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The Effect of Added Salt on the Aggregation of Clay Particles
Journal of Colloid and Interface Science 226, 205–209 (2000) doi:10.1006/jcis.2000.6812, available online at http://www.idealibrary.com on
The Effect of Added Salt on the Aggregation of Clay Particles Ana P. P. Cione, Carla C. Schmitt, Miguel G. Neumann,1 and Fergus Gessner1 Instituto de Qu´ımica de Sao ˜ Carlos, Universidade de Sao ˜ Paulo, Caixa Postal 780, 13560-970 Sao ˜ Carlos SP, Brazil E-mail: [email protected] Received June 24, 1999; accepted March 6, 2000 DEDICATED TO PROFESSOR EDSON RODRIGUES ON HIS 70TH ANNIVERSARY
The spectral time evolution of Methylene Blue (MB) in SWy-1 and in Laponite aqueous clay suspensions are quite different. However, in the presence of added salts the spectral behavior in both clay suspensions becomes similar and approaches that of Laponite. The changes of the spectrum in the Laponite suspensions are due to the different MB species formed as a consequence of the approximation–association processes of the clay particles with time (aging process), whereas for SWy-1 suspensions the spectral variations are due to the migration of the dye molecules over the surfaces of the clay tactoids. The addition of salt assists the particle–particle approximation–association processes in both clays, especially in SWy-1 suspensions. Thus, the absorption spectra of MB in these conditions are altered and show peaks that are ascribed to internal dye species trapped in spaces created by the association of the clay particles. °C 2000 Academic Press Key Words: clays; methylene blue; electrolytes; ionic strength effect.
INTRODUCTION
Spectroscopic techniques have been successfully used to study the behavior of organic molecules adsorbed on clay surfaces (1–6). These methods are specific, very sensitive, and allow the study of different processes, despite the heterogeneity of the clay surfaces and the occurrence of side reactions of the adsorbed probes. The use of cationic dye as spectrophotometric probes has proved to be very successful for the study of systems containing clay particles in aqueous suspensions (7–11). These dyes have very high molar absorption coefficients, allowing their detection even at concentrations as low as 10−7 –10−6 M. In general, these dyes are sensitive to the environment and their absorption spectra may present significant changes when adsorbed on clay particles. When added to clay suspensions, Methylene Blue (MB) may form various different species, like monomers, protonated monomers, dimers, trimers, and higher aggregates, depending on the characteristics of the clay (12, 13). The absorption bands 1
To whom correspondence should be addressed.
of these species are in the visible region of the spectrum and can be easily separated and identified. The first study on the behavior of a cationic dye adsorbed on clay particles was reported by Bergmann and O’Konsky (14). Adding MB to a Wyoming montmorillonite, they observed the presence of four distinct bands near 575, 610, 670, and 760 nm, which were attributed to dye aggregates (trimers and higher), dimers, monomers and J-aggregates, respectively. Since then, many other studies were reported on dye–clay systems with similar results, but sometimes ascribing the absorption bands to different species. Yariv and Lurie (15) assigned the bands at shorter wavelengths (575 and 610 nm) to the interaction of the π -system of the dye with the lone pair of electrons of the oxygen atoms on the clay surface. Later, Cenens and Schoonheydt (13) assumed that the band near 575 nm was due to trimers on the external surface and that the band at 610 nm corresponded to dimers at the outer and inner surfaces of the clay. They confirmed the band at 670 nm as due to the absorption of monomers and showed that the peak at 765 nm is due to the protonated MB adsorbed on the clay. It has to be noted that the monomer absorption at 670 nm presents a shoulder around 615 nm corresponding to a vibronic transition. Care should be taken not to confound this absorption with that due to dimers. The interaction of other cationic dyes like pyronine (16), acridine orange (17), triphenylmethane dyes (10), and rhodamines (7, 18), with different montmorillonite and hectorite clays has also been reported with similar results and showing the same general behavior. The spectra of the adsorbed dye molecules depend on several factors, e.g., nature of the clay and the dye, swelling properties, interlayer cations, particle-size distribution, and ionic strength of the medium. The layer charge density also exerts a significant influence, as noted by Bujd´ak and Komadel (19) in a recent study of the adsorption of MB molecules on clays with varying layer charges. The spectra of the adsorbed dye are not static and significant time-dependent changes can be observed, as reported by Gessner et al. (12) and Neumann et al. (20). These changes were interpreted in terms of the migration of dye molecules initially adsorbed at external surfaces toward the interlamellar regions of the clay particles. Mechanisms involving rearrangement and
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migration of the dye molecules and/or the clay particles were proposed to explain the observed spectral changes. Yariv et al. (8, 21) found that depending on the dye concentration present in a clay aqueous suspension, the clay particles could be in the tactoid form or associated forming flocs (cardhouse or bookhouse flocs). The differences in the spectral behavior of the dye, i.e., the presence of monomers or aggregates, were explained in terms of these different types of clay particle association. In this paper we report the effects of the addition of salt on the behavior of dye–clay suspensions. The spectral changes of the dye molecules, used as spectrophotometric probes, give information about the influence of salts on the aging processes of the clay particles in suspension. EXPERIMENTAL
Methylene Blue (MB, Carlo Erba) was used as received. The clays were a montmorillonite-type Wyoming clay (SWy-1), obtained from the Source Clays Repository of the Clay Minerals Society, University of Missouri, Columbia, Missouri, and Laponite RD, a synthetic hectorite kindly donated by Laporte Industries. The clays were purified as described in an earlier work (12). The properties and elemental composition of the clays are given in Table 1.
Stock suspensions were prepared by dispersing 0.069 g of clay in water (<25 ml) and stirring for 6 h. Afterward, the volume was adjusted to 25 ml. A second clay suspension with a concentration of 0.17 g/L was prepared from the first. This suspension was stirred for at least 90 min (23). MB stock solutions were 6.0 × 10−4 M, to which enough NaCl was added to obtain the required salt concentration in the suspension (0.66 M). Samples were prepared by mixing 2 ml of the clay suspension with the required amount of the salt-containing dye solution and adjusting the total volume to 3 ml. The presence of salt in the dye solution, in the conditions described above, does not alter the absorption spectra. The actual clay and dye concentrations used in the experiments are shown in the corresponding figure captions.
UV–vis spectra were determined on a Hitachi U-2000 spectrophotometer interfaced to a PC computer. All measurements were performed at room temperature (25 ± 1◦ C), using 1-cm pathlength acrylic cuvettes (Sigma). To avoid interference due to light scattering by the clay particles, a reference sample was prepared with 2.0 ml of clay suspension in 4.0 ml of water, using the same NaCl concentration.
RESULTS AND DISCUSSIONS
Adsorption of Methylene Blue on SWy-1 in the presence of NaCl. Figure 1 shows the time evolution spectra of suspensions of SWy-1 montmorillonite containing Methylene Blue in the absence and presence of NaCl 0.66 M. In the absence of salt (Fig. 1a) the changes in the spectra are mainly due to the rearrangement of the dye molecules on the clay particles (12, 20). The dye molecules are initially rapidly adsorbed at the external surfaces of the tactoids, inducing the dye to form higher aggregates, [(MB)+n n , n ≥ 3, λ = 570–580 nm]. As time passes, dye molecules will deaggregate and migrate to the interlamellar region, in which they become protonated [MBH2+ ] due to the presence of acidic sites, giving rise to the absorption band at 760 nm. As this migration progresses, the concentration of dye on the external surfaces decreases, so that the equilibrium is displaced toward monomers and lower aggregates of adsorbed dye molecules (13, 24). Similar experiments performed in the presence of added salt (Fig. 1b) show that immediately after the dye–salt solution is mixed with the clay suspension, an absorption band is observed around 570–610 nm, which corresponds to MB aggregates. A broad absorption band due to MB monomers is also observed around 660–670 nm. As time passes, this band evolves to a well-defined band at 673 nm with a shoulder at 656 nm. The absorption intensities around 570–580 nm also decrease while that in the 615- to 620-nm region increases. The absorption band at 760 nm due to the MB protonated monomers is also observed, but is significantly less intense than that in the systems without salt. The intensity of this band remains practically the same during the time of the experiments. It can be seen, when the initial spectra and their time evolution are compared, that in the presence of added salt other processes different from the rearrangement and migration of the dyes must be occurring. The increase of the ionic strength causes a compression of the double layer, which assists the processes of approximation
TABLE 1 Chemical Composition, Surface Area, and Cation Exchange Capacity of the Used Clays Clay
Type
SiO2 (%)
Al2 O3 (%)
MgO (%)
Fe2 O3 (%)
Area (m2 /g)
CEC (mequiv/100 g clay)
SWy-1 (22) Laponite RD (26)
natural montmor. synthetic hectorite
66.9 66.03
19.6 0.30
3.05 29.03
3.35 0.06
32 360
76.4 73.3
THE EFFECT OF ADDED SALT
207
SCHEME 2. Processes occurring on addition of MB to the clay suspensions. The processes resulting from clay aggregation become dominant in the presence of salt.
FIG. 1. Time dependence of the absorption spectra of Methylene Blue (4.33 × 10−6 M) in SWy-1 aqueous suspensions (0.22 g/L): (a) no added salt; (b) [NaCl] = 0.66 M.
and association between the clay particles (see Scheme 1) (23). As a consequence of these associations, new environments are created in which the dye molecules can be placed. Therefore, the new absorptions appearing around 658 and 615–620 nm can be ascribed to dye species trapped in these internal regions; i.e., the dye that was initially adsorbed on the external surfaces as monomers and aggregates is relocated to internal regions during the process of clay particles association. The adsorbed dye molecules are sensitive to the surroundings and consequently these changes in environment will produce a shift in the absorption wavelengths. The relative lower intensities of the 760-nm peak observed in the presence of salt can be due to a combination of effects. When the ionic strength is increased, particle–particle interactions are enhanced, hindering the reorganization processes of dye molecules on the clay particles. Also, in the presence of
SCHEME 1. Idealized illustration showing the processes of approximation–association of the clay particles.
electrolytes the c-spacing values decrease (25), hindering the migration of the dye molecules to the interlamellar region in which protonation occurs. These processes are illustrated in Scheme 2. When the dye is added to the clay suspension, rapid adsorption to the clay particles occurs. Absorption peaks are detected due to MB monomers at the external surface [(MB+ )ext , 670 nm] and aggregates [(MB+ )n , ∼580 nm]. In the experiments with SWy-1, that forms tactoids in water, some dye monomers migrate to the interlamellar region where they become protonated [MBH2+ , 760 nm]. The latter process does not occur to the same extent with Laponite, as this clay does not form tactoids in suspension. Simultaneously with the dye migration and rearrangement processes, approximation–association of the clay particles will also occur. In the presence of salt, the electrostatic repulsion due to the negative charges on the particles will be screened, reducing the repulsion and favoring the approximation between the tactoids. As a result of these approximation processes, new regions are formed where the dye can be trapped as monomers or aggregates [(MB+ )x ]trapped with absorptions around 615 and 656 nm. The failure to observe the dye rearrangement processes, especially the growth of the peak due to the protonated dye, in buffered aqueous solutions of SWy-1 (13) can be traced to the same effects. Probably, the salts added to buffer the systems increase the ionic strength and hinder the detection of processes other than the ones due to the association of the clay particles. Adsorption of Methylene Blue on laponite RD. The processes observed in systems of MB and SWy-1 in the presence of NaCl are similar to those taking place on Laponite, even in the absence of electrolytes. The only difference between the spectra of MB on Laponite suspensions in the absence (Fig. 2a) and in the presence of salt (Fig. 2b) refers to the band at 656 nm. In the absence of salt, this band grows simultaneously with the decrease of the 668-nm band. When salt was added, only the 656-nm band was observed, even at t = 0. Except for this difference, the spectral time evolution is practically the same for both suspensions, indicating that the same processes are occurring in both systems. It seems that the presence of salt only accelerates the processes occurring in the Laponite clay suspensions. On the other hand, in systems containing SWy-1, the evolution of the systems is completely different with and without salt.
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aggregates (580 nm) always larger than when the salt is added together with the dye. Furthermore, increasing the time interval between the addition of the salt and the addition of the dye results in larger aggregate bands at 580 nm and a decrease in the monomer band at 665 nm. Therefore, it can be assumed that the extent of the approximation–association processes will increase with the aging time of the salt-containing clay suspension, resulting in larger aggregated particles. This will reduce the external area available for adsorption of the dye molecules, so that the local concentration of adsorbed dye molecules increases, leading to the formation of a larger amount of dye aggregates on the clay surface. SUMMARY FIG. 2. Time dependence of the spectra of Methylene Blue (4.33 × 10−6 M) in laponite aqueous suspensions (0.056 g/L): (a) no added salt; (b) [NaCl] = 0.66 M.
In fact, in the absence of salt, the time evolution spectra observed for MB/SWy-1 and MB/Laponite suspensions are quite different. Significant differences in the amount of protonated dye as well as changes at shorter wavelengths are clearly observed. The addition of salt to both clay suspensions will increase the rearrangement of the clay particles “trapping” dye molecules adsorbed on external surfaces in newly formed internal spaces. This effect is already present in Laponite clays, even without the addition of salt, although to a lesser extent. The addition of salt to the clay suspensions, previous to the addition of the dye, gives further evidence for the occurrence of approximation–association processes of the clays particles, as shown in Fig. 3. In this condition the spectra taken immediately after the addition of the dye show absorption bands due to
The temporal evolution of the spectra of basic dyes in clay suspensions can be explained by two different processes: dye adsorption, aggregation and migration, and clay particle rearrangements (aging). Whereas the first prevails in SWy-1, the effects observed in Laponite favor the latter. The addition of salt approaches the observed behavior to that of Laponite dyes and only accelerates the changes in Laponite. In SWy-1, the higher ionic strength will favor the approximation–association processes of the clay particles, giving rise to new sites where dye molecules and dye aggregates can be “trapped”, originating different absorptions. Therefore, the temporal evolution of the spectra will be different in the presence and absence of added salt. The association of the SWy-1 clay particles also precludes the intercalation of dye molecules at the interlamellar sites where they could be protonated, hindering the growth of the 760-nm peak. Furthermore, this peak is practically absent for MB in Laponite clay suspensions. Adding salt to clay suspensions before the addition of the dye favors the approximation–association processes of the clay particles, decreasing the external area for dye adsorption. Thus, larger amounts of dye will be present as aggregates, located probably at the external surface of the associated clay particles.
ACKNOWLEDGMENTS Financial support by FAPESP (94/3505-5) and FINEP (65/92.0063.00) is gratefully acknowledged. A.P.P.C. thanks FAPESP for a graduate fellowship.
REFERENCES
FIG. 3. Absorption spectra of Methylene Blue (6.00 × 10−6 M) in laponite aqueous suspensions (0.028 g/L), obtained just after its addition to the clay suspension. Time “T ” (min) represents the period of aging of the clay suspensions with salt, before the addition of the dye. [NaCl] = 0.66 M.
1. Schramm, L., Yariv, S., Ghosh, D., and Hepler, L., Can. J. Chem. 75, 1868 (1997). 2. Traina, S. J., and Cattopadhyay, S., in “CMS Workshop Lectures, Organic Pollutants in the Environment” (B. L. Sawhney, Ed.), Vol. 8, p. 173. Clay Minerals Society (CMS), 1996. 3. Davies, J. E. D., J. Incl. Phenom. Mol. Recogn. Chem. 24, 133 (1996). 4. Shane, Y., and Bailey, G. W., Clays Clay Miner. 44, 573 (1996). 5. Choren, E., Moronta, A., Arteaga, A., and S´anchez, J., Clays Clay Miner. 45, 221 (1997).
THE EFFECT OF ADDED SALT 6. (a) Thomas, J. K., J. Phys. Chem. 91, 267 (1987). (b) Thomas, J. K., Chem. Rev. 93, 301 (1993). 7. Arbeloa, F. L., Martinez, J. M. H., Arbeloa, T. L., and Arbeloa, I. L., Langmuir 14, 4566 (1998). 8. (a) Yariv, S., and Garfinkel-Shweky, D., Clays Miner. 32, 653 (1997). (b) Yariv, S., and Garfinkel-Shweky, D., Colloid Polym. Sci. 273, 453 (1995). 9. Schoonheydt, R. A, Cenens, J., and de Schryver, F. C., J. Chem. Soc. Faraday Trans. 1 82, 281 (1986). 10. Drobrogowska, C., Hepler, L. G., Gosh, D. K., and Yariv, S., J. Thermal Anal. 37, 1347 (1991). 11. (a) Yariv, S., Ghosh, D. K., and Hepler, L. G., J. Chem. Soc. Faraday Trans. 87, 120 (1991). (b) Yariv, S., Nasser, A., and Bar-on, P. J., J. Chem. Soc. Faraday Trans. 86, 1593 (1990). 12. (a) Gessner, F., Schmitt, C. C., and Neumann, M. G., Langmuir 10, 3749 (1994). (b) Cione, A. P., Neumann, M.G., and Gessner, F., J. Colloid Interface Sci. 198, 106 (1998). 13. Cenens, J., and Schoonheydt, R. A., Clays Clay Miner. 36, 214 (1988). 14. Bergman, K., and O’Konski, C. T., J. Phys. Chem. 67, 2169 (1963). 15. (a) Yariv, S., and Lurie, D., Isr. J. Chem. 9, 537 (1971). (b) Yariv, S., and Lurie, D., Isr. J. Chem. 9, 553 (1971).
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16. Grauer, Z., Grauer, G. L, Avnir, D., and Yariv, S., J. Chem. Soc. Faraday Trans. 1 83, 1685 (1987). 17. Cohen, R., and Yariv, S., J. Chem. Soc. Faraday Trans. 1 80, 1705 (1984). 18. Endo, T., Nakada, N., Sato, T., and Shimada, M., J. Phys. Chem. Solids 49, 1423 (1988). 19. (a) Bujd´ak, J., and Komadel, P., J. Phys. Chem. B 101, 9065 (1997). (b) Bujd´ak, J., Janek, M., Madejova, J., and Komadel, P., J. Chem. Soc. Faraday Trans. 94, 3487 (1998). 20. Neumann, M. G., Schmitt, C. C., and Gessner, F., J. Colloid Interface Sci. 177, 495 (1996). 21. Schramm, L. L., Yariv, S., Ghosh, D. K., and Hepler, L. G., Can. J. Chem. 75, 1868 (1997). 22. Van Olphen, H., and Fripiat, J. J., “Data Handbook for Clay Materials and Other Non-Metallic Minerals.” Pergamon, Oxford, 1979. 23. Van Olphen, H., “An Introduction to Clay Colloid Chemistry,” 2nd ed., p. 86. Wiley-Interscience, New York, 1997. 24. Schoonheydt, R. A., and Heughebaert, L., Clay Miner. 27, 91 (1992). 25. Yariv, S., “Modern Approaches to Wettability, Theory and Applications,” (M. E. Schroder and G. Loeb, Eds.), p. 303. Plenum, New York, 1992. 26. Mao, P., and Thomas, J. K., Langmuir 9, 1504 (1993).
The Effect of Added Salt on the Aggregation of Clay Particles Ana P. P. Cione, Carla C. Schmitt, Miguel G. Neumann,1 and Fergus Gessner1 Instituto de Qu´ımica de Sao ˜ Carlos, Universidade de Sao ˜ Paulo, Caixa Postal 780, 13560-970 Sao ˜ Carlos SP, Brazil E-mail: [email protected] Received June 24, 1999; accepted March 6, 2000 DEDICATED TO PROFESSOR EDSON RODRIGUES ON HIS 70TH ANNIVERSARY
The spectral time evolution of Methylene Blue (MB) in SWy-1 and in Laponite aqueous clay suspensions are quite different. However, in the presence of added salts the spectral behavior in both clay suspensions becomes similar and approaches that of Laponite. The changes of the spectrum in the Laponite suspensions are due to the different MB species formed as a consequence of the approximation–association processes of the clay particles with time (aging process), whereas for SWy-1 suspensions the spectral variations are due to the migration of the dye molecules over the surfaces of the clay tactoids. The addition of salt assists the particle–particle approximation–association processes in both clays, especially in SWy-1 suspensions. Thus, the absorption spectra of MB in these conditions are altered and show peaks that are ascribed to internal dye species trapped in spaces created by the association of the clay particles. °C 2000 Academic Press Key Words: clays; methylene blue; electrolytes; ionic strength effect.
INTRODUCTION
Spectroscopic techniques have been successfully used to study the behavior of organic molecules adsorbed on clay surfaces (1–6). These methods are specific, very sensitive, and allow the study of different processes, despite the heterogeneity of the clay surfaces and the occurrence of side reactions of the adsorbed probes. The use of cationic dye as spectrophotometric probes has proved to be very successful for the study of systems containing clay particles in aqueous suspensions (7–11). These dyes have very high molar absorption coefficients, allowing their detection even at concentrations as low as 10−7 –10−6 M. In general, these dyes are sensitive to the environment and their absorption spectra may present significant changes when adsorbed on clay particles. When added to clay suspensions, Methylene Blue (MB) may form various different species, like monomers, protonated monomers, dimers, trimers, and higher aggregates, depending on the characteristics of the clay (12, 13). The absorption bands 1
To whom correspondence should be addressed.
of these species are in the visible region of the spectrum and can be easily separated and identified. The first study on the behavior of a cationic dye adsorbed on clay particles was reported by Bergmann and O’Konsky (14). Adding MB to a Wyoming montmorillonite, they observed the presence of four distinct bands near 575, 610, 670, and 760 nm, which were attributed to dye aggregates (trimers and higher), dimers, monomers and J-aggregates, respectively. Since then, many other studies were reported on dye–clay systems with similar results, but sometimes ascribing the absorption bands to different species. Yariv and Lurie (15) assigned the bands at shorter wavelengths (575 and 610 nm) to the interaction of the π -system of the dye with the lone pair of electrons of the oxygen atoms on the clay surface. Later, Cenens and Schoonheydt (13) assumed that the band near 575 nm was due to trimers on the external surface and that the band at 610 nm corresponded to dimers at the outer and inner surfaces of the clay. They confirmed the band at 670 nm as due to the absorption of monomers and showed that the peak at 765 nm is due to the protonated MB adsorbed on the clay. It has to be noted that the monomer absorption at 670 nm presents a shoulder around 615 nm corresponding to a vibronic transition. Care should be taken not to confound this absorption with that due to dimers. The interaction of other cationic dyes like pyronine (16), acridine orange (17), triphenylmethane dyes (10), and rhodamines (7, 18), with different montmorillonite and hectorite clays has also been reported with similar results and showing the same general behavior. The spectra of the adsorbed dye molecules depend on several factors, e.g., nature of the clay and the dye, swelling properties, interlayer cations, particle-size distribution, and ionic strength of the medium. The layer charge density also exerts a significant influence, as noted by Bujd´ak and Komadel (19) in a recent study of the adsorption of MB molecules on clays with varying layer charges. The spectra of the adsorbed dye are not static and significant time-dependent changes can be observed, as reported by Gessner et al. (12) and Neumann et al. (20). These changes were interpreted in terms of the migration of dye molecules initially adsorbed at external surfaces toward the interlamellar regions of the clay particles. Mechanisms involving rearrangement and
205
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migration of the dye molecules and/or the clay particles were proposed to explain the observed spectral changes. Yariv et al. (8, 21) found that depending on the dye concentration present in a clay aqueous suspension, the clay particles could be in the tactoid form or associated forming flocs (cardhouse or bookhouse flocs). The differences in the spectral behavior of the dye, i.e., the presence of monomers or aggregates, were explained in terms of these different types of clay particle association. In this paper we report the effects of the addition of salt on the behavior of dye–clay suspensions. The spectral changes of the dye molecules, used as spectrophotometric probes, give information about the influence of salts on the aging processes of the clay particles in suspension. EXPERIMENTAL
Methylene Blue (MB, Carlo Erba) was used as received. The clays were a montmorillonite-type Wyoming clay (SWy-1), obtained from the Source Clays Repository of the Clay Minerals Society, University of Missouri, Columbia, Missouri, and Laponite RD, a synthetic hectorite kindly donated by Laporte Industries. The clays were purified as described in an earlier work (12). The properties and elemental composition of the clays are given in Table 1.
Stock suspensions were prepared by dispersing 0.069 g of clay in water (<25 ml) and stirring for 6 h. Afterward, the volume was adjusted to 25 ml. A second clay suspension with a concentration of 0.17 g/L was prepared from the first. This suspension was stirred for at least 90 min (23). MB stock solutions were 6.0 × 10−4 M, to which enough NaCl was added to obtain the required salt concentration in the suspension (0.66 M). Samples were prepared by mixing 2 ml of the clay suspension with the required amount of the salt-containing dye solution and adjusting the total volume to 3 ml. The presence of salt in the dye solution, in the conditions described above, does not alter the absorption spectra. The actual clay and dye concentrations used in the experiments are shown in the corresponding figure captions.
UV–vis spectra were determined on a Hitachi U-2000 spectrophotometer interfaced to a PC computer. All measurements were performed at room temperature (25 ± 1◦ C), using 1-cm pathlength acrylic cuvettes (Sigma). To avoid interference due to light scattering by the clay particles, a reference sample was prepared with 2.0 ml of clay suspension in 4.0 ml of water, using the same NaCl concentration.
RESULTS AND DISCUSSIONS
Adsorption of Methylene Blue on SWy-1 in the presence of NaCl. Figure 1 shows the time evolution spectra of suspensions of SWy-1 montmorillonite containing Methylene Blue in the absence and presence of NaCl 0.66 M. In the absence of salt (Fig. 1a) the changes in the spectra are mainly due to the rearrangement of the dye molecules on the clay particles (12, 20). The dye molecules are initially rapidly adsorbed at the external surfaces of the tactoids, inducing the dye to form higher aggregates, [(MB)+n n , n ≥ 3, λ = 570–580 nm]. As time passes, dye molecules will deaggregate and migrate to the interlamellar region, in which they become protonated [MBH2+ ] due to the presence of acidic sites, giving rise to the absorption band at 760 nm. As this migration progresses, the concentration of dye on the external surfaces decreases, so that the equilibrium is displaced toward monomers and lower aggregates of adsorbed dye molecules (13, 24). Similar experiments performed in the presence of added salt (Fig. 1b) show that immediately after the dye–salt solution is mixed with the clay suspension, an absorption band is observed around 570–610 nm, which corresponds to MB aggregates. A broad absorption band due to MB monomers is also observed around 660–670 nm. As time passes, this band evolves to a well-defined band at 673 nm with a shoulder at 656 nm. The absorption intensities around 570–580 nm also decrease while that in the 615- to 620-nm region increases. The absorption band at 760 nm due to the MB protonated monomers is also observed, but is significantly less intense than that in the systems without salt. The intensity of this band remains practically the same during the time of the experiments. It can be seen, when the initial spectra and their time evolution are compared, that in the presence of added salt other processes different from the rearrangement and migration of the dyes must be occurring. The increase of the ionic strength causes a compression of the double layer, which assists the processes of approximation
TABLE 1 Chemical Composition, Surface Area, and Cation Exchange Capacity of the Used Clays Clay
Type
SiO2 (%)
Al2 O3 (%)
MgO (%)
Fe2 O3 (%)
Area (m2 /g)
CEC (mequiv/100 g clay)
SWy-1 (22) Laponite RD (26)
natural montmor. synthetic hectorite
66.9 66.03
19.6 0.30
3.05 29.03
3.35 0.06
32 360
76.4 73.3
THE EFFECT OF ADDED SALT
207
SCHEME 2. Processes occurring on addition of MB to the clay suspensions. The processes resulting from clay aggregation become dominant in the presence of salt.
FIG. 1. Time dependence of the absorption spectra of Methylene Blue (4.33 × 10−6 M) in SWy-1 aqueous suspensions (0.22 g/L): (a) no added salt; (b) [NaCl] = 0.66 M.
and association between the clay particles (see Scheme 1) (23). As a consequence of these associations, new environments are created in which the dye molecules can be placed. Therefore, the new absorptions appearing around 658 and 615–620 nm can be ascribed to dye species trapped in these internal regions; i.e., the dye that was initially adsorbed on the external surfaces as monomers and aggregates is relocated to internal regions during the process of clay particles association. The adsorbed dye molecules are sensitive to the surroundings and consequently these changes in environment will produce a shift in the absorption wavelengths. The relative lower intensities of the 760-nm peak observed in the presence of salt can be due to a combination of effects. When the ionic strength is increased, particle–particle interactions are enhanced, hindering the reorganization processes of dye molecules on the clay particles. Also, in the presence of
SCHEME 1. Idealized illustration showing the processes of approximation–association of the clay particles.
electrolytes the c-spacing values decrease (25), hindering the migration of the dye molecules to the interlamellar region in which protonation occurs. These processes are illustrated in Scheme 2. When the dye is added to the clay suspension, rapid adsorption to the clay particles occurs. Absorption peaks are detected due to MB monomers at the external surface [(MB+ )ext , 670 nm] and aggregates [(MB+ )n , ∼580 nm]. In the experiments with SWy-1, that forms tactoids in water, some dye monomers migrate to the interlamellar region where they become protonated [MBH2+ , 760 nm]. The latter process does not occur to the same extent with Laponite, as this clay does not form tactoids in suspension. Simultaneously with the dye migration and rearrangement processes, approximation–association of the clay particles will also occur. In the presence of salt, the electrostatic repulsion due to the negative charges on the particles will be screened, reducing the repulsion and favoring the approximation between the tactoids. As a result of these approximation processes, new regions are formed where the dye can be trapped as monomers or aggregates [(MB+ )x ]trapped with absorptions around 615 and 656 nm. The failure to observe the dye rearrangement processes, especially the growth of the peak due to the protonated dye, in buffered aqueous solutions of SWy-1 (13) can be traced to the same effects. Probably, the salts added to buffer the systems increase the ionic strength and hinder the detection of processes other than the ones due to the association of the clay particles. Adsorption of Methylene Blue on laponite RD. The processes observed in systems of MB and SWy-1 in the presence of NaCl are similar to those taking place on Laponite, even in the absence of electrolytes. The only difference between the spectra of MB on Laponite suspensions in the absence (Fig. 2a) and in the presence of salt (Fig. 2b) refers to the band at 656 nm. In the absence of salt, this band grows simultaneously with the decrease of the 668-nm band. When salt was added, only the 656-nm band was observed, even at t = 0. Except for this difference, the spectral time evolution is practically the same for both suspensions, indicating that the same processes are occurring in both systems. It seems that the presence of salt only accelerates the processes occurring in the Laponite clay suspensions. On the other hand, in systems containing SWy-1, the evolution of the systems is completely different with and without salt.
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aggregates (580 nm) always larger than when the salt is added together with the dye. Furthermore, increasing the time interval between the addition of the salt and the addition of the dye results in larger aggregate bands at 580 nm and a decrease in the monomer band at 665 nm. Therefore, it can be assumed that the extent of the approximation–association processes will increase with the aging time of the salt-containing clay suspension, resulting in larger aggregated particles. This will reduce the external area available for adsorption of the dye molecules, so that the local concentration of adsorbed dye molecules increases, leading to the formation of a larger amount of dye aggregates on the clay surface. SUMMARY FIG. 2. Time dependence of the spectra of Methylene Blue (4.33 × 10−6 M) in laponite aqueous suspensions (0.056 g/L): (a) no added salt; (b) [NaCl] = 0.66 M.
In fact, in the absence of salt, the time evolution spectra observed for MB/SWy-1 and MB/Laponite suspensions are quite different. Significant differences in the amount of protonated dye as well as changes at shorter wavelengths are clearly observed. The addition of salt to both clay suspensions will increase the rearrangement of the clay particles “trapping” dye molecules adsorbed on external surfaces in newly formed internal spaces. This effect is already present in Laponite clays, even without the addition of salt, although to a lesser extent. The addition of salt to the clay suspensions, previous to the addition of the dye, gives further evidence for the occurrence of approximation–association processes of the clays particles, as shown in Fig. 3. In this condition the spectra taken immediately after the addition of the dye show absorption bands due to
The temporal evolution of the spectra of basic dyes in clay suspensions can be explained by two different processes: dye adsorption, aggregation and migration, and clay particle rearrangements (aging). Whereas the first prevails in SWy-1, the effects observed in Laponite favor the latter. The addition of salt approaches the observed behavior to that of Laponite dyes and only accelerates the changes in Laponite. In SWy-1, the higher ionic strength will favor the approximation–association processes of the clay particles, giving rise to new sites where dye molecules and dye aggregates can be “trapped”, originating different absorptions. Therefore, the temporal evolution of the spectra will be different in the presence and absence of added salt. The association of the SWy-1 clay particles also precludes the intercalation of dye molecules at the interlamellar sites where they could be protonated, hindering the growth of the 760-nm peak. Furthermore, this peak is practically absent for MB in Laponite clay suspensions. Adding salt to clay suspensions before the addition of the dye favors the approximation–association processes of the clay particles, decreasing the external area for dye adsorption. Thus, larger amounts of dye will be present as aggregates, located probably at the external surface of the associated clay particles.
ACKNOWLEDGMENTS Financial support by FAPESP (94/3505-5) and FINEP (65/92.0063.00) is gratefully acknowledged. A.P.P.C. thanks FAPESP for a graduate fellowship.
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FIG. 3. Absorption spectra of Methylene Blue (6.00 × 10−6 M) in laponite aqueous suspensions (0.028 g/L), obtained just after its addition to the clay suspension. Time “T ” (min) represents the period of aging of the clay suspensions with salt, before the addition of the dye. [NaCl] = 0.66 M.
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