Electrical Conductivity and Complexation of Sodium Borate in Trehalose and Sucrose Aqueous Solutions

The formation equilibrium constants for the esterification reaction between sucrose or trehalose and borate ion have been determined by measuring the pH changes produced when the corresponding sugar is added to sodium borate solutions. The speciation of the ternary sugar–borate–water system was calculated from this equilibrium information and used to separate the contribution of each ionic species to the electrical conductivity of the system. From the analysis of the results it is concluded that, in the viscosity region studied, the complex borate–trehalose and borate–sucrose ions obey Walden's rule, while the behavior of the smaller borate ion is not clear. The dramatic difference of the infinite dilution molar conductivity, and consequently of the hydrodynamic radius, of the trehalose– and sucrose–borate ester ions is ascribed to the formation of an intramolecular diester in aqueous trehalose.