Wetting phenomenon in the liquid-vapor phase coexistence of a partially miscible Lennard-Jones binary mixture.

We have carried out extensive equilibrium molecular dynamics (MD) simulations to study the structure and the interfacial properties in the liquid-vapor (LV) phase coexistence of partially miscible binary Lennard-Jones (LJ) mixtures. By analyzing the structural properties as a function of the miscibility parameter, $ \alpha $, we found that at relatively low temperatures the system separates forming a liquid A-liquid B interface in coexistence with the vapor phase. At higher temperatures and, $ 0<\alpha\leq 0.5 $, we found a temperature range, $T^{*}_{w}(\alpha) \leq T^{*} < T^{*}_{cons}(\alpha)$, where the liquid phases are wet by the vapor phase. Here, $ T^{*}_{w}(\alpha) $ represents the wetting transition temperature (WTT) and $T^{*}_{cons}(\alpha)$ is the consolute temperature of the mixture. However, for $ 0.5< \alpha < 1$, no wetting phenomenon occurs. For the particular value, $ \alpha=0.25 $, we analyzed quantitatively the $T^{*}$ versus $\rho^{*}$, and $ P^{*} $ versus $ T^{*} $ phase diagrams and found, $ T^{*}_{c}\simeq 1.25 $, and $T^{*}_{cons}\simeq1.25$. We also studied quantitatively, as a function of temperature, the surface tension and the adsorption of molecules at the liquid-liquid interface. It was found that the adsorption shows a jump from a finite negative value up to minus infinity, when the vapor wets the liquid phases, suggesting that the wetting transition (WT) is of first order. The calculated phase diagram together with the wetting phenomenon strongly suggest the existence of a tricritical point. These results agree well with some experiments carried out in fluid binary mixtures.