Density functional approach on wetting behavior of binary associating mixtures

A density functional theory is applied to study wetting behaviors of binary associating mixtures, which are described by the statistical associating fluid theory of Wertheim. When the associating interaction is strong, the phase behavior of the binary associating mixture falls into the type-VI mixtures of the classification scheme of van Konynenburg and Scott. There are two types of closed-loop phase behaviors for the type-VI mixture. That is, a closed-loop phase diagram for vapor-liquid-liquid coexistence (along its triple line) at low pressures and the other one for liquid-liquid coexistence at a relatively high pressure. In this study, the wetting behavior of the lower liquid phase at the surface of the upper liquid phase is carefully examined for both vapor-liquid-liquid coexistence and liquid-liquid coexistence regimes. In the latter regime, a third inert air phase is introduced since wetting behavior always involves three phases. For both regimes the binary associating mixture exhibits a sequence of wetting transitions, complete wetting-->partial wetting-->complete wetting, along with increasing temperature. The order of wetting transitions is carefully examined. It is found that the order of wetting transitions is the consequence of the competition between the attractive interaction range and the associating strength of unlike pair molecules. The most intriguing behavior is that it is possible to observe the sequence of wetting transitions along with increasing temperature at two different orders for air-liquid-liquid coexistence at a high pressure. That is, the upper wetting transition is first order and the lower wetting transition is second order. The pressure effect on the order of wetting transitions for liquid-liquid coexistence is also discussed. (C) 2003 American Institute of Physics.