Impact and structure of water in aqueous octanol mixtures: Hz-GHz dielectric relaxation measurements and computer simulations

Temperature (283 <= T/K <= 323) dependent dielectric relaxation (DR) measurements of aqueous binary mixtures of octanol at three different water mole fractions (XH2O = 0.05, 0.10, 0.20) were performed employing two different frequency windows: 0.00002 <= nu/MHz <= 10 and 0.2 <= nu/GHz <= 50. In addition, DR measurements were carried out for pure octanol and water saturated octanol at 298 K. All these measurements showed, irre-spective of solution temperature, a small decrease of the static dielectric constant (epsilon s) upon addition of water in octanol. Two-step relaxation process with time constants -3-0.3 ns and -120-40 ps characterized the measured DR spectra with no signature of bulk water-like DR time constant (-10 ps). Differential scanning calorimetric (DSC) measurements of octanol + water mixtures reflect only water concentration dependent depression of octanol freezing temperature. Our computer simulations of aqueous octanol mixtures with SPC/E water suggests a dramatic decrease of the average number of water-water H-bonds (NHBw_w) per water molecule and a spectacular increase of water populations possessing O-O-O angles distributed over 0 degrees <= theta <= 60 degrees. Consequently, a negative value for the average tetrahedral order parameter (Q) and an epsilon s as low as -2 for water have been predicted. A severe damage to the global tetrahedrality of water in these mixtures has been predicted and the angle distributions corresponding to negative and positive Q values explicitly shown. These simulation results therefore explain the microscopic origin for small effects of water at low mole fractions on epsilon s of octanol + water binary mixtures.