Xiuquan Sun

Computational studies of aqueous interfaces of SrCl2 salt solutions

The electron density profiles and corresponding surface structures of an aqueous interface of SrCl(2) salt solution were computed by use of molecular dynamics simulations. We used both polarizable and nonpolarizable potential models to describe molecular interactions. The results demonstrate that the polarizable models captured the essential features of the corresponding X-ray reflectivity experimental data while the corresponding nonpolarizable models could not. In addition, we demonstrated that the shape of the X-ray reflectivity curve could be quantitatively reproduced if the simulations were carried out at lower SrCl(2) concentrations, making it likely that the polarizable models used in this study somewhat overestimate the surface concentration of ions. However, significant interfacial enhancement of both Sr(2+) and Cl(-) appears necessary to reproduce the experimental spectra. This is in contrast to systems with monovalent cations, which have generally been found to have a double layer, in which anions are enhanced at the surface but cations are repelled.

Computational study of hydrocarbon adsorption in metal-organic framework Ni2(dhtp).

Enhancing the efficiency of the Rankine cycle, which is utilized for multiple renewable energy sources, requires the use of a working fluid with a high latent heat of vaporization. To further enhance its latent heat, a working fluid can be placed in a metal organic heat carrier (MOHC) with a high heat of adsorption. One such material is Ni\DOBDC, in which linear alkanes have a higher heat of adsorption than cyclic alkanes. We carried out molecular dynamics simulations to investigate the structural, diffusive, and adsorption properties of n-hexane and cyclohexane in Ni\DOBDC. The strong binding for both n-hexane and cyclohexane with Ni\DOBDC is attributed to the increase of the heat of adsorption observed in experiments. Our structural results indicate the organic linkers in Ni\DOBDC are the primary binding sites for both n-hexane and cyclohexane molecules. However, at all temperatures and loadings examined in present work, n-hexane clearly showed stronger binding with Ni\DOBDC than cyclohexane. This was found to be the result of the ability of n-hexane to reconfigure its structure to a greater degree than cyclohexane to gain more contacts between adsorbates and adsorbents. The geometry and flexibility of guest molecules were also related to their diffusivity in Ni\DOBDC, with higher diffusion for flexible molecules. Because of the large pore sizes in Ni\DOBDC, energetic effects were the dominant force for alkane adsorption and selectivity.

Grand Canonical Monte Carlo studies of CO2 and CH4 adsorption in p-tert-butylcalix[4]arene

Grand Canonical Monte Carlo simulations were performed for single component isotherms of CO(2) and CH(4) in the p-tert-butylcalix[4]arene structure. Comparison with literature data for adsorption used the Peng-Robinson equation of state to map simulated fugacities to experimentally determined pressures. CO(2) binding in the high-pressure structure of TBC4 (TBC4-H) occurs in two distinct waves. The cage sites in TBC4 completely fill up, followed by the filling of interstitial sites, resulting in the sum of two Langmuir isotherms being the best way to describe the total absorption isotherms. Our simulation results capture the essential experimental feature that the cage sites are the major contributor to the absorption isotherms, and the contribution of interstitial sites are significantly less. We found that CH(4) does not exhibit the same two-site binding characteristic and has a smaller temperature dependence, which arises from a smaller negative entropy change upon absorption compared with the case for CO(2). Our calculations give higher binding than observed experimentally for the cage site but lower binding for the interstitial site. We also demonstrate that by rescaling the interaction between CO(2) and the lattice, the results can reproduce the experimental data well at low loadings. The rescaled potentials are within the range found in other studies. This makes the discrepancy between experiment and simulation at high loadings greater, which is unexpected for this system. It is postulated that the simulation points to structural changes or defects being partially responsible for the relatively higher absorption found experimentally.

Solvation of dimethyl succinate in a sodium hydroxide aqueous solution. A computational study.

Molecular dynamics simulations were carried out to study dimethyl succinate/water/NaOH solutions. The potential of mean force method was used to determine the transport mechanism of a dimethyl succinate (a diester) molecule across the aqueous/vapor interface. The computed number density profiles show a strong propensity for the diester molecules to congregate at the interface, with the solubility of the diester increasing with increasing NaOH concentration. It is observed that the major contribution to the interfacial solvation free-energy minimum is from electrostatic interactions. Even at higher NaOH concentrations, the increasing electrostatic interaction between the diester and ions is not large enough to favor the solvation of diester in bulk solutions. The calculated solvation free energies are found to be -2.6 to -3.5 kcal/mol in variant concentrations of NaOH aqueous solutions. These values are in qualitative agreement with the corresponding experimental measurements. The computed surface potential indicates that the contribution of diester molecules to the total surface potential is about 25%, with the major contribution from interfacial water molecules.

Understanding ion–ion interactions in bulk and aqueous interfaces using molecular simulations

In addition to its scientific significance, the distribution of ions in the bulk and at aqueous interfaces is also very important for practical reasons. Providing a quantitative description of the ionic distribution, and describing interactions between ions in different environments, remains a challenge, and is the subject of current debate. In this study, we found that interionic potentials of mean force (PMFs) and interfacial properties are very sensitive to the ion-ion interaction potential models. Our study predicted a Sr(2+)--CI- PMF with no contact ion-pair state and a shallow solvent-separated ion-pair state. In addition, we were able to quantitatively capture the experimental X-ray reflectivity results of the aqueous salt interface of the Sr(2+)--Cl- ion-pair, and provided a detailed physical description of the interfacial structure for this system. We also predicted the Xray reflectivity results for SrBr2 and SrI2 systems.

Computational study of ion distributions at the air/liquid methanol interface

Molecular dynamic simulations with polarizable potentials were performed to systematically investigate the distribution of NaCl, NaBr, NaI, and SrCl(2) at the air/liquid methanol interface. The density profiles indicated that there is no substantial enhancement of anions at the interface for the NaX systems, in contrast to what was observed at the air/aqueous interface. The surfactant-like shape of the larger more polarizable halide anions, which is part of the reason they are driven to air/aqueous interfaces, was compensated by the surfactant nature of methanol itself. These halide anions had on average an induced dipole of moderate magnitude in bulk methanol. As a consequence, methanol hydroxy groups donated hydrogen bonds to anions where the negatively charged side of the anion induced dipole pointed, and methyl groups interacted with anions where the positively charged side of the anion-induced dipole pointed. Furthermore, salts were found to disrupt the surface structure of methanol. For the neat air/liquid methanol interface, there is relative enhancement of methyl groups at the outer edge of the air/liquid methanol interface in comparison with hydroxy groups, but with the addition of NaX this enhancement was reduced somewhat. Finally, with the additional of salts to methanol, the computed surface potentials decreased, which is in contrast to what is observed in corresponding aqueous systems, where the surface potential increases with the addition of salts. Both of these trends have been indirectly observed with experiments. The surface potential trends were found to be due to the greater propensity of anions for the air/water interface that is not present at the air/liquid methanol interface.

Computational Studies of Load-Dependent Guest Dynamics and Free Energies of Inclusion for CO2 in Low-Density p-tert-Butylcalix(4)arene at Loadings up to 2:1

The structure, dynamics, and free energies of absorption of CO(2) by a low-density structure (P4/n) of calixarene p-tert-butylalix[4]arene (TBC4) at loadings up to 2:1 CO(2):TBC4 have been studied by using molecular dynamics simulations with two sources of initial TBC4 structures (TBC4-T and TBC4-U). The CO(2)/TBC4 complex structure is very sensitive to the initial lattice spacing of TBC4. From the computed radial distribution functions of CO(2) molecules, a CO(2) dimer is observed for TBC4-T and a cage-interstitial CO(2) structure is suggested for TBC4-U. The dynamics of the CO(2) molecules show little initial TBC4 structural dependency. The free energy of inclusion for a single CO(2) in this TBC4 structure for various loadings is -4.0 kcal/mol at 300 K and -1.8 kcal/mol at 450 K, showing that CO(2) inclusion is favored. The fully loaded 1:1 CO(2):TBC4 system is slightly less favorable at -3.9 and -1.2 kcal/mol at 300 and 450 K, respectively. The first CO(2) added beyond 1:1 loading shows a significant drop in absorption energy to -1.9 and +1.9 kcal/mol at 300 and 450 K. These data are consistent with experimental results showing that low-density structures of TBC4 are able to absorb CO(2) at loadings greater than 1:1 but retention is lower than for 1:1 loaded systems indicating the free energy of inclusion for addition of the CO(2) above 1:1 is less favorable.