Evaporation of water from structured surfactant solutions

Alkyl-propoxy-ethoxylate surfactant aqueous solution films are exposed to air of constant relative humidity, and the water loss from the film is monitored over time, until equilibrium is reached. The surfactants self-assemble into lyotropic liquid crystals with a structure that varies depending on the water concentration at a given time. The water loss data are analyzed to investigate the factors affecting the drying rate of the alkyl-propoxy-ethoxylate surfactants, such as the air relative humidity, microstructure in the surfactant film, and the surfactant degree of hydration. Analytical solutions of the diffusion equation are used to extract the water diffusion coefficient in the film. A diffusion model that accounts for varying film thickness and evaporation at the surface has been used to follow the water loss and thickness data over time, in order to assess the relative importance of diffusion and evaporation under various conditions.