Molecular Mechanisms behind Nonmonotonic Surface Tensions of Binary Aqueous n-Diol Mixtures

The interplay between molecular structure and behavior in aqueous mixtures of amphiphiles is central to understanding microphase formation and macroscopic properties such as surface tension. In this work, we investigate the interfacial behaviors of aqueous mixtures of 1,2-butanediol (12BUT), 1,2-pentanediol (12PENT), 1,2-hexanediol (12HEX), and 1,5-pentanediol (15PENT) across the full composition range using molecular dynamics simulations. Surface tension calculations reproduce the experimentally observed trends, with 12BUT and 12PENT exhibiting a smooth monotonic decrease with increasing diol concentration, whereas 12HEX and 15PENT display a nonmonotonic trend. Analyses of the configurational entropy and conformer populations reveal two distinct aggregation mechanisms: chain flexibility-driven aggregation in 12HEX and rigid geometry-induced stacking in 15PENT. These results link diol structure to interfacial thermodynamics and microphase organization, offering insight into the design and control of amphiphile-water systems.