Effect of interfacial rheology on model emulsion coalescence II. Emulsion coalescence.

In Part I, surface pressure isotherms were measured for model interfaces between a dispersed water phase and a continuous phase of asphaltenes, toluene, and heptane. Here, the coalescence rate of model emulsions prepared from the same components is determined from measured drop size distributions at 23 degrees C. A correlation is found between the initial coalescence rate and the interfacial compressibility. It is shown that the change in coalescence rate as the emulsion ages and coalesces can be predicted from surface pressure isotherm data also obtained at 23 degrees C. The stability of the emulsions was further assessed in terms of free water resolved after a treatment of heating at 60 degrees C and centrifugation. The emulsions were aged up to 24 h prior to treatment. The free water resolution appears to correlate to the "capacity for coalescence" of the interfacial film; that is, to the product of the initial film compressibility and (1-CR), where CR is the film ratio at which the film crumples.

Effect of interfacial rheology on model emulsion coalescence I. Interfacial rheology.

Interfacial elasticity and "dynamic" surface pressure isotherms were measured for interfaces between a dispersed water phase and a continuous phase of asphaltenes, toluene, and heptane. The interfacial modulus is a function of asphaltene concentration and in all cases reached a maximum at an asphaltene concentration of approximately 1 kg/m(3). The modulus increased significantly as the interface aged and slightly as the heptane content increased to a practical limit of 50 vol%. The modulus was approximately the same at 23 and 60 degrees C. The modulus correlated with the inverse of the initial compressibility determined from surface pressure isotherms. The surface pressure isotherms also indicated that a phase transition occurred as the interface was compressed leading to the formation of low compressibility films. Crumpling was observed upon further compression. The phase transition shifted to a higher film ratio with an increase in heptane content and interface age. Asphaltene concentration and temperature (23 and 60 degrees C) has little effect on the surface pressure isotherms. The surface pressure and elasticity measurements are consistent with the gradual formation of a cross-linked asphaltene network on the interface.