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Dynamics of ethanol-hexane and ethanol-water mixtures studied by computer simulations

Like many 1-alkanols, ethanol tend to form hydrogen bonded chain-like aggregates in the neat liquid, as can be seen in computer simulation studies [1]. These aggregates become better characterized when mixing ethanol with alkanes than when mixing with water [2]. Indeed, hydroxyl groups can hydrogen bond better in alkanes than in water, where they are in competition with the hydrogen bonding of water. While the existence of the difference in aggregate structure can be put in evidence by various static properties, such as cluster distribution, pair correlation and structure factors, the corresponding dynamic signature is less obvious to characterize. Indeed, large aggregates can have 2 temporal signatures, a small frequency which would correspond to aggregate dynamics, and a large frequency which could correspond to intra-aggregate motions. The question we ask here is if Classical Molecular Dynamics can provide us with unambiguous signals corresponding to these 2 frequencies, both in the neat ethanol, and the mixtures with hexane and water. In particular, is it possible to extract information on the various types of aggregation scenario which would correspond to the static analysis. Our calculation of the velocity auto-correlation functions and corresponding power spectrum of various atomic groups, show the unambiguous existence of a large frequency, around 400 cm-1, but which appears only under mixing conditions. This seems to contradict the existence of chain-like aggregates in neat ethanol. A similar frequency range has also been reported by other authors, in relation to the influence of the hydrogen bonding dynamics on intra-molecular motions [3, 4]. We discuss the existence of this frequency in terms of the distinction between intra-molecular and intra-aggregate motions, as induced by the hydrogen bonding. In other words, this concerns the ability to distinguish between genuine molecular entitiesand hydrogen bonded supra-molecular entities. [1] L. Zoranić, F. Sokolić and A. Perera, “Microstructure of neat alcohols: a molecular dynamics study”, J. Chem. Phys. 127, 024502-10 (2007) [2] M. Požar, B. Lovrinčević, L. Zoranić, T. Primorac, F. Sokolić, A. Perera, “Micro-heterogeneity versus clustering in binary mixtures of ethanol with water or alkanes”, Phys. Chem. Chem. Phys. 18, 23971-23979 (2016) [3] K. Mazur, I. A. Heisler and S. R. Meech, “THz spectra and dynamics of aqueous solutions studied by the Ultrafast Optical Kerr Effect”, J. Phys. Chem. B 115, 2563-2573 (2011) [4] M. K. Hazra and B. Bagchi, “Collective excitations in liquid dimethyl sulfoxide (DMSO): FIR spectrum, low frequency vibrational density of states, and ultrafast dipolar solvation dynamics”, J. Chem. Phys. 146, 024505-9 (2017)

computer-simulations, aqueous-ethanol, ethanol-alkane, dynamics, power-spectra

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