Journal of Solution Chemistry, Vol. 32, No. 11, November 2003 ( C  2003) Studies of Activity Coefficients for Ternary Systems: Water + 18-Crown-6 + Alkali Chlorides at 298.15 K Kesharsingh Patil 1,∗ and Dilip Dagade 1 Received May 21, 2003; revised September 29, 2003 Osmotic vapor pressure measurements have been carried out for three ternary systems, H 2 O + 0.2 m 18-crown-6 + LiCl, H 2 O + 0.2 m 18-crown-6 + NaCl and H 2 O + 0.2 m 18-crown-6 + KCl at 298.15 K using vapor pressure osmometry. Water activities for each ternary system were measured and used to calculate the activity coefficients of 18-crown-6 (18C6) and its salts following the methodology developed by Robinson and Stokes for isopiestic measurements. In the concentration range studied, it was found that (in NaCl and KCl solutions) there is considerable lowering of activity coefficients of one component in the presence of other solutes that has been attributed to the formation of the complexed 18C6:Na + (or 18C6:K + ) species in solution. The Gibbs energies of transfer of alkali chlorides from water to aqueous 18C6 solutions and that of 18C6 from water to aqueous electrolyte solutions have been calculated. These were further used to evaluate the pair and triplet interaction parameters. The calculation of thermodynamic equilibrium constants using the pair interaction parameter, g NE (i.e., the nonelectrolyte– electrolyte pair interaction) for the studied complexation of cations yields values which are in good agreement with those reported in literature obtained by using ion-selective potentiometry and calorimetry. The results are discussed in terms of water structural effects, complex formation, and hydrophobic interactions. KEY WORDS: 18-Crown-6; alkali chlorides; vapor pressure osmometry; activity co- efficient; Gibbs free energy of transfer; McMillan–Mayer theory; pair and triplet in- teractions; hydrophobic interactions; complex formation; thermodynamic equilibrium constants. 1. INTRODUCTION Understanding of the behavior of nonelectrolytes in aqueous electrolyte so- lutions is of fundamental importance in life sciences, oceanography, and industry. One such nonelectrolyte is 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane), which has been treated as a model compound since its discovery. (1−3) It forms stable complexes with several neutral guest molecules through H bonds and dipolar forces 1 Department of Chemistry, Shivaji University, Kolhapur-416 004, India; e-mail: patilkesharsingh@ hotmail.com ∗ To whom all correspondence should be addressed. 951 0095-9782/03/1100-0951/0 C  2003 Plenum Publishing Corporation