Interface-Induced Concentration Enhancement in Glycine Solutions Investigated Using Surface Plasmon Resonance Spectroscopy and Molecular Dynamics Simulations

Aqueous solutions of small well-soluble molecules, such as glycine, are often considered to be random homogeneous mixtures at molecular scale. However, there is strong experimental evidence that there are mesoscale clusters present in bulk solutions and there are also recent predictions from molecular simulations indicating nanoscale heterogeneity at solution interfaces. In this work, we investigate glycine concentration enhancement in undersaturated aqueous solutions at solid interfaces using a combination of molecular dynamics (MD) simulations and surface plasmon spectroscopy (SPR) measurements. MD simulations predicted a strong concentration enhancement in a nanoscale region of a slightly undersaturated aqueous glycine solution in the vicinity of a Lennard-Jones solid wall representing dispersion interactions of the solid surface with solution molecules. SPR measurements showed for the first time direct experimental evidence of a strong concentration enhancement in the same glycine solution for both gold and polystyrene interfaces. These experiments were also carried out over a series of more dilute concentrations and there was both a clear concentration enhancement observed under all conditions and a difference in the dependence on concentration for the two interfaces investigated. We attribute the interfacial enhancement effect to non-specific dispersion (van der Waals) interactions between solution components and surfaces in contact with solutions. As such interactions are omnipresent in solutions and interfaces, we expect interface-induced concentration enhancement to be a generic phenomenon in liquid solutions, which is likely to have profound effects on interfacial processes, such as heterogeneous nucleation or catalysis.