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Langmuir 1999, 15, 3078-3085
On the Nature of the Binding Sites for Cationic Surfactants on Silica: Studies Using Electron Paramagnetic Resonance Spectroscopy Martin G. Bakker,*,† Gregory L. Turner,† and Claude Treiner‡ Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0336, and Laboratoire Liquides Ioniques et Interfaces Charge´ es, UMR CNRS 7612, Universite´ Pierre et Marie Curie, 4, Place Jussieu, Bat. 74, BP 51, Paris 75005, France Received October 20, 1998. In Final Form: February 5, 1999
Adsorption of the spin-probe 4-[N,N-dimethyl-N-(n-hexadecyl)ammonium]-2,2,6,6-tetramethylpiperidinylN′-oxyl bromide (HTAB*) onto silica in aqueous dispersions has been studied using electron paramagnetic resonance (EPR) spectroscopy. The spin probe has been found to partition between three environments, the aqueous solution, the surface of the silica, and aggregates on the surface. The partitioning between these three environments is reported as a function of available silica surface, pH, and added hexadecyltrimethylammonium bromide (HTAB). At low pH and large silica surface area the spin probe appears to adsorb flat onto hydrophobic areas of the silica surface. At high pH and small surface area the spin probe forms surfactant aggregates.
Introduction Adsorption of surfactants onto particle surfaces is an important phenomenon that impacts areas as diverse as particles separations, paints and inks, enhanced oil recovery, chromatography, pollutant transport, and environmental remediation. Addition of anionic and nonionic surfactants as a method of increasing the solubility of hydrophobic organic contaminants in “pump and treat” techniques for remediating contaminated aquifers has received considerable attention.1 The use of cationic surfactants adsorbed on zeolites as barriers to the transport of organic and inorganic pollutants2-4 is also of interest because of the potential to address difficult to treat “mixed waste” systems. Our particular interest has been in the adsorption of cationic surfactants onto silica surfaces5-7 because silicates are such an important component of natural systems. Surfactants adsorbed onto oxide surfaces can form aggregates capable of solubilizing hydrophobic molecules8 which is utilized in various forms * To whom correspondence should be addressed. E-mail:
[email protected]. † The University of Alabama. ‡ Universite ´ Pierre et Marie Curie. (1) West, C. C.; Harwell, J. H. Environ. Sci. Technol. 1992, 26, 2324. (2) Bowman, R. S.; Haggerty, G. M.; Huddleston, R. G.; Neel, D.; Flynn, M. M. In Surfactant-Enhanced Subsurface Remediation; Sabatini, D. A., Knox, R. C., Harwell, J. H., Eds.; American Chemical Society: Washington, DC, 1995; Vol. 594, p 54. (3) Li, Z.; Roy, S. J.; Zou, Y.; Bowman, R. S. Environ. Sci. Technol. 1998, 32, 2628. (4) Li, Z.; Bowman, R. S. Environ. Sci. Technol. 1998, 32, 2278. (5) Murphy, D. D.; Bakker, M. G.; Spears, D. R. Miner. Metall. Process. 1994, 11, 26. (6) Murphy, D. D.; Spears, D. R.; Bakker, M. G. Colloid Surf. 1995, 96, 143. (7) Bakker, M. G.; Murphy, D. D.; Davis, B. M. In Surfactant Adsorption and Surface Solubilization; Sharma, R., Ed.; American Chemical Society: Washington, DC, 1995; Vol. 615, p 153. (8) O’Haver, J. H.; Harwell, J. In Surfactant Adsorption and Surface Solubilization; Sharma, R., Ed.; American Chemical Society: Washington, DC, 1995; Vol. 615, p 49.
of admicellar chromatography.9 There is some controversy about the structure of such aggregates and a number of different types of aggregates have been postulated.10-17 The different structures are likely to have different abilities to solubilize organic molecules, and so there is considerable interest in determining aggregate structure. This can only be done by spectroscopic or X-ray diffraction techniques. However, apart from the fluorescence studies of the adsorption of nonionic surfactants by Levitz and co-workers,18,19 the neutron reflection study of hexadecyltrimethylammonium bromide (HTAB) on quartz by McDermott et al.20 and our work using electron paramagnetic resonance5-7 (EPR) there has been little work done on characterizing the surfactant aggregate structure on silica by spectroscopic means. Hexadecyltrimethylammonium bromide (HTAB) is a commonly used cationic surfactant for which the adsorption onto silica is well-studied.16,21-23 By replacing one of the methyl groups with the TEMPO spin-label, an EPR(9) Harwell, J. H.; O’Rear, E. A. In Surfactant-Based Separation Processes; Scamehorn, J. F., Harwell, J. H., Eds.; Marcel Dekker Inc.: New York, 1989; Vol. 33, p 155. (10) Kunjappu, J. T.; Somasundaran, P. J. Phys. Chem. 1989, 93, 7744. (11) Somasundaran, P.; Kunjappu, J. T. Miner. Metall. Process. 1988, 5, 68. (12) Kunjappu, J. T.; Somasundaran, P. J. Colloid Interface Sci. 1995, 175, 520. (13) Harwell, J. H.; Yeskie, M. A. J. Phys. Chem. 1989, 93, 3372. (14) Yeskie, M. A.; Harwell, J. H. J. Phys. Chem. 1988, 92, 2346. (15) Rupprecht, H.; Gu, T. Colloid Polym. Sci. 1991, 269, 506. (16) Goloub, T. P.; Koopal, L. K.; Bijsterbosch, B. H.; Sidorova, M. Langmuir 1996, 12, 3188. (17) Behrends, T.; Herrmann, R. Phys. Chem. Earth 1998, 23, 229. (18) Levitz, P. Langmuir 1991, 7, 1595. (19) Levitz, P.; Van Damme, H. J. Phys. Chem. 1986, 90, 1302. (20) McDermott, D. C.; McCarney, J.; Thomas, R. K.; Rennie, A. R. J. Colloid Interface Sci. 1994, 162, 304. (21) Kung, K.-H. S.; Hayes, K. F. Langmuir 1993, 9, 263. (22) Monticone, V.; Treiner, C. J. Colloid Interface Sci. 1994, 166, 394. (23) Wangnerud, P.; Jonsson, B. Langmuir 1994, 10, 3542.
10.1021/la9814743 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/27/1999
EPR Studies of Cationic Surfactants on Silica
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sites on the silica. This in contrast to other metal oxide surfaces which do not show evidence for such a hydrophobic component to surfactant binding. Experimental Section
Figure 1. Simulated EPR spectra of HTAB* showing the effect of rotational correlation time (τ) (a) τ ) 10-10 s, (b) τ ) 10-9 s, (c) τ ) 10-8 s, (d) τ ) 10-7 s, and (e) τ ) 3 × 10-8 s; electron exchange rate of 3 × 105 s-1.
active form of this surfactant (HTAB*, I) with properties similar to
HTAB is formed. The EPR spectrum of HTAB* is sensitive to the local environment and so changes in polarity and in the microviscosity result in observable changes in the EPR spectrum. This can be clearly seen in Figure 1, which shows simulations of the EPR spectrum of HTAB* for various rates of rotation (referred to as the rotational correlation time, τ). As the rate of rotation slows, the lines become broader and the spectrum becomes steadily more asymmetric. Broader lines also result in a decrease in signal amplitude as can be seen from the amplification factors given in Figure 1. The EPR spectrum of HTAB* is also sensitive to the local HTAB* concentration. Exchange of electrons between HTAB* molecules results in the broadening of all three lines of the HTAB* spectrum. At sufficiently high local HTAB* concentration only one broad line is observed, as is simulated in Figure 1e. Kwan et al.24 first used this spin probe to study the formation of HTAB micelles. More recently, we have used changes in the EPR spectrum of HTAB* to show the formation of HTAB7 and HTAB*6 aggregates on silica surfaces. In this investigation we have studied the effect of pH, the HTAB*/silica ratio, and the addition of HTAB on the EPR spectrum of HTAB*. We have observed the partition of HTAB* between solution and two different environments on the silica surface and find that our results can best be explained by the existence of hydrophobic binding (24) Kwan, C. L.; Atik, S.; Singer, L. A. J. Am. Chem. Soc. 1978, 100, 4783.
Materials. Aerosil 200 from Degussa was used throughout this study except as indicated otherwise. The Sipernat 22, Sipernat 50, P25 titania, and alumina were used as supplied by Degussa. The Aerosil 200 is reported to have a BET surface area of 200 m2/g, Sipernat 22 190 m2/g, Sipernat 50 450 m2/g, P25 titania 50 m2/g, and the alumina 100 m2/g. HTAB, purchased from Aldrich, was used without further purification after surface tension measurements found a critical micelle concentration (cmc) of 9.5 × 10-4 M and did not show any evidence for a dip at the cmc. The spin-probe HTAB* was synthesized according to the procedure described by Kwan et al.24 Distilled water was used throughout. Adjustments of pH were carried out by the addition of 0.3 M HCl or NaOH as required. Equipment. The EPR spectrometer used consists of an X-band Varian E-109 Century Series bridge operating at 9.5 GHz, with a IBM 10 in. magnet controlled by a Bruker BH-15 magnetic field controller. EPR spectra of 2000 points were collected on a Mac IIx computer running Labview. A TE-102 rectangular EPR cavity was used throughout. The sample cells used are 0.3 mm flat cells approximately 5 mm wide × 20 mm high. The same sample cell was used throughout because of the superior signalto-noise ratio obtained with it compared to that of commercially available flat cells. The EPR spectra of HTAB* on titania, alumina, and Sipernat 22 and 50 were run on a Bruker ESP 300 system using conditions comparable to those in the remainder of the study. Sample Preparation. All samples were prepared by weighing out the desired amount of silica, adding water and HTAB and/or HTAB* as appropriate, adjusting the pH, and then allowing the sample to equilibrate (with stirring) for 2 days. Earlier studies of HTAB adsorption had shown that on Aerosil 200 equilibrium was reached within 2 days. Data Analysis. Two programs were used to simulate and fit the EPR spectra. The program written by Schneider and Freed25 was modified to run on a PowerMac computer with fitting of the simulated EPR spectra to experimental EPR spectra using the Simplex algorithm taken directly from Press et al.26 This program calculates EPR spectra for slowly tumbling molecules from input tumbling rates, hyperfine coupling matrix, g matrix, Heisenberg exchange rate, and relaxation time. Because the Schneider and Freed program runs very slowly, the FORTRAN program New•SolutionPPC was used to simulate and fit the isotropic and rapidly rotating portions of the EPR spectrum. The program was compiled to run on both PowerMac and PC computers. This program allows simulation of isotropic EPR spectra or spectra in which the molecular motion is sufficiently rapid that the line width can be accurately described by a quadratic equation in the nuclear spin quantum number MI. The program calculates the second-order perturbation to the spin Hamiltonian (necessary for the Nitrogen hyperfine coupling) and will fit a sloping baseline. This program has an option to allow the addition of an EPR spectrum generated experimentally or by another program, the relative position and amplitude of which can then be varied by the Simplex algorithm to give the best fit to an experimental spectrum. For HTAB* in aqueous solution there is considerable hyperfine coupling from the various hydrogen atoms. For degassed, dilute aqueous solutions of HTAB* simulation and fitting of the EPR spectra gave the following set of coupling constants: 0.452 G (six hydrogens), 0.305 G (two hydrogens), and 0.486 G (two hydrogens). These coupling constants were used for the (unresolved) hyperfine coupling constants in the two spectral components simulated using program New•SolutionPPC. In the EPR spectra generated in this study there were two components which could not be satisfactorily simulated using (25) Schneider, D. J.; Freed, J. H. In Spin Labeling. Theory and Applications, Biological Magnetic Resonance, 8; Berliner, L. J., Reuben, J., Eds.; Plenum Press: New York, 1989; Vol. 8, p 1. (26) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numerical Recipes; Cambridge University Press: New York, 1989.
3080 Langmuir, Vol. 15, No. 9, 1999 New•SolutionPPC. One was a background signal in the glass flat cell due to a paramagnetic defect in the glass. This was allowed for by subtracting the EPR spectrum of the empty flat cell. The other component was from HTAB* rotating so slowly that the EPR lines were no longer Lorentzian and the line widths of the EPR lines were not satisfactorily estimated by a quadratic equation. For samples of HTAB* in a 2 wt % silica dispersion at pH ) 7 the EPR consists almost solely of this component. This spectrum was then simulated and fitted using the Schneider and Freed program and holding the hyperfine matrix and g-value matrix to those reported for TEMPO. For other spectra containing this component there were also contributions from two other HTAB* species which were judged to making the fitting of the slowly rotating component of questionable reliability. Instead, the relative amount of this component was determined by using the simulation of the 2 wt % silica sample and letting program New•SolutionPPC add this to the simulated contributions from the other two species. A typical simulation and fit to an experimental spectrum consisted of first removing the contribution of the spectrum from the glass impurity signal by subtracting the spectrum from the empty flat cell. The resulting spectrum was then simulated using New•SolutionPPC. This simulation consisted of a contribution for the slowly rotating spectrum generated either from the Schneider program or from the EPR spectrum for 2 wt % silica, plus contributions generated by New•SolutionPPC for the other species. After the simulated spectrum had been fitted to the experimental spectrum (based on minimizing the least squares of the residuals), each component of the simulated spectrum was then simulated separately, output to a separate file, and numerically integrated twice to obtain the relative amount of each species. For the mixed HTAB*/HTAB samples for which the EPR spectrum of species II was not well-resolved from the other species, the coupling constants and line widths were not allowed to vary but were instead fixed at those of HTAB* in solution.
Results Experiments with HTAB* Only. Figure 2a shows the EPR spectrum for 10-4 M HTAB* adsorbed onto 2 wt % Aerosil 200 silica in an aqueous dispersion. The peak due to the paramagnetic defect in the glass sample cell is marked with an asterix. This background signal is corrected for by subtracting the spectrum from the empty sample cell giving the spectrum in Figure 2b. The EPR spectrum was fitted using the Schneider and Freed program (Figure 2c). The HTAB* is almost certainly in a range of environments and so the fit is to a distribution of spectra rather than a single spectrum. Our aim was to simulate the complete spectrum to enable us to determine the ratios of the various components. The primary concern was therefore to obtain simulated spectra that were in reasonable agreement with the experimental spectra. As can be seen by comparing Figure 2b with Figure 2c, the simulation is in good agreement with the experimental spectrum. When the sample is cooled to 100 K, rotation of the HTAB* slows further, as can be seen in Figure 2d. When HTAB* in solution is cooled to 100 K, it gives the EPR spectrum in Figure 2e. The only apparent difference between the two spectra at 100 K is in the line widths of the EPR peaks which appear somewhat wider in Figure 2e than in Figure 2d. Figure 2f shows the EPR spectrum obtained from a sample of 10-4 M HTAB* adsorbed onto a 2 wt % dispersion of silica, which is then dried. The relatively low water solubility of HTAB* at room temperature, which we estimate to be approximately 2 × 10-4 M, makes it difficult to produce a HTAB* solution concentration sufficiently high to give monolayer surface coverage unless the amount of silica in solution is extremely low. In other words it is not possible to directly determine an adsorption isotherm for HTAB*. To gain some insight into the effect of changing the ratio of the
Bakker et al.
Figure 2. EPR spectra of HTAB*: (a) aqueous dispersion of 2 wt % Aerosil 200 silica at room temperature, 10-4 M HTAB* with paramagnetic defect in glass sample cell shown; (b) EPR spectra after subtraction of signal from paramagnetic defect; (c) simulation/fit to EPR spectrum using Schneider and Freed program; (d) 2 wt % silica dispersion, 10-4 M HTAB* at 100 K; (e) 10-4 M HTAB* at 100 K; (f) 10-4 M HTAB* adsorbed onto 2 wt % silica and then dried.
surface area to the surfactant concentration, we decided to hold the surfactant concentration constant at 10-4 M HTAB* and to vary the amount of silica present. The effect of this can be seen in Figure 3 which shows the EPR spectra for silica concentrations from 2 to 0.1 wt %. At high silica concentrations only one type of EPR spectrum is observed, that of HTAB* rotating slowly. We shall call this species I. As the amount of silica present decreases, a second type of spectrum consisting of three narrow lines begins to grow in. Indeed, the three lines dominate the EPR spectrum at 0.1 wt %. We shall refer to this as species II. The coupling constant (16.75 G) is the same as that in water and the line widths (and hence the microviscosity) are the same as those of HTAB* in water. It is therefore reasonable to assume that species II is HTAB* dissolved in water. Simulated EPR spectra are shown on the right side of Figure 3 (column B) and are in reasonable agreement with the experimental spectra in column A of Figure 3. For the two solutions containing the lowest amount of silica, the agreement between simulated spectra and experimental spectra was substantially improved by including a contribution from a third species. This is in the form of a very broad single Lorentzian line of line width ≈38 G. In previous work on silica of lower surface areas we have observed6 this type of EPR spectrum, which we attributed to aggregates of HTAB* adsorbed on the silica surface. Similar EPR spectra of doxyl stearic acid aggregates were reported by Waterman et al.27on alumina surfaces and for HTAB* micelles by Singer and coworkers.28 We shall refer to this as species III. We have also observed this species clearly on Aerosil 200 silica at (27) Waterman, K. C.; Turro, N. J.; Chandar, P.; Somasundaran, P. J. Phys. Chem. 1986, 90, 6828. (28) Atik, S. S.; Kwan, C. L.; Singer, L. A. J. Am. Chem. Soc. 1979, 101, 5696.
EPR Studies of Cationic Surfactants on Silica
Figure 3. (A) Experimental and (B) simulated EPR spectra of 10-4 M HTAB* adsorbed on silica dispersions, pH ) 7, amount of silica as indicated. Table 1. Distribution of HTAB* between Different Environments wt % silica
species I
species II
species III
2.0 1.5 1.0 0.5 0.25 0.10
950 870, 880 970, 960 690, 640 360, 350 180, 180
4 6, 6 10, 11 13, 13 20, 23 33, 32
