Abstract
The spectral displacement technique was used to determine the 1:1 equilibrium, binding constants (K-2) of the complexes formed between beta-cydodextrin and carboxylate anion guests, including either single components or mixtures in aqueous solution at variable temperatures (25-55 degrees C). A van't Hoff analysis of the results afforded thermodyamic parameters (Delta H degrees, Delta S degrees, and Delta G degrees) of the single-component carboxylate anions and complex mixtures of carboxylic acid species, referred to as naphthenic acids (NM). Three types of single-component examples of NAs with variable hydrogen deficiency (z) values were studied: n-octanoic (z = 0; Si), trans-4-pentylcydohexanecarboxylic acid (z = -2; S2), and dic-ydohexylacetic acid (z = -4; S3). The carboxylate anion mixtures were obtained from commercial suppliers and an industrial source obtained from oil sands process water. The estimated K-2 values decrease with increasing temperature, and the standard Gibbs energy change (Delta G degrees) for complex formation is generally favorable (from -16 to -28 kJ/mol) and largely enthalpy-driven (from -12 to -30 kJ/mol). The change in entropy of complex formation (Delta S degrees) for 51, S2, and S3 varies (-23 and 44 J moll K-1) depending on the guest size and relative lipophilicity. The positive correlation between complex stability, host-guest size-fit complementarity, and lipophilicity of the carboxylate anion reveals the importance of the hydrophobic effect, as evidenced by compensation phenomena for such host-guest complexes.