Pharmaceutics 1 Colloidal Dispersions PDF

15/12/2024 Pharmaceutics 1 Colloidal Dispersions Prof. Rania Hamed Prof. Suhair Sunoqrot 2024/2025 Dispersed Systems Dispersions consist of at least one internal phase (dispersed phase) dispersed in...

15/12/2024 Pharmaceutics 1 Colloidal Dispersions Prof. Rania Hamed Prof. Suhair Sunoqrot 2024/2025 Dispersed Systems Dispersions consist of at least one internal phase (dispersed phase) dispersed in a dispersion medium (continuous phase) Dispersed phase Continuous phase 1  15/12/2024 Classification of Dispersed Systems According to the size of dispersed particles, dispersed systems are classified into two types: Dispersion Particle Size Characteristics Examples Colloidal 1 nm – 1 μm - Visible only in Milk, paint, dispersion electron microscope shaving cream - Diffuse but slower than solutions Coarse Greater than 1 μm - Visible under light Suspensions and dispersion microscope emulsions - Don’t diffuse 1 nanometer;1 nm= 10-9 m 1 micrometer; 1μm= 10-6 m Classification of Dispersed Systems Nanoparticles 2  15/12/2024 Classification of Dispersed Systems Blood is a complex dispersed system containing more than one dispersed phase – The dispersion medium in blood is plasma, which is mostly water – Serum albumin particles (> 1 nm) form a colloidal dispersion – RBCs (about 6 μm in diameter and 2 μm in width) form a coarse dispersion in blood Colloidal Systems What is so special about colloids? Particles of colloidal size range possess an enormous surface area relative to the surface area of an equal volume of larger particles Specific surface area is used to describe the surface area per unit weight or volume of material Large specific surface area results in unique properties (e.g. catalytic, dissolution, electrical, kinetic, optical, etc.) 3  15/12/2024 Colloidal Systems Because of their size, colloidal particles can be separated from molecular particles by a technique known as dialysis In dialysis, a semipermeable membrane is used such as cellophane The membrane should have a pore size that prevents the passage of colloidal particles and permits the passage of small molecules and ions At equilibrium: The colloidal material remains in compartment A The subcolloidal material is distributed equally on both sides of the membrane By continually removing the liquid in compartment B (sink conditions), it is possible to obtain colloidal material in A that is free from subcolloidal contaminants Dialysis 4  15/12/2024 Types of Colloidal Systems 1. Lyophilic (solvent-loving) colloids: – Systems containing colloidal particles that have high affinity to the dispersion medium – Usually obtained by spontaneous dispersion of the material in the solvent being used. – Acacia, gelatin, insulin, and albumin in water → hydrophilic colloids – Rubber and polystyrene in organic solvents → lipophilic colloids Types of Colloidal Systems 2. Lyophobic (solvent-hating) colloids: – Dispersed systems where the dispersed phase has little attraction for the dispersion medium – Ex. inorganic particles such as gold, silver, sulfur, and silver iodide dispersed in water – Lyophobic colloids are prepared by: 1. Dispersion method: that involve the breakdown of larger particles into particles of colloidal dimensions. 2. Condensation method: in which the colloidal particles are formed by aggregation of smaller particles such as molecules. 5  15/12/2024 Types of Colloidal Systems 3. Association (amphiphilic) colloids: – Colloidal aggregates of amphiphiles (surfactants) are formed when the concentration reaches a certain value – These aggregates are called micelles, their size in the nano-range. – The concentration of surfactant monomers at which micelles form is the critical micelle concentration (CMC) Types of Colloidal Systems 3. Association (amphiphilic) colloids: Below CMC, amphiphiles adsorb at the air-water interface, forming monolayer at the surface. Adsorption increases with increasing amphiphile concentration; and surface tension decreases greatly. The interface becomes saturated with monomers and if excess amphiphiles are added, CMC is reached, and micelles start to form in the bulk phase. When reaching the CMC, surface tension remains constant because further additions of surfactant no longer result in a decrease in surface tension 6  15/12/2024 Types of Colloidal Systems 3. Association (amphiphilic) colloids: When surface tension (γ) of a surfactant is plotted against the logarithm of the surfactant concentration (log c), the following plot is obtained: Surface Tension,  Dyne/cm CMC (LogC) Types of Colloidal Systems 3. Association (amphiphilic) colloids (cont’d): – Micelle shapes: a) Micelles in water: hydrocarbon chains face inward and the polar heads are associated with water b) Reverse Micelles in nonpolar liquids: polar heads face inward and the hydrocarbon chains are associated with the continuous nonpolar phase c) Laminar micelles: formed at higher amphiphile concentrations – Spherical micelles exist at concentration close to the CMC – At higher concentrations, laminar micelles are formed and exist in equilibrium with spherical micelles 7  15/12/2024 Types of Colloidal Systems a) Spherical micelle in water b) Reverse micelle in an organic solvent c) Laminar micelle Types of Colloidal Systems 3. Association (amphiphilic) colloids (cont’d): – Amphiphiles may be anionic, cationic, nonionic, or ampholytic (zwitterionic) – If a micelle is formed of an anionic association colloid such as sodium lauryl sulfate, a certain number of sodium ions are attracted to the surface of the micelle, reducing the overall negative charge – The bound ions are termed counter ions or gegenions Micelle of an anionic surfactant in water 8  15/12/2024 Types of Colloidal Systems 3. Association (amphiphilic) colloids (cont’d): Solubilization Association colloids such as micelles increase the solubility of insoluble or slightly soluble materials in the dispersion medium used This phenomena is known as solubilization Absorption, bioavailability, activity, and stability of materials may be modified by solubilization Nonionic surfactants are commonly used as solubilizers because of their low toxicity 9  15/12/2024 Other Colloids Liposomes – Consist of an outer unilamellar or multilamellar membrane and an inner liquid core – Formed with natural or synthetic phospholipids – Can be used to load drugs by two principal ways: Lipophilic compounds can be associated with the liposomal membrane Hydrophilic substances can be dissolved in the inner liquid core Other Colloids Nanoparticles – Nanoparticles are small (1 nm – 1 μm), loaded spheres of natural or synthetic polymers – Nanoparticles have been developed to: 1. Increase drug delivery efficiency 2. Improve drug release profiles and drug targeting 3. Facilitate nontraditional routes of drug administration 10  15/12/2024 Other Colloids Hydrogels – A gel is a colloid with a liquid as the dispersion medium and a solid as a dispersed phase – A hydrogel is a colloid gel in which water is the dispersion medium – Hydrogels are used for wound healing and as sustained- release delivery systems Optical Properties of Colloids The size, shape, and structure of colloidal particles can be observed using the electron microscope The optical microscope uses visible light as its radiation source. Its resolving power is about 20 nm The electron microscope uses a beam of high-energy electrons as its radiation source. Its resolving power is about 0.5 nm (more powerful) 11  15/12/2024 Light Scattering This property is used for determining the molecular weight, the size, and shape of colloidal particles Scattering can be described in terms of turbidity (τ) Light Scattering Turbidity (τ): the fractional decrease in intensity due to scattering as the incident light passes through 1 cm of solution, and is given by: τ = Is/I – Where Is: intensity of light scattered in all directions I: intensity of the incident light At a given concentration of dispersed phase, τ is proportional to the molecular weight of the lyophilic colloid 12  15/12/2024 Kinetic Properties of Colloids The motion of particles with respect to the dispersion medium can be classified into: 1. Thermally induced motion such as Brownian movement, diffusion, and osmosis 2. Gravitationally induced motion such as sedimentation 3. Externally applied force results in viscous flow 4. Electrical induced motion Brownian Motion Random motion of colloidal particles resulting from collisions between the particles and molecules of the dispersion medium The velocity of the particles increases with decreasing particle size Increasing the viscosity of the medium decreases and finally stops the Brownian movement 13  15/12/2024 Sedimentation The velocity (v) of sedimentation of spherical particles is given by Stoke’s law: Where: r: radius of the dispersed particle ρ: density of the particles ρ0: density of the medium η0: viscosity of the medium g: acceleration due to gravity Below 0.5 μm, Brownian movement becomes significant and tends to oppose sedimentation due to gravity and promotes mixing If particles are subjected only to the force of gravity, particles > 0.5 μm obey Stoke’s equation. Ultracentrifugation, which can produce a force one million times that of gravity, is used to bring about the sedimentation of colloidal particles Stability of Colloidal Systems In colloidal dispersions, frequent encounters between the particles occur due to Brownian movement. These collisions may result in: Permanent aggregation of the particles (coagulation), which leads eventually to the destruction of the colloidal system as the large aggregates formed sediment out. Temporary aggregation (flocculation). The particles rebound and remain freely dispersed (a stable colloidal system). 14  15/12/2024 Stability of Lyophilic Colloids Lyophilic sols are thermodynamically stable Stabilization is accomplished by two means: 1. Providing the dispersed particles with an electric charge (repulsion) 2. Surrounding each particle with a protective solvent sheath. Lyophilic colloids may become unstable in two ways: 1. When oppositely charged lyophilic colloids are mixed → neutralized particles form a layer rich in colloidal aggregates → Coacervation 2. When high amounts of salt (electrolyte) are added → agglomeration and sedimentation → Salting out (i.e., particularly with a salt whose ions become strongly hydrated, the colloidal material loses its water of solvation to these ions and coagulates) Stability of Lyophobic Colloids Lyophobic sols are thermodynamically unstable Particles can be stabilized by the presence of electric charges on their surfaces. Similar charges produce a repulsion that prevents coagulation of the particles In the absence of sufficient electric charge, particles agglomerate to reduce the surface area → increase in particle size → sedimentation Lyophobic colloids are very sensitive to low amounts of electrolytes. 15