Volumetric Studies of 2,2,2-Cryptand in Aqueous and Aqueous KBr Solutions at 298.15 K: An Example Involving Solvent-Induced Hydrophilic and Hydrophobic Interactions

Density measurements of good precision are reported for aqueous and aqueous salt (KBr) solutions containing 2,2,2-cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) (similar to 0.009 to similar to 0.24 mol center dot kg(-1)) for the binary systems and for the ternary system with similar to 0.1 mol center dot kg(-1) 2,2,2-cryptand and varying KBr concentrations (similar to 0.06 to similar to 0.16 mol center dot kg(-1)) at 298.15 K. The density data have been used to study the variation of apparent molar volume () of 2,2,2-cryptand and of KBr as a function of concentration. 2,2,2-Cryptand is a diamine and hence it is hydrolyzed in aqueous solutions and needs an appropriate methodology to obtain meaningful thermodynamic properties. We have adopted a method of hydrolysis correction developed initially by Cabani et al. and later by Kaulgud et al. to analyze our volumetric data for the aqueous solutions. The method is described and we were successful in obtaining the limiting partial molar volume of the bare (free) cryptand in water at 298.15 K. Volumes of ionization as well as volumes of complexation (with KBr) are calculated. Estimations of the apparent molar volume of 2,2,2-cryptand in CCl4 are also reported. There is a loss in volume for the cryptand on transferring it from CCl4 to water. The volume changes due to ionization for the cryptand in water are calculated to be -20.5 and -0.6 cm(3)center dot mol(-1) for the mono- and di-protonation equilibria respectively, while the volume of complexation for K+ is +24.5 cm(3)center dot mol(-1). The results are discussed in terms of conformation, protonation equilibria and selective encapsulation of K+ ions in cryptand cavities. The solution volume properties seem to depend upon water-solute interaction as well on the solute-solute association because of hydrophobic interactions caused by lowering of the charge density on formation of cryptand-K+ species in solution.