Effect of hydrogen bonding on the mixing behaviour of ternary aqueous mixtures

Abstract

This study correlates the solution behaviour of an aqueous mixture to hydrogen bonding between its three components (water, liquid Pluronic L31 and either tetrahydrofuran, THF or dimethyl-sulfoxide, DMSO). Pluronic L31 is miscible in either THF, DMSO or water, but separates from ternary mixtures, at specific solvent to water ratios. Water species can either donate a single (SD) or two hydrogens (DD), and accept a single (SA) or two (DA) hydrogens, depending on the other components in the mixture. This is reflected in the OH stretch band, as probed using attenuated total reflectance-Fourier transform infrared spectroscopy. Pluronic L31 and THF both compete for hydrogen bonding with DD-DA, with up to 30% THF (v/v, relative to water), where separation occurs. Above 30% THF, Pluronic L31 and THF do not compete for the same water species and mix freely. Pluronic L31 separates from DMSO-water mixtures with 10–80% DMSO. Competition for similar water species explains separation up to 50% DMSO. Above 50% DMSO, we must also consider two-way interactions between all components. At low DMSO concentrations, DMSO mainly interacts with water trough the SO group, while methyl groups from DMSO can interact with Pluronic L31. At higher DMSO contents, the methyl groups of DMSO interact more markedly with water. The second derivative of the νasCH3 peak displays a split with ≤ 50% DMSO, which disappears at higher DMSO percentages. The four peaks in the second derivative of the of the νasCH3 peak correlate to the interactions between DMSO and the four water species.