Thermodynamic expression of surface reconstruction impact on surface wettability

Abstract

In this work we demonstrated noticeable change in the surface wettability in response to environmental changes for self-assembled monolayers (SAM) of functional alkanethiols as a result of the orientation of surface polar groups as well as the permeation of the environmental phase into the SAM surface region with the aid of sum frequency generation spectroscopy and quartz microcrystal balance. Yet the revealed surface reconstruction caused unexpectedly subtle change in SAM surface energy (within ± 8 %) due to checks and balances of the dispersive and polar components of surface energy. By introducing the work of wetting as the difference between the work of adhesion of a fluid on a solid and its surface tension, here we managed to consistently express surface reconstruction impact in SAM surface wettability as an exponential function of the relative change in work of wetting for the wetting fluid. Furthermore, we also succeeded in harnessing the relative change ratio of the work of wetting of water to that of oil to demarcate surface polar groups, which are > 1 for weakly polar groups (e.g., Br and pyrrole), <1 for small, highly polar groups (e.g., OH, NH2 and COOH), and environmentally sensitive for large, highly polar groups (e.g., H2PO3).