Water of hydration in coagels

Abstract

Vitamin C-based alkanoyl-6-O-ascorbic acid esters, ASCn, are a class of surfactants, interesting both on account of their phase behaviour, and of the properties of the supramolecular assemblies they form. When dispersed in water at room, or lower, temperatures above ca. 5% w/w concentration, they form coagels. At higher temperatures, the microstructure changes to micellar solutions for surfactants of low hydrocarbon chain length n. The longer chained systems form gel phases. The transition enthalpy change is dominated by rearrangements in hydrophobic chain packing. On the other hand the Krafft point seems instead to be dictated mainly by interactions between the polar headgroups and the solvent. The coagel phase is usually thought of as formed of surfactant lamellae separated by thin interlayers of strongly bound, essentially "frozen", water molecules. In this work, DSC measurements were performed to explore the interactions between water and the surfactant molecules. Two kinds of water were detected: interlayer hydration water and bulk water. The number of hydration water molecules per polar headgroup was inferred from the experimental results. Further insights into the coagel structure were gained from X-ray diffraction and optical microscopy. The effects of glycerin (GLY), propylene glycol (PG), and poly(ethylene glycol) (PEG), as co-solvents were investigated by conductivity and DSC experiments. Glycerin seems to stabilize the coagel, probably through the formation of hydrogen bonds that compete for the polar headgroups with the strongly bound water molecules. By contrast, PG and PEG decrease the compactness of the lamellar structure. This is most likely because these compounds can penetrate into the lipid portion of the lamellae, and reduce the hydrophobic interactions that hold the compact assemblies together.