Variation and decomposition of the partial molar volume of small gas molecules in different organic solvents derived from molecular dynamics simulations

Abstract

The partial molar volumes, V(i), of the gas solutes H2, CO, and CO2, solvated in acetone, methanol, heptane, and diethylether are determined computationally in the limit of infinite dilution and standard conditions. Solutions are described with molecular dynamics simulations in combination with the OPLS-aa force field for solvents and customized force field for solutes. V(i) is determined with the direct method, while the composition of V(i) is studied with Kirkwood-Buff integrals (KBIs). Subsequently, the amount of unoccupied space and size of pre-formed cavities in pure solvents is determined. Additionally, the shape of individual solvent cages is analyzed. Calculated V(i) deviate only 3.4 cm(3) mol(-1) (7.1%) from experimental literature values. Experimental V(i) variations across solutions are reproduced qualitatively and also quantitatively in most cases. The KBI analysis identifies differences in solute induced solvent reorganization in the immediate vicinity of H2 (<0.7 nm) and solvent reorganization up to the third solvation shell of CO and CO2 (<1.6 nm) as the origin of V(i) variations. In all solutions, larger V(i) are found in solvents that exhibit weak internal interactions, low cohesive energy density and large compressibility. Weak internal interactions facilitate solvent displacement by thermal solute movement, which enhances the size of solvent cages and thus V(i). Additionally, attractive electrostatic interactions of CO2 and the solvents, which do not depend on internal solvent interactions only, partially reversed the V(i) trends observed in H2 and CO solutions where electrostatic interactions with the solvents are absent. More empty space and larger pre-formed cavities are found in solvents with weak internal interactions, however, no evidence is found that solutes in any considered solvent are accommodated in pre-formed cavities. Individual solvent cages are found to be elongated in the negative direction of solute movement. This wake behind the moving solute is more pronounced in case of mobile H2 and in solvents with weaker internal interactions. However, deviations from a spherical solvent cage shape do not influence solute-solvent radial distribution functions after averaging over all solvent cage orientations and hence do not change V(i). Overall, the applied methodology reproduces V(i) and its variations reliably and the used V(i) decompositions identify the underlying reasons behind observed V(i) variations.

What Determines CO2 Solubility in Ionic Liquids? A Molecular Simulation Study

“…Overall, ̅ values of CO 2 in ILs at saturation concentration were found between 30-40 cm 3 /mol, which was substantially smaller than in conventional organic solvents, where ̅ ranged from 45-53 cm 3 /mol in the limit of infinite dilution. 59 In other words, CO 2 apparently displaces solvent molecules much more in organic solvents than in ILs. This suggests in case of ILs a substantially weaker perturbation of the overall solvent structure caused by CO 2 adsorption, which is consistent with the observation of unchanged RDFs above.…”

An Odor Interaction Model of Binary Odorant Mixtures by a Partial Differential Equation Method

“…When investigating the solution volume of liquid mixtures, similar interactions also exist, but by using a set of partial differential equations, these interactions are successfully explained with partial molar volume [10]. For example, the influence of constituents' mixing ratio to the volume of an ethanol-water solution can be analyzed from the change of corresponding partial molar volumes of both ethanol and water.…”

CHAPTER 9. Partial Molar Volumes of Gases Dissolved in Liquids

“…At infinite dilution, in the grand canonical ensemble, V LN 2 is given by the Kirkwood-Buff expression 192,193 as is determined only by the spatial solvent structure around the solute and the isothermal compressibility of the pure solvent. Klähn et al 182 now suggest decomposition of the integral into physically reasonable and intuitively appealing contributions that will facilitate the discussion of the observed variations of the partial molar volume in different solutions. Specifically, Equation (9.57) may be written as follows:…”

“…179 The popularity of SPT is partly due to the fact that its predictions can be easily connected with experiment, and partly because it is based on an intuitively appealing interpolative view of the connection between known microscopic, molecular and macroscopic limits. 40,180,181 Recently, Klähn et al 182 determined computationally partial molar volumes at infinite dilution of H 2 , CO and CO 2 dissolved in liquid heptane (b L* T;1 ¼ 14:6 Â 10 À10 Pa À1 ), diethylether (b L* T;1 ¼ 19:3 Â 10 À10 Pa À1 ), acetone (2-propanone; b L* T;1 ¼ 12:9 Â 10 À10 Pa À1 ) and methanol (b L* T;1 ¼ 12:5 Â 10 À10 Pa À1 ), where all data refer to 298.15 K and ambient pressure. Using molecular dynamics simulations [183][184][185] in combination with solvent OPLS-AA (Optimized Potentials for Liquid Simulations-All Atom) force field parameters 186,187 and customised force field parameters for the solutes, V LN 2 is computed by the direct method, that is, the volume difference between the pure liquid solvent and a solution containing a single solute molecule is calculated.…”

“…The combined approach of Klähn et al 182 (that is, direct molecular dynamics simulations and decomposition of KirkwoodBuff integrals obtained via pair distribution functions) indeed provides valuable insights into the physical-chemical factors that influence V LN 2 .…”