Colloid Lect 5 PDF

Dr. Doaa Hussien Hassen Department of Pharmaceutics and Industrial Pharmacy, College of Pharmacy & Pharmaceutical Manufacture Misr University or Science & Technology...

Dr. Doaa Hussien Hassen Department of Pharmaceutics and Industrial Pharmacy, College of Pharmacy & Pharmaceutical Manufacture Misr University or Science & Technology Email: [email protected] Dr. Doaa Hussien Hassen 4/6/2022 1 4/6/2022 Dr. Doaa Hussien Hassen 2  PROPERTIES OF COLLOIDS Optical Kinetic Electric Ultra- Electron Brownian Osmotic Diffusion Viscosity Sedimentation microscopy microscopy motion pressure 3 4/6/2022 Dr. Doaa Hussien Hassen  Light scattering: When a beam of light is passed through a colloidal sol some of the light may be absorbed (when light of certain wavelengths is selectively absorbed a colour is produced), some is scattered and the remainder is transmitted undisturbed through the sample. Because of the scattered light the sol appears turbid: this is known as the Tyndall effect. 4/6/2022 Dr. Doaa Hussien Hassen 4 4/6/2022 Dr. Doaa Hussien Hassen 5 4/6/2022 Dr. Doaa Hussien Hassen 6  Light scattering measurements are of great value for estimating particle size, shape and interactions, particularly of dissolved macromolecular materials, as the turbidity depends on the size (molecular weight) of the colloidal material involved. Measurements are simple in principle but experimentally difficult because of the need to keep the sample free from dust, the particles of which would scatter light strongly and introduce large errors. 4/6/2022 Dr. Doaa Hussien Hassen 7  Ultra-microscopy: Colloidal particles are too small to be seen with an optical microscope. Light scattering is made use of in the ultramicroscope, in which a cell containing the colloid is viewed against a dark background at right angles to an intense beam of incident light. The particles, which exhibit Brownian motion, appear as spots of light against the dark background. 4/6/2022 Dr. Doaa Hussien Hassen 8  Ultra-microscopy: 4/6/2022 Dr. Doaa Hussien Hassen 9  Ultra-microscopy: Although the particles cannot be seen directly, the bright spots corresponding to particles can be observed and counted. The average particle size can be calculated using: The conc. of colloidal solution. Dimension of cell used. The number of particle per unit weight of dispersion 4/6/2022 Dr. Doaa Hussien Hassen 10 4/6/2022 Dr. Doaa Hussien Hassen 11  Electron microscopy: The electron microscope, capable of giving actual pictures of the particles, is used to observe the size, shape and structure of colloidal particles. The success of the electron microscope is due to its high resolving power, defined in terms of d, the smallest distance by which two objects can be separated yet remain distinguishable. 4/6/2022 Dr. Doaa Hussien Hassen 12  Electron microscopy: The smaller the wavelength of the radiation used the smaller is d and the greater the resolving power. The radiation source of the electron microscope is a beam of high energy electrons having wavelengths in the region of 0.01 nm, d is thus about 0.5 nm 4/6/2022 Dr. Doaa Hussien Hassen 13  Electron microscopy: 4/6/2022 Dr. Doaa Hussien Hassen 14  These include several properties of colloidal systems that relate to the motion of particles with respect to the dispersion medium 4/6/2022 Dr. Doaa Hussien Hassen 15  Colloidal motions are divided into: Thermal motions include: Brownian motion, diffusion and osmosis. Gravity (or a centrifugal field) leads to sedimentation. Electrically induced motions. Applied externally (viscosity). 4/6/2022 Dr. Doaa Hussien Hassen 16  Colloidal particles are subject to random collisions with the molecules of the dispersion medium, with the result that each particle pursues an irregular and complicated zigzag path. 4/6/2022 Dr. Doaa Hussien Hassen 17  If the particles (up to about 2 um diameter) are observed under a microscope or the light scattered by colloidal particles is viewed using an ultramicroscope, an erratic motion is seen. This movement is referred to as Brownian motion. The velocity of the particles increase, with decreasing the particle size and viscosity 4/6/2022 Dr. Doaa Hussien Hassen 18  As a result of Brownian motion colloidal particles spontaneously diffuse from a region of higher concentration to one of lower concentration. The rate of diffusion is expressed by Fick's first law: 4/6/2022 Dr. Doaa Hussien Hassen 19  Fick's first law: dm is the mass of substance diffusing in time dt across an area A under the influence of a concentration gradient dC/dx (the minus sign denotes that diffusion takes place in the direction of decreasing concentration). D is the diffusion coefficient and has the dimensions of area per unit time. 4/6/2022 Dr. Doaa Hussien Hassen 20  If the colloidal particles are approximately spherical, the following equations can be used to obtain radius and molecular weight: Where: D = diffusion coefficient from Fick’s law R = molar gas constant η = = viscosity of solvent T = absolute temp r = radius of spherical particles N = Avogadros' number 4/6/2022 Dr. Doaa Hussien Hassen 21  Equation to obtain molecular weight: Where: M = mol WT V = partial specific volume (approx. equal to volume in cm3 of 1 gm solute obtained from density measurement). 4/6/2022 Dr. Doaa Hussien Hassen 22  If a solution and a solvent are separated by a semipermeable membrane, there will be tendency to equalize chemical potentials (and hence concentrations) on either side of the membrane results in a net diffusion of solvent across the membrane. The pressure necessary to balance this osmotic flow is termed osmotic pressure. 4/6/2022 Dr. Doaa Hussien Hassen 23 4/6/2022 Dr. Doaa Hussien Hassen 24  For a colloidal solution the osmotic pressure π can be described by: Where: C: the concentration of the solution, M: molecular weight of the solute and B:constant depending on the degree of interaction between the solvent and solute molecules. Thus a plot of π/C versus C is linear, with the value of the intercept as C — > 0 giving RT/M, enabling the molecular weight of the colloid to be calculated. The molecular weight obtained from osmotic pressure measurements is a number average value 4/6/2022 Dr. Doaa Hussien Hassen 25 4/6/2022 Dr. Doaa Hussien Hassen 26  The diffusion of small ions through a membrane will be affected by the presence of a charged macromolecule that is unable to penetrate the membrane because of its size. At equilibrium the distribution of the diffusible ions is unequal, being greater on the side of the membrane containing the non-diffusible ions. Consequently, unless precautions are taken to correct for this effect or to eliminate it, the results of osmotic pressure measurements on charged colloidal particles such as proteins will be invalid. 4/6/2022 Dr. Doaa Hussien Hassen 27  Viscosity is an expression of the resistance to flow of a system under an applied stress. An equation of flow applicable to colloidal dispersions of spherical particles was: η= ηo(1+2.5Φ) Where: η0 : the viscosity of the dispersion medium and η: the viscosity of the dispersion Φ: volume fraction of colloidal particles which equal to the volume of the particles divided by the total volume of the dispersion. 4/6/2022 Dr. Doaa Hussien Hassen 28  A number of viscosity coefficients may be defined with respect to that equation. These include: Relative viscosity: ηrel =η/ ηo=1+2.5Φ Specific viscosity: ηsp= η/ ηo -1=2.5Φ 4/6/2022 Dr. Doaa Hussien Hassen 29  The viscosity of colloidal dispersion is affected by the shapes of the particles of dispersed phase (as shape affect the degree of solvation) thus sphero-colloids form low viscosity dispersion while linear particles form high viscosity dispersion. Viscosity can detect changes in the shape of colloid partricles (e.g viscosity falls when linear colloids are placed in a poor solvent assuming spherical shape) 4/6/2022 Dr. Doaa Hussien Hassen 30  The characteristic of polymers used as substitutes for plasma (plasma extender) depend on:- 1. Molecular weight. 2. Size and shape of macromolecule. 3. The ability of polymer to impart the proper viscosity and osmotic pressure to the blood. 4/6/2022 Dr. Doaa Hussien Hassen 31  Consider a spherical particle of radius (a) and density (σ) falling in a liquid of density (ρ) and viscosity (η). The velocity v of sedimentation is given by Stokes' law: V= 2a2g(σ-ρ)/9η Where g is acceleration due to gravity. If the particles are subjected only to the force of gravity, then as a result of Brownian motion, the lower size limit of particles obeying Stock’s low is about 0.5 μm. 4/6/2022 Dr. Doaa Hussien Hassen 32  A stronger force than gravity is needed for colloidal particles to sediment, and use is made of a high-speed centrifuge, usually termed an ultracentrifuge, which can produce a force of about 106g. Ultracentrifugation is useful for:- 1- Determining the mol. Wt of polymers. 2- Determining the degree of homogeneity of the sample. 4/6/2022 Dr. Doaa Hussien Hassen 33  Particles dispersed in liquid may be charged due to: Ion dissolution: Ionic substances can acquire a surface charge due to unequal dissolution of the oppositely charged ions of which they are composed. 4/6/2022 Dr. Doaa Hussien Hassen 34  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. Because the concentrations of Ag+ and I-determine the electric potential at the particle surface, they are termed potential determining ions. In a similar way H+ and OH-are potential determining ions for metal oxides and hydroxides such as magnesium and aluminium hydroxides. 4/6/2022 Dr. Doaa Hussien Hassen 35  Ionization The charge is controlled by the ionization of surface groupings; For example, amino acids and proteins acquire their charge mainly through the ionization of carboxyl and amino groups to give –COO- and NH3+ ions. 4/6/2022 Dr. Doaa Hussien Hassen 36  The ionization of these groups and hence the net molecular charge depends on the pH of the system: At low pH : the protein will be positively charged because of the protonation of this group, -COO —> COOH, and the ionization of the amino group -NH2 —> -NH3 + At higher pH: the protein will be negatively charged because of carboxylic group exist as caroxylate –COOH —> COO- 4/6/2022 Dr. Doaa Hussien Hassen 37  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. This may be represented as follows: 4/6/2022 Dr. Doaa Hussien Hassen 38 4/6/2022 Dr. Doaa Hussien Hassen 39  A protein is least soluble (the colloidal sol is least stable) at its isoelectric point and is readily desolvated by very water- soluble salts such as ammonium sulphate. Thus insulin may be precipitated from aqueous alcohol at pH 5.2. Erythrocytes and bacteria usually acquire their charge by ionization of surface chemical groups such as sialic acid. 4/6/2022 Dr. Doaa Hussien Hassen 40  Ion adsorption: A net surface charge can be acquired by the unequal adsorption of oppositely charged ions. Surfaces of solutions in water are more often negatively charged than positively charged, because cations are generally more hydrated than anions. Consequently, the former 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. 4/6/2022 Dr. Doaa Hussien Hassen 41  I. The electrical double layer: Origin of electric charges on silver iodide solutions: Silver iodide solutions can be prepared by the reaction: AgNO3 + NaI AgI +NaNO3 In the bulk of silver iodide particles, there is a stoichiometric ratio of Ag+ to I-. If the reaction is carried out in an excess of I- over Ag+ ions, the particles will be negatively charged and they will attract the positively charged ions (counter-ions) from the solution and repels negative ions (co-ions). 4/6/2022 Dr. Doaa Hussien Hassen 42  I. The electrical double layer: 4/6/2022 Dr. Doaa Hussien Hassen 43  I. The electrical double layer: The solution in vicinity of the surface contains a much higher concentration of Na+, which will be the counter-ions. The number of Na+ions equal to the number of excess I- ions in the surface and equivalent to the net negative surface charge of the particle. The Na +concentration is highest in the immediate vicinity of the negative surface, where they form a compact layer called Stern Layer or tightly bound layer, and decrease with distance from the surface, throughout a diffuse layer called Gouy Chapman layer or more diffuse layer. The combination of the two layers of the oppositely charged ions constitutes an electric double layer. The thickness of double layer usually ranges from 10 to 1000 A. 4/6/2022 Dr. Doaa Hussien Hassen 44 4/6/2022 Dr. Doaa Hussien Hassen 45 The electric potential of the plane is equal to the work against electrostatic forces required to bring a unit electric charge from the infinity (in this case from the bulk of the solution) to the plane 4/6/2022 Dr. Doaa Hussien Hassen 46 4/6/2022 Dr. Doaa Hussien Hassen 47  The potential at the actual surface is the electrothermodynamic (Nernset) potential E , and it is defined as the difference in potential between the actual surface and the electroneutral region of the solution The decrease in the potential across diffuse layer is gradual. The diffuse layer gradually comes to an end as the composition approaches that of the bulk liquid where the anion concentration equals the concentration of cations and the potential approaches zero. 4/6/2022 Dr. Doaa Hussien Hassen 48  elelectrokinetic Zeta (ζ) potential is defined as the difference in potential between the surface of the tightly bound layer (shear plane) and the electroneutral region of the solution. The ζ potential governs the degree of repulsion between adjacent, similarly charged, dispersed particles. If ζ potential is reduced below a certain value, the attractive forces exceed the repulsive forces, and the particles come together (Flocculation). 4/6/2022 Dr. Doaa Hussien Hassen 49  Lyophobic colloids: They are thermodynamically unstable, as the particles are stabilized only by the electrical charges on their surface which lead to repulsion between particles. If the ions removed from the system by dialysis, the particles will agglomerate and increase in size, leading to sedimentation. The addition of traces amounts of electrolytes to lyophobic colloids, will stabilize the system, as they giving charge to the system, leading to repulsion. However, if excess of electrolyte is added, this may reduce the thickness of double layer, lowering zeta potential below the critical value. 4/6/2022 Dr. Doaa Hussien Hassen 50  Ions of charge opposite to that of particles are most effective in coagulating colloids The valency of the ions is important. The coagulation effect of bivalent ions is not just twice that of monovelant ions but the activity increases between ten to hundred fold on increasing the valency of the acting ions by one. Coagulation of lyophobic colloid can be achieved by: 1. Addition of high concentration of electrolytes. 2. Mixing of oppositely charged colloids. 4/6/2022 Dr. Doaa Hussien Hassen 51  Lyophilic colloids: They are thermodynamically stable and the system constitutes one phase. The stability of the system depends mainly on interaction of the colloid with the solvent, the so called salvation. If the solvent is water, this interaction is called hydration. There is an enormous variety of electrically charged lyophilic colloids, e.g. proteins, pectins, agar, acacia, alginic acid and their stability depend to a large extend on hydration and lesser extent on electric charge. The electrolyte concentrations needed for the precipitation of hydrophilic colloids are many times greater than those needed for coagulation of hydrophobic ones. 4/6/2022 Dr. Doaa Hussien Hassen 52  Coagulation of hydroophilic colloid can be achieved by: 1- Salting out by large concentration of electrolyte. (Salting out phenomena can be reversed by dialysis, where ions will be removed). 2- Additions of alcohol and acetone solvents, which have a high affinity to water, cause dehydration of colloids and decrease their stability. So addition of small amount of electrolytes will cause coagulation. 3- Addition of less polar solvent renders the solvent mixture unfavorable for colloid, so electrolyte can salt out colloid with relative ease. 4/6/2022 Dr. Doaa Hussien Hassen 53  4- Coacervation, which is a phenomenon in which macromolecular solutions separate into two liquid layers Coacervation occurs when negatively and positively charged hydrophilic colloids are mixed together, where separation of particles occur forming a colloid- rich layer (known as coacervate). e.g. gelatin and acacia. Gelatin at a pH below its isoelectric point is positively charged, acacia above about pH 3 is negatively charged; a combination of solutions at about pH 4 results in coacervation 4/6/2022 Dr. Doaa Hussien Hassen 54