Coalescence stability of emulsion-sized droplets at a planar oil–water interface and the relationship to protein film surface rheology

Abstract

An experimental technique for measuring coalescence times of emulsion sized oil droplets at a planar oil–water interface is described. Distributions of coalescence times as a function of droplet size are presented for systems with n-hexadecane as the oil phase and a protein solution of β-casein, κ-casein or lysozyme as the aqueous phase (10^–4 wt% protein, pH 7, ionic strength 0.1 mol dm^–3, 25 °C). When the planar interface is aged for only 20 min there is no measurable difference in the distribution of coalescence times for the three proteins. However, when the planar interface is aged for 72 h there is a sizeable fraction of droplets, different for the three proteins, which does not coalesce at all, and the relative efficiencies of the adsorbed proteins in preventing coalescence lie in the order: lysozyme > κ-casein > β-casein. Time-dependent surface shear viscosities at the oil–water interface are reported for adsorbed films of the same three proteins under exactly the same experimental conditions. After 20 min the surface viscosity values for all three proteins are low ( < 1 mN m^–1 s), but after 72 h their values are distinctly different: 200 mN m^–1 s (lysozyme), 60 mN m^–1 s (κ-casein) and < 1 mN m^–1 s (β-casein). These results are consistent with the view that there is a positive correlation between coalescence stability and the mechanical strength of protein films adsorbed at the oil–water interface.