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Langmuir 2001, 17, 2346-2356
Studies of Cetylpyridinium Chloride and Cetylpyridinium Salicylate in Solution and Adsorbed on Silica Surfaces Using X- and W-Band Electron Paramagnetic Resonance Spectroscopy Martin G. Bakker* and Edward L. Granger Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0336
Alex I. Smirnov† College of Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received October 27, 2000. In Final Form: January 29, 2001 The electron paramagnetic resonance (EPR) spin probe 4-[N,N-dimethyl-N-(n-hexadecyl)ammonium]2,2,6,6-tetramethylpiperindinyl-N′-oxyl (HTAB*) has been used to study the adsorption and aggregation of cetylpyridinium chloride (CPC) and cetylpyridinium salicylate (CPSa) on silica particles. In CPC micelles, the local viscosity, as determined from the rotational correlation times of HTAB*, was found to be approximately constant, as was the local polarity determined from Aiso, giso, and HTAB* equilibrium constants. In CPSa micelles at concentrations above the sphere-to-rod transition, significant decreases in polarity and local viscosity were observed. The affinity of CPC for the strong binding site on the silica surface was found to be higher than the affinities of CPSa and HTAB. The polarities and local viscosities of the surfactant aggregates and the corresponding micelles were found to be similar. From analysis of the line widths of the EPR spectra, it was concluded that CPSa on silica surfaces forms two coexisting aggregate phases, one which excludes HTAB* and one in which HTAB* is concentrated. The HTAB* is believed to be excluded from the former phase because of ordering of the salicylate counterion and pyridinium headgroups as proposed by Favoriti and Treiner.1
Introduction The adsorption of surfactants onto surfaces is central to areas as diverse as detergency, particle separations,2,3 enhanced oil recovery,4,5 chromatography,6-10 formation of mesoporous materials,11 and modification of electrode properties.12 For many of these applications, the formation of surfactant aggregates on these surfaces is critical to the function of the surfactant. A wide variety of aggregate structures have been suggested13 including monolayers,14 * To whom correspondence should be addressed. Phone: 205348-9116. Fax: 205-348-9104. E-mail:
[email protected]. † Present address: Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204. (1) Favoriti, P.; Treiner, C. Langmuir 1998, 14, 7493. (2) Fuerstenau, M. C.; Miller, J. D.; Kuhn, M. C. Chemistry of Flotation; Society of Mining Engineers: New York, 1985; p 177. (3) Fuerstenau, D. W.; Herrera-Urbina, R. In Surfactant-Based Separation Processes; Scamehorn, J. F., Harwell, J. H., Eds.; Marcel Dekker Inc.: New York, 1989; Vol. 33, p 259. (4) Lake, L. W. Enhanced Oil Recovery; Prentice Hall: Englewood Cliffs, NJ, 1989. (5) Pope, G. A.; Baviere, M. Basic Concepts in Enhanced Oil Recovery Processes; Elsevier Science Publishing: New York, 1991; Vol. 33. (6) Berthod, A. J. Chromatogr., A 1997, 780, 191. (7) Cox, G. B.; Stout, R. W. J. Chromatogr. 1987, 384, 315. (8) 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. (9) Hansen, S. H.; Helboe, P.; Thomsen, M. J. Chromatogr. 1991, 544, 53. (10) Khaledi, M. G. J. Chromatogr., A 1997, 780, 3. (11) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (12) Rusling, J. F. Colloids Surf., A 1997, 123-124, 81. (13) Bakker, M. G.; Turner, G. L.; Morris, T.; Granger, E. J. Chromatogr., B 2000, 743, 65. (14) Fuerstenau, D. W. J. Phys. Chem. 1956, 60, 981.
bilayers,14 hemimicelles,14,15 and admicelles.16,17 Kunjappu and Somasundaran18 have suggested the collective term “solloids” for such aggregates, because the structure is often poorly understood. Because the chemical properties of a solloid are believed to be dependent upon the solloid structure, there is interest in a better understanding of this structure and the relationship of the structure to chemical and physical properties. One particular aspect of solloid structure that to date has received little attention is the incorporation of counterions and their effect on solloid structure and properties. Only a handful of studies have dealt with this topic,19-22 despite the knowledge that the type of counterion has an important effect on the adsorption of the surfactant.23 Recently, Favoriti and Treiner1,24,25 have investigated the effect of salicylate counterions on the adsorption of cetylpyridinium onto silica surfaces and on the physical properties of the solloids formed. They observed that salicylate is strongly coadsorbed with (15) Somasundaran, P.; Healy, T. W.; Fuerstenau, D. W. J. Phys. Chem. 1964, 68, 3562. (16) Yeskie, M. A.; Harwell, J. H. J. Phys. Chem. 1988, 92, 2346. (17) Harwell, J. H.; Hoskins, J. C.; Schecter, R. S.; Wade, W. H. Langmuir 1985, 1, 251. (18) Kunjappu, J. T.; Somasundaran, P. J. Colloid Interface Sci. 1995, 175, 520. (19) Bijsterbosch, B. H. J. Colloid Interface Sci. 1974, 47, 186. (20) Bitting, D.; Harwell, J. H. Langmuir 1987, 3, 500. (21) Hankins, N. P.; O’Haver, J. H.; Harwell, J. H. Ind. Eng. Chem. Res. 1996, 35, 2844. (22) 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. (23) Leimbach, J.; Sigg, J.; Rupprecht, H. Colloids Surf. 1995, 94, 1. (24) Bury, R.; Favoriti, P.; Treiner, C. Colloids Surf. 1998, 139, 99. (25) Favoriti, P.; Mannebach, M. H.; Treiner, C. Langmuir 1996, 12, 4691.
10.1021/la001517r CCC: $20.00 © 2001 American Chemical Society Published on Web 03/16/2001
CPC and CPSa in Solution and on Silica Surfaces
cetylpyridinium cations onto silica surfaces and results in a 6-fold increase in the plateau value for adsorption compared to cetylpyridinium chloride. Similar behavior was observed for cetylpyridinium with 4- and 5-aminosalicylate as counterions. Favoriti and Treiner suggested that the aromatic pyridinium headgroups of the surfactant and the aromatic salicylate ions might be stacked in a fashion that allowed closer packing. We were interested in testing this hypothesis using electron paramagnetic resonance (EPR) spectroscopy. The spin probe 4-[N,N-dimethyl-N-(n-hexadecyl)ammonium]-2,2,6,6-tetramethylpiperindinyl-N′-oxyl (HTAB*, 1) is a cationic surfactant which contains the tetramethylpiperindinyl-N′-oxyl (TEMPO) spin label as a part of the headgroup. (We have previously used HTAB* as an abbreviation for the bromide salt of this spin probe, but both the bromide and the iodide salts (used in this work) will dissociate to give the cation. In the interests of compatibility with our previous work, we will continue to use this abbreviation.) Because of the location of the spin label, we would expect this probe to be particularly sensitive to the type of stacking phenomenon suggested by Favoriti and Treiner. We have previously used this spin probe to study the binding of the cationic surfactant trimethylammonium bromide (HTAB) on silica particles.13,22,26,27 The HTAB* spin probe is partitioned between solution, a strongly binding site on the silica surface, and HTAB solloids. The EPR spectrum of HTAB* in aqueous solution is characterized by three symmetric lines spaced 1.675 mT apart. When HTAB* is bound onto the surface, the rate of rotation of the spin label decreases markedly and becomes insufficient to average the anisotropic components of the g matrix and the nitrogen hyperfine coupling tensor. This gives rise to an EPR spectrum consisting of asymmetric peaks, with approximately 6 mT separating the EPR peaks at high and low magnetic fields (compared with 3.3 mT separation for HTAB* in solution). HTAB* incorporated in micelles or solloids rotates more slowly than HTAB* in solution but still sufficiently rapidly to partially average the anisotropic components. This gives an EPR spectrum consisting of three symmetric lines of almost equal spacing, but with different heights and widths. The EPR spectra of HTAB* in each of these three environments are therefore characteristic, and from numerical analysis of the spectra the relative amounts of each component can be determined.
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microenvironment.28-30 If the probe is tumbling rapidly, the isotropic nitrogen hyperfine coupling constant drops from 1.675 mT in water to about 1.60 mT in nonpolar media. The widths of the three lines can provide information about the rate at which the probe tumbles and local spin-probe concentration. For a nitroxide radical in the fast tumbling limit, the homogeneous (Lorentzian) components of the widths of the three lines are given by
Γ(MI) ) A + BMI + CMI2
(1)
where Γ is the line width, MI is the nuclear spin quantum number, and A, B, and C are functions of the g matrix, the nitrogen hyperfine tensor, and the diffusion tensor of the spin probe. If the g matrix and nuclear hyperfine tensor are known, then the A, B, and C terms in eq 1 can be used to determine the rates of rotation of the spin probe. This is often reported as the rotational correlation time, the time required for the probe to rotate once. There are in general three rotational correlation times τx, τy, and τz, which are for motion about the different axes. These correlation times are related to the local viscosity and the shape of the rotating molecule. It is found that the local viscosity of water is generally lower than that in the interiors of micelles; hence, the rotational correlation time(s) can be used to detect the formation of micelles and to give insight into the physical properties of the micelles. For EPR experiments at 9.5 GHz and below, the A line width parameter is, to a good approximation, independent of the rate of rotation and is the width of the Lorentzian component of the center EPR line. However, the value of the parameter A is affected by the relaxation rate of the spin probe. In our system, the major contribution to the relaxation rate is due to spin exchange when two spin probes collide. The rate of these collisions is determined by the local concentration and the rate of translational diffusion. If the diffusion rate is constant over a set of experiments, then A can be used as a measure of the local spin-probe concentration. In this paper, we report the results of a set of experiments using HTAB* to study micelles and solloids of cetylpyridinium chloride (CPC) and cetylpyridinium salicylate (CPSa). From accurate least-squares simulation analysis of the EPR spectrum of HTAB*, we have obtained information about partitioning, local concentration of the probe, and solloid properties. A preliminary account of this work is included in work recently published.13 Experimental Section
Appropriate analysis of the EPR spectra can give information about the system beyond how the probe is partitioned. The value of the nitrogen hyperfine coupling constant is a measure of the polarity of the HTAB*
Materials. Cetyl pyridinium chloride was purchased from Aldrich and was used without further purification. Cetyl pyridinium salicylate was a gift from Prof. C. Treiner1 of Universite´ Pierre et Marie Curie, Paris, France. 4-[N,N-Dimethyl-N-(nhexadecyl)ammonium]-2,2,6,6-tetramethylpiperindinyl-N′oxyl iodide was purchased from Molecular Probes and was used as received unless otherwise indicated. The silica particles used throughout this study are Aerosil 200 silica, a gift from Degussa Corporation, Germany. Aerosil 200 is a fumed nonporous silica with an average particle size of 12 nm and a surface area of 200 m2/g. Equipment. The majority of the 9 GHz EPR spectra were collected on an X-band EPR spectrometer consisting of an E-109 console, a Century Series bridge, and an IBM ER 073 10 in. magnet controlled via a Bruker B-H15 field controller. The data on the effect of HTAB* concentration were collected at Universite´
(26) Bakker, M. G.; Turner, G. L.; Matthews, J.; Zhang, K. In Surfactant Based Separations; Scamehorn, J., Harwell, J., Eds.; American Chemical Society: Washington, DC, 1999; Vol. 740, p 260. (27) Bakker, M. G.; Turner, G. L.; Treiner, C. Langmuir 1999, 15, 3078.
(28) Knauer, B. R.; Napier, J. J. J. Am. Chem. Soc. 1976, 98, 4395. (29) Mukerjee, P.; Ramachandran, C.; Pyter, R. A. J. Phys. Chem. 1982, 86, 3189. (30) Schartz, R. N.; Peric, M.; Smith, S. A.; Bales, B. L. J. Phys. Chem. 1997, 101, 8735.
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Pierre et Marie Curie (University of Paris 6, France) on a Bruker ESP 360 in the laboratory of Prof. M. Breysse. The X-band samples were pipetted into homemade 0.3 mm thick flat cells27 and placed in the rectangular cavity of the particular spectrometer used. The W-band data were collected on the W-band (95 GHz) instrument at the EPR Research Center at the University of Illinois at Urbana-Champaign. Field calibration of the W-band instrument was carried out using a sample of peroxylaminedisulfonate in water and a Metrolab NMR Gaussmeter. Sample Preparation. Silica samples were stirred for a minimum of 2 days before EPR measurements. We have previously found that this is sufficient to achieve equilibrium for both pH and surfactant adsorption.27 Samples were prepared with distilled water as 2 wt % dispersions, unless stated otherwise. Typically, HTAB* concentrations were 10-5 M for experiments on micelles and 4 × 10-5 M for experiments with silica. X-Band EPR spectra were collected at 2 mW microwave power, 100 kHz modulation, and 0.02 mT modulation amplitude, except for CPC/ HTAB*/silica for which the modulation amplitude was 0.05 mT. Data Analysis. Solution spectra were simulated and fitted using our Fortran program New•Soln running on both MacIntosh PPC and PC platforms.27 The program has been modified to incorporate the fast Fourier transforms in Numerical Recipes31 to carry out Gaussian convolution of the line shape and to enable appropriate simulation of the effect of a nonzero time constant used for data acquisition. The Levenberg-Marquardt leastsquares fitting algorithm in Numerical Recipes was added to New•Soln. Estimates of 95% confidence limits were determined by the procedure discussed in more detail in the Supporting Information. As described previously,26,27 HTAB* in the presence of cationic surfactants and silica particles can be partitioned between two or more chemical environments and can lead to an EPR spectrum characteristic of each environment. The experimental EPR spectrum can therefore consist of several (typically two or three) overlapping spectral components. Simulation and fits involving one, two, or three different spectral components were therefore generated as appropriate. For samples containing silica, a portion of the HTAB* is often strongly absorbed, leading to a “powder” spectrum. Despite the relatively good agreement between simulated and experimental powder spectra in this work (below), there are systematic differences substantially exceeding those found for mobile radicals. Because of the large widths of the features of powder spectra, the differences between simulation and experiment are also likely to be of significant width. It is therefore likely that the least-squares fitting procedure would preferentially adjust the broader spectral component to compensate for these errors. Therefore, using a simulated powder spectrum could possibly bias the fitting of the other components of the spectra. Accordingly, as we have described previously, for experimental EPR spectra in which the immobilized spectral component occurred we added an experimental spectrum consisting of solely this component. This allowed accurate determination of the relative amount of this spectral component and optimum fitting of the remaining components. From accurate analysis of approximately 20 different spectra in which a spectral component corresponding to HTAB* in solution was well resolved, it was concluded that the unresolved hydrogen hyperfine coupling in HTAB* could best be simulated using a Gaussian envelope with a width of 0.09 mT (full width at half-maximum). Changes in the hydrogen hyperfine with environment are expected to be reasonably minor, and so this value was used for all HTAB* species (except for HTAB* strongly bound on silica surfaces). Likewise, the rate of entry and exit of HTAB* into cationic micelles is known to be slow32,33 and is expected to be comparable in cationic solloids. The parameters for HTAB* in solution were therefore held fixed in the various simulations. Powder EPR spectra were simulated using computer program EPR-NMR which was purchased from Prof. J. A. Weil, Depart(31) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numerical Recipes; Cambridge University Press: New York, 1989. (32) Fox, K. K. In Chemical and Biological Applications of Relaxation Spectroscopy; D. Reidel Pub. Co.: Hingham, MA, 1974; p 215. (33) Fox, K. K. J. Chem. Soc., Faraday Trans. 1 1978, 74, 220.
Bakker et al.
Figure 1. Experimental and fitted EPR spectra of HTAB* in solutions of CPC (A) and CPSa (B). The left column shows experimental spectra. The right column shows fitted spectra with residuals plots (difference between experimental and fitted) shown above the fitted spectra: (a) 6.25 × 10-2 M CPC, (b) 3.13 × 10-3 M CPC, (c) 7.85 × 10-4 M CPC, (d) 2.39 × 10-2 M CPSa, (e) 2.99 × 10-3 M CPSa, (f) 3.74 × 10-4 M CPSa, and (g) 4.67 × 10-5 M CPSa. ment of Chemistry, University of Saskatchewan, Canada,34 and was run on a Pentium II computer without modification. After simulation, the center magnetic field of the spectra was adjusted so as to give gzz ) 2.0023. According to the g-factor theory of Stone, the out-of-plane component of the g matrix in aromatic and conjugated systems is equal to the free electron value. So in nitroxides, where the unpaired electron is in a π* orbital, gzz is expected to be close to 2.0023 and is generally found to be so.35
Results Four sets of EPR experiments were carried out using the HTAB* spin probe: (1) experiments on CPC and CPSa micelles to provide a point of comparison for (2) the experiments for CPC and CPSa solloids formed on silica, (3) a set of experiments in which the ratio of HTAB* to CPC was systematically varied to allow the local concentration of HTAB* to be determined, and (4) experiments in frozen solutions to provide values of the g matrix and nitrogen hyperfine tensor needed to give accurate rotational correlation times. CPC and CPSa Micelles. Figure 1 shows representative EPR spectra of HTAB* in solutions of CPC and CPSa (34) Clark, F.; Dickson, R. S.; Fulton, D. B.; Isoya, J.; Lent, A.; McGavin, D. G.; Mombourquette, M. J.; Nuttall, R. H. D.; Rao, P. S.; Rinneberg, H.; Tennant, W. C.; Weil, J. A. EPR-NMR; University of Saskatchewan: Saskatoon, Saskatchewan, Canada, 1993. (35) Lebedev, Y. S. In Electron Spin Resonance; Royal Society of Chemistry: Cambridge, UK, 1994; Vol. 14, p 63.
CPC and CPSa in Solution and on Silica Surfaces
Figure 2. Dependence of A component of HTAB* line width on concentration of CPSa (b) and CPC (9). The two arrows indicate the cmc’s of CPC and CPSa. Trend lines drawn are to guide the eye and do not correspond to any particular model.
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Figure 4. Equilibrium constant, Ks, for partitioning of HTAB* between solution and micelles: CPSa (b) and CPC (0). The arrows indicate the cmc values of CPSa and CPC.
(Supporting Information) and do not show any significant trends, except immediately above the cmc where there is some suggestion of higher B and C values. However, the increase is only a little larger than the error bars, and so we are not convinced that it is significant. Figure 4 shows how HTAB* is partitioned between solution and micelles. The equilibrium constant, Ks, is defined36,37 as
Ks ) [HTAB*mic]/([HTAB*sol]Cmic)
Figure 3. Dependence of B (O) and C (9) line width components for HTAB* in CPSa micelles. The rotational correlation times are as indicated on the right-hand scales.
of indicated concentrations. Included are the corresponding fitted spectra using two components above the critical micellar concentration (cmc) and one below. The complete sets of experimental and fitted spectra are included in the Supporting Information as Figures S7 and S8 for CPC and CPSa, respectively. The fitted parameters are included as Tables S1 and S2. Figure 2 shows the value of A for HTAB* in CPC and CPSa micelles as a function of surfactant concentration. At lower surfactant concentrations, we found that a simulation using two HTAB* species did not give χ2 values significantly lower (
