Adsorption of surfactants at liquid interfaces: thermodynamics

The thickness and hence material content of a surface is generally unknown, and there are two common definitions of a surface/interface. In one the surface is treated as a phase distinct from the surrounding bulk phases, and in the other, due to Gibbs, the Gibbs dividing surface is supposed to be a plane, parallel to the physical interface. The former model gives rise to the surface concentrationΓ‎s of a surfactant, and the Gibbs model introduces the surface excess concentration, Γ‎σ‎. Some thermodynamic quantities for surfaces (e.g. surface chemical potential and Gibbs free energy for surfaces) are defined. Adsorption lowers interfacial tension by an amount termed the surface pressure, and the Gibbs adsorption equation allows the calculation of Γ‎s or Γ‎σ‎ for a surfactant from the variation of interfacial tension of a liquid/fluid interface with surfactant concentration in bulk solution.

Gibbs Thermodynamics of the Renninger-Wilemski Problem

The Renninger-Wilemski problem in nucleation is analyzed. The Gibbs dividing surfaces method with external parameters is used to enrich the initial model. It is shown that both the traditional (Doyle) model and the Renninger-Wilemski model are not complete ones and, namely, the Gibbs dividing surface approach can solve this problem. It is shown that the application of the Gibbs approach also requires some model constructions. The simplified Gibbs model is proposed. It is shown that the simplified Gibbs model gives for the height of activation barrier the same numerical results as the Renninger-Wilemski model.

Experimental Study on Water Evaporation Enhanced by Surface Heating

The average evaporation flux was significantly higher while water was heated at a flat surface by two aligned heating elements than that while the water surface was heated 5 mm below in the designed experiments under the similar conditions. The observation is contrary to the Stefan condition. A thermodynamic model is derived from the Gibbs dividing-surface approximation at a flat evaporating surface to demonstrate that an interfacial flow can enhance the evaporation by transporting energy from a high temperature position to a low temperature position. The measures showed that the interfacial liquid temperature was up to 6.9°C higher around the heating wires than that at the centerline between two heating wires as water was heated at the interface. The induced interfacial flow can transport the energy to maintain the evaporation by overtaking the negative thermal conduction to the interface globally.