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Microheterogenity of alcohol-water mixtures: experimental and simulation study

Liquid water and liquid alcohols are both hydrogen-bonded. However, while water possesses a three-dimensional hydrogen-bonded network, alcohols show chain and ring association. Thus, investigation of mixtures of these two liquids are of considerable interest, in particular, when results are subsequently compared with those obtained for aqueous solutions of higher alcohols, where phase separation may be observed. Indeed, the anomalous negative excess entropy characteristic for aqueous mixtures of lower alcohols containing hydrophobic groups may be successfully discussed by invoking molecular scale segregation of the components across the entire composition range [1]. Such local immiscibility or microheterogeneity, is observed in the experimental as well as in the simulations results [2][3]. In this study we investigate the microheterogeneity in the water-alcohol system focusing on the clustering properties. Previous Brillouin scattering measurements of the alcohol-water mixtures were mostly done with a single-pass Fabry-Perot spectrometer [4-5]. Here, we present Brillouin scattering measurements over the full composition range, using a 6-pass tandem Fabry-Perot interferometer with a finesse of about 80, and FWHH of about 90 MHz of the instrumental function. The molecular dynamics simulations were done using the Dlpoly2.14 package. We calculated: the clustering parameters, such as cluster distribution for the clusters that are made of each component and the clusters that are mixed ; and differentiate the hydrogen - bonded clusters and the sterical clusters. We also studied the static correlations such as sites radial distribution functions and Kirkwood-Buff integrals, which are directly connected with the compressibility. At the end we compared the simulation results with the results of the experiments. [1] S. Dixit, J. Crain, W. C. K. Poon, J. L. Finney and A. K. Soper, Nature 416, 829 (2002) ; (b) A. K. Soper, L. Dougan, J. Crain and J. L. Finney, J. Phys. Chem. B 110, 3472 (2006) [2] Y. Seshimo, Y. Ike and S. Kojima, Jpn. J. of Appl. Phys. 47. No 5 (2008) [3] L. Zoranić, F.Sokolić, A.Perera, J. Chem. Phys, 127, 024502 (2007) [4] K. Negishi, M. Yamazaki, and Y. Torikai, Japan. J. Appl. Phys. 6, 1016 (1967) [5] A. Asenbaum and M. Sedlacek, Acustica 30, 109 (1974)

alcohol-water mixture; MD simulation; Brillouin scattering measurments

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