Non-additive symmetric mixtures at selective walls

Abstract

The results of Monte Carlo simulation of adsorption and wetting behaviour of a highly non-additive symmetric mixture at selective walls is discussed. We have concentrated on the interplay between the surface induced demixing in the adsorbed films and the properties of the bulk mixture, which exhibits a closed immiscibility loop. It has been shown that the wetting behaviour depends on the absolute values of the parameters determining the strengths of interaction between the mixture components and the surface, as well as on their difference. In general, an increase of the difference between the adsorption energies of the components leads to a decrease of the wetting temperature. In the cases when the wetting of non-selective walls occurs at the temperatures above the onset of demixing transition in the bulk, an increasing wall selectivity leads to a gradual decrease of the wetting temperature towards the triple point, in which the vapour coexists with the mixed and demixed liquid phases. When the wetting temperature at the non-selective wall is located below the onset of the demixing transition in the bulk mixture, an increase of the adsorption energy of the selected component causes the developing adsorbed films to demix and to show the reentrant mixing upon approaching the bulk coexistence. At the temperatures above the onset of the demixing transition in the bulk, the adsorbed films remain demixed up to the bulk coexistence and undergo the first-order wetting transition. A rather unexpected finding has been the observation of a gradual increase of the wetting temperature at highly selective walls.

The interplay between wetting and demixing in non-additive symmetric mixtures at selective walls

“…In particular, we have concentrated on the mixtures with the bulk phase diagram like that shown in Figure 1. It should be mentioned that we have already discussed the effects of wall selectivity on the wetting of the mixture with e = 0.6 and s = 0.73 [35], which also show the closed immiscibility loop. Here, we have considered the mixtures of the lower tendency towards demixing, with s = 0.7 and 0.72, and demonstrated how the wall selectivity affects their wetting behavior.…”

“…Although, we have not carried out calculations for the values of ∆ε * gs greater than 1.0, nevertheless it can be anticipated that the wetting temperature should gradually increase when ∆ε * gs becomes high enough. This prediction is based on the results obtained for the the previously discussed systems with s = 0.7 and ε * gs,A = 6.0, as well as on the results obtained for the system with s = 0.73 and ε * gs,A = 6.0 [35]. There is, however, a difference in the behavior of the systems with s = 0.7, 0.72 and 0.73, resulting from a different tendency towards demixing.…”

“…However, this phenomenon has not been observed for the largest value of ∆ε * gs = 1.0 considered here. The suppression of reentrant mixing occurs in the mixture with s = 0.73, which exhibits still higher tendency towards demixing [35], and results in a gradual increase of the wetting temperature when ∆ε * gs is higher than about 0.4.…”

“…We recall that when ε * gs,A and ε * gs,B are both equal to 5.0 (a non-selective wall), the wetting occurs below T * tr,v−ml−dl , at the wetting temperature T w,1 ≈ 0.97. In the previous work [35], we have presented the results for a similar situation, but for the mixture characterized by s = 0.73. It has been shown that when the wetting transition at non-selective wall occurs at the temperature between T * tr,v,ml−ms and T * tr,v−ml−dl , an increase of ∆ε * gs leads to non-monotonous changes of T * w,1 , and that a complete wetting also occurs at any temperature exceeding T * tr,v−ml−dl .…”

“…The model applied here has been the same as that used in reference [35]. Thus, the potential describing the interaction between the components of the binary symmetric mixture, consisting of the components A and B, has been represented by the truncated (12,6) Lennard-Jones potential…”