Solubility of Nitrogen in Methane, Ethane, and Mixtures of Methane and Ethane at Titan-Like Conditions: A Molecular Dynamics Study

Abstract

We have studied the temperature dependence of the solubility of nitrogen in methane, ethane, and mixtures of methane and ethane using vapor-liquid equilibrium simulations of binary and ternary mixtures of nitrogen, methane and ethane for a range of temperatures between 90K and 110K at a pressure of 1.5 atm, thermodynamic conditions that may exist on the Saturn's giant moon, Titan. We find that -(i) the solubility of nitrogen in both methane and ethane decreases with increasing temperature; (ii) the solubility of nitrogen in methane is much larger compared to that in ethane at low temperatures, (iii) solubility of nitrogen in a ternary mixture of methane, ethane, and nitrogen increases upon increasing mole-fraction of methane. Our results are in quantitative agreement with the recent experimental measurement of the solubility of nitrogen in methane, ethane, and a mixture of methane and ethane. Furthermore, we find a strong temperature-dependent surface adsorption of nitrogen at the nitrogen-hydrocarbon interface, previously unknown. We have also investigated surface tension of the gas-liquid interface and find that it decreases upon decreasing temperature. Moreover, we find that the interfacial layer of adsorbed nitrogen and ethane show a preferential orientational ordering at the interface.

Pressure and temperature dependence of solubility and surface adsorption of nitrogen in the liquid hydrocarbon bodies on Titan

“…Theoretical work using thermodynamic models utilizing available experimental data have focused on the same process [14,15,18]. We have recently investigated the phase-equilibria of nitrogen and hydrocarbon liquids on Titan using vapor-liquid equilibrium simulations at a pressure of 1.5 atm and temperatures between 90 K and 110 K [11]. We find that the solubility of nitrogen decreases with increasing temperature and the the solubilities values are in quantitative agreement with available experimental data.…”

“…Moreover, the surface tension of the nitrogen-methane binary mixture is about half the value of the surface tension of the nitrogen-ethane binary mixture at the same temperature at a pressure of 1.5 atm, suggesting that it would be easier to form a critical nucleus of a bubble in the nitrogen-methane mixture as compared to nitrogen-ethane mixture. Furthermore, in these studied we found a strong temperature-dependent surface adsorption where nitrogen forms a dense surface layer of about 2 nm thickness [11]. The complex and rich chemistries on Titan with the presence of liquid hydrocarbon lakes on the surface poses many challenging questions.…”

“…Recent experiments, thermodynamics models, and molecular simulations suggest that the liquid hydrocar-bon bodies on the surface are major sinks of nitrogen [8][9][10][11]. Most of these experiments were motivated by the detection of transient bright features by Cassini RADAR system, also known as "Magic Islands" [12,13].…”

Modeling of nitrogen solubility in normal alkanes using machine learning methods compared with cubic and PC-SAFT equations of state

“…The N 2 –N 2 repulsive force decreases with an increase in temperature, which results in increased solubility of N 2 at higher temperatures. However, increasing the solubility of N 2 with an increase in temperature may not be true for all normal alkanes and literature survey shows that the N 2 solubility in methane and ethane decreases with increasing temperature 117 . Normal alkanes are nonpolar, as they contain nothing but C–C and C–H bonds.…”

Vertical compositional variations of liquid hydrocarbons in Titan’s alkanofers

“…In order to investigate the actual influence of these molecular diffusion coefficients more precisely, we also evaluated the diffusion coefficients of N 2 and C 2 H 6 in CH 4 by molecular dynamics (MD) simulations. A similar technique has already been employed, in the context of Titan, by Kumar & Chevrier (2020). Here, we have employed the well-known open-source package GROMACS 2,3 2018 version (Abraham et al 2015).…”