FAST AND SLOW DYNAMIC PROBES IN SOLVENT STRUCTURAL INVESTIGATIONS

The ability of water molecules to hydrate a proton from a photon-initiated weak acid depends on the microscopic concentration and structure of water in the aqueous solvent. Such acids thus can serve as dynamic probes of the local solvent environment in aqueous solvents perturbed by salts, surfaces, and dilution effects. In this paper, the microscopic local structures of pure liquid water and of two-component mixed aqueous solvents were examined by means of this technique. Aqueous solutions of methanol, ethanol, acetonitrile, dioxane, and glycerol served as solvents. Two different probes were used: 1-naphthol (a "fast" probe) and 2-naphthol (a "slow" probe). A single-exponential decay was observed from these two probes in each of the neat solvents and also in low-viscosity mixed solvents. In these cases local solvent homogeneity is attained during the probe lifetime. The single-exponential result was also found for 2-naphthol in mixed aqueous solvents of dioxane and glycerol. However, when 1-naphthol was dissolved in these mixed solvents, and for water concentrations higher than 40%, a single exponential no longer provided a good fit to the decays. This observation indicates that solvent inhomogeneity is being probed during the short fluorescent lifetime of the 1-naphthol probe. 0 © 1990 American Chemical Society.