Dynamics of an excited acid in 1-propanol—water mixtures

The excited state proton transfer reaction of 4-hydroxy-1-naphthalenesulphonate in 1-propanol—water mixtures has been studied at six different temperatures ranging from 5 to 60°C. Because of the hydrophobic interaction in these mixtures, the dissociation of the probe cannot be explained by a simple water dimer model which we found to be applicable to methanol—water and ethanol—water mixtures. In 1-propanol—water mixtures the proton acceptor concentration, however, shows a linear dependence on the water concentration above a certain critical concentration of water. This behavior is explained by a new model assuming a water dimer within a water cluster to be the effective proton acceptor.