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Enhanced density fluctuations in water-ethanol mixtures at low ethanol concentrations: Temperature dependent studies

Since the structural transformations observed in water-ethanol binary mixtures are apparently driven by relatively weak intermolecular forces (like hydrophobicity and hydrogen bonding) that often cooperate to form self assembled structures, one expects the aggregation properties to show strong temperature dependence. We study the effect of temperature on the formation of transient ethanol clusters as well as on the dynamic density heterogeneity induced in the system due to such clustering. The dynamic heterogeneity is expected to occur on small length scales with short lifetime and both are expected to be temperature dependent. Indeed, a major finding of the work is strong temperature dependence of the extent of structural heterogeneity. Distinct signature of static and dynamic heterogeneity of ethanol molecules is also found to appear with lowering of temperature. This is attributed to the formation of transient ethanol clusters that are known to exhibit considerably small lifetime (order of a few picosecond). The transient dynamical features of dynamic heterogeneity are expected to affect those relaxation processes occurring at sub-picosecond time scales. On the other hand, strong temperature dependence of micro-structure formation can be anticipated to be due to enhanced structural order stimulated in the system with lowering of temperature. Present analyses reveal a number of interesting features which were not explored beforehand in this widely studied binary mixture.