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Studies on the Interaction of Surfactants with Cationic Dye by Absorption Spectroscopy
Journal of Colloid and Interface Science 221, 181–185 (2000) doi:10.1006/jcis.1999.6573, available online at http://www.idealibrary.com on
Studies on the Interaction of Surfactants with Cationic Dye by Absorption Spectroscopy Mitali Sarkar1 and Swapan Poddar Department of Chemistry, University of Kalyani, Kalyani 741235, India Received April 8, 1999; accepted October 8, 1999
The interaction of methyl violet, a cationic dye, with various surfactants, viz. anionic (SDS), nonionic (Triton X-100), and cationic (CTAB), has been investigated spectrophotometrically in submicellar and micellar concentration range. While in the submicellar concentration region of SDS the higher aggregates of the dye are found, in the micellar concentration region the monomer of the dye predominates. With nonionic surfactant the dye is solubilized primarily as the monomer. CTAB produces no perturbation to the visible spectra of the dye. In the presence of strong electrolytes such as NaNO3 and NaCl the dye aggregates are formed at a much lower SDS concentrations. °C 2000 Academic Press Key Words: methyl violet; surfactants; submicellar and micellar concentration region; aggregation and deaggregation.
INTRODUCTION
Extensive research carried out recently has confirmed the ability of surfactants to affect the electronic absorption spectra of solutions of many dyes, viz., triphenylmethane (1, 2), azo (3), and phenothazine (4). This property has been utilized in spectrophotometric determination of metal ions (5–7) and to improve spectral characteristics of colored systems (8–10). The principle of complex formation in the metal–dye binary system and ion association followed by micelle solubilization in the metal–dye– surfactant ternary system has been studied in detail, while the study and description of the mechanism of interaction between the dye and the surfactant is still of interest (11–15). It is observed that a large number of cationic dyes, when in concentrated solution, show deviations from Beer’s law. This is attributable to the formation of aggregates of these species. These aggregates are held together by two different kinds of forces, dispersion forces due to interaction between the π -systems of the dye (16) and forces resulting from hydrophobic effects (17). The sum of these forces must be larger than the electrostatic repulsion between the positive charges of the dye molecules. Methyl violet, one of the dyes in the aminotriphenylmethane series, is known to be used in histochemistry and dermatology (18). It possesses interesting spectroscopic properties, viz., metachromatism, non-Beer’s-law behavior, formation of 1
To whom correspondence should be addressed.
a twisted charge-transfer excited state, and concentrationdependent self-association in aqueous solution (19). In continuation of our previous report (20) on the interaction of methyl violet and surfactants in supermicellar concentrations, the present communication deals with the effect of tensides on the spectroscopic properties of methyl violet using the anionogenic tenside sodium dodecyl sulfate (SDS), the nonionogenic tenside Triton X-100 (TX-100), and the cationogenic tenside cetyltrimethylammonium bromide (CTAB) in submicellar concentrations. EXPERIMENTAL
All the chemicals were of analytical reagent grade. A stock solution of 1.0 × 10−3 mol dm−3 methyl violet was prepared by dissolving purified dye in distilled water. The solution was stored under cold and dark conditions. The stability was monitored by measuring the absorption spectra in the visible region. A stock solution of 1.0 × 10−2 mol dm−3 surfactants was prepared in distilled water. Sodium nitrate solution (1.0 × 10−1 mol dm−3 ) was used to adjust the ionic strength of the solution. The spectrophotometric measurements were carried out on a Perkin–Elmer spectrophotometer with a matched pair of silica cuvets having an internal thickness of 10 mm. The spectral measurements were done in a constant-temperature water bath accurate to within ±0.1◦ C. RESULTS AND DISCUSSION
In an aqueous solution the dye methyl violet exists in cationic form, having the structure shown in Fig. 1. It exhibits a maximum absorption band at 590 nm and a shoulder at 540 nm (Fig. 2) at 298 K. Earlier it was reported that the shoulder originates from the 0 → 1 vibrational transition (21). Lueck et al., in a recent paper (22) based on the semiemperical molecular orbital calculation, claim that the shoulder in methyl violet is due to symmetry-lowering solvent interactions which give rise to two visible electronic transitions for the monomer. The optical properties of aqueous solutions of various dyes have drawn the attention of chemists for quite some time (23–30). The present communication deals with the interaction of methyl violet with surfactants in both submicellar and micellar concentrations with and without NaNO3 , a strong electrolyte.
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FIG. 1. Structure of methyl violet.
Effect of Anionic Surfactant The effect of anionic surfactant on the absorption spectrum of methyl violet was studied at pH 6.9. The SDS concentration for this purpose was varied from 1.0 × 10−4 to 1.0 × 10−2 mol dm−3 for a fixed dye concentration of 1.5 × 10−5 mol dm−3 (Fig. 2). As the surfactant concentration increased gradually, the monomer absorbance decreased and some characteristic new bands appear. For SDS concentrations up to 1.5 × 10−4 mol dm−3 (curve 2) absorbance at 590 nm decreases and the shoulder becomes somewhat prominant after being blue-shifted to 538 nm. The band at 538 nm is assigned as the dimer of the dye. Such dimers occur with many dyes, particularly those having strong aggregating character, e.g., dimethylmethylene blue and pinacyanol. Formation of dimers of some cationic dyes such as thionine, acridine orange, azure B, and rhodamine (4, 31–33) was observed and explained in terms of dye–surfactant interaction. It may be thought that association of SDS anions with dye cations suppresses their mutual repulsion forces and thus favors the dye polymerization (32) viz. the dimer. The associates in turn can further induce the formation of premicellar surfactant aggregate with solubilized dye content (34, 11) and may form other higher dye aggregate. In the present case the monomer absorbance is found to decrease gradually with a concomitant appearance of a peak at 512 nm for a surfactant concentration of 8.5 × 10−4 mol dm−3 . It may be argued that the monomer itself may be perturbed and the band at 512 nm may be either a perturbed band (due to the presence of counterion) or one due to a higher oligomer. Study of aggregation of triphenylmethane dyes (22) indicates that the counterion has little or no influence on the dye aggregation process. Moreover, Lueck et al. (19), by multiwavelength linear regression analysis, confirmed the bands for methyl violet at 538 and 512 nm as due to the dimer and the trimer, respectively, for methyl violet. Principal component analysis (PCA) (35, 36), as applied by several authors to the spectral data, seems to be useful for precise identification of methyl violet–SDS aggregates. In the present case, it appears, therefore, that during the dye concentration process with premicellar aggregates, methyl violet forms a higher oligomer, i.e., a trimer, contrary to the early prediction of Stork (37). As the surfactant concentration is increased, more and more micelles are gradually formed in which first dimers predominate and trimers disappear gradually (up to a surfactant concentration of 3.2 × 10−3 mol dm−3 ) and finally dimers converted into monomers. The concentration (6.0 × 10−3 mol dm−3 ) at
which monomers begin to predominate may be assumed to be the resultant cmc and is somewhat different from the true cmc of SDS (8.0 × 10−3 mol dm−3 ) in the aqueous solution. This low value of the cmc results as the electrostatic repulsion within the anionic moiety of SDS is reduced by the positive charge of the added dye cation (38). The monomer band shifts slightly, finally appearing at 587 nm. In the cases of azure B (4) and methylene blue (39), a shifting of the monomer peak was reported. Monomer absorbance becomes even more pronounced in micelles with surfactant concentration increased further above the cmc (curve 7) than that in the absence of surfactant * (curve 1) for the same dye content, indicating that monomer ) * trimer equlibrium is in favor of the monomer in this dimer ) situation. The dye (D)–surfactant (S) aggregate formation therefore occurs in the following manner. At very low concentrations, far below the cmc, there is formation of a dye–surfactant salt
FIG. 2. Visible absorption spectra of methyl violet and SDS at SDS concentrations (mol dm−3 ) of (1) 0.0, (2) 1.5 × 10−4 , (3) 5.2 × 10−4 , (4) 8.5 × 10−4 , (5) 3.2 × 10−3 , (6) 6.0 × 10−3 , and (7) 1.0 × 10−2 .
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tions the dimer maxima diminish gradually and the micelles contain predominantly monomers. The monomer maximum shifts to 587 nm. The formation of premicellar and micellar aggregates with lower SDS concentration in the presence of NaNO3 is in conformity with the lower cmc value of SDS in the presence of a strong electrolyte. The experimental result indicates that while the cmc of SDS in aqueous solution is 7.3 × 10−3 mol dm−3 , those in the presence of 1.0 × 10−1 mol dm−3 of NaNO3 and NaCl are 1.3 × 10−3 and 1.4 × 10−3 mol dm−3 , respectively, and are close to the reported data (40). The change in the monomer absorbance with SDS was studied (Fig. 4) in the presence of NaNO3 (curve 1) and in the absence of NaNO3 (curve 2). The monomer absorbance in both cases decreases first and as the SDS concentration is increased gradually the monomer absorbance increases rapidly above 2.8 × 10−3 mol dm−3 in the absence of NaNO3 and above 6.9 × 10−4 mol dm−3 in the presence of NaNO3 . These points are chosen as the starting point of the classical micelle formation where the dye molecules number much fewer than the surfactant molecules. Effect of Ionic Strength FIG. 3. Visible absorption spectra of methyl violet and SDS in presence of NaNO3 (1.0 × 10−1 mol dm−3 ) at SDS concentrations (mol dm−3 ) of (1) 0.0, (2) 3.2 × 10−4 , (3) 4.3 × 10−4 , (4) 6.9 × 10−4 , and (5) 1.0 × 10−3 .
starting with the ion pair (D+ · S− ) and continuing with dye– surfactant aggregates represented as (D+ · S− )n . Near and just below the cmc the progress of the reorganization of (D+ · S− )n aggregates into premicelles with a monomeric D+ content results in an increase in the absorbance in this high premicellar region (just below the cmc), showing that the presence of premicelles provides the dye with a micellar-like environment. On further increase in SDS concentration, the absorbance reaches its limiting value and all dye molecules are compartmentalized into normal micelles as monomeric molecules.
The effect of ionic strength of the solution on the absorption spectra of methyl violet was studied in detail with NaNO3 ranging from 2.0 × 10−2 to 1.0 mol dm−3 at two preselected SDS concentrations, i.e., 6.9 × 10−4 mol dm−3 (Fig. 5) and 2.8 × 10−3 mol dm−3 (Fig. 6), the points corresponding to classical micelle formation. Analysis of the curves in Fig. 5 indicates that at lower NaNO3 concentration (up to 4.0 × 10−2 mol dm−3 ) dyerich premicelle aggregates are formed containing dimers and trimers. As the aggregation ability of tensides increases with NaNO3 concentration, more and more micelle aggregates are formed. The trimers disappear and the dimers predominate in solution. With further increase in ionic strength monomers become
Effect of Strong Electrolyte In order to find out the effect of an electrolyte on the absorption spectra of methyl violet an experiment similar to the previous one was performed, adding NaNO3 (Fig. 3). It is observed that in the presence of NaNO3 (1.0 × 10−1 mol dm−3 ) and with increasing SDS concentration the monomer absorbance at 590 nm here also decreases, dimer absorbance at 538 nm increases, and trimer absorbance at 512 nm appears (cf. Fig. 2), but at a much lower surfactant concentration, i.e., at 3.2 × 10−4 mol dm−3 (curve 2), than the previous one. The monomer and trimer maxima were found to decrease first with an increased dimer peak for SDS concentrations up to 6.9 × 10−4 mol dm−3 (curves 3 and 4). The more micelles are generated as a result of increased surfactant concentration, the less prominent is the dye polymerization. Up to a SDS concentration of 1.4 × 10−3 mol dm−3 (curve 5) the trimer maxima disappear and the micelles contain relatively large amounts of dimer and at greater SDS concentra-
FIG. 4. Effect of SDS concentration with and without NaNO3 on the monomer absorbance of methyl violet.
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FIG. 5. Effect of strength of NaNO3 on visible absorption spectra of methyl violet and SDS (6.9 × 10−4 mol dm−3 ) at NaNO3 concentrations (mol dm−3 ) of (1) 0.0, (2) 4.0 × 10−2 , (3) 2.5 × 10−1 , and (4) 1.0.
predominant in the micelle. Thus the effect of methyl violet upon absorbance behavior, by adding both salt and surfactant, is similar although the origin might be different. More surfactant increases the solubilization of dye, while more salt will tend to produce larger micelles and sort out the dye from the water into the micelle. The absorption spectra of methyl violet with higher SDS concentration (2.8 × 10−3 mol dm−3 ) is presented in Fig. 6
FIG. 7. Visible absorption spectra of methyl violet and Triton X-100 at Triton X-100 concentrations (mol dm−3 ) of (1) 0.0, (2) 5.0 × 10−3 , (3) 5.0 × 10−3 , with NaNO3 (1.0 × 10−2 mol dm−3 ).
with varying NaNO3 concentrations. The nature of the curves is almost similar to those in Fig. 5 except in the absorbance value. As the surfactant concentration is kept much higher micelles are expected to form at very low salt concentrations. Effect of Cationic Surfactant The effect of CTAB on the absorption spectra of methyl violet was studied in submicellar and micellar concentration regions and no change in absorption spectra of methyl violet was observed. The result indicates that due to repulsive electrostatic interaction methyl violet does not solubilize in CTAB micelles but remains in the water phase. Effect of Nonionic Surfactant With the increase in concentration of Triton X-100 from the submicellar to the micellar region, only a slight increase in monomer absorbance, but no aggregated peak, is observed (Fig. 7). The spectral characteristic is similar except in absorbance value when NaNO3 is added to the aqueous solution. The dye is thought to be present in the micelle primarily as the monomer. CONCLUSION
FIG. 6. Effect of strength of NaNO3 on visible absorption spectra of methyl violet and SDS (2.8 × 10−3 mol dm−3 ) at NaNO3 concentrations (mol dm−3 ) of (1) 0.0, (2) 2.0 × 10−2 , and (4) 1.0.
Methyl violet forms dimers and trimers with SDS in submicellar concentration regions. Beyond the cmc the dye gradually reverts to the monomer state. With Triton X-100 the dye
INTERACTION OF SURFACTANTS WITH CATIONIC DYE
solubilizes in the micelles as monomers. CTAB has no interaction with the dye. REFERENCES 1. Colichman, E., J. Am. Chem. Soc. 73, 3385 (1951). 2. Rosendorfova, J., and Cermakova, L., Talanta 27, 705 (1980). 3. Savvin, S. B., Chernova, R. K., and Shtykov, S. N., Zh. Anal. Khim. 33, 88 (1978). 4. Nemcova, I., and Duskeva, M., Collect. Czech. Chem. Commun. 51, 296 (1992). 5. Hsu, Z. F., and Liao, X. M., Talanta 35, 1007 (1988). 6. Qi, W. B., and Zhu, L. Z., Talanta 32, 1013 (1985). 7. Mari, I., Fujita, Y., Fujita, K., Nakahashi, Y., Kato, K., and Nakamua, T., Anal. Lett. 21, 2339 (1989). 8. Nemcova, I., Hrachovska, J., Mikova, J., and Suk, V., Microchem. J. 30, 39 (1984). 9. Seidel, A., and Hoyer, E., J. Prakt. Chem. 324, 299 (1982). 10. Vesugi, K., Miyawaki, M., and Nagahiro, T., Microchem. J. 32, 332 (1985). 11. Diaz-Garcia, M. E., and Sanz-Medel, A., Talanta 33, 255 (1986). 12. Kapoor, R. C., J. Ind. Chem. Soc. 65, 541 (1986). 13. Havel, J., Buresova-Jancarova, I., and Kuban, V., Collect. Czech. Chem. Commun. 48, 1290 (1983). 14. Kuban, V., Hedvabny, J., Jancarova, I., and Vrchlabsky, M., Collect. Czech. Chem. Commun. 54, 622 (1989). 15. Ortona, O., Vitagliano, V., and Robinson, B. H., J. Colloid Interface Sci. 125, 271 (1988). 16. London, F., Z. Physik. Chem. B 11, 222 (1930). 17. Jordan, D. O., Kuruocsev, T., and Martin, M. L., Trans. Faraday Soc. 65, 612 (1969). 18. Basu, P. S., Biswas, B. B., and Pal, M. K., Int. Rev. Cytol. 29, 1 (1970).
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19. Lueck, H. B., Rice, B. L., and McHale, J. L., Spectrochim. Acta A 48A, 819 (1992). 20. Sarkar, M., and Poddar, S., Spectrochim. Acta A 55A, 1737 (1999). 21. Pal, M. K., and Choudhury, M., Makromol. Chem. 133, 151 (1970). 22. Lueck, H. B., and Mc.Hale, J. L., J. Am. Chem. Soc. 114, 2342 (1992). 23. Pramanick, D., and Mukherjee, D., J. Colloid Interface Sci. 157, 131 (1993). 24. Joshi, H. D., and Desai, C. M., Ind. J. Chem. A 33, 965 (1994). 25. Guo, L. N., Petitramel, M., Asnaud, I., Gauthier, R., and Chevalier, Y., J. Soc. Dyers Colour 110, 149 (1994). 26. Gehlen, M. H., Ferreira, M., and Neumann, M. G., J. Photochem. Photobiol. 87, 55 (1995). 27. Banerjee, D., Das, P. K., Mondal, S., Ghosh, S., and Bagchi, S., J. Photochem. Photobiol. 98, 183 (1996). 28. Mandal, A. K., and Pal, M. K., J. Colloid Interface. Sci. 192, 98 (1997). 29. Karukstis, K. K., Savin., D. A., Loftus, C. T., and D’Angeles, N. D., J. Colloid. Interface. Sci. 203, 157 (1998). 30. Hirata, H., and Eimura, N., J. Colloid. Interface. Sci. 199, 111 (1998). 31. Sakata, T., Suda, Y., and Tanaka, J., J. Phys. Chem. 81, 537 (1977). 32. Das, A. R., and Moulik, S. P., Colloid. Polymer Sci. 257, 665 (1979). 33. Pal, P., Zeng, H., Durocher G., and Girad, D., J. Photochem. Photobiol. A 98, 65 (1996). 34. Neumann, M. G., and Gehlen, M. H., J. Colloid Interface. Sci. 135, 209 (1990). 35. Takatsuki, M., and Yamaoka, K., J. Sci. Hiroshima Univ. A 40(3), 387 (1976). 36. Yamaoko, K., and Takatsuki, M., Bull. Chem. Soc. Jpn. 51, 3182 (1978). 37. Stork, W. H. J., Lippits, G. J. M., and Mandal, M., J. Phys. Chem. 76, 1772 (1972). 38. Yamagishi, A., J. Colloid Interface Sci. 81, 511 (1981). 39. Bergmann, K., and O’Konski, G. T., Talanta 30, 617 (1984). 40. Coirin, M. J., and Harkins, W. D., J. Am. Chem. Soc. 69, 683 (1971).
Studies on the Interaction of Surfactants with Cationic Dye by Absorption Spectroscopy Mitali Sarkar1 and Swapan Poddar Department of Chemistry, University of Kalyani, Kalyani 741235, India Received April 8, 1999; accepted October 8, 1999
The interaction of methyl violet, a cationic dye, with various surfactants, viz. anionic (SDS), nonionic (Triton X-100), and cationic (CTAB), has been investigated spectrophotometrically in submicellar and micellar concentration range. While in the submicellar concentration region of SDS the higher aggregates of the dye are found, in the micellar concentration region the monomer of the dye predominates. With nonionic surfactant the dye is solubilized primarily as the monomer. CTAB produces no perturbation to the visible spectra of the dye. In the presence of strong electrolytes such as NaNO3 and NaCl the dye aggregates are formed at a much lower SDS concentrations. °C 2000 Academic Press Key Words: methyl violet; surfactants; submicellar and micellar concentration region; aggregation and deaggregation.
INTRODUCTION
Extensive research carried out recently has confirmed the ability of surfactants to affect the electronic absorption spectra of solutions of many dyes, viz., triphenylmethane (1, 2), azo (3), and phenothazine (4). This property has been utilized in spectrophotometric determination of metal ions (5–7) and to improve spectral characteristics of colored systems (8–10). The principle of complex formation in the metal–dye binary system and ion association followed by micelle solubilization in the metal–dye– surfactant ternary system has been studied in detail, while the study and description of the mechanism of interaction between the dye and the surfactant is still of interest (11–15). It is observed that a large number of cationic dyes, when in concentrated solution, show deviations from Beer’s law. This is attributable to the formation of aggregates of these species. These aggregates are held together by two different kinds of forces, dispersion forces due to interaction between the π -systems of the dye (16) and forces resulting from hydrophobic effects (17). The sum of these forces must be larger than the electrostatic repulsion between the positive charges of the dye molecules. Methyl violet, one of the dyes in the aminotriphenylmethane series, is known to be used in histochemistry and dermatology (18). It possesses interesting spectroscopic properties, viz., metachromatism, non-Beer’s-law behavior, formation of 1
To whom correspondence should be addressed.
a twisted charge-transfer excited state, and concentrationdependent self-association in aqueous solution (19). In continuation of our previous report (20) on the interaction of methyl violet and surfactants in supermicellar concentrations, the present communication deals with the effect of tensides on the spectroscopic properties of methyl violet using the anionogenic tenside sodium dodecyl sulfate (SDS), the nonionogenic tenside Triton X-100 (TX-100), and the cationogenic tenside cetyltrimethylammonium bromide (CTAB) in submicellar concentrations. EXPERIMENTAL
All the chemicals were of analytical reagent grade. A stock solution of 1.0 × 10−3 mol dm−3 methyl violet was prepared by dissolving purified dye in distilled water. The solution was stored under cold and dark conditions. The stability was monitored by measuring the absorption spectra in the visible region. A stock solution of 1.0 × 10−2 mol dm−3 surfactants was prepared in distilled water. Sodium nitrate solution (1.0 × 10−1 mol dm−3 ) was used to adjust the ionic strength of the solution. The spectrophotometric measurements were carried out on a Perkin–Elmer spectrophotometer with a matched pair of silica cuvets having an internal thickness of 10 mm. The spectral measurements were done in a constant-temperature water bath accurate to within ±0.1◦ C. RESULTS AND DISCUSSION
In an aqueous solution the dye methyl violet exists in cationic form, having the structure shown in Fig. 1. It exhibits a maximum absorption band at 590 nm and a shoulder at 540 nm (Fig. 2) at 298 K. Earlier it was reported that the shoulder originates from the 0 → 1 vibrational transition (21). Lueck et al., in a recent paper (22) based on the semiemperical molecular orbital calculation, claim that the shoulder in methyl violet is due to symmetry-lowering solvent interactions which give rise to two visible electronic transitions for the monomer. The optical properties of aqueous solutions of various dyes have drawn the attention of chemists for quite some time (23–30). The present communication deals with the interaction of methyl violet with surfactants in both submicellar and micellar concentrations with and without NaNO3 , a strong electrolyte.
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FIG. 1. Structure of methyl violet.
Effect of Anionic Surfactant The effect of anionic surfactant on the absorption spectrum of methyl violet was studied at pH 6.9. The SDS concentration for this purpose was varied from 1.0 × 10−4 to 1.0 × 10−2 mol dm−3 for a fixed dye concentration of 1.5 × 10−5 mol dm−3 (Fig. 2). As the surfactant concentration increased gradually, the monomer absorbance decreased and some characteristic new bands appear. For SDS concentrations up to 1.5 × 10−4 mol dm−3 (curve 2) absorbance at 590 nm decreases and the shoulder becomes somewhat prominant after being blue-shifted to 538 nm. The band at 538 nm is assigned as the dimer of the dye. Such dimers occur with many dyes, particularly those having strong aggregating character, e.g., dimethylmethylene blue and pinacyanol. Formation of dimers of some cationic dyes such as thionine, acridine orange, azure B, and rhodamine (4, 31–33) was observed and explained in terms of dye–surfactant interaction. It may be thought that association of SDS anions with dye cations suppresses their mutual repulsion forces and thus favors the dye polymerization (32) viz. the dimer. The associates in turn can further induce the formation of premicellar surfactant aggregate with solubilized dye content (34, 11) and may form other higher dye aggregate. In the present case the monomer absorbance is found to decrease gradually with a concomitant appearance of a peak at 512 nm for a surfactant concentration of 8.5 × 10−4 mol dm−3 . It may be argued that the monomer itself may be perturbed and the band at 512 nm may be either a perturbed band (due to the presence of counterion) or one due to a higher oligomer. Study of aggregation of triphenylmethane dyes (22) indicates that the counterion has little or no influence on the dye aggregation process. Moreover, Lueck et al. (19), by multiwavelength linear regression analysis, confirmed the bands for methyl violet at 538 and 512 nm as due to the dimer and the trimer, respectively, for methyl violet. Principal component analysis (PCA) (35, 36), as applied by several authors to the spectral data, seems to be useful for precise identification of methyl violet–SDS aggregates. In the present case, it appears, therefore, that during the dye concentration process with premicellar aggregates, methyl violet forms a higher oligomer, i.e., a trimer, contrary to the early prediction of Stork (37). As the surfactant concentration is increased, more and more micelles are gradually formed in which first dimers predominate and trimers disappear gradually (up to a surfactant concentration of 3.2 × 10−3 mol dm−3 ) and finally dimers converted into monomers. The concentration (6.0 × 10−3 mol dm−3 ) at
which monomers begin to predominate may be assumed to be the resultant cmc and is somewhat different from the true cmc of SDS (8.0 × 10−3 mol dm−3 ) in the aqueous solution. This low value of the cmc results as the electrostatic repulsion within the anionic moiety of SDS is reduced by the positive charge of the added dye cation (38). The monomer band shifts slightly, finally appearing at 587 nm. In the cases of azure B (4) and methylene blue (39), a shifting of the monomer peak was reported. Monomer absorbance becomes even more pronounced in micelles with surfactant concentration increased further above the cmc (curve 7) than that in the absence of surfactant * (curve 1) for the same dye content, indicating that monomer ) * trimer equlibrium is in favor of the monomer in this dimer ) situation. The dye (D)–surfactant (S) aggregate formation therefore occurs in the following manner. At very low concentrations, far below the cmc, there is formation of a dye–surfactant salt
FIG. 2. Visible absorption spectra of methyl violet and SDS at SDS concentrations (mol dm−3 ) of (1) 0.0, (2) 1.5 × 10−4 , (3) 5.2 × 10−4 , (4) 8.5 × 10−4 , (5) 3.2 × 10−3 , (6) 6.0 × 10−3 , and (7) 1.0 × 10−2 .
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tions the dimer maxima diminish gradually and the micelles contain predominantly monomers. The monomer maximum shifts to 587 nm. The formation of premicellar and micellar aggregates with lower SDS concentration in the presence of NaNO3 is in conformity with the lower cmc value of SDS in the presence of a strong electrolyte. The experimental result indicates that while the cmc of SDS in aqueous solution is 7.3 × 10−3 mol dm−3 , those in the presence of 1.0 × 10−1 mol dm−3 of NaNO3 and NaCl are 1.3 × 10−3 and 1.4 × 10−3 mol dm−3 , respectively, and are close to the reported data (40). The change in the monomer absorbance with SDS was studied (Fig. 4) in the presence of NaNO3 (curve 1) and in the absence of NaNO3 (curve 2). The monomer absorbance in both cases decreases first and as the SDS concentration is increased gradually the monomer absorbance increases rapidly above 2.8 × 10−3 mol dm−3 in the absence of NaNO3 and above 6.9 × 10−4 mol dm−3 in the presence of NaNO3 . These points are chosen as the starting point of the classical micelle formation where the dye molecules number much fewer than the surfactant molecules. Effect of Ionic Strength FIG. 3. Visible absorption spectra of methyl violet and SDS in presence of NaNO3 (1.0 × 10−1 mol dm−3 ) at SDS concentrations (mol dm−3 ) of (1) 0.0, (2) 3.2 × 10−4 , (3) 4.3 × 10−4 , (4) 6.9 × 10−4 , and (5) 1.0 × 10−3 .
starting with the ion pair (D+ · S− ) and continuing with dye– surfactant aggregates represented as (D+ · S− )n . Near and just below the cmc the progress of the reorganization of (D+ · S− )n aggregates into premicelles with a monomeric D+ content results in an increase in the absorbance in this high premicellar region (just below the cmc), showing that the presence of premicelles provides the dye with a micellar-like environment. On further increase in SDS concentration, the absorbance reaches its limiting value and all dye molecules are compartmentalized into normal micelles as monomeric molecules.
The effect of ionic strength of the solution on the absorption spectra of methyl violet was studied in detail with NaNO3 ranging from 2.0 × 10−2 to 1.0 mol dm−3 at two preselected SDS concentrations, i.e., 6.9 × 10−4 mol dm−3 (Fig. 5) and 2.8 × 10−3 mol dm−3 (Fig. 6), the points corresponding to classical micelle formation. Analysis of the curves in Fig. 5 indicates that at lower NaNO3 concentration (up to 4.0 × 10−2 mol dm−3 ) dyerich premicelle aggregates are formed containing dimers and trimers. As the aggregation ability of tensides increases with NaNO3 concentration, more and more micelle aggregates are formed. The trimers disappear and the dimers predominate in solution. With further increase in ionic strength monomers become
Effect of Strong Electrolyte In order to find out the effect of an electrolyte on the absorption spectra of methyl violet an experiment similar to the previous one was performed, adding NaNO3 (Fig. 3). It is observed that in the presence of NaNO3 (1.0 × 10−1 mol dm−3 ) and with increasing SDS concentration the monomer absorbance at 590 nm here also decreases, dimer absorbance at 538 nm increases, and trimer absorbance at 512 nm appears (cf. Fig. 2), but at a much lower surfactant concentration, i.e., at 3.2 × 10−4 mol dm−3 (curve 2), than the previous one. The monomer and trimer maxima were found to decrease first with an increased dimer peak for SDS concentrations up to 6.9 × 10−4 mol dm−3 (curves 3 and 4). The more micelles are generated as a result of increased surfactant concentration, the less prominent is the dye polymerization. Up to a SDS concentration of 1.4 × 10−3 mol dm−3 (curve 5) the trimer maxima disappear and the micelles contain relatively large amounts of dimer and at greater SDS concentra-
FIG. 4. Effect of SDS concentration with and without NaNO3 on the monomer absorbance of methyl violet.
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FIG. 5. Effect of strength of NaNO3 on visible absorption spectra of methyl violet and SDS (6.9 × 10−4 mol dm−3 ) at NaNO3 concentrations (mol dm−3 ) of (1) 0.0, (2) 4.0 × 10−2 , (3) 2.5 × 10−1 , and (4) 1.0.
predominant in the micelle. Thus the effect of methyl violet upon absorbance behavior, by adding both salt and surfactant, is similar although the origin might be different. More surfactant increases the solubilization of dye, while more salt will tend to produce larger micelles and sort out the dye from the water into the micelle. The absorption spectra of methyl violet with higher SDS concentration (2.8 × 10−3 mol dm−3 ) is presented in Fig. 6
FIG. 7. Visible absorption spectra of methyl violet and Triton X-100 at Triton X-100 concentrations (mol dm−3 ) of (1) 0.0, (2) 5.0 × 10−3 , (3) 5.0 × 10−3 , with NaNO3 (1.0 × 10−2 mol dm−3 ).
with varying NaNO3 concentrations. The nature of the curves is almost similar to those in Fig. 5 except in the absorbance value. As the surfactant concentration is kept much higher micelles are expected to form at very low salt concentrations. Effect of Cationic Surfactant The effect of CTAB on the absorption spectra of methyl violet was studied in submicellar and micellar concentration regions and no change in absorption spectra of methyl violet was observed. The result indicates that due to repulsive electrostatic interaction methyl violet does not solubilize in CTAB micelles but remains in the water phase. Effect of Nonionic Surfactant With the increase in concentration of Triton X-100 from the submicellar to the micellar region, only a slight increase in monomer absorbance, but no aggregated peak, is observed (Fig. 7). The spectral characteristic is similar except in absorbance value when NaNO3 is added to the aqueous solution. The dye is thought to be present in the micelle primarily as the monomer. CONCLUSION
FIG. 6. Effect of strength of NaNO3 on visible absorption spectra of methyl violet and SDS (2.8 × 10−3 mol dm−3 ) at NaNO3 concentrations (mol dm−3 ) of (1) 0.0, (2) 2.0 × 10−2 , and (4) 1.0.
Methyl violet forms dimers and trimers with SDS in submicellar concentration regions. Beyond the cmc the dye gradually reverts to the monomer state. With Triton X-100 the dye
INTERACTION OF SURFACTANTS WITH CATIONIC DYE
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