Temperature-driven molecular aggregation and phase behavior in sec-butyl alcohol/water mixtures

Abstract

The miscibility and phase behavior of binary liquid mixtures are strongly regulated by temperature, often leading to liquid-liquid phase separation under conditions defined by an upper critical solution temperature (UCST), lower critical solution temperature (LCST), or closed-loop miscibility gaps. In this work, the temperature-driven aggregation behavior and spatial distribution of alcohol and water molecules in sec-butyl alcohol (SBA)/water mixtures, exhibiting a closed-loop miscibility gap across LCST and UCST points, were examined utilizing molecular dynamics (MD) simulations, graph theory, and spatial inhomogeneity analysis. The radial distribution function (RDF) and graph theoretical analysis in the binary liquid mixture reveal that small SBA aggregates at lower temperatures are formed due to the strong interaction of alcohol with water, which is well-mixed with liquid water, promoting a homogeneous mixture. However, at intermediate temperatures, enhanced alcohol-alcohol interaction causes the formation of larger SBA aggregates, which are incompatible with water, triggering liquid-liquid phase separation in the aqueous mixture. At higher temperatures, these large alcohol aggregates are fragmented into small ones due to increased thermal motion, which is compatible with the water H-bond network, restoring homogeneity in the single-phase state. The spatial distribution analysis highlights that at lower temperatures, low h -values reflect a uniform distribution of constituent molecules with small-sized alcohol aggregates. In contrast, as temperature rises, large SBA aggregates with high h -values indicate spatial inhomogeneity and the emergence of water-incompatible aggregates, resulting in the phase separation in binary liquid mixtures. At elevated temperatures, decreasing h -values signify restored homogeneity as alcohol aggregates disassemble, re-establishing a single-phase state in the two-component system. These investigations on temperature-induced aggregation behaviors are anticipated to critically contribute to the broader understanding of LCST, UCST, and reentrant phase behaviors in binary liquid mixtures as well as the liquid-liquid phase separation phenomena in biological systems. © 2025 Elsevier B.V.