Solubility enhancement effect of cyclodexin on groundwater pollutants

Cyclodextrin (CD) molecules are polycyclic glucose oligomers that have a hydrophilic exterior and a hydrophobic cavity. This structure provides CD the characteristic of enhancing the solubility of groundwater pollutants. The degree to which CD increases the apparent aqueous solubility of certain chemicals has been defined as the solubility enhancement factor (SEF). In this study, a novel and experimentally simple method has been developed to determine the SEF, which can be mathematically obtained by ratio of apparent and traditional Henry's law constants. The effects of temperature and CD concentration on the SEFs and CD cavity occupation have been investigated. Our results show that SEF values are inversely related to temperature for most examined chemicals, which is consistent with the assertion that the CD-chemical complexes are less stable at higher temperatures. The exception is toluene that shows the least SEF fluctuation within the temperature range studied (5 to 65 °C). This may indicate that the toluene-CD complex is particularly stable. As the definition of SEF predicted, linear relationships were found between the SEFs and CD concentrations for all the subject chemicals. The CD cavity occupation fraction at 5 °C were 3.14, 2.55, 2.04, 1.60, and 1.67 times greater than the values at 65 °C for of trichloroethylene, perchloroethylene, bezene, ethylbenze, and o-xylene, respectively. The fraction of CD cavities occupied was found to be inversely related to the CD concentration for all tested chemicals when pollutant mass are held constant. This study provides important information to accurately evaluate the performance of CD when used for aquifer remediation.