Self-Assembled glycerol monooleate demixes miscible liquids through selective hydrogen bonding to water

Abstract

Water is miscible with dimethylsulfoxide (DMSO), with which it interacts through hydrogen bonds (H-bonds). Glycerol monooleate (GMO, 53 g/L) demixes water and DMSO, yielding kinetically stable DMSO in water emulsions with up to ≈75 % DMSO (relative to water, v/v), as demonstrated by Fourier Transform Infrared (FTIR) spectromicroscopy, ultra small angle X-ray scattering (USAXS) and light scattering experiments, and optical microscopy. With > 75 % DMSO (relative to water) emulsions reverse and DMSO becomes the continuous phase (as shown by electrical conductivity measurements). The size of emulsified droplets decreases with increasing DMSO ratios (relative to water), as demonstrated by USAXS. Synchrotron X-ray Diffraction (XRD) measurements conducted in the small (SAXS) and wide angle (WAXS) regions show that the DMSO:water ratio affects GMO self-assembly. GMO (53 g/L) self-assembles into cubic crystal mesophases with 40 % DMSO (relative to water) and into disordered lamellar phases with 50 % DMSO. At even higher DMSO percentages, GMO self-assembles into reverse micelles, which contain water and are surrounded by DMSO (as shown by analyzing XRD data with the Steubner and Strey model). Although GMO mainly partitions in the DMSO rich phase, attenuated total reflectance (ATR)-FTIR measurements show that GMO interacts with water through H-bonds. Importantly, it increases the proportion of single H-bond donors (SD, which structure water the most) relative to double donors (DD, which structure water less). H-bonding between the hydrophilic head of GMO and initiates separation between DMSO and water, which would be H-bonded to one another in the absence of GMO. While this study focuses on DMSO, GMO also emulsifies water and tetrahydrofuran (THF), dimethylformamide (DMF) and dioxane. Our findings redefine which solvents can be emulsified with water, and have potential implications for water treatment.