Host–guest inclusion complex of β-cyclodextrin and benzoic acid in water–ethanol solvents: spectroscopic and thermodynamic characterization of complex formation

Abstract

In this study, an inclusion complex of benzoic acid with β-cyclodextrin (BA-βCD) was obtained from water–ethanol solvents. The yield of complex synthesis in binary mixtures is greater than in water and reaches maximum value at 0.10 mol fraction of ethanol. Results of FTIR spectroscopy analysis showed that the main difference in the spectra of the acid and inclusion complex was observed in the frequency ranging from 2500 to 3100 cm−1, corresponding to aromatic hydrogen vibrations. These vibrations are highly attenuated in complex. Phase solubility and differential scanning calorimetry studies revealed that the inclusion complex was obtained with 1:1 stoichiometric ratio and the solubility of benzoic acid increased with an increase in β-cyclodextrin concentrations in water. The logarithm of stability constant in water was found to be lgK = 1.99. The thermodynamic parameters for the reaction of (BA-βCD) complex formation in H2O–EtOH solvents were determined from calorimetric experiments carried out by means of the calorimetric titration system TAM III (TA Instruments) at T = 25 °C. The heat effects of mixing β-cyclodextrin solutions with benzoic acid were obtained from water–ethanol mixed solvents containing X(EtOH) = (0.00, 0.10, 0.20 and 0.30) mole fraction at pH = 3.6 and T = 25 °C. However, at X(EtOH) = 0.30 mol fraction, according to the calorimetric titration data, no complex formation occurs. When transferring from H2O to H2O–EtOH solvents, complex stability decreases from lgK = 2.4 to lgK = 0.7, wherein the reaction exothermicity increases from − 12.2 kJ mol−1 to − 44.3 kJ mol−1. An increase in the exothermicity of complexation is accompanied by a decrease in the entropic contribution to the change in the reaction Gibbs energy.