Insight into the Effect of pH on Component Structure and Dynamics in Glycerol–Water Mixture from NMR and Molecular Dynamics Simulations

Abstract NMR experiments and molecular dynamics simulations have been performed to study the assemblage of the components in a binary mixture of glycerol and water by systematically varying the glycerol content and the pH (acidity) of the medium. NMR studies show that chemical shifts of glycerol alkyl protons move upfield (low‐frequency shift or shielding) and water protons move downfield (high‐frequency shift or deshielding). Molecular dynamics studies provide evidence for an increase in CH─OH interactions between glycerol molecules, an increase in cluster size of glycerol, and spatial localization of water with an increase in glycerol concentration or a decrease in the pH (from neutral to acidic, pK a ∼ 5) of the medium for a given concentration of glycerol. The intra‐ and intercomponent interaction energies vary systematically with glycerol concentration across the pH scale. Translational diffusion of clusters suggests that the average size of the dynamic clusters of glycerol increases from 2 to 8 monomers as the glycerol level in the binary mixture is raised from ∼ 5% to 60%, irrespective of the medium acidity. The preferential interactions of glycerol and water are explained by invoking the concept of composition fluctuation.