The interaction of solid water with methyl-terminated self-assembled monolayers of alkanethiols containing odd and even numbers of carbon atoms
The odd – even effect in the wetting of methyl-terminated self-assembled monolayers (SAMs) have been previously observed on a microscopic level and in ambient conditions. However, fundamental understanding of the interaction of a water adsorbate with the surface remains inconclusive. Using infrared reflection-absorption spectroscopy (IRRAS), the average structural orientations of water molecules deposited on SAMs with even and odd number of carbon atoms, were diagnosed under cryogenic and UHV conditions. A peak deconvolution method was developed in this work to unambiguously monitor changes in the broad O-H stretch band of solid water as the system was manipulated. This analysis confirmed that the geometry of the methyl terminal groups at the interface influences the as-deposited water molecules’ interactions with the substrate, and more specifically, the peak positions. After the transition from amorphous solid water (ASW) to crystalline ice (CI), molecular orientations of water adsorbed on the surface were not affected by the interaction with the even and odd SAMs. Moreover, the peak analysis revealed a transformation of the dominant width broadening component from Gaussian (heterogeneous broadening factors) to Lorentzian (homogeneous broadening factors) during crystallization of water deposited on all SAMs.