Environ. Sci. Technol. 1993, 27,2 168-2 173
Partitioning and UV Absorption Studies of Phenanthrene on Cationic Surfactant-Coated Silica Tohren C. G. Klbbey and Kim F. Hayes'
Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48 109-2125 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, KO,, has been estimated for phenanthrene partitioning to CTAB- and CPC-coated silica for fractional organic coverages,f,,, ranging from 0.003 to 0.18. In the case of CPC-coated silica, KO,was found to be nearly constant with pH and coverage, having a value of approximately 150 000 cm3/g. For CTAB-coated silica, KO, was found to be greater than 250000 cm3/g. Because CTAB and CPC have identical 16-carbon hydrophobic tails, the difference in KO,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 KOCvalues between CPC- and CTAB-coated silicas, in view of the similarities of the UV absorption spectra between the two surfactant coatings, indicate that accessibilityof surface coatings to phenanthrene may differ between the two systems.
Introduction Ionic surfactants have been shown in many instances to significantly enhance the sorptive capacity of oppositely charged soil constituents for hydrophobic organic contaminants (HOCs) (1-4). Because soils are predominantly negatively charged, it has been suggested that cationic surfactants could be used to immobilize HOCs in contaminated areas, preventing contaminant plumes from spreading. To make effective use of cationic surfactants for HOC containment, however, it may be important to understand how the sorptive capacity of a surfactantmodified soil is influenced by changes in pH, type of surfactant, and extent of surfactant coating. To date, little work has been reported which systematically examines the influence of sorbed surfactant structure on the sorptive capacity of surfactant-modified soil materials. Earlier studies of ionic surfactant sorption on oppositely charged surfaces used sorption isotherm shapes and electrophoretic mobility data to make predictions about sorbed surfactant structure (5-7). More recently, techniques such as fluorescence spectroscopy (8,9),FTIR (IO), electron spin resonance (9),and neutron specular reflection (11)have been used to probe the sorbed structure of ionic surfactants. While the specific results of these studies have varied with the surfactants and surfaces studied, most have tried to relate sorbed surfactant structure to the
* To whom correspondence should be addressed. 2188
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structure of surfactant micelles. In dilute surfactant solutions, surfactant molecules do not associate significantly with one another, they remain in a monomeric state. If the temperature is greater than the Krafft temperature (approximately 20 OC for the surfactants used here) and surfactant concentration is increased beyond the critical micelle concentration (cmc), the hydrophobic surfactant tails associate with one another to escape the hydrophilic water environment, forming clusters known as micelles. Similarly, the generally accepted model for ionic surfactant sorption on oppositely charged surfaces suggests that surfactant molecules will sorb as individual monomers only at very low coverages; as coverage is increased, surfactant molecules are expected to form patches of one- or twolayered surface clusters known as hemimicelles and admicelles, respectively (Figure 1). Depending on a number of factors, such as surface charge density, electrostatic affinity of the surfactant for the surface, surfactant tail length, and extent of surface coverage, either hemimicellar or admicellar structures may be favored (12). In addition, because of surface heterogeneity in real systems, patches of hemimicelles and admicelles may be present in different regions of the same soil grain (12,13). In micellar surfactant solutions, HOCs are able to escape the unfavorable water environment by partitioning either partially or entirely into the hydrophobic core of the micelles. This phenomenon, known as solubilization, has been studied extensively (14-1 7). Similarly, when surfactant-coated soil constituents are present, HOCs are able to escape the unfavorable water environment by partitioning to the hydrophobic portion of the sorbed surfactant coating. If the sorbed surfactant structure changes significantly with changes in pH or extent of surfactant coverage (e.g., if the amount of hemimicellar structure changes relative to the amount of admicellar structure, or if the overall sorbed surfactant packing density or order changes), then the sorptive capacity of the surfactantmodified soil may change. This change in sorptive capacity can be due to either a change in the hydrophobicity of the coating (Le., from a change in the ability of the surfactant coating to exclude water from the local environment of the partitioned HOC molecules) or a change in the accessibility of the coating to the HOC molecules. The experiments described here examine the partitioning behavior of phenanthrene, a hydrophobic, threering polycyclic aromatic hydrocarbon (PAH), in the presence of cationic surfactant-coated silica. Two surfactants were used in this study, cetyltrimethylammonium bromide (CTAB, C H ~ ( C H ~ ) I ~ N + ( CBr-) H &and cetylpyridinium chloride (CPC, C H ~ ( C H ~ ) ~ ~ NC1-). + C ~CTAB HE and CPC were selected because both have identical 16carbon hydrophobic tails, but different cationic, hydrophilic head groups with differing affinities for the silica surface. The hypothesis of this research was that the different affinities of CTAB and CPC for the silica surface could lead to different sorbed structures with different sorptive capacities for phenanthrene. 0013-936X/93/0927-2188$04.00/0
0 1993 Amerlcan Chemlcal Soclety
Admicellar Structure
f
Hemimicellar Structure
Silica Surface
b-
Hydrophobic Surfactant Tail Charged Head Group
Figure 1. Hemimicellar and admicellar sorbed structures for ionic surfactants on oppositely charged surfaces.
For both CTAB- and CPC-coated silica, experiments were performed to quantitatively examine the effect of changes in extent of surfactant surface coverage on phenanthrene partitioning behavior. For CPC-coated silica, these experiments were performed at two pH values to assess whether variations in pH would have an effect on sorptive capacity independent of their effect on surface coverage. In addition, ultraviolet absorption spectra of phenanthrene partitioned to the surface of CTAB- and CPC-coated silica were examined for peak shifts which would be indicative of changes in the partitioned phenanthrene's local environment. Experimental Section
Chemicals and Silica. CTAB and CPC were purchased from Fluca Chemical Co. with purities greater than 98% and were used as received. Phenanthrene was purchased from Aldrich Chemical Co. with purity greater than 98% and was used as received. Radiolabeled[l4C1cetyltrimethylammonium iodide (CTAI) and [14Clphenanthrene were obtained from American Radiolabeled Chemicals Inc. and Sigma Chemical Co., respectively. Both had purities greater than 98% and were used as received. Fumed, nonporous silica was purchased from Sigma with purity greater than 99.8%. This high-purity silica was reported to be made up of primary particles with a mean particle size of 11nm and an external surface area of 255 m2/g,with little or no internal surface area. The silica was washed with 0.01 N hydrochloric acid and 15%hydrogen peroxide to remove any metal or organic impurities. Each step in the silica-washing procedure was followed by exhaustive rinsing with Milli-Q water (Millipore Corp.). Surfactant Sorption on Silica. CTAB- and CPCcoated silica samples were prepared by combining pHadjusted stock solutions of surfactant, NaC1, Milli-Q water, and a 3 g/L silica slurry in 40-mL Teflon tubes, with each tube corresponding to a different initial surfactant concentration. For CTAB-coated silica samples, a small amount (
