Compressibility studies of aqueous and CCl4 solutions of 18-crown-6 at T = 298.15 K

The experimental measurements of sound velocities of 18-crown-6 (18C6) in dilute aqueous as well as in dilute CCl4 solutions at T = 298.15 K are reported and these data are combined with density, expansivity and specific heat data to yield isentropic and isothermal compressibility properties in solutions. The compressibility and expansivity data are further used to obtain apparent molar compressibility values (ϕKS,ϕKT)(ϕKS,ϕKT) of 18C6 in solutions as well as the parameter energy volume coefficient (δU/δV)T in both solvent media. The calculations of hydration number (h) in aqueous solutions are made following the method developed by Shiio et al  . At infinite dilution the ϕKS0 and h values are obtained as −17 mm3 · MPa−1 · mol−1 and ∼4, respectively. The hydration number for 18C6 is found to be ∼2 when isothermal compressibility data (ϕKT0=-8mm3·MPa-1·mol-1) are used. These observations have been explained on the basis of occupation of crown cavity by four water molecules, two of them (bridged) are firmly held while the other two are exchangeable. The corresponding studies in CCl4 solutions provide further information about transfer compressibility of 18C6 molecules and the effect due to conformational status of 18C6 as well as the solvent effect. It has been noted that the geometrical relaxational properties of Ci and Cs conformations contributes largely towards the isothermal compressibility behaviour exhibited in CCl4 solutions.