Orthokinetic coalescence of protein-stabilized emulsions

Abstract

A phenomenological model of orthokinetic stability, which combines a second-order coalescence process and a first-order disruption process, is tested against experimental results for β-lactoglobulin oil-in-water emulsions. Numerical data in the form of plots of average volume—surface diameter as a function of shearing time are presented for different values of the various model parameters: the coalescence and disruption rate constants, the critical droplet size for disruption, and the fractal dimensionality characterizing the ‘reactivity’ term in the orthokinetic rate equation. Satisfactory agreement between theory and experiment requires a low fractal dimensionality ( D ≈ 2) similar to that encountered for perikinetic reaction-limited cluster—cluster aggregation, and much less than the classical Smoluchowski value ( D = 3) for immediate full coalescence. Adjusting the pH of the β-lactoglobulin emulsion after formation but before shearing leads to changes in the protein surface coverage and the degree of flocculation. Experimental results are presented for the effects of shearing intensity and pH on the destabilization time of the β-lactoglobulin emulsions, and the trends are compared with those predicted by the phenomenological model. Adjusting the pH towards the isoelectric point of the protein induces substantial perikinetic flocculation of the emulsions and a correspondingly enhanced rate of orthokinetic destabilization. Changes in interdroplet interactions during the shearing process due to ageing of the adsorbed protein layer may have to be included in any refinement of the model.