Intermolecular interactions in complex liquids: effective fragment potential investigation of water-tert-butanol mixtures.

Structure and bonding patterns in tert-butanol (TBA)-water mixtures are investigated by using molecular dynamics simulations with the effective fragment potential (EFP) method. EFP is a model potential in which all parameters are obtained from a set of ab initio calculations on isolated fragment molecules. Mixed-basis EFP potentials (called "EFPm") for water and TBA molecules were prepared and tested in this work. The accuracy of these EFP potentials is justified by comparison of structures and binding energies in water, TBA, and water-TBA dimers with MP2/6-311++G(d,p) data. It has been found that the discrepancies between EFP and MP2 do not exceed 0.1 Å in intermolecular distances and 1 kcal/mol in binding energies. Structures of TBA-water solutions with 0.0, 0.06, 0.11, 0.16, and 0.50 TBA mole fractions were analyzed by using radial distribution functions (RDFs) and coordination numbers. These results suggest that, at low TBA concentrations, the structure of water is enhanced and water and TBA are not homogeneously mixed at the molecular level. In the equimolar TBA-water solution, the microscopic mixing is more complete. Analysis of the energy components in TBA-water solutions shows that, while the electrostatic and exchange-repulsion terms provide the largest contributions to the total potential energy, the relative importance of the polarization and dispersion terms depends on the concentration of TBA. With an increase of TBA concentration, the fraction of the dispersion energy increases, while the fraction of polarization energy diminishes. However, both polarization and dispersion terms are essential for accurate description of these systems.