(204h) Programming Pore Uptake and Demixing of Surfactant Solutions in Spatially Confined Pores

The self-assembly of non-ionic surfactants in confined geometries plays an important role in chemical, pharmaceutical and petroleum technologies. The ability of a porous material to uptake and release molecules is determined by the thermodynamic state of adsorbed surfactants in pores. However, the effect of spatial confinement on the thermodynamic state of adsorbed surfactants is poorly understood. This lack of insight has led to a significant knowledge gap between the surfactant self-assembly in bulk and in nanoconfined pores. In this study, we use a combination of small angle neutron scattering (SANS) experiments and all-atom molecular dynamics (MD) simulations to develop a better understanding of the effect of confinement on self-assembly and temperature-induced liquid-liquid phase separation of surfactant solutions. Here we study the demixing of a model non-ionic surfactant Triethyleneglycol monohexyl ether (C₆E₃) in tubular nanopores of SBA-15 silica material (pore diameter of 8.6 nm). We find that the non-ionic surfactant shows an in-situ aggregation behaviour in silica pores upon increasing temperature above the lower solution critical temperature (LCST). The surfactant molecules directly bound to the pore walls act as anchor sites for subsequent adsorption, resulting in the increased uptake of non-anchored surfactants upon increasing temperature and driving an increase in the size of assemblies. The non-anchored surfactant molecules can be released into bulk solvent by decreasing the temperature below LCST. This ability of C₆E₃ surfactant allows reversible partitioning of secondary molecules (here dye) dissolved in micellar cores between the pore space and bulk solvent. The findings presented in the article provide fundamental insights into the surfactant assembly in confinement, which is crucial in the development of new multi-responsive materials for isolating and recovering molecules.