Deuterium NMR Investigation of the lyotropic phases of alkyl β-glycoside/D2O systems

We have investigated the phase behavior of four glycosides (βC8OGlc, βC8SGlc, βC10OGlc, βC8OGal) in water and D2O by optical polarizing microscopy and deuterium NMR. Previously published phase diagrams were evaluated by deuterium NMR, via monitoring D2O spectra, and confirmed the presence of the hexagonal, bicontinuous cubic, and lamellar phases in these glycosides. We have also shown the presence of the gel phase in (βC10OGlc) and observed the extensive supercooling of the lamellar phase to temperatures well below the Kraft line. While the main features of the phase diagrams were confirmed, some phase boundaries were found to be slightly different. Magnetically aligned spectra were also observed for relatively dilute samples for the hexagonal phase (βC8OGlc and βC8OGal) and the lamellar phase (βC8SGlc and βC10OGlc). The average number of bound water molecules per headgroup in the lamellar phase for the glycosides was determined by the systematic measurement of the quadrupolar splitting of D2O over a wide range of values of the (glycoside/water) molar ratio. The number of water molecules bound to the headgroup was found on average to be about 1.6-1.7 water molecules with no significant differences in this value for the different glycosides (and over the temperature range investigated), indicating that the bound water content is predominately influenced by the number of hydroxyl groups of the headgroup only. However, this bound water content of only 1.6-1.7 water molecules per sugar headgroup is surprisingly low, suggesting strong intermolecular interactions of the OH groups of headgroup sugars. The results are in line with computational results reported earlier for the octyl-β-glucoside and β-galactoside, which show the presence of strong intralayer hydrogen bonding.