Theoretical Studies On Molecular Interactions In Solution And Interface

The knowledge on the relationship between the microscopic structure and the macroscopic thermodynamic properties of the solution and interface systems is significant. Molecular dynamics (MD) simulation can further contribute to the understanding on the microscopic nature of the macroscopic and mesoscopic phenomena for the solution and interface systems. Basing on the characteristic of molecular interactions, typical systems for solution and interface have been studied by using MD simulation and related theories, aiming of establishing the relationship between the microscopic structure and the macroscopic thermodynamic properties. The basic conceptions and phenomena, such as hydrogen-bonding (HB) interactions,π-π interactions, solvation effect, hydrophobic effect, surface propensity, wetting and spreading, are mainly involved in our researches.The solvation differences between cis-and transplatin molecules in water have been studied by using MD simulations. Firstly, the potential energy curves of transplatin-water systems are obtained by using high-level quantum chemistry calculation. Then the force field parameters for interactions between platin and water molecules are optimized with these potential energy curves. Due to the distinct different arrangement of the Cl and NH3ligands, the solvation effects for cis-and transplatin in water are significant different. Moreover, in comparison with cisplatin, there are more HBs around transplatin and these HBs show the longer lifetime.Choosing the appropriate force field is one critical success in MD simulations. Eight sets of the standard force fields, OPT-FF, AMBER03, GAFF, OPLS-AA, OPLS-CS, CHARMM27, GROMOS53A5, and GROMOS53A6, are used in the MD simulations study of liquid benzene properties, aiming of evaluting the superiority of these force fields for the π-aromatic system. The local structure, the long-range structure, and thermodynamic properties of liquid benzene are examined. Comparing the simulation results with the experimental data, the OPLS-AA force field is promising for the applications to the aromatic π-π interaction systems.Based on the above study, implying employing the OPLS-AA force field and TIP4P model, all-atom MD simulations for the benzene-water mixtures are performed, aiming of exploring the relationship between microscopic structures and the thermodynamic properties. We find that both of the aggregation process of hydrophilic units in hydrophobic solvent and that of hydrophobic units in hydrophilic solvent are primarily driven by the enthalpy variance. Moreover, the molecular aggregations show the double-scaled features:firstly assembling in a quasi-plane at the low concentration, then bulking in three dimensions with the concentration increase. Another significant finding is that the former process is further classified into:a reaction-limited aggregation model for the clustering of benzene in the water-rich mixture, due to the short-distanced intermolecular interactions; while a diffusion-limited aggregation model for the formation of water cluster in the benzene-rich mixture, triggered by the long-range dipole-dipole interactions.In addition, two types of interface systems have also been investigated. Firstly, all-atom MD simulations are performed with different solute concentrations and at various temperatures, in order to an understanding of the microscopic structures and thermodynamic properties of NH3and NH4+(along with halogen anion Cl-, Br-, or I-) at the aqueous solution-air interfaces. The surface propensities of NH3, Br-, and I-species are predicted, while the is repelled from the interface. Moreover, the surface propensities of NH3, Br-, and I-clearly decline when the temperature rises, indicating that the entropic contribution disfavors the presence of these species at the solution-air interface.Secondly, aiming at the wetting process on solid surface, the wetting and spreading dynamics of the liquid droplet of water, benzene, methanol, ethanol, propanol, butanol, phenyhnethanol,2-phenyl-ethanol,3-phenyl-l-propanol, and4-phenyl-l-butanol on graphene are investigated by using all-atom MD simulations. The spreading velocity of alcohols and phenylated alcohols are reduced with the increase of the length of carbon chains. With the same length of carbon chains, the spreading velocity of alcohols are much larger than that of phenylated alcohols. Moreover, the self-diffusion coefficient is found to be significant for the spreading velocity. Considering the temperature effects on the spreading exponent, it is found that the spreading exponent increases with the rise of temperature.