Colloids that have strong affinity for water Hydrophilic colloids Colloids that have strong affinity for lipid/oil Lipophilic colloids Colloids that have affinity for water and lipid/oil Amphiphilic colloids Amphiphilic colloids having high HLB values are soluble in aqueous phase Amphiphilic colloids having low HLB values are soluble in lipid/oily phase PROPERTIES OF COLLOIDS Kinetic properties : Temperature change Gain of energy Random movement of particles (colloids). Gravity leads to sedimentation. Viscous flow (in liquids) is the result of an externally applied force. Brownian movement : 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. Why true colloidal particles follow Brownian movement? Diffusion : Because of Brownian movement, colloidal particles spontaneously diffuse from a region of higher concentration to one of lower concentration. Sedimentation : Colloidal particles settle due to the force of gravitation following Stoke's Law V = 2r 2 g ( 1 - 2 ) 9 where, v = velocity of sedimentation r = radius of particle g = acceleration due to gravity 1 = particle density 2 = density of medium = viscosity of medium
Copyright rules apply 13 of 24 FYI: Properties of colloids -
Copyright rules apply 14 of 24 ELECTRICAL PROPERTIES OF COLLOIDS Colloidal particles usually possess an electric charge. This charge contributes significantly to the stability of the colloidal dispersion. Origin of the Charge Ionization Ionizing groups on the surface dissociate to give the particle a positive or negative charge 3 . R - NH 2 - COO - in alkaline solution R - NH 3 + - COO - isoelectric point (zwitterion) R - NH 3 + - COOH in acidic solution Ion Adsorption Some colloids acquire a surface charge due to unequal adsorption of oppositely charged ions. Cellulose normally do not carry any charge but become negatively charged in aqueous solution due to adsorption of hydroxyl ion – normally, cations are more hydrated, stay in the bulk, while anions are less hydrated and stay on the surface. Na + Electrical Charge on Colloids Ionization and/or ion adsorption imparts some charge to the particle surface. Consideration of this charge is essential because, Similar charge of the particles could produce repulsion (stability problems in suspension) Permeation of drugs through membranes 3 Protein particles possess both acidic and basic functional groups. In aqueous solution at low pH, the
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- Fall '09
- Surface tension, Colloids and Rheology, Surface and Interfacial Tension in Therapeutics