Mechanisms of solvent separation using sugars and sugar alcohols

Abstract

This study uses sugars (dextrose, sucrose, ribose, fructose and mannose) and sugar alcohols (maltitol, erythritol, sorbitol, xylitol) to separate water from tetrahydrofuran (THF) and acetonitrile (AN). Bottle tests and nuclear magnetic resonance (NMR) show that above 0.25 M all sugars and sugar alcohols effectively separate water from either THF or AN. At 0.5 M concentrations, maltitol and sucrose yield a non-negligible interfacial tension between AN and water, while the interfacial tension is negligible with all other compounds. This indicates that while all compounds have similar separation effectiveness above a benchmark concentration, maltitol and sucrose (which are dimers) have a stronger effect on water structure compared to the other compounds tested (which are monomers). Attenuated Total Reflectance – Fourier Transform Infrared spectroscopy (ATR-FTIR) was used to explain solvent separation, based on the effect of sugars on hydrogen bonding (H-bonding) and on the nitrile band. The H-bonding peak was deconvolved into peaks representative of different water clusters, comprised of water molecules donating and accepting a different number of H-bonds. Principal component analysis (PCA) shows that single H-bond donors (SD) (at approximately 3200 cm−1) and double H-bond donor (DD) (at approximately 3400 cm−1) are most affected by maltitol and sucrose. All sugars tested induce a blue shift of the H-bonding of absorbance peaks for DD and SD, in either water or in mixtures of water-AN and water-THF. This indicates that they strengthen H-bonding in these clusters. The effect of all sugars is comparable when concentrations are expressed as OH equivalents. Sugars increase the ratio between the amplitude A of SD relative to DD, and SD are most effective at structuring water. The difference between the ratio A(SD)/A(DD) after and before sugar addition is lowest in water (≈0.9), followed by THF-water mixtures (≈1.1) and AN-water mixtures (≈1.2). This indicates that solvents enhance the effect of sugars on H bonding. This is likely because sugars are not soluble in either THF or AN, which therefore excludes them and promotes their interactions with water. In turn, water-sugar H-bonding weakens interactions between AN-water or THF-water, leading to solvent separation. The analysis of the nitrile band shows that sugars and sugar alcohols increase the relative amount of free nitrile, which is correlated to weaker interactions between AN and water. Maltitol and sucrose display approximately two times the relative amount of free nitrile compared to dextrose and erythritol. Our study confirms that sugars and sugar alcohols weaken solvent-water interactions, and reveals that they separate solvents by increasing the proportion of SD relative to DD.