Electrophoretic mobility, dynamic light scattering and solution-depletion measurements have been combined to study t h e influence of electrostatic interactions on the thickness and structure of adsorbed layers of p-casein at the solid/liquid interface (polystyrene latex particles). Charge effects are investigated by removing phosphate groups from the protein, by varying the ionic strength and by including divalent calcium ions which are known to bind to the protein in solution. The observations on hydrodynamic thickness of the bound protein layer are interpreted in terms of a previously proposed model where most of the protein molecule lies close to the surface, leaving the highly charged N-terminal portion forming a loop extending into solution. The extension of this loop into solution is dominated by electrostatic interactions. Moderating these by diminishing protein charge via dephosphorylation or calcium-binding or by reducing the range of repulsion by increasing ionic strength, causes the loop to relax and decreases the effective thickness of the protein layer.

In an attempt to characterize the steric stabilizing sheath around the casein micelles of bovine milk, photon correlation spectroscopy techniques have been used to measure the micellar radius on exposure to ethanolic buffers of varying pH, ionic strength and calcium concentration. It is shown that on exposure to alcohol, the stabilizing protein sheath undergoes dimensional collapse and that immediately prior to aggregation, a minimum or core radius is reached, characteristic of the diluting buffer conditions. Defining barrier thickness as the difference between the micellar radius in alcoholfree buffer and this minimum radius, the same linear relationship is observed between barrier thickness and the critical ethanol concentration required to reach the core radius and induce subsequent aggregation, whether those variations in barrier thickness were achieved by altering the pH, ionic strength or calcium level of the buffer. Considering the initial rate of response to added ethanol as a measure of barrier strength, it is observed that thicker barriers are weaker whereas thinner barriers are more resistant to collapse and hence intrinsically stronger. This paradox is qualitatively resolved by considering the stabilizing sheath to possess some of the characteristics of a weak or soft gel, whose rigidity or extent of cross-linking is influenced by the variations in buffer conditions.

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