Molecular dynamics simulation of diffusion coefficients and structural properties of some alkylbenzenes in supercritical carbon dioxide at infinite dilution

Abstract

The binary infinite dilute diffusion coefficients, D₁₂(∞), of some alkylbenzenes (Ph-C(n), from Ph-H to Ph-C12) from 313 K to 333 K at 15 MPa in supercritical carbon dioxide (scCO2) have been studied by molecular dynamics (MD) simulation. The MD values agree well with the experimental ones, which indicate MD simulation technique is a powerful way to predict and obtain diffusion coefficients of solutes in supercritical fluids. Besides, the local structures of Ph-C(n)/CO2 fluids are further investigated by calculating radial distribution functions and coordination numbers. It qualitatively convinces that the first solvation shell of Ph-C(n) in scCO2 is significantly influenced by the structure of Ph-C(n) solute. Meanwhile, the mean end-to-end distance, the mean radius of gyration and dihedral angle distribution are calculated to gain an insight into the structural properties of Ph-C(n) in scCO2. The abnormal trends of radial distribution functions and coordination numbers can be reasonably explained in term of molecular flexibility. Moreover, the computed results of dihedral angle clarify that flexibility of long-chain Ph-C(n) is the result of internal rotation of C-C single bond (σ(c-c)) in alkyl chain. It is interesting that compared with n-alkane, because of the existence of benzene ring, the flexibility of alkyl chain in Ph-C(n) with same carbon atom number is significantly reduced, as a result, the carbon chain dependence of diffusion behaviors for long-chain n-alkane (n ≥ 5) and long-chain Ph-C(n) (n ≥ 4) in scCO2 are different.

Molecular dynamics simulation of diffusion coefficients and structural properties of ketones in supercritical CO2 at infinite dilution

“…In recent years, some authors successfully performed classical MD simulation for calculation of organic molecules in SC-CO 2 [21,28,[32][33], taking advantage of the accurate potential functions available in the literature. More than replacing or incrementing measurements, MD is also very desired to unveil and interpret experimental trends observed.…”

“…During the last decades, the MD simulation techniques opened avenues in the prediction of thermo-physical properties, under equilibrium and non-equilibrium conditions, of a wide variety of systems [21,[28][29][30][31]. The information acquired from microscopic computations is 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 vital for the development of reliable and robust macroscopic models, which means that MD simulation and phenomenological modeling may complement each other.…”

“…With respect to computer simulations, they are very useful for investigating dynamic, structural and thermodynamic properties, allowing researchers to complement other existing studies and get valuable information not directly obtained by experiment [21,[27][28][29] as, for instance, radial distribution functions, coordination numbers, and energetic data.…”

“…Therefore, infinite dilution or tracer diffusion coefficients, , have been investigated for decades through experimental measurement [7][8][9][10][11], phenomenological modeling [9,[12][13][14][15][16][17][18][19], and molecular dynamics (MD) simulations [12,14,[20][21]. For measuring diffusion coefficients of solutes in supercritical carbon dioxide (SC-CO 2 ), the most adopted techniques are the geometric and chromatographic methods [9,11,22] despite the inherent technical difficulties and equipment costs associated to the last one [22].…”

Molecular dynamics analysis of compositional effects in hydrocarbon systems property calculations

“…They made the interesting argument that molecular simulation can be employed as a tool for the determination of Fick diffusivities in high pressure systems, like in oil reservoirs, without the need to construct complicated and expensive experiments. Wang et al [9] extended these diffusion calculations to supercritical CO 2 /alkyl benzene binary mixtures emphasizing the structural aspects. They also made a similar argument that molecular simulation technique is a powerful way to predict diffusion coefficients of solutes in supercritical fluids.…”