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Vibrational, rotational and hydrogen bond dynamics of ethanol molecules in binary aqueous and non-aqueous mixtures

Ethanol-water mixtures have been widely analyzed in the past by both experiments and computer simulations. This attention comes from the fact that ethanol is one of the smallest amphiphilic molecules and a deeper understanding of its interaction with water may lead to valuable insight for many biological processes, such as the formation of lipid membrane in aqueous environment [1]. It should also be noted that many thermodynamic quantities, such as excess entropy, heat capacity, molar volume and speed of sound, show anomalous behavior depending on the composition of the mixture [2-5]. These effects are generally elucidated by the structural changes of the water hydrogen bonded network, which gets disrupted with the insertion of ethanol molecules. Computer simulations [6] and experiments [7] have indeed shown that in the low-ethanol regime, ethanol molecules form small clusters in water by shielding their methyl groups. When it comes to study of ethanol molecules in a lipid-like milieu, hexane is the perfect solvent. For this binary system spectroscopic experiments have provided evidence of the formation of ethanol clusters, consisting of mostly 4 to 6 molecules [8]. Surely, hydrogen bond plays the leading role for this scenario, since hexane is inert, unlike water. However, less is known about the dynamics of ethanol molecules in these mixtures, especially in view of the difference in the interaction with the solvent. We compare the results for vibrational and orientational dynamics of ethanol in water and hexane, by monitoring power spectra and orientational autocorrelation functions, respectively. Hydrogen bond dynamics is analyzed through hydrogen bond autocorrelation functions and probabilities for hydrogen bond breaking. Ethanol diffusion coefficients are also compared, as well as the self-part of the van Hove functions. References [1] P. Ball, Chem. Rev. 108, 74−108 (2008). [2] H. S. Frank and M. W. Evans, J. Chem. Phys. 13, 507–532 (1945). [3] R. F. Lama and B. C. Y. Lu, J. Chem. Eng. Data 10, 216–219 (1965). [4] K. Nakanishi, N. Kato, and M. Maruyama, J. Phys. Chem. 71, 814–818 (1967). [5] G. D’Arrigo and A. Paparelli, J. Chem. Phys. 88, 405 (1988). [6] M. Mijaković, B. Kežić, L. Zoranić, F. Sokolić, A. Asenbaum, C. Pruner, E. Wilhelm and A. Perera, J. Mol. Liq. 164, 1-2, 66-73 (2011). [7] K. Egashira and N. Nishi, J. Phys. Chem. B 102, 4054-4057 (1998). [8] K. M. Murdoch, T. D. Ferris, J. C. Wright and T. C. Farrar, J. Chem. Phys. 116, 5717 (2002).

Ethanol-water mixtures ; Ethanol-hexane mixtures ; Computer simulations

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