The premolten layer of ice next to a hydrophilic solid surface: correlating adhesion with molecular properties

Abstract

In order to establish the potential correlation between the macroscopic ice adhesion and the molecular properties of the premolten layer (PML), the adhesion strength between ice and hydrophilic silica has been measured as a function of temperature. In addition, temperature-dependent molecular properties have been determined using techniques that are sensitive to different aspects of the PML, specifically total internal reflection (TIR) Raman, vibrational sum frequency (VSFS) and NMR spectroscopies. The ice shear adhesion strength was observed to increase linearly with decreasing temperature until -25 °C, where a plateau marked the adhesive strength having reached the cohesive strength of ice. Interestingly, at temperatures higher than -20 °C the ice samples slid on smooth (Ra < 0.4 nm) silica surfaces. This sliding behavior was not observed on rougher silica surfaces (Ra ~ 6 nm). By varying the penetration depth of the evanescent field, TIR Raman was used to establish an upper limit to the thickness of the PML in contact with silica (<3 nm even at -0.3 K below the bulk melting temperature). Additional quantitative determination of the temperature-dependent thickness of the PML was obtained from 2H NMR measurements in mesoporous silica particles. Finally, the inherently surface specific technique, VSFS, which probed changes in the hydrogen bond environment, indicated at approximately -25 °C the onset of PML, followed by a marked structural change occurring just a fraction of a degree below the melting temperature. Jointly, the experimental approaches link, strongly and consistently, ice adhesion to the PML properties. Specifically, it is inferred that the premolten layer facilitates sliding and contributes to the observed friction behavior, provided its thickness is comparable to the surface roughness of the underlying silica substrate.