Monte Carlo simulations of equilibrium solubilities and structure of water in n-alkanes and polyethylene

Abstract

Gibbs ensemble Monte Carlo methods based on a force field that combines the simple point charge [Berendsen et al., in Intermolecular Forces, edited by Pullman (Reidel, Dordrecht, 1981), p. 331] and transferable potentials for phase equilibria [Martin and Siepmann, J. Phys. Chem. B 102, 2659 (1998)] models were used to study the equilibrium properties of binary systems consisting of water and n-alkanes with chain lengths from hexane to hexadecane. In addition, systems where extended linear alkane chains (up to 300 carbon units long) were used to represent amorphous polyethylene were simulated in the presence of water using a connectivity altering osmotic Gibbs ensemble. In these simulations the equilibrium between a liquid water phase and a polymer phase into which water was inserted was studied. The predicted solubilities, which were determined between 350 and 550 K, are in good agreement with experiment, where experimental results are available, and the density of water molecules in the hydrocarbons is approximately 63% as high as in saturated water vapor under the same conditions. At the lower temperatures most of the water exists as monomers; increasing the temperature leads to an increase in the density of water in the alkane phase and hence in the fraction of molecules that participate in clusters. Dimers are the most prevalent clusters in all hydrocarbons and at all temperatures studied, and the fraction of clusters of given size decrease with increasing cluster size. A large fraction of trimers, tetramers, and pentamers, which are the cluster sizes for which topologies have been studied, are cyclic at low temperatures, but at higher temperatures linear structures predominate. The same properties are observed for pure water vapor clusters in equilibrium with the liquid phase, showing that the cluster topologies are not significantly affected by the surrounding hydrocarbon.

Friction, tribochemistry and triboelectricity: recent progress and perspectives

“…Electrostatic potential dissipation rates are slower at lower relative humidity but negative potentials generally decay at a slower rate than positive ones, in agreement with Baum et al 150 A surprising finding was the negative equilibrium potential at 50 LDPE (ca. -6V), which can be also be explained assuming that negative ions are partitioned between the polymer and the atmosphere, as observed or suggested in other situations 151,152 . Specific OHadsorption at water-oil and water-air interfaces is well established in the literature [153][154][155] and an analogous specific 55 adsorption can also be considered in the LDPE-humid atmosphere interface thus accounting for the excess negative potential at equilibrium.…”

Molecular modelling of oxygen and water permeation in polyethylene

“…Previous studies [43] have shown that this number of chains is sufficiently large, and that the chains are sufficiently long, to provide converged results for PE. These structures were constructed using the grand canonical Monte Carlo (GCMC) method with a chemical potential that was sufficiently large to induce the six PE chains into the box while at the same time avoiding large atomic overlaps.…”

“…This system size has successfully been used in previous simulations of the solubility of water in PE. [43] Since oxygen and water have low solubilities in PE under the conditions studied here, the PE matrix is not expected to show significant swelling, and the volume of the PE box was fixed to decrease the simulation time. For comparison, some simulations were also performed with flexible volume of the PE box.…”

“…mg(penetrant) / g(polymer) or mol(penetrant) / mol(polymer)) [43]. The solubility coefficient that is used to determine the permeation coefficient is the partition function of the penetrant molecules in the polymer, i.e., the ratio between the number of penetrant molecules in the polymer box and the number of these molecules that would have been in this box in the absence of the polymer.…”

Molecular dynamics simulations of the diffusion coefficients of light n-alkanes in water over a wide range of temperature and pressure

“…Despite the significant effort that was put into the prediction of phase equilibria of mixtures of H 2 O with n-alkanes [36][37][38][39][40][41][42][43][44] by many research groups, only one molecular simulation study exists in the literature for the diffusion coefficient of CH 4 in H 2 O, while none for higher n-alkanes. Specifically, Shvab and Sadus [45] have performed an extensive series of molecular dynamics (MD) simulations for the calculation of a number of thermodynamic properties and diffusion coefficients of the H 2 O-CH 4 mixture for various CH 4 compositions.…”