The association of aromatic anions in water

Molar conductances of dilute solutions of sodium 2-naphthalenesulfonate at 25°C and of sodium 2-anthraquinonesulfonate have been measured at 25 and 37°C. These data are interpreted to show that the anthraquinonesulfonate anion is dimerized in solution. Dimerization constants and the enthalpy and entropy of dimerization are calculated. Spectrophotometric absorbancies have been measured at 25, 30, 35 and 40°C for a series of aqueous solutions containing both sodium 4-dimethylaminobenzenesulfonate and sodium 3-nitrobenzenesulfonate. Equilibrium constants and enthalpy and entropy changes for formation of the 1 1 complex between these two ions have been calculated from these data. The formation of these complexes between like-charged ions, and of dimers of other aromatic solutes in water is discussed in terms of a two-state model of hydrophobic hydration.