Hydrophobic effects: A computer simulation study of the temperature influence in dilute O-2 aqueous solutions

We present a computer simulation study of the temperature dependence of the structural and dynamical properties of dilute O-2 aqueous solutions. A cla thrate-like solvation shell, in line with other apolar gas solutions, emerg ed from the present simulations. The average number of water molecules in t he first hydration shell decreases with temperature, and, in the investigat ed temperature range (291-348 K), a net transfer of one water molecule from the hydration shell to the bulk has been detected. We have found oscillati ons of both water density and electrostatic charges in the neighborhood of the apolar solute, which is surrounded by shells of water at different dens ity, and with water molecules oriented in such a way as to form shells with alternating net electrostatic charges. In the O-2, first hydration shell w ater-water interactions are stronger and water diffusional and rotational d ynamics slower than in the bulk. A hydrogen bond's mean lifetime is affecte d by the apolar solute as well, being shorter in the first hydration shell. Differences between shell and bulk water properties are smoothed by increa sing temperature, Suggestions for the molecular mechanism relevant to the m ore general problems of the hydrophobic effects are deduced from the simula tions. A possible microscopic explanation for the lowering of solubility of oxygen in water with temperature is given. (C) 1999 American Institute of Physics. [S0021-9606(99)50502-3].