Association of hydrophobic ions in aqueous solution: A conductometric study of symmetrical tetraalkylammonium cyclohexylsulfamates

Abstract

Tetraalkylammonium salts of cyclohexylsulfamic acid were used as model systems to study the ion-pairing process of hydrophobic ions. The electric conductivities of aqueous solutions of tetramethyl-, tetraethyl-, tetrapropyl-, tetrabutyl- and tetrapentylammonium salts of cyclohexylsulfamic acid were measured from 278.15 K to 303.15 K (in steps of 5 K) in the concentration range ∼ 0.2 ∙ 10 − 3 < c (mol dm − 3 ) < ∼ 6 ∙ 10 − 3 . Evaluation of the limiting molar conductivity Λ ∞ and the association constant K A was based on the low concentration chemical model of electrolyte solutions, that includes short-range forces. From the temperature dependence of the limiting molar conductivities Eyring's enthalpy of activation of charge transport was estimated. The standard Gibbs free energy, enthalpy and entropy of the ion-pairing process were calculated from the temperature dependence of the ion-association constants. It was found that in the investigated systems the ion association can be interpreted as strongly enthalpy driven process that does not include any important release of water molecules from the hydration shells of ions. The non-Coulombic contribution to the Gibbs free energy was evident and favours the association process.