Abstract
Interfacial tension and rheology of water/acidic oil (w/o) interfaces at neutral and alkaline pH conditions have been investigated by means of an oscillating drop tensiometer. The acidic oil phases used for this study were a West-African crude oil and a heavy distilled fraction of this oil, both diluted in cyclohexane to decrease oil viscosity and allow interfacial rheology measurements. In contact with water of initial pH of 6.5, both oils formed a 2D-gel near its gelation point at the w/o interface. For initial pH beyond 8, ionized molecules like naphthenates formed and adsorbed massively to the w/o interface, modifying the interfacial tension and rheology. We found that naphthenates did not impede but rather delayed the formation of the 2D-gel, and their presence could significantly reduce the gel strength, decreasing the stability of the corresponding w/o emulsions. Because the gel formation was not found to be an instantaneous process, it more surely contributes to the strengthening of emulsions with aging rather to their initial stability.
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