Molecular Simulation Study of Vapor−Liquid Critical Properties of a Simple Fluid in Attractive Slit Pores: Crossover from 3D to 2D

Effect of Surface Chemistry on Confined Phase Behavior in Nanoporous Media: An Experimental and Molecular Modeling Study

It is well accepted that nanopore size is a controlling parameter in determining the phase behavior of confined adsorbate molecules. Despite this knowledge, the quantitative effect of surface chemistry on the confined phase behavior is a factor that remains obfuscated. Obtaining a complete understanding of the variables controlling confined phase behavior is a critical step in developing more complete equations of state for predictive modeling. To this end, a combined experimental and molecular modeling study was conducted to investigate the effects of surface chemistry and wetting on the confined phase behavior of propane and n-butane in modified and unmodified silica MCM-41. Isotherms were measured in four types of silica MCM-41 modified with varying sizes of alkyl groups to determine the effects of increasing surface modification. Results showed that increased pore surface coverage of carbon resulted in a notable change in the capillary condensation pressures, adsorption enthalpy, and confined critical temperature of the adsorbate. Correlations between the surface coverage of carbon and the confined critical temperature were presented and supported by thermodynamic arguments. The primary conclusions were partially supported by hybrid molecular dynamics-Monte Carlo simulations of propane adsorption in models of the four types of experimental adsorbents. Several differences were noted and explained between the experimental and modeling results. Energetic heterogeneity on the surface of the modified MCM-41 adsorbents as well as differences in adsorbate entropy induced by surface features and chemistry were suggested as primary driving factors for the observed trends. The results of this work have direct implications for improving understanding of confined phase behavior in materials of varying surface chemistries.

Methane storage in nanoporous material at supercritical temperature over a wide range of pressures

The methane storage behavior in nanoporous material is significantly different from that of a bulk phase, and has a fundamental role in methane extraction from shale and its storage for vehicular applications. Here we show that the behavior and mechanisms of the methane storage are mainly dominated by the ratio of the interaction between methane molecules and nanopores walls to the methane intermolecular interaction, and a geometric constraint. By linking the macroscopic properties of the methane storage to the microscopic properties of a system of methane molecules-nanopores walls, we develop an equation of state for methane at supercritical temperature over a wide range of pressures. Molecular dynamic simulation data demonstrates that this equation is able to relate very well the methane storage behavior with each of the key physical parameters, including a pore size and shape and wall chemistry and roughness. Moreover, this equation only requires one fitted parameter, and is simple, reliable and powerful in application.

Effects of Confinement on the Dielectric Response of Water Extends up to Mesoscale Dimensions

The extent of confinement effects on water is not clear in the literature. While some properties are affected only within a few nanometers from the wall surface, others are affected over long length scales, but the range is not clear. In this work, we have examined the dielectric response of confined water under the influence of external electric fields along with the dipolar fluctuations at equilibrium. The confinement induces a strong anisotropic effect which is evident up to 100 nm channel width, and may extend to macroscopic dimensions. The root-mean-square fluctuations of the total orientational dipole moment in the direction perpendicular to the surfaces is 1 order of magnitude smaller than the value attained in the parallel direction and is independent of the channel width. Consequently, the isotropic condition is unlikely to be recovered until the channel width reaches macroscopic dimensions. Consistent with dipole moment fluctuations, the effect of confinement on the dielectric response also persists up to channel widths considerably beyond 100 nm. When an electric field is applied in the perpendicular direction, the orientational relaxation is 3 orders of magnitude faster than the dipolar relaxation in the parallel direction and independent of temperature.

Nanostructured water and carbon dioxide inside collapsing carbon nanotubes at high pressure

We present simulations of the collapse under hydrostatic pressure of carbon nanotubes containing either water or carbon dioxide.

Effect of confinement on the solid-liquid coexistence of Lennard-Jones fluid

The solid-liquid coexistence of a Lennard-Jones fluid confined in slit pores of variable pore size, H, is studied using molecular dynamics simulations. Three-stage pseudo-supercritical transformation path of Grochola [J. Chem. Phys. 120(5), 2122 (2004)] and multiple histogram reweighting are employed for the confined system, for various pore sizes ranging from 20 to 5 molecular diameters, to compute the solid-liquid coexistence. The Gibbs free energy difference is evaluated using thermodynamic integration method by connecting solid-liquid phases under confinement via one or more intermediate states without any first order phase transition among them. Thermodynamic melting temperature is found to oscillate with wall separation, which is in agreement with the behavior seen for kinetic melting temperature evaluated in an earlier study. However, thermodynamic melting temperature for almost all wall separations is higher than the bulk case, which is contrary to the behavior seen for the kinetic melting temperature. The oscillation founds to decay at around H = 12, and beyond that pore size dependency of the shift in melting point is well represented by the Gibbs-Thompson equation.