Dynamic interfacial properties of aqueous surfactant solutions

This thesis reports an investigation of dynamic interfacial properties of aqueous surfactant systems using experimental and molecular simulation techniques. Active control of dynamic interfacial properties of aqueous solutions is demonstrated using 4,4′-bis(trimethylammoniumhexyloxy)azobenzene bromide, BTHA, a light-sensitive surfactant, in combination with sodium dodecyl sulfate, SDS. Light affects the dynamic interfacial properties of the mixed surfactant solution through its influence on the state of aggregation within bulk solution and the rate of diffusive transport of surfactant to the surface of the solution. Measurements of dynamic surface tension show that relaxation of the interface to equilibrium occurs through two processes with distinct timescales. The fast process with a characteristic time of <1 s is associated with the adsorption of monomeric SDS. The slower process is associated with the adsorption of BTHA and possesses a characteristic time (∼50s–1500s) that can be controlled by illumination with UV or visible light. Illumination changes the state of aggregation of the BTHA within bulk solution and thus the effective diffusion coefficient of the surfactant.; The dynamic interfacial properties of aqueous surfactant systems are also investigated using molecular dynamics simulations and Transition State Theory. In contrast to past experimental studies of dynamic surface tension, the methodology developed in this thesis does not require an assumption of the form of the sorption rate expression. The desorption rate constant calculated as a function of excess surface concentration is not monotonic, but possesses a minimum value at intermediate excess surface concentrations. This behavior contrasts rate expressions used in past experimental investigations. The potential of mean force calculated for the process of adsorption of decanol reveals the presence of a free energy barrier. The calculated sorption rate constants agree with previous experimental studies. These results validate a methodology by which dynamic interfacial properties can be calculated from knowledge of the molecular structure of surfactants.