Abstract:

The equilibrium amount of water that hydrates amphiphilic molecules (surfactants, lipids, or copolymers), as well as the kinetics of water sorption or desorption (swelling or drying, respectively) by amphiphiles, are of great fundamental (e.g., interplay between solvent diffusion and self-assembled nanostructure) and technological (e.g., product shelf-life and dissolution of detergent powders or pharmaceutical tablets) interest. This thesis presents results of studies where we expose films containing polyethoxylated surfactants to air of controlled relative humidity, and monitor the water loss over time until equilibrium is reached. The surfactants self-assemble into lyotropic liquid crystals with structure that may be different depending on the surfactant type and water concentration. The equilibrium water concentration data are presented in the form of an adsorption isotherm and are further analyzed in conjunction with data on the spacing between self-assembled structures (obtained from small angle neutron scattering) to reveal intermolecular interactions. The data on the time course of water loss by the surfactant films are fitted with a model that accounts for water diffusion in the film and evaporation at the surface, in order to assess the relative importance of diffusion and evaporation under various conditions, and to extract diffusion coefficients for water in the film as a function of surfactant type and concentration.