Aqueous Binary Mixtures: A Study Using NMR Measurements and ab initio Molecular Dynamics Simulations

Abstract

Binary mixtures having a combination of hydrophobic and hydrophilic groups, such as waterdimethyl sulfoxide (DMSO), water-methanol, water-ethanol, water-tertiary butyl alcohol (TBA), water dioxane, and water-glycerol, have been widely studied. These mixtures often show non-ideal mixing behaviour with properties such as electrical conductivity, permittivity, speci fic volume, enthalpy of mixing and light-scattering behaviour exhibiting non-monotonic behaviour as a function of the composition of the mixture. The waterethanol system that is miscible over the entire composition range is perhaps one of the most elementary but nontrivial model system that exhibits non-ideal behaviour. The mixtures exhibit negative excess entropy and a strong increase in heat capacity as compared to an ideal solution, with properties such as, molar volume, excess entropy, compressibility, viscosity, diffusion coeffcient, and sound attenuation coeffcient exhibiting composition dependent anomalies. The occurrence of different solvation regimes in water-alcohol binary mixtures has been widely reported, although the values of the transition points, the mole fraction of alcohol in the mixture, show minor differences depending on the experimental technique. It is generally accepted that the anomalous properties of aqueous binary mixtures is a consequence of the perturbation of the local and global hydrogen bond network due to the presence of the second component. Despite a large number of investigations encompassing both theories and experiments, a molecular understanding of the anomalous properties of the water-ethanol system has remained elusive. The focus of this thesis is on the role of hydrogen bonding and the modi fication of the hydrogen bonding network due to the presence of the second component in water-ethanol and water-ethylene glycol systems using solution Nuclear Magnetic Resonance (NMR) spectroscopy aided by ab initio Molecular Dynamics (AIMD) simulations for interpreting the experimental observations. In summary, this thesis has attempted to show how 1H NMR measurements in combination with ab initio MD simulations can be used to probe H-bonding and the nature of association in aqueous-binary mixtures of ethanol and ethylene glycol. The strategy was to first establish a geometrical de nition of H-bonds in pure ethanol and ethylene glycol before attempting the same for aqueous-binary mixtures. It is shown here that the secondary isotope effect can be used to distinguish intra and intermolecular interactions in the 1H NMR of mixtures of ethanol and ethylene glycol along with deuterted counter parts. It is shown how 1D transient NOE measurements of these mixtures, in conjuction with AIMD simulations can establish the geometry of H-bonds in pure ethanol and ethylene glycol. A simple procedure to determine the geometry of H-bonds between different donor-acceptor pairs ethanol-ethanol, water-ethanol and water-water in water-ethanol mixture from fragment obtained from AIMD is outlined. Having established the geometry of H-bonds it is possible to determine the thermodynamics of hydrogen bond formation in water-ethanol system at different compositions. The nature of association in the aqueous-binary mixtures could be established by 1H NMR measurements. The fact that there are water-rich and ethanol-rich clusters in dynamic equilibrium could be established from these measurements. The results presented in this thesis could pave the way for a better understanding, at the molecular level, intriguing properties of binary mixtures using a combination of NMR measurements and molecular dynamics simulations.