Studies of Osmotic and Activity Coefficients in Aqueous and CCl 4 Solutions of 18-Crown-6 at 25 °C Kesharsingh Patil,* Rajesh Pawar, and Dilip Dagade Department of Chemistry, ShiVaji UniVersity, Kolhapur-416 004, India ReceiVed: March 29, 2002 The osmotic coefficients and densities of aqueous and CCl 4 solutions involving 18-crown-6 (18C6) as solute in the concentration range 0.1 to 2.0 m at 25 °C were measured. The data obtained is used to calculate activities of solvent and activity coefficient of solute and solvent at 25 °C as a function of concentration. It has been observed that activity coefficient of 18C6 increases with increase in concentration of 18C6 while the reverse is true for CCl 4 solutions. Using the partial molar volume of the solute at infinite dilution, the solute-solvent cluster integral values were evaluated which yielded information on solute-solvent interaction. Application of McMillan-Mayer theory enabled us to obtain second and third virial coefficients for solute, which have been decomposed into attractive and repulsive contributions to solute-solute interactions. These are compared with other solutes such as sucrose, glucose, urea, etc. On the basis of attractive and repulsive contributions to solute-solvent and solute-solute interactions in aqueous solutions, the results are interpreted in terms of hydrogen-bonding of water molecules in the crown cavity and hydrophobic stacking interactions mediated through water molecules. The results of CCl 4 solutions are further examined from the point of view of effect of conformational characteristics of 18C6 on the properties in a nonaqueous solvent. 1. Introduction In recent years, the studies of crown ethers and cryptands have yielded valuable information about selective binding of alkali metal cations. It is well-known that the 18-crown-6 (18C6) forms stable complexes with several neutral molecules, such as water, acetonitrile, methanol, etc., through H-bonds and dipolar forces (“molecular recognition”). The interactions of crown ethers with such neutral molecules are important from the point of view of understanding the mechanism of biological transport, molecular recognition, and enzyme specific binding activities as well as extraction abilities of crown ethers. 1-3 The most prominent feature of 18C6 is its capability to complex alkali metal ions in the polar cage of oxygen atoms through undirectional Coulombic forces (“spherical recogni- tion”) and to transport them eventually into lipophilic phases. 3 It has been found that the stability of 18C6:K + is higher in methanol than in water, meaning thereby that 18C6-H 2 O interactions are comparatively stronger. 4 X-ray diffraction 5 and Raman spectroscopic 6 studies have revealed that 18C6 forms various hydrates (1:4, 1:8, and 1:12) in solid state which exists in D 3d conformation. The conformation of 18C6 in pure solid state is C i (along with a small amount of C s ); thus, some energy is expended in conformational change from C i to D 3d during complexation. The NMR relaxation, diffusion coefficient, NIR, FTIR, and partial molar volume studies in water have proved that the macrocyclic effect associated with such a neutral molecule requires the presence of bridged water molecules to have a cyclic structure. 7-11 It was shown that at least four water molecules are H-bonded with the oxygen atoms of 18C6. 9-11 This most probable hydration structure of 18C6 is composed of two bridging water molecules (doubly H-bonded), which forms H-bonds simultaneously with two oxygen atoms of 18C6 and, the two other water molecules that are singly H-bonded to the ring. The molecular dynamics study of Kowall and Gieger 12 shows that the hydrophobic hydration of 18C6 plays an important role in governing the conformational dynamics of 18C6 in aqueous solutions, while the ab initio 13 studies indicate that the cooperative electrostatic interactions governs the hydration pattern of 18C6 in aqueous solutions. To understand the thermodynamic behavior of aqueous 18C6 solutions, from which the solute-solute as well as solute- solvent interactions can be investigated, precise and accurate activity coefficient data, along with partial molar volume data, are required. In this context, we are reporting the osmotic and activity coefficient study of aqueous 18C6 solutions. Also some measurements were made in CCl 4 medium since 18C6 exists in C i conformation in CCl 4 . The data are subjected to the analysis using the McMillan-Mayer theory 14 and approach evolved by Kozak et al. 15 Using activity coefficients, calculated from osmotic coefficient data, the minimum attractive and repulsive contributions to the second and third osmotic virial coefficients are estimated. The information about pairwise and triplet interactions is derived. Also the hydration number of 18C6 in aqueous solutions is determined using activity data following the method described by Robinson and Stokes. 16 The results are compared with other hydrogen bonding nonelectro- lytes and discussed in terms of solute-solute and solute-solvent interactions. 2. Experimental Section 18C6 (99% pure) procured from Merk-Schuchardt was used without further purification. The contact with atmospheric moisture was avoided by handling 18C6 in a drybox fabricated in our laboratory. The salt NaCl of AR grade (BDH) was dried under vacuum for 24 h before use. All the solutions, either in water (doubly glass distilled) or CCl 4 (HPLC grade, Merck), were prepared on molality basis and converted to molarity scale using the density data at 25 °C. 9606 J. Phys. Chem. A 2002, 106, 9606-9611 10.1021/jp020839w CCC: $22.00 © 2002 American Chemical Society Published on Web 09/21/2002