Evolution of the micro-structure of aqueous alcohol mixtures with cooling: A computer simulation study

We study by computer simulations aqueous ethanol and tert-butanol mixtures for temperatures below room temperature, from T=250 K to T=150 K, in order to monitor how concentration fluctuations and micro-segregation evolve with the lowering of temperature. Similarly to our recent study of aqueous-methanol mixtures [J. Chem. Phys. 145, 144502 (2016)], we find that concentration fluctuations tend to decrease - as monitored by the lowering of the k = 0 value of the structure factors, while micro-segregation becomes more specific, in particular with a marked tendency of water to form short chain-like clusters, instead of large bulky nano-domains at higher temperatures. This clustering translates into a growing pre-peak in the water-water structure factors at k ≈ 0.5Å −1. The decrease of concentration fluctuations is also in agreement with the fact that calculated Kirkwood-Buff integrals tend to become ideal, in sharp contrast with the data for T=300 K, in particular for aqueous-tbutanol mixtures. This near-ideality is a consequence of water forming supra-molecular linear clusters. We conjecture that the cooling of nano-structured mixtures produces a “transfer” of k = 0 concentration fluctuation mode towards k≠0 mode, leading in particular to these supra-molecular water clusters.