Interfacial Phenomena in Ionic Fluids

摘要

Frequently the control of technological processes in molten salt or metal-molten salt chemistry depends on a precise knowledge of the wetting properties at the fluid-solid (wall) or fluid-vapour interface, respectively. In the simplest case of a pure liquid drop in contact with a solid substrate a transformation from partial wetting (contact angle Θ>0) to complete wetting (Θ=0) may occur with varying pressure P or temperature T. For binary fluid mixtures a variety of wetting phenomena have to be considered both at the fluid-substrate and the fluid-vapour interface. A detailed classification of the wetting behaviour of simple binary liquid mixtures on the basis of simple theoretical models, including the role of atomic interactions, has recently been given by Dietrich and Latz [1], Only in recent years it has been shown by experiment and theory, that changes of the wetting characteristics e.g. from partial to complete wetting are accompanied by surface phase transitions, so called wetting transitions - for recent articles and reviews see [2-5] -. Going from a state of being not wet to one of being wet they manifest by a singular change of the surface excess density Γ_A and a corresponding increase of the wetting film towards macroscopic thickness. For a two component system of A und B molecules, the Gibbs dividing surface here is chosen at Γ_B = 0. The loci of singularity of Γ_A define the surface phase diagram. Considering binary mixtures with liquid-liquid immiscibility and an upper critical point, different types of wetting transitions may occur. If at coexisitence Γ_A increases continuously for T < T_W and diverges at T_W, this is called a critical wetting transition. On the other hand, if Γ_A is finite and low for T < T_W and diverges at T_W, the wetting transition is of first order. This first order wetting transition necessarily is connected with prewetting transitions in the adjacent homogeneous A-rich liquid phase, whereby a microscopic jump from thin to thick adsorption films takes place. These prewetting transitions define the prewetting line in the surface phase diagram. It merges tangentially at T_W into the coexistence curve and ends in a critical prewetting point. It can occur only on one side of the miscibility gap (see e.g. [6]).