size : Class Particle Size* Colloidal dispersion From 1 nm to 0.5 µm Characteristics of System 1. Not resolved by ordinary microscope (although may be detected under ultramicroscope) Visible in electron microscope 2. Pass through filter paper 3. Do not pass semipermeable membrane 4. Diffuse very slowly Examples Colloidal silver sols, natural and synthetic polymers, cheese, butter, jelly, paint, milk, shaving cream, etc. chapter 16 | Colloidal Dispersions Classification of Dispersed Systems Based on Particle Size : Class Particle Size* Characteristics of System 1. Visible under microscope 2. Do not pass through normal Coarse Greater than dispersion 0.5µm filter paper 3. Do not pass through semipermeable membrane 4. Do not diffuse 1 *nm (nanometer) = 10-9 m; 1 µm (micrometer) = 10-6 m. Examples Grains of sand, most pharmaceutical emulsions and suspensions, red blood cells chapter 16 | Colloidal Dispersions Diffusion through a semipermeable membrane : Because the size of colloidal particles, they can be separated from molecular particles by the use of semipermeable membrane. This technique is known as Dialysis. As shown in the following figure chapter 16 | Colloidal Dispersions Conditions on the two sides, A and B, of the membrane are shown at the start and at equilibrium. The open circles are the colloidal particles that are too large to pass through the membrane. The solid dots are the electrolyte particles that pass through the pores of the membrane. chapter 16 | Colloidal Dispersions Dialysis occurs in vivo. Thus, ions and small molecules pass readily from the blood, through a natural semipermeable membrane, to the tissue fluids; the colloidal components of the blood remain within the capillary system. The principle of dialysis is utilized in the artificial kidney, which removes low– molecular-weight impurities from the body by passage through a semipermeable membrane. Types of Colloidal Dispersion chapter 16 | Colloidal Dispersions Types of Colloidal Dispersions* (According to the type of dispersed phase and dispersion medium) Dispersed Phase Dispersion Medium Colloid Type Examples Solid Solid Solid sol Pearls, opals Liquid Solid Solid emulsion Gas Solid Solid foam Pumice, marshmallow Solid Liquid Sol, gel Jelly, paint Cheese, butter chapter 16 | Colloidal Dispersions Dispersed Phase Dispersion Medium Colloid Type Examples Liquid Liquid Emulsion Gas Liquid Foam Solid Gas Solid aerosols Whipped cream, shaving cream Smoke, dust Liquid Gas Liquid aerosols Clouds, mist, fog Note: a gas in gas always produce a solution Milk, mayonnaise chapter 16 | Colloidal Dispersions Types of Colloidal Dispersion : (According to the shape of dispersed particles) The shape adopted by particles in dispersion is important because the more extended the particle, the greater is its specific surface and the greater is the opportunity for attractive forces to develop between the particles of the dispersed phase and the dispersion medium. Some representative shapes of sphero colloids and fibrous colloids are shown in Figure below chapter 16 | Colloidal Dispersions chapter 16 | Colloidal Dispersions Some shapes that can be assumed by colloidal particles: A. spheres and globules, B. short rods and prolate ellipsoids, C. oblate ellipsoids and flakes, D. long rods and threads, E. loosely coiled threads, and F. branched threads. chapter 16 | Colloidal Dispersions Types of Colloidal Systems (According to the interaction between the dispersed phase and dispersion medium) Lyophilic Colloids Systems containing colloidal particles that interact to an appreciable extent with the dispersion medium are referred to as lyophilic (solvent-loving) colloids. Owing to their affinity for the dispersion medium, such materials form colloidal dispersions, or sols, with relative ease. chapter 16 | Colloidal Dispersions Thus, lyophilic colloidal sols are usually obtained simply by dissolving the material in the solvent being used. For example, the dissolution of acacia or gelatin in water leads to the formation of a sol. The various properties of this class of colloids are due to the attraction between the dispersed phase and the dispersion medium, which leads to solvation ( the attachment of solvent molecules to the molecules of the dispersed phase). In the case of hydrophilic colloids, in which water is the dispersion medium, this is term is known as hydration. chapter 16 | Colloidal Dispersions (( One of the most important property of lyophilic colloid is the presence of solvent sheath)) Most lyophilic colloids are organic molecules, for example, gelatin, acacia, insulin, albumin, rubber, and polystyrene. Of these, the first four produce lyophilic colloids in aqueous dispersion media (hydrophilic sols). Rubber and polystyrene form lyophilic colloids in nonaqueous, organic solvents. chapter 16 | Colloidal Dispersions These materials accordingly are referred to as lipophilic colloids. These examples illustrate the important point that the term lyophilic has meaning only when applied to the material dispersed in a specific dispersion medium. A material that forms a lyophilic colloidal system in one liquid (e.g., water) may not do so in another liquid (e.g., benzene). chapter 16 | Colloidal Dispersions Lyophobic Colloids : The second class of colloids is composed of materials that have little attraction, if any, for the dispersion medium. These are the lyophobic (solvent-hating) colloids and, so, their properties differ from those of the lyophilic colloids. This is primarily due to the absence of a solvent sheath around the particle. chapter 16 | Colloidal Dispersions Lyophobic colloids are generally composed of inorganic particles dispersed in water. Examples of such materials are gold, silver, sulfur, arsenous sulfide, and silver iodide. In contrast to lyophilic colloids, it is necessary to use special methods to prepare lyophobic colloids. chapter 16 | Colloidal Dispersions Association Colloid : Association or amphiphilic colloids form the third group in this classification. They are certain organic molecules or ions, termed amphiphiles or surfaceactive agents, are characterized by having two distinct regions of opposing solution affinities within the same molecule or ion. chapter 16 | Colloidal Dispersions When present in a liquid medium at low concentrations, the amphiphiles exist separately and are of such a size as to be subcolloidal. As the concentration is increased, aggregation occurs over a narrow concentration range. These aggregates, which may contain 50 or more monomers, are called micelles. Because the diameter of each micelle is of the order of 50 Å, micelles lie within the size range of colloidal particles , such compounds are called association colloidal. chapter 16 | Colloidal Dispersions Lyophilic Lyophobic Association (Amphiphilic ) Dispersed phase consists generally of large organic molecules lying within colloidal size range Dispersed phase Dispersed phase consists of ordinarily consists of aggregates (micelles) of small inorganic particles, such organic molecules or ions as gold or silver whose size individually is below the colloidal range Molecules of dispersed phase are solvated, i.e., they are associated with the molecules comprising the dispersion medium Little if any interaction occurs between particles and dispersion medium (no solvation ) Hydrophilic or lipophilic portion of the molecule is solvated, depending on whether the dispersion medium is aqueous or nonaqueous chapter 16 | Colloidal Dispersions Lyophilic Molecules disperse spontaneously to form colloidal solution Lyophobic Material does not disperse spontaneously, and special procedures therefore must be adopted to produce colloidal dispersion Association (Amphiphilic) Colloidal aggregates are formed spontaneously when the concentration of amphiphile exceeds the critical micelle concentration chapter 16 | Colloidal Dispersions Lyophilic Lyophobic Association (Amphiphilic) Viscosity of the dispersion medium ordinarily is increased greatly by the presence of the dispersed phase; at sufficiently high concentrations, the sol may become a gel; viscosity and gel formation are related to solvation effects and to the shape of the molecules, which are usually highly asymmetric Viscosity of the dispersion medium is not greatly increased by the presence of lyophobic colloidal particles, which tend to be unsolvated and symmetric Viscosity of the system increases as the concentration of the amphiphile increases, as micelles increase in number and become asymmetric chapter 16 | Colloidal Dispersions Lyophilic Dispersions are stable generally in the presence of electrolytes; they may be salted out by high concentrations of very soluble electrolytes; effect is due primarily to desolvation of lyophilic molecules Lyophobic Lyophobic dispersions are unstable in the presence of even small concentrations of electrolytes; effect is due to neutralization of the charge on the particles; lyophilic colloids exert a protective effect Association (Amphiphilic) In aqueous solutions, the critical micelle concentration is reduced by the addition of electrolytes; salting out may occur at higher salt concentrations Kinetic Properties of Colloids chapter 16 | Colloidal Dispersions Kinetic Properties of Colloids There are several properties of colloidal systems that relate to the motion of particles with respect to the dispersion medium. The motion may be:1. Thermally induced 2. Gravitationally induced 3. Applied externally 4. Electrically induced motion chapter 16 | Colloidal Dispersions 1. Thermally induced (increased by increasing temp) : Brownian motion : Brownian motion describes the random movement of colloidal particles. The erratic motion, resulting from the bombardment of the particles by the molecules of the dispersion medium. chapter 16 | Colloidal Dispersions The motion of the molecules cannot be observed, of course, because the molecules are too small to see. The velocity of the particles increases with decreasing particle size, while increasing the viscosity of the medium decreases and finally stops the Brownian movement. chapter 16 | Colloidal Dispersions Diffusion : Particles diffuse spontaneously from a region of higher concentration to one of lower concentration until the concentration of the system is uniform throughout. Diffusion is a direct result of Brownian movement, therefore (all factors affect Brownian movement will affect diffusion) chapter 16 | Colloidal Dispersions According to Fick's first law, the amount (dq) of substance diffusing in time (dt) across a plane of area (S) is directly proportional to the change of concentration, dc, with distance traveled, dx. Fick's law is written as dq = - g=-D 𝑔/𝑐𝑚3 2 cm sec 𝑐𝑚 𝒅𝒄 DS 𝒅𝒙 dt →→ D = cm2 / sec. (Units) D is the diffusion coefficient, the amount of material diffusing per unit time across a unit area when dc/dx, called the concentration gradient, is unity. chapter 16 | Colloidal Dispersions It is normal for the concentration curve to increase or decrease sharply at the boundaries of the barrier because, in general, C1 is different from Cd, and C2 is different from Cr chapter 16 | Colloidal Dispersions D thus has the dimensions of area per unit time. Negative sign in the equation, signifies that the diffusion occurs in decreasing concentration of diffusion The passage of a substance may be through porous membrane or through tortuous pores or channels (example. Skin). Smaller particles diffuse faster in a given media Molecular > colloid > coarse chapter 16 | Colloidal Dispersions Passive diffusion caused by a concentration gradient and carried out through Brownian motion It is important for the release of drug from topical preparation and in the GIT absorption of drugs chapter 16 | Colloidal Dispersions chapter 16 | Colloidal Dispersions Osmosis : It is the action in which only the solvent is transferred, while the diffusion involve the passage of solute. The osmotic pressure, π, of a dilute colloidal solution is described by the van't Hoff equation: π = cRT where c is molar concentration of solute , R, gas constant = 0.082 atm.mole/ L.deg This equation can be used to calculate the molecular weight of a colloid in a dilute solution. chapter 16 | Colloidal Dispersions chapter 16 | Colloidal Dispersions 2. Gravitationally induced : Sedimentation : The velocity, v, of sedimentation of spherical particles having a density ρ in a dispersion medium of density ρ0 and a viscosity η0 is given by Stokes's law : V= 𝟐𝒓𝟐 ( 𝝆 –𝝆𝟎) 𝒈 𝟗 𝜼𝟎 r = radius of particle, ρ = density of particle, ρ0 = density of the dispersion medium η0 = viscosity of the dispersion medium, g = is the acceleration due to gravity chapter 16 | Colloidal Dispersions Factors affecting sedimentation : Particle size ( ↓ particle size → ↓ sedimentation ). Viscosity ( ↑ viscosity → ↓ sedimentation). Difference between the densities of the dispersed particles and the dispersion medium ( ↑ difference → ↑ sedimentation ). chapter 16 | Colloidal Dispersions If the particles are subjected only to the force of gravity, then the lower size limit of particles obeying Stokes's equation is about 0.5 µm. This is because Brownian movement becomes significant and tends to offset sedimentation due to gravity and promotes mixing instead. Consequently, a stronger force must be applied to bring about the sedimentation of colloidal particles in a quantitative and measurable manner. chapter 16 | Colloidal Dispersions This is accomplished by use of the ultracentrifuge, which can produce a force one million times that of gravity. 3. Applied externally Viscosity : Viscosity is an expression of the resistance to flow of a system under an applied stress. The more viscous a liquid is, the greater is the applied force required to make it flow at a particular rate. chapter 16 | Colloidal Dispersions Viscosity study can give us information about The shape of particles in dispersion. The molecular weight of the dispersed phase. The shape of particles in dispersion : The shapes of particles of the disperse phase affect the viscosity of colloidal dispersions. Spherocolloids form dispersions of relatively low viscosity, whereas systems containing linear particles are more viscous. chapter 16 | Colloidal Dispersions The relationship of shape and viscosity reflects the degree of solvation of the particles. If a linear colloid is placed in a solvent for which it has a low affinity, it tends to “ball up,” that is, to assume a spherical shape, and the viscosity falls. This provides a means of detecting changes in the shape of flexible colloidal particles and macromolecules. chapter 16 | Colloidal Dispersions The molecular weight of the dispersed phase : The molecular weight of the is measured by using viscometer and the molecular weight obtained by this technique is called viscosity average molecular weight. The molecular weight of the polymer solution is very high so the viscosity of polymer solution is very high compared to that of pure solvent. From the Mark-Houwink equation the relationship between the molecular weight and viscosity is given below [ɳ] =KMα chapter 16 | Colloidal Dispersions Where *ɳ+ is the intrinsic viscosity, M is the molecular weight, K and α are constants for a particular polymer solvent system. If we know the k and α values for a given polymer solution, the intrinsic viscosity and molecular weight can be calculated using the above equation. 4. Electrically induced motion : It is the movement of charged surface with respect to an adjacent liquid phase. One of the most important application for such movement is the electrophoresis.