Colloidal Dispersions Lecture Notes PDF

Colloidal dispersions Introduction- properties Aulton’s Pharmaceutics The Design and Manufacture of Medicines 3rd Ed Ch. 6 Ansel’s Pharmaceutical dosage form 9th Ed. Ch. 14 American Pharmacy  Disperse systems system Particle size...

Colloidal dispersions Introduction- properties Aulton’s Pharmaceutics The Design and Manufacture of Medicines 3rd Ed Ch. 6 Ansel’s Pharmaceutical dosage form 9th Ed. Ch. 14 American Pharmacy  Disperse systems system Particle size True solution Less than 1nm Colloidal dispersions 1nm – 500nm = 0.5μm Fine dispersion 500nm= 0.5μm – 10μm Coarse dispersions 10 – 50μm  Colloidal Dispersion Colloidal system or colloidal dispersion is a heterogeneous system which is made up of Dispersed phase, dispersed as very fine particles in another substance called dispersion medium (Continuous phase). Classification of colloidal dispersions Physical state of the dispersed phase 1 and the continuous medium Particle size 2 Interaction between the dispersed 3 phase and the continuous medium  Dispersed phase Medium / Phases Gas Liquid Solid NONE Liquid aerosol Solid aerosol Gas (All gases are mutually Examples: fog, mist, Examples: smoke, dust miscible) hair sprays particulates Continuous medium Foam Emulsion Sol Example: whipped Examples: milk, Liquid Examples blood, antacid cream, Shaving cream, mayonnaise, hand suspensions carbonated soft drinks cream ,lotions Solid foam Gel Solid sol & suspension Solid Examples: aerogel, Examples: agar, Example: colored glass, styrofoam, pumice gelatin, jelly, pastes like toothpastes  Particle size Size of colloidal particles are in range of 1– 1000nm while size of suspension is >1000nm and that of true solution is < 1nm. Thus size of colloids lies between that of true solution and suspension. The colloidal particles can’t be seen with naked eye. This is why, colloidal system appears as homogeneous mixtures, but in reality are heterogeneous mixtures. Differentiation according to particle size Solution Colloidal dispersion Suspension 1000 nm (10 – 50 µm) Electron microscope (lower Visible in electron Ordinary microscope limit) microscope, ultra microscope High rate of diffusion Low rate of diffusion No diffusion nor (Brownian motion ) Brownian motion Pass through Pass filter paper Retained by filter paper 1. filter paper , Settle down or cream 2. dialysis membrane a)collodion; cellulose nitrate b)cellophane ultrafiltration ( with suction) or electro dialysis (electric current) High osmotic pressure Low osmotic pressure No osmotic pressure Do not scatter light Scatter light (Tyndall effect) Absorb light NaCl, glucose Surfactant micelle Pharmaceutical Suspensions  Particle size and size distribution Particle size can affect absorption behavior  Solid aerosols; particle size must be in the range of 1- 5μm and no particle above 10μm  Measured by osmotic pressure (no. of particles)  Measured by sedimentation or light scattering the weight rather than no. is important  Spherical particles; The size is determined by the diameter  Asymmetrical particles are measured by Stoke’s diameter dst which describes an equivalent sphere undergoing sedimentation at the same rate as sample particle Interaction between dispersed phase and dispersion medium lyophilic lyophobic Liquid loving Liquid hating Association colloids Surfactant  Dispersion medium is water Disperse Hydrophilic spontaneously High affinity (swell) Un stable dispersions Hydrophobic Need stabilizing agent Lyophobic Association(Amphiphilic) Lyophilic Weak interaction/ Liquid hating Dispersed phase consists of Strong forces of interaction Lyophobic colloid is a colloidal Aggregates (micelles) of small /Liquid loving, dispersion in which there are organic molecules Lyophilicity is the tendency of little interaction between the particles, surfaces, or functional or ions whose size dispersed phase and the groups to become extensively continuous phase individually is below the wetted, solvated, swollen, or Dispersed phase in lyophobic colloidal range dissolved by solvents colloids is not solvated by the dispersion media Hydrophilic or lipophilic Molecules of dispersed If the continuous phase is water, portion of the molecule is phase are solvated, i.e., they it is also called hydrophobic solvated, depending on are associated with the colloids whether the dispersion molecules comprising the medium is aqueous or Non- dispersion medium aqueous Water based colloids with Dispersed phase consists inorganic dispersed phase are Tween generally of large organic lyophobic ex: Span molecules lying within Gold Au198 injection USP colloidal size range Mild silver protein NF Poly saccharides Sulfur Proteins, acacia, tragacanth, Some of lyophobic colloids methylcellulose possess lyophilic properties (eg. hydrosols of silica and alumina). Lyophobic Association(Amphiphilic) Lyophilic Viscosity of the dispersion Viscosity of the system Viscosity of the dispersion medium is not greatly increases as the medium ordinarily is increased by the presence of concentration of the increased greatly by the lyophobic colloidal particles, amphiphile increases, as presence of the dispersed which tend to be Un- micelles increase in number phase; at sufficiently high solvated and symmetrical and become asymmetric concentrations, the sol may Sols of low viscosity, become a gel; viscosity and even at high concentration, gel formation are related to because of low solvation and solvation effects and to the low attraction between the shape of the molecules, particles compared to high which are usually highly repulsion asymmetric Lyophobic colloidal particles Usually high at sufficiently are not readily solvated high concentration of because the continuous disperse phase a gel may be phase prefer to interact with formed. one another than be Lyophilicity is the tendency involved in solvating the of particles, surfaces, or dispersed particles functional groups to become extensively wetted, solvated, swollen, or dissolved by solvents Lyophobic Association(Amphiphilic) Lyophilic Material does not disperse Colloidal aggregates are Molecules disperse spontaneously, and special formed spontaneously spontaneously to form procedures therefore must when the concentration of colloidal solution be adopted to produce amphiphile exceeds the colloidal dispersion critical micelle concentration Lyophobic sols with high lyophilic sols form gels on net inter-particle attraction coagulation leads to coagulation forming distinct granules and the system cannot easily be restored to its colloidal state. At high concentration lyophobic systems turn into pastes Lyophobic preparation Lyophilic Cannot be prepared directly by mixing colloid with liquid. Prepared directly by Special methods are employed to prepare them need stabilizers like mixing colloid with SAA liquid. Preparation may be either easy to prepare 1. Mechanical reduction of larger particles to colloidal size by 1) Disperse either spontaneously a) Colloidal mill, hammer, ball jet, roller in the b) Ultrasonic vibration appropriate solvent 2. Condensation 2) The resultant Condensation of smaller particles to form a colloid usually involves dispersion are a) Physical (add hot water to acetone solution of ppt. sulfur) intrinsically b) Chemical (strong acid and sod thio-sulphate) produce typically stable displacement, hydrolysis, or oxidation and reduction 3) Molecularly dissolved lyophilic materials are in colloidal range  Lyophobic lyophilic Formation *Dispersions usually of metals, inorganic * Generally proteins, of crystals etc., with macromolecules etc., which dispersion Never form spontaneously * disperse spontaneously in a * high interfacial surface-free energy solvent. due to large increase in surface area on * Interfacial free energy is low. formation. There is a large increase in Positive ΔG of formation, entropy when rigidly held *thermodynamically unstable. chains of a polymer in the dry *Particles of sol remain dispersed due to state unfold in solution. electrical repulsion * free energy of formation is negative, *thermodynamic stable system Stability Controlled by charge on particles. Controlled by charge and The particles in such sols are stabilized solvation of particles only by the presence of electric charges surrounding each particle with on their surfaces. The like charges a protective solvent sheath that produce a repulsion that prevents prevents mutual adherence coagulation of the particles, it can be when the particles collide as a Stabilized by additives such as surfactant result of Brownian movement (lowering the interfacial energy of the system) or by protective colloids (Steric stabilization - protective colloid action) Effect of electrolytes addition Lyophobic Lyophilic Very sensitive to added * Dispersions are stable generally in the presence of electrolyte, leading to electrolytes. * aggregation in an * If sufficient salt is added, agglomeration and irreversible manner. sedimentation of the particles may result. This * Depends on phenomenon, referred to as “salting out,”. (a) The type and valency * Effect is due to desolvation of the lyophilic molecules of counter ion of and depends on the tendency of the electrolyte ions to electrolyte, become hydrated. (b) Concentration of * The precipitating power is directly related to the electrolyte. hydration of the ion and hence to its ability to separate * At a particular water molecules from the colloidal particles concentration sol passes * Proteins are more sensitive to electrolytes at their from disperse to isoelectric points. aggregated state. * Lyophilic colloids when salted out may appear as amorphous droplets known as a coacervate  Effect of addition of macromolecular material to lyophobic colloidal sols Diagram of flocs 1. When added in small amounts, many polyelectrolyte and polymer molecules (lyophilic colloids) can adsorb simultaneously on to two particles and are long enough to bridge across the energy barrier between the particles. This can even occur with neutral polymer when the lyophobic particles have a high zeta potential 2. (Steric stabilization - protective colloid action) if larger amounts of polymer are added, sufficient to cover the surface of the particles, then a lyophobic sol may be stabilized to Polymer bridging coagulation even in the absence of significant zeta potential.  Properties of colloids 1-Kinetic properties Thermal motion manifests itself in the form of Brownian motion, diffusion (low) and osmosis (low). Brownian motion :Colloidal particles are subject to random collisions with the molecules of the dispersion medium, with the result that each particle in a complicated zigzag path. Responsible for the diffusion of colloidal particles. Gravity (or a centrifugal field) leads to sedimentation, by ultra-centrifugal force about 𝟏𝟎𝟔 g. Low osmotic pressure Measurement of these properties enables molecular weight or particle size to be determined.  2-Optical properties When a strong beam of light is passed through a colloidal sol, a visible cone, resulting from the scattering of light by the colloidal particles, is formed. This is the Faraday–Tyndall effect. Low solvation of the lyophobic colloidal dispersion leads to large difference in refractive index between the liquid medium and the dispersed phase which produces marked light scattering and strong Tyndall beams Optical properties ( Tyndall effect) why is the sky blue? * The intensity of scattered, Is, light is inversely proportional to the fourth power of the wavelength, λ (Rayleigh law): 𝟏 𝑰𝒔 ~ 𝝀𝟒 * Thus, shorter-wavelength light (blue) (λ=450nm) is scattered more intensely than longer-wavelength light (yellow and red), (λ=650 nm) and so the scattered light is mostly blue, whereas transmitted light has a yellow or reddish color The scattering of short λ light gives the sky its blue color. In contrast, transmitted light has a yellow color. At sunrise and sunset, sunlight has to travel a longer distance through the atmosphere than at noon. Colloid properties used for measuring the molecular weight determination Tyndall effect Viscosity Osmotic pressure  3- Electrical property of colloids Most surfaces acquire a surface electric charge when brought in contact with an aqueous medium due to 1. Ion dissolution e.g. AgI dispersion , Mg(OH)₂ 2. Ionization of the surface grouping -COO¯ and NH₃⁺ ions net charge depend on the pH like proteins 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. 3. Ion adsorption at the interfaces , SAA The particles of a colloid selectively adsorb ions and acquire an electric charge. All of the particles of a given colloid take on the same charge (either positive or negative) and thus are repelled by one another. If the charge on the particles is neutralized, they may precipitate out of the dispersion.  Electrophoresis If an electric potential is applied to a colloid through a liquid, the charged colloidal particles move toward the oppositely charged electrode; this migration is called electrophoresis. Electrophoresis; movement of a charged particle plus attached ions relative to a stationary liquid under the influence of an applied electric field. Used in measurement of zeta potential  Terminology Imbibtion: taking up of certain amount of liquid like water without considerable increase in volume Swelling taking up of liquid by a gel with an increase in volume Syneresis great interaction between dispersed phase particles ,upon standing dispersed medium is squeezed out in droplets and gel shrink  Xerogels are gels in which the vehicle has been removed, leaving a polymer network, e.g. polymer films. Xerogel :formed when liquid removed from the gel and only framework remains Example. Sheet gelatin, acacia tears, Tragacanth flakes Dispersion of Lyophobic sols may be lyophobic sols flocculated , it is a 2 phase system where the sol can be looked upon as a continuous floccule Examples are Aluminum Hydroxide Gel, USP and magnesium hydroxide gels (Milk of Magnesia) Dispersion of Clays such as bentonite, aluminum magnesium silicate lyophobic sols (Veegum) and to some extent kaolin form gels by flocculation in a special manner. They are hydrated aluminum (aluminum/magnesium) silicates whose crystal structure is such that they exist as flat plates The flat part or 'face' of the particle carries a negative charge due to O~ atoms and The edge of the plate carries a positive charge due to Al3+/Mg2+ atoms. As a result of electrostatic attraction between the face and the edge of different particles a gel structure is built up, forming what is usually known as a 'card house floc’ Ex Bentonite Magma, NF The forces holding the particles together in this type of gel are relatively weak - van der Waals forces in the secondary minimum flocculation of aluminum hydroxide, electrostatic attraction in the case of the clays - and because of this these gels show the phenomenon of thixotropy  Shape of colloidal particles(hydrophobic) Many colloidal systems, including emulsions, liquid aerosols and most dilute micellar solutions, contain spherical particles, Small deviations from sphericity are often treated using ellipsoidal models. Clay suspensions are examples of systems containing plate-like particles  Shape of (hydrophilic) colloidal particles The shape of colloidal 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. High molecular weight polymers and naturally occurring macromolecules often form random coils in aqueous solution. A colloidal particle is something like a hedgehog— in a friendly environment, it unrolls and exposes maximum surface area. Under adverse conditions, it rolls up and reduces its exposed area. Properties as flow, sedimentation, and osmotic pressure are affected by changes in the shape of colloidal particles. Particle shape may also influence pharmacological action. Thank You