Vapor-Liquid Phase Coexistence, Critical Properties, and Surface Tension of Confined Alkanes

Configurational-bias grand-canonical transition-matrix Monte Carlo simulations are conducted to investigate various thermophysical properties, such as phase coexistence, critical properties, density and orientation profiles of liquid and vapor phases, and vapor-liquid surface tension of methane, ethane, propane, n-butane, and n-octane in bulk and slit pores of graphite and mica surfaces. An exponential-6 (exp-6) model is used for the investigation of normal alkanes with a cutoff radius, 15 Å. It is found that the surface tension of the bulk n-alkane, C1-C4, based on a truncated exp-6 model, agrees reasonably well with the experimental data. Critical properties are reported by means of the rectilinear diameter approach and least-squares technique. The shift in the critical temperature under confinements follows more than two linear regimes with an inverse in the slit width, as the slit width approaches the two-dimensional limit. This is contrary to what has been previously reported in the literature. The behavior of the critical temperature shift is sensitive to the nature of the surface. The critical density, on the other hand, fluctuates with a decrease in the slit width. The shift in the critical vapor pressure continuously increases with a decrease in the slit width toward the two-dimensional value and becomes constant for pore sizes less than 5 Å for the fluid studied in this work. Corresponding state plots suggest that the deviation of the saturation vapor pressure from the bulk saturation pressure under confinement is positive for large pores and negative for smaller pores. Vapor-liquid surface tension values for n-alkanes, in both types of slit pores, are computed via the finite-size scaling method of Binder and compared with their bulk values. Our investigation reveals that under slit-pore confinement the vapor-liquid surface tension decreases many fold.