Dichotomous behavior of water in binary mixtures with a turning point between 40 and 60 °C

Abstract

Water is known for its anomalous properties; however, changes in its properties between the 40–60 °C temperature range remain poorly understood, particularly in complex aqueous systems. In this study, α-cyclodextrin, urea, poly(vinyl alcohol), and an inclusion complex of β-cyclodextrin with an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄mim]PF₆), were found to exhibit dichotomous behavior, rather than just an anomaly or a progressive shift in water and its binary mixtures. Using temperature-dependent dynamic light scattering (DLS) analysis, physicochemical measurements, and near-infrared (NIR) spectroscopy, a sharp and additive-independent transition in hydrodynamic particle size was observed, which was centered around 50 °C. This bifurcation indicated an intrinsic reconfiguration of the hydrogen-bonding network of water. Principal component analysis (PCA) and two-dimensional correlation spectroscopy (2DCoS) of the NIR spectra revealed coordinated spectral changes across this window, confirming a discrete restructuring process rather than a gradual thermal response. Notably, features below and above ∼50 °C were spectrally and dynamically distinct, supporting a two-state-like-model of water. These results demonstrated that the 40–60 °C range marked a thermally induced, cooperative transition in the structural organization of water, which was modulated but not dominated by solute interactions. This dichotomous behavior offers critical insights into the unique responsiveness of water and highlights the importance of this temperature window in understanding solvation dynamics, reaction kinetics, and aqueous-phase behavior across scientific disciplines. Such structural duality may inspire novel thermal switches, responsive solvents, and smart hydration systems for future applications in drug delivery, nanofluidics, and green chemistry.