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Langmuir 2008, 24, 11514-11517
Interaction of Fe(III) Tetrakis(4-N-methylpyridinium)porphyrin with Sodium Dodecyl Sulfate at Submicellar Concentrations O. Yaffe, E. Korin, and A. Bettelheim* Department of Chemical Engineering, Ben-Gurion UniVersity of the NegeV, P.O. Box 653, Beer-SheVa 84105, Israel ReceiVed July 6, 2008. ReVised Manuscript ReceiVed August 7, 2008 Interaction of water soluble Fe(III) tetrakis(4-N-methylpyridinium)porphyrin (Fe(III)TMPyP) with sodium dodecyl sulfate (SDS) in submicellar concentrations has been studied by surface tension, optical absorption, resonance light scattering (RLS), ζ-potential, and energy dispersive X-ray spectroscopy (EDS) measurements. Measurements were conducted for a fixed concentration of Fe(III)TMPyP (6 × 10-5 M) and SDS in various concentrations ranging between 6 × 10-6 and 6 × 10-2 M. Two macroscopic phase transitions, precipitation and redissolution, were observed as function of SDS concentration. The presence of a new surface active porphyrin-surfactant complex was detected. Furthermore, the presence of two oppositely charged Fe(III)TMPyP-SDS bulk moieties has been demonstrated. Possible structures for the different moieties are suggested, and the phase transitions are discussed.
Introduction Porphyrins and related macrocyclic compounds have been studied in recent decades for a wide variety of applications such as therapeutic drugs and targeting agents,1,2 energy converters in photovoltaic cells,3 and catalysts in many chemical reactions, such as for oxygen reduction in fuel cells.4 It has been known for many years that dissolved porphyrins tend to form molecular complexes through noncovalent bonding.5-7 In many cases, such mesoscopic aggregates possess special properties compared to dissolved porphyrin monomers. Among the primary parameters that affect porphyrin aggregation are temperature, pH, ionic strength, and solvent dielectric constant.6 The photophysical properties of porphyrins are known to depend on their aggregation state.8 Aggregation has been shown to change their adsorption and fluorescence properties.9 This phenomenon may also decrease their activity as sensitizers10 and contrast agents.11 The existence of strong interactions between dye molecules and different surfactants has been known for almost 50 years.12 In the last three decades, several publications have discussed the nature of intermolecular interactions. Some of these articles described solely the interaction of porphyrins with surfactant (1) Jones, R. D.; Summerville, D. A.; Basolo, F. Chem. ReV. 1979, 79, 139– 179. (2) Bonnett, R. Chem. Soc. ReV. 1995, 24, 19–33. (3) LaVan, D. A.; Cha, J. N. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 5251– 5255. (4) Chen, J.; Zhang, W.; Officer, D.; Swiegers Gerhard, F.; Wallace Gordon, G. Chem. Commun. 2007, 335, 3–5. (5) Abraham, R. J.; Eivazi, F.; Pearson, H.; Smith, K. M. J. Chem. Soc., Chem. Commun. 1976, 1976, 698–699. (6) Dolphin, D., Ed. The Porphyrins, Vol. IV: Physical Chemistry, Pt. B; Academic Press: New York, 1979; pp 527-542. (7) Forshey, P. A.; Kuwana, T. Inorg. Chem. 1981, 20, 693–700. (8) Ricchelli, F. J. Photochem. Photobiol., B 1995, 29, 109–118. (9) Gandini, S. C. M.; Borissevitch, I. E.; Perussi, J. R.; Imasato, H.; Tabak, M. J. Lumin. 1998, 78, 53–61. (10) Keene, J. P.; Kessel, D.; Land, E. J.; Redmond, R. W.; Truscott, T. G. Photochem. Photobiol. 1986, 43, 117–120. (11) Lyon, R. C.; Faustino, P. J.; Cohen, J. S.; Katz, A.; Mornex, F.; Colcher, D.; Baglin, C.; Koenig, S. H.; Hambright, P. Magn. Reson. Med. 1987, 4, 24–33. (12) Mukerjee, P. Anal. Chem. 1956, 28, 870–873. (13) Kadish, K. M.; Maiya, B. G.; Araullo-McAdams, C. J. Phys. Chem. 1991, 95, 427–431. (14) Kadish, K. M.; Maiya, G. B.; Araullo, C.; Guilard, R. Inorg. Chem. 1989, 28, 2725–2731.
micelles,13,14 while others examined the premicellar region as well.15,16 The interaction between ionic and nonionic surfactants and free base meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and its Zn(II) and Fe(III) derivatives, as well as Fe(III) mesotetrakis(4-N-methyl-pyridinium)porphyrin (Fe(III)TMPyP), has been reported by Tabak et al.15,17,18 These studies suggest that charged porphyrins and metalloporphyrins, cationic as well as anionic, interact with ionic micelles. On the basis of data obtained by UV-vis spectroscopy, light scattering, and NMR, three species were observed upon titration of FeTMPyP with sodium dodecyl sulfate (SDS): a free porphyrin, a premicellar porphyrin-surfactant aggregate, and porphyrin solubilized in SDS micelles.18 It seems that there is no controversy regarding the fact that surfactants in premicellar concentrations interact with porphyrins to form colloids with unique electronic properties. However, the exact nature of the involved species as well as the mechanism and driving force of the observed aggregation processes are still unclear. The present study focuses on the aggregation behavior of Fe(III)TMPyP in the presence of SDS, well below its formal critical micelle concentration (cmc). Using surface tension and ζ-potential measurements, techniques not usually used in this context, as well as other techniques such as UV-vis spectroscopy, energy dispersive X-ray spectroscopy (EDS), and resonance light scattering (RLS), it was possible in the present work to reveal new species existing at submicellar surfactant concentrations. This is important in view of the biomimetic, medical, and catalytic applications of porphyrins and metalloporphyrins.
Experimental Section Materials. The sodium salt of dodecyl sulfate (SDS) was purchased from Sigma Chemicals. The chloride salt of Fe(III) tetrakis(4-Nmethylpyridinium)porphyrin was purchased from Midcentury (Chicago, IL). Both were used as obtained. Sample Preparation. Porphyrin and surfactant batch solutions were prepared by dissolving their salts in deionized water (18 MΩ/ (15) Gandini, S. C. M.; Yushmanov, V. E.; Borissevitch, I. E.; Tabak, M. Langmuir 1999, 15, 6233–6243. (16) Maiti, N. C.; Mazumdar, S.; Periasamy, N. J. Phys. Chem. B 1998, 102, 1528–1538. (17) Gandini, S. C. M.; Yushmanov, V. E.; Tabak, M. J. Inorg. Biochem. 2001, 85, 263–277. (18) Santiago, P. S.; Gandini, S. C. M.; Tabak, M. J. Porphyrins Phthalocyanines 2005, 9, 94–108.
10.1021/la802122q CCC: $40.75 2008 American Chemical Society Published on Web 09/13/2008
Interaction of Fe(III)TMPyP with SDS
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Figure 1. Photographs of 6 × 10 -5 Μ Fe(III)TMPyP aqueous solutions with increasing SDS concentration from left to right.
cm, Millipore). Porphyrin-surfactant mixtures were obtained by mixing diluted solutions of both species. All experiments were conducted in isothermal conditions (25 °C) following an incubation period of 18 h. UV-Vis spectroscopy and ζ-potential measurements were conducted with porphyrin concentrations well below their solubility limit (∼10-3 M) and in conditions (pH ) 6 in the absence of buffers) where self-aggregation is assumed to be negligible.7 Surfactant concentrations were in the range of 0.05-120% of the cmc. Methods. Surface Tension. The surface tension was measured by using a PC controlled KSV SIGMA 70 tension balance with Du Nouy platinum ring. The ring was thoroughly cleaned and flamed before each measurement. The measurements were automatically corrected to the actual values by means of the Hug and Mason compensation for interface distortion.19 The estimated error was (0.01 mN/m. UV-Vis Spectroscopy. UV-Vis spectra were recorded on a JASCO V-530 UV/vis dual-beam spectrophotometer, using quartz cells of 0.1 cm path length. ζ-Potential. The ζ-potential was measured with a ZetaPlus ζ-potential and particle size analyzer (Brookhaven Instruments Corporation, Holtsville, NY) equipped with a 30 mW, 657 nm laser (Hamamatsu Photonics K.K., Hamamatsu City, Japan). Resonance Light Scattering (RLS). The RLS profiles were recorded on an Edinburgh Co. (Edinburgh, Scotland, U.K.) FL920 spectrofluorimeter in synchronous map mode. The wavelength of the recorded scattering intensities (detector in 90° in respect to incident light) was identical to the excitation wavelength, and therefore, only elastic scattering was taken into account.19 Energy DispersiVe X-ray Spectroscopy (EDS). Structural characterization was carried out using scanning electron microscopy (SEM). The electron micrographs and atomic analysis were obtained using a FEI Quanta 200 scanning electron microscope equipped with energy dispersive X-ray spectroscopy (EDS). Samples were prepared by air drying solid precipitates for 48 h and then placing them on gold stubs using double sided carbon tape.
Results Macroscopic Observations. Aqueous solutions of SDSFe(III)TMPyP with the concentration of SDS in the range 2.4 × 10-6-9.6 × 10-3 M and a constant Fe(III)TMPyP concentration of 6 × 10-5 Μ were visually examined 18 h after being mixed. Solutions with SDS concentrations of up to 2 × 10-4 M showed a greenish color which became lighter as the concentration of SDS increased (Figure 1). Finally, a colorless solution with a solid precipitate was obtained. Compositional characterization using EDS indicated that the precipitate was composed of carbon, oxygen, sulfur, and iron. Although the sulfur to iron concentration ratio was 3.5 instead of 5 (which should have been obtained if all of the chloride ions were replaced by SDS molecules), no chlorine or sodium (porphyrin or surfactant counterions) was detected in the precipitate. This is probably due to poor resolution of this technique, resulting in the small amounts of the analyzed elements compared to carbon. Further increase of the SDS concentration seemed to cause redissolution, resulting in a yellowish solution. The solutions darkened as the concentration of the surfactant approached the cmc value (8 × 10-3 M).
Figure 2. Surface tension of aqueous solutions of SDS as function of concentration (curve a) and of SDS solutions containing 6 × 10-5 M Fe(III)TMPyP (curve b). The circle indicates the concentration of SDS at which precipitation is observed. Dashed vertical lines indicate regions with different surface behavior.
Surface Tension Measurements. Measurements of surface tension for solutions with increasing concentration of SDS, in the absence and presence of Fe(III)TMPyP, are presented in Figure 2. The surface tension behavior of the SDS solutions devoid of metalloporphyrin (curve a) follows previous reports.20 However, the curve describing the dependence of surface tension versus SDS concentration at constant Fe(III)TMPyP concentration (6 × 10-5 M) shows four distinct regions (curve b). A sharp decrease in surface tension was observed at very low concentrations of SDS up to 10-5 Μ (region I). The surface tension in the second region (II), extending from 10-5 to ∼10-4 Μ SDS, was almost constant: ∼ 50 mN/m, until macroscopic precipitation was observed at [SDS] ∼ 2 × 10 -4 M. A further increase of SDS concentration causes a sharp increase of surface tension, up to a value of 69 mN/m (region III). This takes place at [SDS] ∼ 4 × 10 -4 M, where the precipitate disappears. Both the surface tension increase and the redissolution are sharp transitions which occur almost instantaneously. At higher SDS concentration, the surface tension-SDS concentration curves obtained in the absence and presence of metalloporphyrin (curves a and b, respectively) superimpose, suggesting that the presence of Fe(III)TMPyP does not alter the surface tension of the SDS solutions. It also should be noted that no change of the surfactant cmc is observed in the presence of the metalloporphyrin. ζ-Potential Measurements. Figure 3 depicts the dependence of ζ-potential on SDS concentration for a solution of 6 × 10-5 M Fe(III)TMPyP. No measurements could be obtained for very low SDS concentrations (∼10-5 M) corresponding to region I in the surface tension-SDS concentration curve (curve b, Figure 2). This is reasonable in view of the lack of light scattering (on which the ζ-potential method is based) for these clear solutions. However, a ζ-potential with an approximate value of 30 mV is obtained for solutions within the SDS concentration region II (∼2 × 10-5-2 × 10-4 M). Further addition of SDS to the region where precipitation is observed (region III) causes an inversion of surface charge, and the ζ-potential diminishes until a nearly constant ζ-potential of -50 mV is obtained in the redissolution region (IV). (19) Pasternack, R. F.; Collings, P. J. Science 1995, 269, 935–939. (20) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; John Wiley & Sons: New York, 2004; pp 464-482.
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Figure 3. ζ-Potential measurements of aqueous solutions of SDS containing Fe(III)TMPyP at a fixed concentration (6 × 10-5 M) as function of SDS concentration.
Figure 4. Characteristic resonance light scattering profiles for solutions of Fe(III)TMPyP (6 × 10-5 M) in the absence (a) and presence (b-d) of various SDS concentrations: 3.5 × 10 -6 M (b), 5 × 10 -5 M (c), and 2.4 × 10 -3 M (d).
Resonance Light Scattering Behavior. Figure 4 presents representative light scattering measurements for a Fe(III)TMPyP solution in the absence and presence of various SDS concentrations. As expected, almost no scattering was observed for the Fe(III)TMPyP solution devoid of surfactant (curve a) as well as for a low SDS concentration (curve b) corresponding to region I in Figure 2. However, metalloporphyrin solutions containing SDS concentrations corresponding to region II exhibited significant scattering. Although the scattering intensity was not corrected for light adsorption effects, the signal qualitatively showed a significant increase in this case (curve c) and was approximately 3 orders of magnitude higher than that of solutions devoid of surfactant (curve a). The scattering profile for SDS concentrations in the redissolution range (IV) showed a scattering profile (curve d) in the same order of magnitude as that obtained for range II (curve c). However, the dependence of scattering intensity on SDS concentration for the redissolution process was opposite to that preceding precipitation. While scattering intensity increased with increasing SDS concentration in range II, it decreased in range IV (curves a and b, respectively, Figure 5). This could imply that either the concentration or size of the particles increases as a function of SDS concentration in the region preceding the precipitation region, while the opposite
Yaffe et al.
Figure 5. Dependence of scattering intensity (excitation wavelength: 420 nm) on SDS concentration for regions II (a) and IV (b).
Figure 6. Spectra of aqueous solutions of Fe(III)TMPyP (6 × 10-5 Μ) in the absence (a) and presence (b-d) of various SDS concentrations: 2.1 × 105 M (b), 1.9 × 10-3 M (c), and 1.2 × 10-2 M (d), corresponding to regions preceding precipitation (II), redissolution (IV), and beyond the cmc. Inset: zoom of the Q-band region.
occurs when the metalloporphyrin redissolves at high SDS concentrations. UV-Vis Spectra. UV-Vis spectra for Fe(III)TMPyP solutions in the absence and presence of various surfactant concentrations are shown in Figure 6. The Fe(III)TMPyP solution devoid of SDS showed a Soret band with λmax ) 421 nm and Q-bands at λmax ) 597 and 631 nm, characteristic of this metalloporphyrin.7 No significant change was noticed for spectra obtained with SDS concentrations within range I (curve b). However, spectra for SDS concentrations within range II exhibited a decrease of the Soret band intensity and a new Q-band at 569 nm, while the bands at 631 and 597 nm disappeared (curve c). Figure 7 shows the location of the Soret band in the different SDS regions. While λmax is identical for regions I and II, the Soret band exhibits a blue-shift from 414 to 403 nm in the redissolution region (IV). This band red-shifts in SDS concentrations exceeding the cmc.
Summary and Discussion The present study deals with the aggregation behavior of Fe(III)TMPyP in aqueous solutions containing low concentrations (premicellar region) of SDS as surfactant. Various experimental methods were used for probing the different aspects of the SDS-Fe(III)TMPyP interactions. It was established that different phenomena occur, depending on the surfactant concentration,
Interaction of Fe(III)TMPyP with SDS
Figure 7. Location of λmax of the Soret band of Fe(III)TMPyP versus SDS concentration. Scheme 1. Schematic Illustration of the Aggregation Behavior of the SDS-Fe(III)TMPyP System in the Submicellar Region
and these are qualitatively illustrated in Scheme 1. At very low SDS concentrations (region I), the Fe(III)TMPyP-SDS solutions are characterized by a constant, sharp decrease of surface tension. This implies that the species obtained in this SDS concentration region are surface active. From the insignificant changes in light scattering and UV-vis spectra of these solutions, as compared to Fe(III)TMPyP solutions devoid of SDS, it can be concluded that almost no aggregation occurs in this region. However, after further increase of SDS concentration (region II), both the ζ-potential and light scattering measurements indicate the formation of bulk colloidal particles. These techniques indicate that positively charged particles are formed and these lead to increased scattering intensities which are 2-3 orders higher than those observed for metalloporphyin solutions or for solutions of surfactant in concentrations below the cmc. The decrease of dissolved metalloporphyrin concentration is evidenced by the
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gradual decrease of the Soret band intensity, while the transformations occurring in the Q-bands seem to indicate that the two species (metalloporphyrin and surfactant) have interacted. Since the RLS measurements in this SDS concentration region (II) show that the scattering intensity gradually increases as the concentration of SDS increases, it can be deduced that either the particle size or concentration increases. This finally leads to massive aggregation and precipitation (region III). The results obtained by EDS analysis indicate the absence of chloride and sodium ions in the precipitate. As these are counterions of the metalloporphyrin and surfactant, their absence suggests that the precipitate is mainly a Fe(III)TMPyP-SDS complex. This is in agreement with the results obtained by ζ-potential, RLS, and UV-vis spectroscopy measurements in which region III extends to surfactant concentrations corresponding to a stoichiometry of ∼5 surfactant molecules for each metalloporphyrin molecule. Further addition of SDS causes “redissolution”, since solutions regain their color. However, ζ-potential measurements show that these are colloidal solutions with negatively charged particles instead of the positively charged ones in region II. This indicates that the structures of the aggregates in regions II and IV are different. This is also in agreement with the results obtained with the other experimental techniques. The surface tension profile in region IV is similar to that of the surfactant solutions devoid of metalloporphyin. This indicates that the predominant species in this region has surface characteristics which are different from those existing in regions I and II. RLS measurements for solutions in range IV exhibit scattering profiles which are similar in shape and magnitude to those for samples in region II. However, the scattering intensities in region IV decrease as function of SDS concentration, as opposed to those in region II. This indicates that the concentration or diameter of the colloids decreases progressively as the concentration of SDS increases. This is in agreement with previous studies that report that, in high concentrations of surfactant (well over the cmc),18,21 porphyrin solutions exhibit spectroscopic properties that are similar to those of the free dissolved porphyrin.17,18 In the case of the Fe(III)TMPyP-SDS system and in SDS concentrations below the cmc (region IV), the Soret band is blue-shifted compared to that of the metalloporphyrin. Unlike the metalloporphyin, the Soret peak of the metalloporphyrin-SDS solutions in region IV is asymmetric. This seems to imply that there is a dispersion of particles with different size and structure in these solutions and that the peak of the Soret band is a superposition of their light absorption properties. Two possible mechanisms are consistent with the above results: (a) the concentration of complexes in solution is constant and the added SDS molecules continuously adsorb to the colloids and change their properties, and (b) the addition of SDS molecules causes the colloids in solution to break up to smaller and smaller particles with higher SDS/ FeTMPyP molecular ratios. Acknowledgment. The authors would like to thank Robert F. Pasternack from the Department of Chemistry and Biochemistry, Swarthmore College, and Rachel Yerushalmi-Rozen from the Engineering Chemistry Department, Ben-Gurion University of the Negev, for fruitful discussions. LA802122Q (21) Kwak, J. C. T. Polymer-Surfactant Systems; CRC Press: New York, 1998; pp 482-520.
