Density functional approach to wetting behavior of water on solid surfaces modified by grafting of chains and their mixtures

Abstract

We apply a classical density functional theory for associating fluids to study wetting of a simple model of water and graphite-like solid surface chemically modified by the grafting of chains and their mixtures. The grafting points are randomly distributed on the surface. The chain molecules consist of tangentially bonded spherical segments. We explore adsorption of water on such complex substrates and evaluate different projections of the wetting phase diagrams dependent on grafting density, chains length and composition of grafted chains mixture. Besides, the contact angle dependence on temperature is obtained and discussed. If the total grafting density is high enough, the prewetting line splits into two lines that join at a triple point. Each of the lines is associated with changes in the predominant configuration of segments of grafted chains. The role of attractive interaction of water with segments of chains is explored. Despite the qualitative similarity of the phase diagrams of one- and two-component grafted chains, the behavior of chain molecules during phase transitions may be different. The chains of one kind can stretch, while those of the second - can collapse. Such behavior corresponds to the “vertical separation” of the chains. Trends of this phenomenon are studied in detail.