PNNL

Abstract

Combining time-resolved vibrational sum frequency generation (TR-vSFG) spectroscopy with ab initio DFT-based molecular dynamics simulations (AIMD/DFT-MD), we rationalize how electrolytes affect the structure and vibrational dynamics of water at a silica surface as a function of the surface charge density. The interplay between theory and experiments allows us to disentangle the dynamics of the truly interfacial water, also known as binding interfacial layer (BIL) from the subsequent SFG-active “bulk like” water of the diffuse layer (DL), which also contribute to the TR-vSFG signal and has caused controversies in rationalizing the pH/electrolytes effect on interfacial vibrational relaxation. We here identify the relaxation time of the true interface, and provide a molecular picture of how ions change the interfacial hydrogen bonding environment and as a result affect the interfacial vibrational relaxation. Inner-sphere adsorbed ions alter the balance between competing water-water and water-surface interactions, leading to the transition from an inhomogeneous water coordination landscape towards a highly interconnected in-plane HB (hydrogen bonding) network in the BIL. When vSFG probes the BIL at neutral and moderately charged silica-water interfaces (pH~2 & 6), ions accelerate the vibrational relaxation of the water via formation of an efficient, highly-interconnected 2D-HBNetwork; when vSFG probes the DL at highly negative silica-water interfaces (pH~12) the relaxation is fast and independent of ion concentration (up to 0.5 M) due to probing of the bulklike DL structure. The model proposed in this work to separate the relaxation times of water in BIL and DL layers should be widely applicable to other aqueous/charged interfaces.