Effect of varying water content on the structure of butyl alcohol/water mixtures: FT-NIR two-dimensional correlation and chemometric studies

Effect of varying water content on the structure of butyl alcohol/water mixtures has been examined by Fourier-transform near-infrared (FT-NIR) spectroscopy. The experimental spectra were analyzed by two-dimensional (2D) correlation and chemometric methods. Our results do not provide evidences for the presence of significant amounts of water free from any specific interactions. The molecules of water predominantly form two hydrogen bonds between different associates of alcohol. This way, the structure of liquid alcohols becomes more stable. Our previous and current studies did not reveal a meaningful reduction in the population of the free OH groups of alcohols on addition of water. The free OH groups of water are present in the mixtures at all studied water contents and they originate mostly from the molecules acting as a single proton donor. The population of molecules of water that act as a proton acceptor is limited. The hydrogen bonds between water and alcohol are broken more easily in s-BuOH and t-BuOH and hence, the number of the free OH groups of water in these alcohols is higher than that in the primary alcohols. This results from smaller degree of self-association in s-BuOH and t-BuOH relative to that in the primary butanols. At higher water content the water–water interactions become more important. The clusters of water are created around the molecules of water attached to the terminal molecules of alcohol leading to decrease in the population of the free OH groups of water.