Hydrophilic and hydrophobic competition in water-methanol solutions

We study the thermal behavior of the longitudinal spin-lattice, T-1, and the transverse spin-spin, T-2, relaxation times of the macroscopic magnetization in water / methanol solutions. Our aim is to investigate the reciprocal influence of hydrophobic effects on water properties and of hydrophilicity (via hydrogen bond, HB, interactions) on the solute. Using classical Nuclear Magnetic Resonance spectroscopy, we find a single characteristic correlation time tau(c) that reflects all local structural con figurations and characterizes the thermal motion effects of the magnetic nuclei on the spin-spin interaction. We find that in the supercooled regime the correlations are stronger, with respect to ambient temperature, because the HB interactions have a lifetime long enough to sustain a stable water network. However, increasing the temperature, progressively decreases the HB interaction lifetime and destroys the water clusters with a consequent decoupling in the dynamic modes of the system. In addition, at temperatures higher than about 265 K, the hydrophobicity becomes gradually stronger and governs the physical properties of the solutions.