Spatial inhomogeneity and molecular aggregation behavior in aqueous binary liquid mixtures

Abstract

The component molecules in aqueous mixtures tend to aggregate with increasing concentration, and it has been conclusively reported that their spatial distribution is no longer uniform. Here, we attempt to measure the spatial inhomogeneity of constituent molecules in the cosolvent-water mixture by calculating the h value wherein the time-varying molecular configurations are obtained from molecular dynamics (MD) simulations conducted at various concentrations and temperatures. The results reveal that the miscible methanol solutions have a small h value, indicating the homogeneous distribution of components, while the immiscible dichloromethane-water mixtures have a relatively large h value, representing the localization of components due to the formation of two separated liquid phases. In the butanol-water mixture, the h value decreases significantly with increasing temperature, which reflects a temperature-dependent phase change from two liquid phases to a single phase, resulting in uniform distribution of molecular components at high temperatures. Meanwhile, in the graph theoretical analysis of cosolvent aggregates and water H-bond network, two prominent types of aggregation behavior are shown in the investigated binary liquid system, i.e., self-associated or spatially extended aggregates. The growth of self-associated cosolvent aggregates facilitates the liquid-liquid phase separation, resulting in a heterogeneous distribution of components with high h values. The development of a spatially extended network enables fully mixing with water H-bond network, bringing about the homogeneous distribution of constituent molecules and thus resulting in low h values. The concept of spatial inhomogeneity and bifurcating hypothesis on aggregation behavior is anticipated to contribute significantly to understanding the underlying issues, such as miscibility and liquid-liquid phase separation in aqueous mixtures.