Critical temperature, surface tension and vapor density estimations of methanol using the scaled model

Abdalla Obeidat,Mohammad Badarneh

doi:10.1016/j.heliyon.2019.e01595

Abstract

In the present work, we applied scaled model to calculate surface tension, vapor densities and the critical temperatures of four different models of methanol: namely, H1, J1, J2 and L1 models. The scaled model is based on calculating the free energy of the system. Free energy calculations were performed by applying the Bennet acceptance ratio (BAR) using Monte-Carlo simulations at low temperature range of 220K–280K. The BAR is based on calculating the free energy difference of n-molecules and (n-1)-molecules plus a free probe on methanol. Estimations of vapor densities are based on extrapolating the intercept of the scaled free energy linear line as number of molecules approaches infinity, which requires a pre-known values for liquid densities. To accomplish this, a series of molecular dynamic simulations were performed at low temperature range of 200K–300K with steps of 10K. All the estimated properties were in excellent agreement with experimental published data.

Highlights

Due to importance of methanol in our daily life and its applications in engineering, science, medicine and industry [1, 2, 3, 4], scientists investigated and will continue investigate this substance experimentally and theoretically at macro and micro levels [5, 6, 7, 8, 9, 10, 11, 12, 13]
Many scientists propose different potential functions for methanol, some of these potentials are based on six-site atoms such as OPLS-AA [14] and GROMOS96 [15], and some are based on three-site atoms by assuming the methyl group as a unite atom and in this case the methanol computationally looks like water with different parameters such as H1, H2 [16], J1, J2 [17], B3 [18], L1 [19] and TraPPE-UA [20]
The model depends on the method of simulation and these simulations could be standard to computational community, or might be a rigid model based on applying a statistical mechanics theory in nonstandard method

Summary

Introduction

Due to importance of methanol in our daily life and its applications in engineering, science, medicine and industry [1, 2, 3, 4], scientists investigated and will continue investigate this substance experimentally and theoretically at macro and micro levels [5, 6, 7, 8, 9, 10, 11, 12, 13]. The critical temperature Tc, is not easy to estimate, one of the best methods to estimate it is by calculating the vapor-liquid phase diagram as a function of temperature T and making a fit to the points using Wegner expansion with constants depends on the substance [30]. It is worth mentioning that the scaled model is a phenomenological model based in assuming capillarity approximation for the cluster free energy for large enough cluster sizes, with a surface tension that is linear in temperature and vanishing at the critical temperature. This method is not applicable for all substance, but with those follow capillarity approximation such as water and alcohols

Theory

Methodology

Results and discussion

Conclusions

Funding statement