Distributed charge models of liquid methane and ethane for dielectric effects and solvation

Abstract

Liquid hydrocarbons are often modelled with fixed, symmetric, atom-centred charge distributions and Lennard-Jones interaction potentials that reproduce many properties of the bulk liquid. While useful for a wide variety of applications, such models cannot capture dielectric effects important in solvation, self-assembly, and reactivity. The dielectric constants of hydrocarbons, such as methane and ethane, physically arise from electronic polarisation fluctuations induced by the fluctuating liquid environment. In this work, we present non-polarisable, fixed-charge models of methane and ethane that break the charge symmetry of the molecule to create fixed molecular dipoles, the fluctuations of which reproduce the experimental dielectric constant. These models can be considered a mean-field-like approximation that can be used to include dielectric effects in large-scale molecular simulations of polar and charged molecules in liquid methane and ethane. We further demonstrate that solvation of model ionic solutes and a water molecule in these fixed-dipole models improve upon dipole-free models.

GRAPHICAL ABSTRACT

Keywords:

• Liquid methane
• liquid ethane
• dielectric constant
• solvation
• Titan

Acknowledgments

We thank Pratip Chakraborty and Ryan Nival for helpful discussions. Input files necessary to reproduce the simulations in this work are available at github.com/remsing-group/MethaneEthaneDCModels. We acknowledge the Office of Advanced Research Computing (OARC) at Rutgers, The State University of New Jersey for providing access to the Caliburn cluster and associated research computing resources that have contributed to the results reported here. This article is dedicated to Mike Klein on the occasion of his 80th birthday.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work is supported by the National Aeronautics and Space Administration under grant number 80NSSC20K0609 issued through the NASA Exobiology Program.