Formation of trapped meta-stable liquid–vapour interfaces in polar liquids in presence of excess gas-like molecules: Anomalous heat capacities and emergence of microscopic bubbles

Abstract

We have elucidated the wetting behaviour of a polar-liquid in contact of an attractive planar surface. The formation of meta-stable liquid column-like structures, separated by vapour phase molecules, forming liquid–vapor interfaces, has been investigated in confined planar geometry. We have used lattice based Monte Carlo simulation for both canonical and grand-canonical systems. The presence of excess gas-like molecules has been reported to alter partial wetting conditions due to excess pressure in canonical ensemble. The system is driven towards formation of microscopic liquid–vapor interfaces, and the distinct liquid–vapor interface forming bridge-like structures disappear at high temperatures. The existence of meta-stable interfaces and meso- and nano-scale liquid structures under confinement appears to be a direct manifestation of the restricted movement of the molecules. We conclude that associated irreversibility due to stacking of atoms in extremely narrow geometries results into formation of meta-stable equilibrium structures. The negative heat capacities reduce at lower temperatures. Further it also depends on the extent of vapor phase as well as presence of gas-like molecules. Adsorption isotherm for liquid molecules adsorbed at the first layer of the attractive surface confirms that the negative heat capacities are rooted into the formation of trapped meta-stable states due to complete wetting of the surface due to adsorption of liquid molecules at first (few) layers of the surface. Due to the inaccessibility of the microstates, the system is trapped into a meta-stable or quasi-equilibrium state. The final structure is an outcome of the reconciliation between molecular interactions, interfacial energies and the extent of physical confinement. The micrographs and density profiles recorded at different temperatures T = 1.0, 1.2, 1.4 and 1.6, phase-coexistence, specific heat capacity and configurational entropy curves confirm for the same. The computational model for our case study fully explores the origin of the interfaces and successfully accounts definite reasons for the associated negative heat capacities, which has been riddling scientist for many decades.