Ionic Strength and Temperature Dependence of the Dilution Heat of Hexametaphosphate Sodium Salt.

Small cyclic polyphosphates, like sodium hexametaphosphates (HMP), behave as polyanions since they can be incorporated into aggregates with oppositely charged polycations or incorporated in to polyelectrolyte multilayer films. Their dilution heat in small electrolyte solutions, has however never been characterized. Isothermal titration calorimetry was used to determine the dilution enthalpy of HMP as a function of both the temperature and the NaCl concentration in the electrolyte solutions. A transition between exothermic and endothermic heats of dilution is found when the salt concentration is increased or when the temperature is decreased. A phenomenological model is developed to explain those results. It is based on an estimation of a translational entropy increase of the polyanion upon its dilution. The entropy changes due to counterion release are estimated to be negligible. [ABSTRACT FROM AUTHOR]