Membrane formation by preferential solvation of ions in mixture of water, 3-methylpyridine, and sodium tetraphenylborate

The structure and dynamics of a ternary system composed of deuterium oxide (D₂O), 3-methylpyridine (3MP), and sodium tetraphenylborate (NaBPh₄) are investigated by means of small-angle neutron scattering (SANS) and neutron spin echo (NSE) techniques. In the SANS experiments, a structural phase transition is confirmed between a disordered-phase and an ordered-lamellar-phase upon variation of the composition and/or temperature of the mixture. The characteristic lengths of the structures is on the sub-micrometer scale. A dispersion relation of the structure is measured through NSE experiments, which shows that the relaxation rate follows a cubic relation with momentum transfer. This implies that the dynamics of the system are determined predominantly by membrane fluctuations. The present results indicate that 3MP-rich domains are microscopically separated from bulk water in the presence of NaBPh₄, and that the layers behave as membranes. These results are interpreted that preferential solvation of salt in each solvent induces a microphase separation between the solvents, and the periodic structure of 3MP-rich domains is stabilized by the long-range electrostatic interaction arising from Na⁺ ions in D₂O-rich domains.