چكيده لاتين
This study investigates the application of the OPLS force field with a virtual site (OPLS-VSIL) for calculating various volumetric, structural, and dynamical properties of imidazolium-based dicationic ionic liquids, comparing the results with those obtained from two force fields, OPLS-2009IL and 0.8*OPLS-2009IL. It can be said that both the effect of charge reduction and the type of charge distribution were examined for calculating the properties of these ionic liquids. Initially, the study focused on volumetric properties, including density and isothermal compressibility. The comparison of simulated densities with experimental results for the ionic liquids under study indicated that the average deviation of density from experimental values is nearly the same for all three types of OPLS-based force fields. Additionally, in the 0.8*OPLS-2009IL and OPLS-VSIL force fields, it was observed that the densities decreased linearly with increasing temperature, while such a systematic trend was not observable in the OPLS-2009IL force field. Subsequently, molecular dynamics simulations based on the three types of OPLS-based force fields were employed to calculate structural properties, including radial distribution functions (RDF), spatial distribution functions (SDF), combined distribution functions (CDF), neighborhood analysis, and domain analyses. Initially, the results of the RDFs obtained from the three OPLS-based force fields were compared with those from AIMD simulations. The results showed that the OPLS-VSIL force field provided a better match in terms of the location and intensity of RDF peaks compared to the OPLS-2009IL and 0.8*OPLS-2009IL force fields. Furthermore, the results from the CDFs for intermolecular orientation between the two cation rings of two ionic liquids with different alkyl chain lengths indicated that the parallel arrangement of the two rings in the two different cations was more clearly observed in the simulations using the OPLS-VSIL force field than those in the OPLS-2009IL and 0.8*OPLS-2009IL force fields. The results also indicated that the reduction of the total charge of the system and the manner of charge distribution did not significantly affect the outcomes of the Domain and Voronoi analyses. Finally, this study examined the results of molecular dynamics simulations using various OPLS-based force fields for calculating dynamic properties such as diffusion, viscosity, van Hove correlation functions, hydrogen bond dynamics, ion pair dynamics, ion cage dynamics, and vector reorientation. The calculations related to diffusion coefficients for the two ionic liquids showed that the 0.8*OPLS-2009IL and OPLS-VSIL force fields, with a total charge of ±1.6 e, exhibited faster dynamics compared to the OPLS-2009IL force field, which had a total charge of ±2.0 e. Additionally, the dynamics of hydrogen bonding, ion pairs, ion cages, and vector reorientation were faster in the 0.8*OPLS-2009IL and OPLS-VSIL force fields than those in the OPLS-2009IL force field, as the total charge of the system was reduced by 20% in these two force fields. Overall, it can be concluded that the OPLS-VSIL force field offers better capabilities for accurately predicting the various properties of imidazolium-based dicationic ionic liquids compared to the OPLS-2009IL and 0.8*OPLS-2009IL force fields.
