Hydrophobic effects:: A computer simulation study of the temperature influence in dilute O2 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 clathrate-like solvation shell, in line with other apolar gas solutions, emerged from the present simulations. The average number of water molecules in the first hydration shell decreases with temperature, and, in the investigated 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 oscillations of both water density and electrostatic charges in the neighborhood of the apolar solute, which is surrounded by shells of water at different density, 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 water-water interactions are stronger and water diffusional and rotational dynamics slower than in the bulk. A hydrogen bond's mean lifetime is affected by the apolar solute as well, being shorter in the first hydration shell. Differences between shell and bulk water properties are smoothed by increasing temperature, Suggestions for the molecular mechanism relevant to the more general problems of the hydrophobic effects are deduced from the simulations. 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].