Cubic mesophases of self-assembled amphiphiles separate miscible solvents

Abstract

Sodium lauroyl lactylate (SLL) is soluble in water and insoluble in organic solvents, while glycerol monooleated (GMO) is soluble in organic solvents and insoluble in water. These amphiphiles separate miscible solvents (e.g., water and either dimethylsulfoxide, DMSO, dimethylformamide, DMF, acetonitrile, AN, or tehtrahydrofuran, THF). Separation segregates water and the organic solvents into either microdomains (emulsified droplets) or free phases. With mixtures containing 3:7–7:3 DMF:water ratios (v/v), SLL kinetically stabilizes DMF-water emulsions for over a week (longer times were not investigated). Emulsions are DMF in water with DMF:water ratios ≤ 2:3, and water in DMF for DMF:water ratios ≥ 1:1. Optical microscopy and SEM illustrate emulsification, and confocal microscopy qualitatively shows segregation between DMF (dyed with fluorescein) and water (which appears dark in confocal images). Water droplets in DMF are kinetically stable because they are surrounded by self-assembled SLL cubic mesophases, either gyroid or primitive, depending on the SLL and DMF concentrations (as demonstrated by x-ray diffraction, XRD). SLL also stabilizes DMSO-water emulsions (for over a week), thereby segregating the two solvents with similar mechanisms. DMSO-water separation is quantitatively demonstrated by mid-infrared (mid-IR) spectromicroscopy. Separation between AN and water occurs for AN-water mixtures in which SLL has intermediate solubility, i.e., with 3:2 and 4:1 AN:water ratios (v/v). In this range, SLL yields emulsions which destabilize overnight, separating into AN-rich and water-rich phases, as demonstrated using nuclear magnetic resonance (NMR). In water, SLL self-assembles into primitive cubic liquid mesophases and it affects hydrogen bonding (H-bonding) of water, as shown by deconvolving the H-bond peak into peaks representative of different water clusters, comprised of water molecules donating and accepting a different number of H-bonds. In water, SLL induces a blue shift of the hydrogen bonding (H-bonding) of absorbance peaks for double (DD) and single (SD) H-bond donors, indicating that it strengthens H-bonding. Importantly, it increases the ratio between the amplitude A of SD relative to DD, and SD are most effective at structuring water. As a result, SLL would inhibit interactions between organic solvents and water, initiating separation. Similar to SLL, GMO is known for its ability to form cubic mesophases. GMO stabilizes emulsions of water miscible solvents (THF, DMSO and DMF) and water. This result indicates that selected amphiphiles self-assembled into cubic mesophases can emulsify miscible solvents.