Pressure dependence of adsorption isotherm parameters and transport dynamics of a nonionic surfactant, LS-36, at the carbon dioxide (CO2) gas-liquid water interface

Surfactants are important to a wide variety of elevated pressure processes; however, little is understood about the effect of pressure on surfactant interfacial thermodynamics or transport. While some experimental studies exist, concrete conclusions are difficult to infer due to experimental artifacts, e.g., non-equilibrium conditions. We recently showed that few experiments are capable of achieving bulk fluid saturation, which is essential to careful measurements of surfactant thermodynamics and transport dynamics. In other words, the accurate study of surfactant adsorption equilibrium and transport parameters first and foremost requires bulk fluids that are otherwise in equilibrium. This work uses our recently developed high pressure microtensiometer (HPMT), which ensures bulk phase saturation, to measure thermodynamic isotherms for tri(ethylene glycol)-hexa(propylene glycol)-monododecyl ether (LS-36) surfactant (concentrations between 10-7 and 10-3 mol/L and pressures between 1 and 57 bar) at the CO2 gas-liquid water surface. All measurements were conducted at 22.5 degrees C. Note that in this study, the surfactant was dissolved in the liquid water phase and is unable to partition into the gaseous CO2 phase. Isotherms at different pressures were determined from surface tension data, analyzed, and compared to determine trends in thermodynamic parameters. Dynamic surface tension measurements were performed to assess transport dynamics in terms of the well-studied diffusion timescale. We find that surface tension depends non-linearly on pressure and surfactant concentration, such that surfactants are less effective at lowering surface tension as pressure increases. Furthermore, surfactant adsorption is a non -monotonic function of pressure and goes through a maximum at intermediate pressures. This result suggests that surfactant adsorption is strongly dependent on the free energy of the CO2-water surface. Overall, this work shows that the thermodynamics of surfactants at equilibrium surfaces under pressure are non-trivial and need additional investigation to quantify the driving forces for surfactant adsorption and to develop models for high pressure processes.