Intermolecular Interactions in Complex Liquids: Effective Fragment Potential Investigation of Water<italic>tert</italic>-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. ■ INTRODUCTION Broader use of bioalcohols, i.e., alcohols produced from biomass rather than petroleum sources, could result in energy security and lower emissions of green-house gases. Apart from the high production cost, a stumbling block for achieving the extensive use of alcohols is liquid−liquid phase separation in hydrocarbon−alcohol mixtures that becomes even more exacerbated by the presence of water. While different impurities (water, ions, organic molecules) can either induce or inhibit phase separation in hydrocarbon−alcohol systems, there is little known about the underlying mechanisms of these phenomena, since molecular level studies of these systems are scarce. In contrast, there is considerable interest in the nature of hydrophobic and hydrophilic hydration, with many recent controversial findings. For example, until recently, hydration of alcohols has been interpreted in terms of the Frank− Evans classical “iceberg” model. However, recent experimental and theoretical studies provide strong evidence of incomplete mixing at the molecular level and retention of the network structure of bulk water. The level of mixing, morphology, and other microscopic and macroscopic properties of liquids are governed by the nature of the underlying interactions between molecules in the liquid. Tertiary butanol (TBA) is considered to be a promising candidate for use as a biofuel. TBA is the largest monohydric alcohol that is fully soluble in water. TBA−water systems have been investigated in a number of studies. Neutron diffraction experiments on small concentrations of TBA in water suggest that the primary association between alcohol molecules is through interactions of the hydrophobic groups. Even at a high TBA concentration (0.86 mol fraction), direct polar interactions between alcohol molecules were not prevalent. At this concentration, a few waters coordinate several TBA molecules, similar to what was reported by Guo et al. for water−methanol solutions. Computer simulations of TBA−water mixtures have been performed by Lee and van der Vegt, with results differing from experiments. In particular, significant TBA−TBA hydrogen bonding for all concentrations above 0.04 mol fraction alcohol has been observed. In this study, the molecular structure of TBA−water mixtures is investigated by simulations with the general effective fragment potential (EFP), with particular attention paid to hydrogen bonding. The EFP method is a quantum mechanics based potential that is a computationally inexpensive way of modeling intermolecular interactions in noncovalently bound systems. Absence of fitted parameters and natural partitioning of the interaction energy into electrostatic, polarization, dispersion, and exchange-repulsion terms make it an attractive choice for analysis and interpretation of intermolecular forces. Previously, general EFP has been successfully applied for investigation of the noncovalent Received: August 12, 2011 Revised: January 22, 2012 Published: February 10, 2012 Article