Interfacial Acidity on Oxide Surfaces: A Scaling Paradigm and the Role of the Hydrogen Bond

A fundamental understanding of acidity at an interface, as mediated by structure and molecule-surface interactions, is essential to elucidate the mechanisms of a range of chemical transformations. While the strength­­­­ of an acid in the gas and solution phases is conceptually well understood, how acid-base chemistry works at an interface is notoriously more complicated. Using density functional theory and nonlinear vibrational spectroscopy, we have developed a method to determine the interfacial Brønsted-Lowry acidity of aliphatic alcohols adsorbed on the {100} surface of the model perovskite, strontium titanate. Here we show that, while shorter and less branched alkanols are less acidic as a gas and more acidic in solution, shorter alcohols are less acidic whereas less substituted alkanols are more acidic at the gas-surface interface. Hydrogen bonding plays a critical role in defining acidity, whereas structure-acidity relationships are dominated by van der Waals interactions between the alcohol and the surface.