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

Recent molecular simulations indicate nanoscale heterogeneity at liquid-solution interfaces, which is expected to significantly impact interfacial processes, such as catalysis and nucleation. We investigate the enhancement of glycine concentration in aqueous solutions at solid interfaces using molecular dynamics (MD) simulations and surface plasmon spectroscopy (SPR) measurements. MD predicts an interfacial concentration enhancement in a nanoscale solution region at the solid surface. SPR shows for the first time direct experimental evidence of such an enhancement in small molecule liquid mixtures. The excess interfacial mass density on gold and polystyrene surfaces for undersaturated glycine solutions reaches up to ∼50 ng/cm2, corresponding to a 1 nm layer with over double the glycine saturation concentration. We attribute the interfacial enhancement to omnipresent van der Waals interactions between solution components and surfaces. We expect this effect to be a common phenomenon in solutions that is likely to have profound effects on both natural and industrial interfacial processes.