1 EMULSIONS STABILITY Abd Karim Alias, 2013© Prof. Abd Karim Alias Universiti Sains Malaysia Photo courtesy of Beatriz Bonal on Flickr
2 } Emulsion stability – ability to resist changes in its physicochemical properties with time. Mechanisms of emulsion breakdown : } Creaming – the process in which droplets move upwards (droplets density < density of continuous phase) } Sedimentation - the process in which droplets move downwards (droplets density > density of continuous phase) Mechanisms of Emulsion Instability Abd Karim Alias, 2013©
3 Mechanisms of emulsion breakdown (.. cont ’) : } Flocculation – the process in which two or more droplets “stick” together to form an aggregate (but the droplets still retain their individual integrity) } Coalescence - the process in which two or more droplets merge together to form a single larger droplet. } Phase inversion – the process in which o/w emulsion changes to w/o emulsion, or vice versa. Mechanisms of Emulsion Instability Abd Karim Alias, 2013©
4 Flocculation Phase inversion Creaming Sedimentation Coalescence Mechanisms of Emulsion Instability Abd Karim Alias, 2013©
5 • Types of electrostatic (interactive/repulsive) forces • DLVO theory • Electrical double layer • Zeta potential (what it is, its significance, how to measure) • Effect of ionic strength & pH Forces between colloidal particles Abd Karim Alias, 2013©
6 • The effects of the intermolecular forces on the stability of disperse systems can be described in terms of the forces between the colloidal particles determined by the summation of the intermolecular forces over many molecules. INTERACTIONS BETWEEN DISPERSED DROPLETS Abd Karim Alias, 2013©
7 • Two main interacting forces in affecting colloid systems: (1) van der Waals attractive forces and (2) electrostatic repulsive forces. • Attractive forces tend to destabilise colloids whereas repulsive forces generally impart stability. INTERACTIONS BETWEEN DISPERSED DROPLETS Abd Karim Alias, 2013©
8 • Originates from the dipole-dipole interactions. • The force between two droplets arising from van der Waals interactions is always attractive for like droplets. • The attractive force increases more and more rapidly as the droplets approach. van der Waals Interactive Forces Abd Karim Alias, 2013©
9 _ + _ + F F = - β /r 6 β = constant; r = distance in between van der Waals Interactive Forces Abd Karim Alias, 2013©
10 Particle with negative surface charge Slipping plane Stern layer Diffuse layer Distance from parBcle surface Surface potenBal Stern potenBal Zeta potenBal 0 -‐100 mV Electrical Double Layer & Zeta Potential Abd Karim Alias, 2013©
11 ELECTRICAL DOUBLE LAYER _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + } Oppositely charged ions (counterions) are preferentially attracted towards the surface, and ions of the same charge (coions) tend to be repelled away.
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- Fall '09