Water Solubility Enhancement of Phthalates by Cetyltrimethylammonium Bromide and β-Cyclodextrin

Water solubility enhancements of six phthalates (five aliphatic phthalates and one phenyl phthalate) by cetyltrimethylammonium bromide (CTAB) and β-cyclodextrin (β-CD) were studied at 25 °C. The solubilities of these phthalates are remarkably enhanced by CTAB solutions above the critical micelle concentration (cmc). Only marginal enhancement of phthalate solubility was observed in solutions containing CTAB below its cmc and β-CD at low concentrations (less than 5 mM). The solubility enhancements of the phthalates are proportional to the added amount of CTAB and β-CD. Partition coefficients of the phthalates between monomeric CTAB surfactant and water ( K MN ) and between CTAB micelle and water ( K MC ) were estimated from the experimental data. The mechanisms of solubility enhancements by CTAB and β-CD were discussed. A log-linear equation was proposed and evaluated for the solubilization by CTAB below cmc, while the previously proposed linear partitioning model was questioned. The structures of the complexes formed between phthalates and β-CD were proposed, and the formation constants were estimated. The values of log K MC, log K MN, and log K β - CD of the phthalates were found to correlate linearly with the log K OW of phthalates, with the exception of the solid phenyl phthalate.