PARTITIONING AND UV ABSORPTION STUDIES OF PHENANTHRENE ON CATIONIC SURFACTANT-COATED SILICA

Partitioning of phenanthrene to cetylpyridinium chloride (CPC)- and cetyltrimethylammonium bromide (CTAB)-coated silica has been investigated as a function of surfactant surface coverage and pH at 0.1 M ionic strength. On the basis of isotherm data, a carbon-normalized partition coefficient, K(oc), has been estimated for phenanthrene partitioning to CTAB- and CPC-coated silica for fractional organic coverages, f(oc), ranging from 0.003 to 0.18. In the case of CPC-coated silica, K(oc) was found to be nearly constant with pH and coverage, having a value of approximately 150 000 cm3/g. For CTAB-coated silica, K(oc) was found to be greater than 250 000 cm3/g. Because CTAB and CPC have identical 16-carbon hydrophobic tails, the difference in K(oc) between the two coated silicas indicates that the different surfactant head groups have an effect on the sorptive capacity of the surfactant coatings. UV absorption spectra obtained for phenanthrene partitioned to aqueous micelles and surfactant-coated silica indicate that the two surfactants form coatings with similar solvation environments to one another and to aqueous micelles, independent of coverage. The differences in the phenanthrene K(OC) values between CPC- and CTAB-coated silicas, in view of the similarities of the UV absorption spectra between the two surfactant coatings, indicate that accessibility of surface coatings to phenanthrene may differ between the two systems.