title: Dynamical properties of water-methanol solutions.
abstract: We study the relaxation times tα in the water-methanol system. We examine new data and data from the literature in the large temperature range 163 < T < 335 K obtained using different experimental techniques and focus on how tα affects the hydrogen bond structure of the system and the hydrophobicity of the alcohol methyl group. We examine the relaxation times at a fixed temperature as a function of the water molar fraction XW and observe two opposite behaviors in their curvature when the system moves from high to low T regimes. This behavior differs from that of an ideal solution in that it has excess values located at different molar fractions (XW = 0.5 for high T and 0.75 in the deep supercooled regime). We analyze the data and find that above a crossover temperature T ∼ 223 K, hydrophobicity plays a significant role and below it the water tetrahedral network dominates. This temperature is coincident with the fragile-to-strong dynamical crossover observed in confined water and supports the liquid-liquid phase transition hypothesis. At the same time, the reported data suggest that this crossover temperature (identified as the Widom line temperature) also depends on the alcohol concentration.
authors.affiliation: Dipartimento MIFT, Sezione di Fisica, Università di Messina, I-98166 Messina, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Mallamace
authors.affiliation: Dipartimento MIFT, Sezione di Fisica, Università di Messina, I-98166 Messina, Italy.
authors.forename: Carmelo
authors.initials: C
authors.lastname: Corsaro
authors.affiliation: Consorzio per lo Sviluppo dei Sistemi a Grande Interfase, Unità di Catania, I-95125 Catania, Italy.
authors.forename: Domenico
authors.initials: D
authors.lastname: Mallamace
authors.affiliation: Consiglio Nazionale delle Ricerche-IPCF Messina, I-98158 Messina, Italy.
authors.forename: Cirino
authors.initials: C
authors.lastname: Vasi
authors.affiliation: Dipartimento MIFT, Sezione di Fisica, Università di Messina, I-98166 Messina, Italy.
authors.forename: Sebastiano
authors.initials: S
authors.lastname: Vasi
authors.affiliation: Center for Polymer Studies and Department of Physics, Boston University, Boston, Massachusetts 02215, USA.
authors.forename: H Eugene
authors.initials: HE
authors.lastname: Stanley