Three-dimensional Morphology and X-ray Scattering Structure of Aqueous tert-Butanol Mixtures: A Molecular Dynamics Study

It is well established that water-alcohol mixtures exhibit anomalous properties at very low as well as at very high alcohol concentrations. Almost all the studies in this regard intend to link these anomalies to the microscopic structural changes as water (or alcohol) concentration increases in the mixture. However, it is important to note that the nature of these structural changes could be different at the water- and TBA-rich concentrations. In this article, our goal is to address such structural change overs, if really present, in the mixtures of water and tert-butanol (TBA) by using simulated X-ray scattering structure function, S(q), real space radial and spatial distribution functions and heterogeneity order parameter. By using a judicial partitioning scheme, we show that structural characteristic of pure water is qualitatively retained for X (TBA) < 0.1. The simulated S(q) peaks at around q = 2 and q = 2.8 (-1), which correspond to water oxygen correlations, begin to fade away only after X (TBA) a 0.1. This is a clear indication of microscopic structural transition at X (TBA) ae 0.1. Beyond X (TBA) = 0.1, the TBA structural features begin to take over to that of water. The peak at q =1.3 (-1) which primarily corresponds to nonpolar-nonpolar correlations in pure TBA begin to rise at X (TBA) ae 0.1. However, the pre-peak at around q = 0.75 (-1), which is due to polar-polar and nonpolar-polar correlations in pure TBA, seems to appear at lower q value only at the equi-molar concentration of the mixture. From the solvent cage surrounding the TBA molecules, we observe that while the aggregation of TBA alkyl groups, due to hydrophobic interaction, is maximum at 10% TBA, the intervening hydrogen bonding interactions between water and TBA molecules tend to lower the hydrophobic interactions between the alkyl groups of alcohol with increasing concentration of TBA. In addition to this, we also observe dimers and small clusters of water molecules in the TBA-rich regime. The computed heterogeneity order parameters for the individual components of the mixture reveal enhanced non-uniform distribution of the TBA molecules near X (TBA)ae 0.1 to 0.3. These results are also supported by the radial distribution functions and nearest neighbour coordination numbers of water and TBA oxygen atoms around TBA oxygen.