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CHARACTERISATION OF COLLOIDS
Physical Pharmacy 2 CHARACTERISATION OF COLLOIDS Kausar Ahmad Physical Pharmacy 2 KBA
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OBJECTIVES OF CHARACTERISATION
Physical Pharmacy 2 OBJECTIVES OF CHARACTERISATION To determine stability To formulate emulsions or suspensions with higher stability Take precautions Physical Pharmacy 2 KBA
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Macroscopic observations
Physical Pharmacy 2 HOW TO CHARACTERISE? Microscopy Macroscopic observations Turbidity Rheology Particle sizing Zeta potential Macroscopic observations Extent of aggregation, phase separation, creaming, sedimentation Physical Pharmacy 2 KBA
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PROPERTIES OF COLLOIDAL SYSTEMS
Physical Pharmacy 2 PROPERTIES OF COLLOIDAL SYSTEMS Optical Kinetic Physical Pharmacy 2 KBA
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OPTICAL PROPERTIES OF COLLOIDS
Physical Pharmacy 2 4/25/2017 OPTICAL PROPERTIES OF COLLOIDS LIGHT SCATTERING caused by fine particles Faraday Tyndall effect a visible cone is formed Turbidity Molecular weight Results from a suspension of fine particles that obscures light rays - requires many days for sedimentation because of the small particle size. In industries: 1. light-interference method is classified as nephelometric. Unit of measurement is nephelometric turbidity unit, NTU. 2. When formazin is used to prepare turbidity standards for comparator tube determinations, the unit is formazin turbidity unit, FTU. Turbidity, t, is the fractional decrease in intensity of the incident light, which passes through 1 cm of solution. It is expressed as the intensity of light scattered in all direction (Is) divided by the intensity of the incident light, I. t = Is / I At a given concentration of a dispersed phase, turbidity is proportional to the molecular weight of the lyophilic colloid: M Physical Pharmacy 2 KBA
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KINETIC PROPERTIES OF COLLOIDS
Physical Pharmacy 2 4/25/2017 KINETIC PROPERTIES OF COLLOIDS arises from bombardment of dispersed particles by molecules of dispersion medium. Brownian motion particles diffuse spontaneously from region of high to low concentration Diffusion allows the calculation of molecular weight of colloid Osmotic pressure given by Stoke’s law Sedimentation/Creaming resistance to flow under applied stress. Viscosity Brownian motion – arises from bombardment of dispersed particles by molecules of dispersion medium. Diffusion – particles diffuse spontaneously from region of high to low concentration, according to Fick’s law the amount of substance dq diffusing in time dt across a plane with surface area S: dq = -DS(dc/dx)dt where D is the diffusion coefficient and dc/dx is the concentration gradient. Osmotic pressure: allows the calculation of molecular weight of colloid Sedimentation: as given by Stoke’s law Viscosity – resistance to flow under applied stress Brownian motion – arises from bombardment of dispersed particles by molecules of dispersion medium. Viscosity – resistance to flow under applied stress Physical Pharmacy 2 KBA
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COLLOIDAL VS COARSE Particles cannot be seen using ordinary microscope
Physical Pharmacy 2 COLLOIDAL VS COARSE Particles cannot be seen using ordinary microscope Visible only in electron microscope Particles diffuse (slowly) Colloidal Particles visible under microscope Do not pass through filter paper Particles do not diffuse Coarse Physical Pharmacy 2 KBA
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KINETICS OF COAGULATION RATE AT WHICH A SOL COAGULATES
Physical Pharmacy 2 KINETICS OF COAGULATION RATE AT WHICH A SOL COAGULATES Temperature increases, thermal energy increases, kinetic energy increases. REPULSIVE POTENTIAL ENERGY? HIGH STABLE Thus, When concentration of dispersed phase increases, probability to encounter one another is higher. LOW COAGULATION Physical Pharmacy 2 KBA
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RATE OF PARTICLE AGGREGATION
Physical Pharmacy 2 RATE OF PARTICLE AGGREGATION The rate at which particles aggregate is given by: -dn/dt = k2n2 n - number of particles per unit volume of sol at time t k2 is a second-order rate constant Physical Pharmacy 2 KBA
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PARTICLE CONCENTRATION
Directly: by visual particle counting microscopy Indirectly: from turbidity spectrophotometric or light scattering measurements. Physical Pharmacy 2
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MICROSCOPIC EXAMINATION Aggregation of Solids
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Physical Pharmacy 2 PHASE SEPARATION Phase separation occurs as a result of creaming or sedimentation. The volumes of the different phases that separated out are recorded as a function of time. Physical Pharmacy 2 KBA
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Phase separation of emulsions in 100 ml measuring cylinder.
Physical Pharmacy 2 Phase separation of emulsions in 100 ml measuring cylinder. Oil emulsions with different emulsifiers at 10% concentration; emulsified at 6000 rpm for 30 min. at 30C. Physical Pharmacy 2 KBA
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TURBIDITY Time END OF LECTURE 1/2 Physical Pharmacy 2
Why %T increase with time? Time END OF LECTURE 1/2 Physical Pharmacy 2 KBA
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PARTICLE SIZE ANALYSIS
Physical Pharmacy 2 PARTICLE SIZE ANALYSIS Common method to determine overall stability of colloid The particle size distribution (PSD) of a colloid can be determined using: Microscope Coulter counter Instrument based on laser diffraction technique Instrument based on photon correlation spectroscopy For monitoring stability the PSD is taken as a function of time Physical Pharmacy 2 KBA
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PARTICLE SIZE DISTRIBUTION
Physical Pharmacy 2 PARTICLE SIZE DISTRIBUTION Effect of Span®20/Tween®20 concentration on size and polydispersity of oil droplet Physical Pharmacy 2 KBA
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Physical Pharmacy 2 VISCOSITY An obvious change in viscosity as a function of time, is observed in gel formation. Partially flocculated system will also show an increase in viscosity. Depending of the type of particles, viscosity may decrease or increase following coagulation. Gel formation - increase Sedimentation - decrease Physical Pharmacy 2 KBA
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Rotational Viscometer
Physical Pharmacy 2 MEASURING VISCOSITY Rotational Viscometer Cone and plate U-tube Flow cup GPC Physical Pharmacy 2 KBA
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VISCOSITY OF EMULSIONS
Physical Pharmacy 2 VISCOSITY OF EMULSIONS Effect of oil content on oil emulsion with 12% E906; emulsified at 6000 rpm for 30 min. at 40C. Physical Pharmacy 2 KBA
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Physical Pharmacy 2 ZETA POTENTIAL Physical Pharmacy 2 KBA
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ZETA POTENTIAL FACTORS
Physical Pharmacy 2 ZETA POTENTIAL FACTORS pH Ionic strength Type of electrolyte Concentration of additives A zeta potential value is a meaningless number if quoted without a definition of its environment: Physical Pharmacy 2 KBA
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Physical Pharmacy 2 Zeta potential of Intralipid as a function of electrolyte concentration and type of electrolyte CaCl2 NaCl From Florence & Attwood Physical Pharmacy 2 KBA
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STATE OF AGGREGATION VS ZETA POTENTIAL
Physical Pharmacy 2 STATE OF AGGREGATION VS ZETA POTENTIAL Physical Pharmacy 2 KBA
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Change in Viscosity in relation to change in zeta potential
Physical Pharmacy 2 Change in Viscosity in relation to change in zeta potential If ZP remains constant viscosity of the system will also remain constant. If ZP is lowered by cationic electrolytes or polyelectrolytes stability of the system will reduce progressively from simple agglomeration to fluid gel formation or a rigid gel. Physical Pharmacy 2 KBA
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Physical Pharmacy 2 SEDIMENTATION Difference in density between particles in dispersed phase and medium accumulate under the influence of gravity at the bottom -SEDIMENTATION On top -CREAMING Physical Pharmacy 2 KBA
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TYPE & VOLUME OF SEDIMENT
dense sediment difficult to redisperse particles that form bridges, give high volume BUT form loose sediment which is more easily dispersed. Physical Pharmacy 2
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INTENTIONAL REDUCTION OF ZETA POTENTIAL
Physical Pharmacy 2 INTENTIONAL REDUCTION OF ZETA POTENTIAL add polyelectrolytes allow ‘bridging’ result in loose aggregates cake can redisperse easily Physical Pharmacy 2 KBA
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CONTROLLED FLOCCULATION
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ACCELERATED STABILITY TESTS
Physical Pharmacy 2 ACCELERATED STABILITY TESTS Phase inversion, destabilise, precipitation of SAA Heating at elevated temperatures Compression of electrical double layer High salt or electrolyte concentration Severe acidity or alkalinity Physical Pharmacy 2 KBA
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Physical Pharmacy 2 REFERENCES RJ Hunter, Foundations of Colloid Science Volume 2 Chapter 16, Clarendon Press Oxford (1989) Lab data The aggregation pix from internet but I have lost the link. I acknowledge the contribution of authors. Thank you. Physical Pharmacy 2 KBA
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