Molecular dynamics study of the processes in the vicinity of the n-dodecane vapour/liquid interface

Abstract

Abstract:Molecular dynamics simulation is used to study the evaporation and condensation of n-dodecane (C 12 H 26 ), the closest approximation to Diesel fuel. The interactions in chain-like molecular structures are modelled using an optimised potential for liquid simulation (OPLS). The thickness of the transition layer between the liquid and vapour phases at equilibrium is estimated. It is shown that molecules at the liquid surface need to obtain relatively large translational energy to evaporate. The vapour molecules with large translational energy can easily penetrate deeply into the transition layer and condense in the liquid phase. The evaporation/condensation coefficient is estimated and the results are shown to be compatible with the previous estimates based on the molecular dynamics (MD) analysis and the estimate based on the transition state theory. The velocity distribution functions of molecules at the liquid-vapour equilibrium state are found in the liquid phase, interface and the vapour phase. These functions in the liquid phase and at the interface are shown to be close to isotropic Maxwellian for all velocity components. The velocity distribution function in the vapour phase is shown to be close to bi-Maxwellian with the temperature for the distribution normal to the interface being larger than the one for the distribution parallel to the interface.

Molecular dynamics study on evaporation and reflection of monatomic molecules to construct kinetic boundary condition in vapor–liquid equilibria

“…Moreover, the velocity distribution functions of the evaporating and condensing molecules were almost the same as the Maxwell distribution at a liquid temperature of 85 K. However, the temperatures of the velocity distribution functions were slightly higher than that of the Maxwell distribution: a temperature of approximately 89.5 K was obtained from the nonlinear least square method, which leads to the increase of average velocity of evaporating or condensing molecules. According to the recent studies [16][17][18], when molecules evaporate or condense at the interface, the translational energy of the molecules becomes higher than that of molecules in the bulk phase. Also, the increasing of the temperature obtained from the velocity distribution function of evaporating molecules was reported [11,18], which agrees with the present results.…”

“…Xie et al [16,18], Cao et al [17] and Louden et al [19] investigated the phase change of polyatomic molecules (n-dodecane or water molecules). Gu et al [15] studied unsteady nonequilibrium phase change using the multiscale molecular simulation.…”

“…[9][10][11][12][13][14][15][16][17][18][19][20]). As one of the methods to avoid the ambiguities introduced in the process of assigning evaporating or reflecting molecules based on MD simulations, a concept of spontaneous evaporation was assumed [12], and the molecular mass flux due to spontaneous evaporation was confirmed with MD simulation based on virtual-vacuum evaporation [10,12].…”

The effects of internal molecular dynamics on the evaporation/condensation of n-dodecane

“…water, [8][9][10][11][12] n-alkanes, [13][14][15] diethylene glycol 16 ) and of  T , 6,7,17,18 the underlying physics of the processes is still not fully understood. For example, Varilly and Chandler 8 estimated the  m of water to be close to unity using the simple point charge (extended) (SPC/E) solvent model and transition path sampling technique.…”

“…All the aforementioned values were obtained using MD simulations at 298 K. The experimentally observed values of  m were reported to be 0.62±0.09 (Raman spectroscopy) 11 and 0.25 to 1.0 (Tensimeter Jet) 12 . Xia and Landman 13 predicted a constant value (0.9) of  m for hexane and hexadecane molecules using an empirical potential at 334-554 K. Cao et al 14 and Xia et al 15 obtained the values of  m for ndodecane using a modified OPLS (Optimised Potentials for Liquid Simulations) force field. It was shown that this coefficient decreases from 0.9 to 0.3 when T/T c increases from 0.45 to 0.9, where T c = 658.2 K is the critical temperature for n-dodecane.…”