A Volumetric Study of Cyclodextrin-α,ω-Alkyl Dicarboxylate Anion Complexes in Aqueous Solutions

Abstract

The apparent molar volumes (Vφ,S) of a homologous series of sodium α,ω-alkyl dicarboxylate [(CH2)x(CO2Na)2, x = 4, 6, 8, 10, 12] salts (S) have been determined in water (w) and in aqueous cyclodextrin (CD) solutions at 25 °C. The magnitude of Vφ,S in ternary (w+S+CD) solutions containing β-CD and DM-β-CD (2,6-di-O-methyl β-cyclodextrin) are greater than in the binary (w+S) aqueous systems. However, in ternary solutions containing α-CD, the magnitude of Vφ,S is lower than in the binary systems. The apparent molar volume (AMV) of the α,ω-alkyl dicarboxylate salts in ternary aqueous solutions can be related to the formation of inclusion complexes between the CD and the anions (Cx2- where x = 6, 8, 10, 12). The transfer volume of the Cx2- guests at infinite dilution from water to aqueous CD solutions are well described by a simple molecular additivity scheme which accounts for the number of methylene groups of the guest that are included and the number of water molecules displaced from the CD interior. The volumetric properties of the ternary systems have been quantitatively described in terms of a two- or three-site model that accounts for the AMV of bound and unbound species. In cases where CD-S inclusion complexes are formed, β-CD and DM-β-CD form 1:1, α-CD forms 1:1 (x ≤ 8) and 2:1 complexes (x ≥ 10). The binding mode involves threading of the methylene chain of the guest through the CD interior while the carboxylate groups remain hydrated in the bulk solvent or adjacent to the CD annuli. The calculated 1:1 binding constants (K1:1) vary between 101−103 kg mol-1 and the 2:1 binding constants (K2:1) are ca. 102 kg2 mol-2. The relative binding affinity of the host systems with a common Cx2- guest are as follows: α-CD > β-CD > DM-β-CD, and these differences are mainly related to the diameter of the host cavity.