Reflection on the horizontal and vertical organization of water at oxide interfaces and their SFG vibrational signatures

Abstract

The hydrophilic and hydrophobic wetting behaviors of \ce{SiO2} and \ce{Al2O3} surfaces at PZC conditions are examined at the molecular level using graph theory. Hydrophobic wettability is manifested by a remarkable, collectively organized horizontal 2D hydrogen-bonded network (2DN) of water, anchored to the oxide surface through a dense array of vertically oriented hydrogen-bonded motifs. In contrast, hydrophilic wettability is characterized solely by vertical hydrogen-bonded motifs, similar to those that anchor the 2DN at hydrophobic aqueous interfaces. Analysis of these vertical motifs—considering both their structural features and SFG spectroscopic signatures—provides a direct molecular-level connection between the microscopic organization of interfacial water and the macroscopic manifestations of hydrophilic and hydrophobic behavior.