The surface roughness, but not the water molecular orientation varies with temperature at the water–air interface

Abstract

We examine the temperature dependence of the interfacial molecular structure at the water–air interface by combining experimental and simulated sum-frequency generation (SFG) spectroscopy. The experimental SFG spectra of the OH-stretching mode show a decrease in the amplitude at ∼3300 cm⁻¹ with increasing temperature, while the 3700 cm⁻¹ ‘free OH’ SFG feature is insensitive to temperature changes. The simulated spectra are in excellent agreement with experiment. A comparison between interfacial SFG spectra and bulk infrared/Raman spectra reveals that the variation of the SFG signal due to the temperature change is not caused by a temperature-dependent OH bond orientation of the interfacial water molecules, but can be fully accounted for by the temperature dependence of the optical response of water. These results indicate that while the thickness of the interfacial region varies with temperature, the molecular organization of interfacial water at the water–air interface is surprisingly insensitive to temperature changes.