Spontaneous transfer of amphiphilic molecules from evaporating water to involatile oil and its effects on the rate of water evaporation

Abstract

We have investigated the evaporation kinetics of water through an involatile oil layer with amphiphilic molecules dissolved in both the water and oil phases. As water evaporated, the concentration of amphiphilic molecules in the water phase increased, which induced spontaneous transfer of the amphiphilic molecules to the oil phase. The volume of the oil phase increased owing to the phase transfer, which caused a decrease in the rate of water evaporation. However, the evaporation flux of water was not explained by only the thickness of the oil layer, and the amount of dissolved amphiphilic molecules in the oil phase also affected the evaporation flux. We propose that the amphiphilic molecules in the oil phase affected transport of water molecules through the oil layer because there would be a strong interaction between the amphiphilic molecules and water molecules in the oil phase. A simple mathematical model based on retarded diffusion reasonably explained the experimental data. Phase transfer of the amphiphilic molecules is important to understand the evaporation kinetics of water in phase-separated solutions, such as emulsions. The kinetics of water evaporation and the compositions of the amphiphilic molecules compared with those at equilibrium are discussed.