Thermodynamic studies for aqueous solutions involving 18-crown-6 and alkali bromides at 298.15 K

Osmotic vapor pressure measurements have been carried out for three ternary systems, H2O + 0.2 m 18-crown-6 + NaBr, H2O + 0.2 m 18-crown-6 + KBr and H2O + 0.2 m 18-crown-6 + CsBr at 298.15 K using vapor pressure osmometry. The concentration of salts was varied between 0.04 and 0.6 m. The measured water activities were used to calculate the activity coefficient of water, 18-crown-6 and mean molal activity coefficients of ions. The lowering of activity coefficients of one component in presence of other is attributed to the existence of host-guest type complex equilibria in solution phase. The Gibbs transfer free energies, which have been calculated using the activity data, were used to estimate the McMillan-Mayer pair and triplet interaction parameters and are compared with that of alkali chlorides reported recently by us using similar studies. The pair interaction parameters, gNE (non-electrolyte-electrolyte interaction coefficient), are used to obtain the thermodynamic equilibrium constant values for 18-crown-6:M+ complexes, which on comparison with alkali chlorides indicate that the counter anions plays a definite role in stabilizing such complexes in solution phase. Sign and the magnitude of triplet interaction parameters (gNNE or gNEE) show that along with electrostatic interactions hydrophobic effects also play an important role in stabilizing the host-guest type complexes.