Adsorption, structure, and stress in binary interfaces

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Abstract
The physics of fluid–fluid interfaces is investigated with the gradient theory of inhomogeneous fluid. The theory is used to predict component density and principal pressure (negative of stress) profiles in flat and curved interfaces formed in carbon dioxide and decane mixtures. In some cases the component density (and total density) profiles are not monotonic, with CO2 being prominently surface active. In composition regions near where a perfect‐wetting third phase splits out the composition and pressure profiles are especially structured, a thin‐film appearing which is akin to the perfect‐wetting phase and which almost separates two distinct interfaces. This precursor to the third phase gives rise to Antonov’s rule for the interfacial tension between the two phases that are nearly in equilibrium with the third, perfect‐wetting phase. Below a perfect‐wetting transition temperature Tcw, such thin‐films do not arise near compositions of the three phase region. Curvature of sufficiently small drops or bubbles significantly affects adsorption at interfaces. The theory predicts that the Young–Laplace equation as traditionally applied overestimates the pressure drop across the interface in some cases (as found earlier in the case of one‐component drops), but underestimates the pressure drop across interfaces related to perfect‐wetting structures. The minimum work of forming critical nuclei is predicted.