Langmuir 199410, 3749-3753
3749
Time-Dependent Spectrophotometric Study of the Interaction of Basic Dyes with Clays. 1. Methylene Blue and Neutral Red on Montmorrillonite and Hectorite Fergus Gessner,* Carla C. Schmitt, and Miguel G. Neumann* Instituto de Quimica de Sdo Carlos, Universidade de ,960Paulo, Caixa Postal 369, 13560-970 Sdo Carlos SP, Brazil Received February 17, 1994. In Final Form: August 1, 1994@ The variation with time of the electronic spectra of the basic dyes methylene blue and neutral red in the presence of montmorillonite and hectorite clays has been studied to obtain information about the time-dependent processes involved in these interactions. M e r being mixed, the dyes are immediately adsorbed on the external surface of the clay particles, as induced aggregates or as monomers. Depending on the structure, conformation,and counterions ofthe clays, a slower desorption process takes place, which tends to take dye monomers to the interlamelar region of the clay particles. A n acid-base equilibrium is rapidly established for the monomers due to their interaction with acid sites present in this region. There is no need to postulate J-aggregates, interactions of the 0-layer of the clays with monomers, or dimers in the interlamelar regions to explain the observed behavior. Introduction Spectroscopic, photophysical, and photochemical techniques have been successfully used to study many types of heterogeneous systems, from micelles and membraned to silica and alumina surfaces and zeolite cavities, as well as clay surface and interlayer properties.2 The importance of clay minerals, especially swelling clays, has been known for a long time because of the peeuliar properties of these materials. Their catalytic activity has been studied for a large number of reaction^,^ including many industrial processes. More recently, pillared clays have attracted the attention of many researchers due to their potential applications as selective ~ a t a l y s t s . ~ Clays can be classified according to their layered s t r u ~ t u r e .Each ~ layer formed by condensation of sheets of linked Si(O,OHI4 tetrahedra with sheets of linked M2-3(OH)6octahedra, where M is a trivalent or divalent cation, usually Al or Mg. Different materials are formed depending on the way that these sheets condense. Clays like montmorrillonite and hectorite, used in this work, are formed by layers that result from the condensation of two tetrahedral sheets with one octahedral sheet and are classified as 2:l layered clays. The separation between these layers is variable and depends on various factors, such as water content, counterions, and nature of the adsorbed material. These layers are negatively charged due to the isomorphous replacement of some cations by others of similar size but with lower charge. The negative charge is balanced by hydrated cations placed in the interlayer spaces. When cationic dyes are placed in solutions containing anionic polyelectrolyte^,^^^ nucleic acids,8 or clay suspenAbstract published in Advance ACS Abstracts, September 15, 1994. (1)(a)Kalyasundaraman, K. Photochemistry in Microheterogeneous Systems, Academic Press: New York, 1987. (b)Neumann, M. G.;Tiera, M . J. Quim. Noua 1993,16,280. (2) (a) Thomas, J. K. Chem. Rev. 1993,93,301. (b)Thomas, J. K. J . Phys. Chem. 1987,91,267.(c)Thomas,J. K. The Chemistry ofEzcitation ut Interfaces; ACS Monograph 191; American Chemical Society: Washington, DC, 1984. (3) Pinnavaia, T. J. Science 1983,220, 365. (4) Mittchell, I. V., Ed. Pillared Layered Structures: Current Trend and Applications; Elsevier: Amsterdam, 1990, and references therein. ( 5 ) Theng, B. K. G . The Chemistry of Clay Organic Reactions, Wiley: New York, 1974; Chapter 1. (6) Neumann, M. G.; Hioka, N. J . Appl. Polym. Sci. 1987,34,2829. @
0743-7463/94/2410-3749$04.50l0
sions, they present the same metachromatic behavior observed in pure aqueous solutions a t higher concentrations, i.e., a decrease in the monomer absorption band (a-band) and the appearance of bands a t lower wavelengths CP- and y-bands) due to the formation of dimers and higher aggregate^.^ For polyelectrolytes and nucleic acids, it is assumed that the chain acts as a pattern for the dye molecules,forcingthem into a geometry that favors aggregatiom6t8 In the case of clays, the dye molecules are adsorbed on the particle internal and/or external surfaces, creating high-concentration regions, which facilitate the aggregation of the dye. The first report on the metachromatic behavior of a cationic dye (methylene blue) on a clay (Wyoming montmorrillonite) was published by Bergmann and O’Konski nearly 30 years ago.1° They observed the presence of four distinct absorption bands around 575,610,670, and 760 nm, which were ascribed to dimers and higher aggregates (two peaks), monomers, and the J-band of the dimer, respectively. Several other authors also studied the same system with similar result^,^^-^^ but in some cases, giving other origins to the peaks. Yariv and Lurie12ascribed the spectral changes a t the shorter wavelengths to the interaction of the n system of the dye with the lone pair of electrons of the oxygen atoms on the internal clay surface. From work of the adsorption on Hectorite and Barasyn, Cenens and Schoonheydt14 assumed that the 575 nm band corresponded to trimers on the external surface and the 610 nm to dimers a t the outer and internal surfaces of the clay. The origin of the peak a t longer wavelengths has also been a matter of controversy: Bergmann and O’KonskilO supposed that it was due to the J-component of the dimer, whereas Yariv and Lurie12 ascribed it to either the protonated or the semireduced form of the dye. Cenens and Schoonheydt14demonstrated, (7) (a)Shirai, M.; Nagatsuka, T.; Tanaka, M.Makromo1. Chem. 1978, 179, 173. (b) Shirai, M.; Hanatani, Y.; Tanaka, M. J . MacromoE. Sci., Chem. Ed. 1985, A22,279. (c) Schubert, M.; Pal, M. K. J . Phys. Chem. 1963,67,1821.(d) Schubert, J.;Levine, A. J. Am. Chem. SOC.1966,77, 4197. (8) Bradley, D. F.; Wolff, M . K. Proc. Natl. Acad. Sci. U S A . 1969, 45, 944. (9) Rabinowitch, E.; Epstein, L. F. J. Am. Chem. SOC.1941, 63, 69. (10) Bergmann; K.; O’Konski, C. T. J . Phys. Chem. 1963,67,2169. (11) Bodenheimer, W.; Heller, L. Isr. J. Chem. 1968, 6, 307. (12)Yariv, S.; Lurie, D. Isr. J . Chem. 1971, 9, 537. (13) Yariv. S.; Lurie, D. Isr. J. Chem. 1971, 9, 553. (14) Cenens, J.;Schoonheydt, R. A. Clays Clay Miner. 1988,36,214.
0 1994 American Chemical Society
3750 Langmuir, Vol. 10, No. 10, 1994
clay SWY-1 SHCa-1
type nat. montmor. nat. hectorite
Si02 (%) 66.9 34.7
Gessner et al. Table 1. Properties of the Clays A 1 2 0 3 (%) MgO (%) Fez03 (%) 19.6 3.05 3.35 0.69 15.3 0.02
by calculations, that the J-component of the dimer should be around 720 nm and ascribed the longer wavelength peak a t 765 nm to the protonated form of the dye. Later work by Cenens and co-workers,15on the adsorption of proflavine, led them to postulate the presence of a protonated form of the dye a t the external surface of the particles. The interaction of the montmorillonite and hectorite clays with other basic dyes, like p r ~ f l a v i n e , ' ~acridine J~ orange,17 thionine derivatives,ls pyronine,lg triphenylmethane dyes,2O and rhodamine^,^^^,^^^^^^ has been studied by several authors with similar results. Also, the absorption of these dyes by other clays was studied, showing the same general characteristics, except for the lack of the long-wavelength peak in some of them. The behavior of the same class of dyes, especially thionine and its analogues, in the presence of zeolites has also been s t ~ d i e d . Evidence ~ ~ , ~ ~ was found for the protonation of thionine in the internal channel ofzeolite L. On the other hand, the protonated form of larger dyes like methylene blue and ethylene blue was not observed in the same systems, due to their inability to penetrate into the channelsaZ3In the presence of zeolite Y, which has larger internal cages, thionine forms internal dimers not observed with zeolite Lez2 In this paper we report a spectroscopic study on the time evolution of the absorption of cationic dyes by montmorrillonite clays. These results allow a better understanding of the properties of the clay particles in aqueous suspension as well as a more accurate assignment of the peaks to the different species present in the systems.
Experimental Section The dyes for which the adsorption on clays was studied were methylene blue (MB, Carlo Erba) and neutral red (NR, Merck). They were used as received. Two clays were used, a montmorrillonite type Wyoming (SWy-1) and hectorite (SHCa-l), from the Clay Minerals Society, University of Missouri, Columbia, MO. Some physical properties of these clays a r e shown in Table 1.24 (15)(a) Cenens, J.;Vliers, D. P.; Schoonheydt, R. A.; De Schryver, F. C. In Proc. Int. Clay Conf.Denver 1985; Schultz, D. G., Van Olphen, H., Mumpton, F. A., Eds.; The Clay Minerals SOC.:Bloomington, IN, 1987;p 352.(b) Schoonheydt, R.A.; Cenens, J.; De Schryver, F. C. J. Chem. SOC.,Faraday Trans. 1 1986,82,281. (16)Cenens, J.;Schoonheydt, R. A.; De Schryver,F. C. Spectroscopic Characterizationof Minerals and Their Surfaces.ACSSymp. Ser. 1989, 415,378. (17)Cohen, R.;Yariv, S. J. Chem. SOC.,Faraday Trans. 1 1984,80, 1705. (18)(a) Bose, H.S.; Sunwar, C. B.; Chakravarti, S. K Indian J. Chem. 1987,26A,944.(b)Sunwar, C. D.; Bose, H. S. J.Colloid Interface Sci. 1990,136,54. (19)(a)Grauer, Z.; Grauer, G. L.; Avnir, D.; Yariv, S. J. Chem. SOC., Faraday Trans. 1 1987,83,1685.(b) Endo, T.; Nakada, N.; Sato, T.; Shimada, M. J. Phys. Chem. Solids 1988,49,1423. (20)(a)Yariv, S.;Ghosh, D. K.; Hepler, L. G. J.Chem. Soc., Faraday Trans. 1991,87, 1201.(b) Yariv, S.;Nasser, A.; Bar-on, P. J. Chem. Soc., Faraday Trans. 1990,86,1593.(c) Goshal, D. N.; Mukhejee, S. K. Indian J . Chem. 1972,10,835. (d) Dobrogowska, C.; Hepler, L. G.; Ghosh, D. K.; Yariv, S. J. Thermal Anal. 1991,37,1347. (21)(a)Grauer,Z.;Avnir,D.;Yariv,S.Can. J . Chem. 1984,62,1889. (b)Tapia EstBvez, M. J.; L6pez Arbeloa, F.; L6pez Arbeloa; T.; L6pez Arbeloa, I. Langmuir 1993,9, 3629.(c) Tapia EstBvez, M. J.; L6pez Arbeloa, F.; L6pez Arbeloa; T.; L6pez Arbeloa, I. J. Colloid Interface Sei. 1994,162,412. (221Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. J.Am. Chem. SOC.1993,115, 10438. (23)Calzafemi, G.; Gfeller, N. J.Phys. Chem. 1992,96,3428. (24)Van Olphen, H., Fripiat, J. J., Eds. Data Handbook for Clay Materials and Other Non-Metallic Minerals; Pergamon: Oxford, 1979.
"."
500
area (m2/g)
CEC (meq/100 g clay)
32 63
76.4 43.9
600
700
800
Wavelength, nm
Figure 1. Spectra of methylene blue (1.73 x M) in t h e presence of SWy-1 (0.11 g/L, 21% CEC). Times expressed in minutes after mixing. The procedure for the purification of the clays was as follows: (i)30 g ofthe clay was suspended in 1.5 L of deionized water until a homogeneous suspension was obtained. This suspension was taken to pH 3.5 (HC1) and centrifuged. The same procedure was applied twice to the obtained solid; (ii) the solid was suspended again in 1.5L of water at pH 8 (NaOH) and left for 12 h, after which the first 1 0 cm of the supernatant liquid was siphoned and stocked. The remaining solid was treated again i n the same way and the liquid added to the former; (iii) this solution was acidified to pH 3.5, and a saturated solution of NaCl was added. After flocculation (2-3 days), the supernatant liquid was rejected. (iv) The solution containing the flocculated clay was dialyzed with Millipore water until a negative chloride ion test was obtained. (v) Finally, the purified clay was dried by "freeze drying". To exchange the counterions of SWy, 0.5 g of the sodium salt was suspended in 10 mL ofwater and mixed with 20 mL of a 2.5 M solution of the chloride of the desired counterion (LiCl, CsCl, or CaC12). The mixture was stirred for 12 h a n d centrifuged, and the procedure was repeated twice. Afterward, the clay was suspended, dialyzed, and freeze-dried. All the clay suspensions were prepared in Millipore deionized water and stirred until a colloidal suspension was obtained. After about an hour, sufficient to stabilize the conformation of the clay particles, the appropriate amount of a dye stock solution (5.2 x M) was added under stirring. UV-vis spectra were recorded on a Hitachi U-2000 spectrophotometer, and fluorescence spectra on an Aminco-Bowman 58 spectrofluorimeter. All experiments were performed at room temperature, 25 C! 1 "C.
-
-
-
Results and Discussion Methylene Blue on SWy-1. As can be seen in Figure 1, the initial spectrum of MB in the presence of SWy-1 presents a very intense peak in the region of 570 nm, corresponding to trimers and higher aggregates of the dye, although a t these concentrations the proportion of aggregated dye in homogeneous solution is minimal. Also, a smaller peak can be observed in the region around 670 nm, ascribed to the monomer. The initial ratio between the aggregate and monomer absorption peaks, A57dA670, is constant, independent of the time elapsed between the stabilization of the clay suspension and the addition of the dye. An isosbestic region is found a t ca. 600 nm, indicating that the decrease with time of the lowerwavelength absorption corresponds to the transformation ofthe aggregates into the monomer ofthe dye. This region extends over approximately 15 nm, which is an indication that there are two or more transformations going on, i.e.,
Langmuir, Vol. 10, No. 10, 1994 3751
Interaction of Basic Dyes with Clays higher aggregates and trimers into dimers and monomers, and dimers into monomers. The maximum wavelength of the monomer peak is observed at 670 nm. This represents a red shift by about 10 nm when compared with the maximum observed for dilute aqueous solutions.1° Similar shifts were observed previously for MB in solutions containing polyelectrol y t e ~and ~ , ~DNA,9 suggesting that these monomers are placed in a region in which they are interacting with charged or polarizable sites of the clay layer, probably the oxygen atoms. Therefore, it seems not probable that the peak observed around 600 nm corresponds to internal dye monomers.12 Interactions ofn electrons will generally lower the energy of the ground state less than that of the excited state, which is more polarizable. This will result in a red shift of the peak. Thus, it seems not to be very likely that this peak arises from dye monomers placed in the interlayer spaces. The growing with time of a peak a t larger wavelengths is also observed. Absorptions in the same region are observed when methylene blue is dissolved in very acid aqueous solutions,25 so that this new peak, with a maximum a t around 765 nm, is attributed to the protonated form of the dye.14 This protonation may occur at acidic sites in the interlamelar region, which arise from the existence of unbalanced charges of the ~ 1 a y . Similar l~~ enhanced protonation of methylene blue in laponite has been attributed to the high dissociation constants of the water molecules adsorbed in the monolayer in the interlamelar region.26 No fluorescence is observed a t any time, indicating that none of the species present in the system is placed in the bulk of the solution. This means that all the absorbing species are in the domain of the clay. The processes occurring during the first 3-4 h after the mixing of the dye with the clay can be described by MB+(soln)
-
\1 fast
MB+(ext)
-
670 nm
665m
slow
MB+(int)
670 nm
fast
MBHZ+(inl)
765 nm
] slow
(W 570-600
(MB'),
where MB+(soln)corresponds to the dye in the bulk ofthe solution, MB+(ext)and (MB+),(ext) to the dye monomer and induced aggregates on the external surface of the clay tactoids, MB+(int)to monomers in the interlayers of the clay, and MBH2+(int)to the protonated dye, also in the interlayer space. As observed in Figure 2, the peaks a t 670 and 765 nm grow a t approximately the same rate and simultaneous to the decrease of the aggregates' peak a t 573 nm. At very long times, over 24 h, the ratio between the peaks corresponding to the protonated and deprotonated dyes starts to increase, possibly due to the onset of a new process (see below) or to a slow hydrolysis of the internal sites. The simultaneous increase of the monomer peak with the decrease of the aggregate peak is a demonstration that the aggregates, mostly dimers and trimers, must be placed in the external surface of the particles. If not, one would not observe the transformation of this peak into the monomer peak, as there is no reason for a deaggregation in the interlamelar region, when no deaggregation occurs on the external surface of the tactoids. These (25) Meyer, H. W.; Treadwell, W. D. Helv.Chim. Acta 1968, 183, 1461. (26) Schoonheydt, R. A.; Heughebaert, L. Clay Miner. 1992,27,91.
765nm
-------
o'2k-----50
1W lime, min
150
Figure 2. Time evolution of the intensities of absorption of the peaks corresponding to monomers (670 nm), aggregates (573 nm), and protonated (765 nm) species of methylene blue in the presence of SWy-1.Experimental conditionsas in Figure 1.
unprobable interlamelar dimers should have a larger aggregation constant than the external dimers, as the interaction of the dye molecules with both negatively charged layers will reduce the electrostatic repulsion between the dye molecules. Furthermore, the clay layers are close enough to avoid the formation of dimers in the interlamelar region, similar to what was found with zeolites,22where this internal aggregation only occurs in the supercages of zeolite Y. Thus, only monomeric dye molecules will be inside the tactoids, ruling out the possibility of dimers on the internal surfaces of the clay.14,"%17,19 The rate constant for the deaggregation processes can be calculated from the decrease of the absorbance a t 570 nm (A570), corrected for the residual contribution of the monomers to the absorbance a t this wavelength A~570, which is assumed to be proportional to the absorption peak a t 675 nm (A675): AAg570
=
-
=
-
The calculated constant corresponds to approximately 0.011 min-', which compares reasonably well with the value of 0.02 min-' (time constant 50 min) reported by Bergmann and O'Konski'O for the growing rate of dye monomers. Effect of Changes of Clay and Dye Concentrations. The ratio between dye and clay was changed from 48 to 0.6% CEC by varying the amount of added dye to a clay suspension of 0.11 g/L. For low ratios, the initial intensity of the aggregate band at 570 nm becomes weaker because of the low dye concentration. Still, the initial temporal behaviour is similar to that described above and shown in Figure 1for 21% CEC; i.e., the bands corresponding to the monomer and the protonated monomer increase, showing a n isosbestic point around 600 nm. However, after a time, a new isosbestic point can be observed around 700 nm as shown in Figure 3 for a system with ratio of 5.9% CEC. This isosbestic point indicates that a t that time practically all aggregates have been consumed, so that the only acting process is the protonation of monomers MB+(ext)
-
-
MB+(int) MBH2+(int)
The relationship between dye and clay was also varied from 85 to 2.6% CEC by keeping the amount of dye constant (1.73 x M) and changing that of the clay. The only effect that was observed was that in the systems with lower clay content the aggregate peak was larger, as expected from the lower external area available for dye adsorption. Also, experiments were performed, keeping the dye to clay ratio constant a t 21% CEC and varying the amount of both components in the solution, from 0.01 to 0.25 g/L of clay. For the lowest amount of clays, the initial ratio
Gessner et al.
3752 Langmuir, Vol. 10, No. 10, 1994
1
0.25
120
0.3
0 20 w
0
2
5 0.15
m
g
fc
0.2
B a
0.10 QL
01
0.05 0.0
0.00
500
800
700
600
Wavelength, nm
Wavelength I nm
M in the Figure 3. Spectra of methylene blue 0.49 x presence of SWy-1 (0.11 g/L, 5.9%CEC). Times expressed in
minutes after mixing.
1
1.4
1,3{
00
02
01
0.4
03
[clay] / g
05
L"
Figure 4. Ratio between the absorbances of the aggregate and monomer bands of methylene blue in the presence of Swy1, as a function of clay concentration,keeping the ratio of dye t o clay at 21% CEC.
0.f
,
,
,
50
100
150
Time. min
Figure 5. Increase ofthe absorptionintensities ofthe monomer peaks (670 nm) of methylene blue in the presence of SWy-1 with differentcounterions.Experimentalconditions as in Figure 1.
of the peaks corresponding to aggregate and monomer was low and increased up to a constant value around a clay concentration of 0.075 g L , as can be seen in Figure 4. This effect can be assigned to the fact that a t high clay concentrations the aggregation of the clay will be larger, and relatively less external surface is available for adsorbing dye aggregates. Effect of Counterions. The use of SWy-1 with other counterions modifies the kinetics, as well as the equilibrium concentration of the MB species present in the system, as shown in Figure 5, though it does not change the overall behavior from that observed with the Na+ salt of the clay. There seems to be no large difference between clays with sodium or lithium counterions. When the equilibrium is reached, there are nearly 20%more dye molecules in the Li clay than in the Na clay, as already observed by Bodenheimer and Heller." The rate a t which the equilibrium is reached is the same in both cases, suggesting that the swelling of the clays is the ratedetermining process, and no differential restrictions are
Figure 6. Spectra ofneutral red 1.73 x M in the presence of SWy-l(O.11g/L).Times expressed in minutes after mixing.
imposed on the dye molecules for entering into the interlayer region. On the other hand, the substitution of sodium for the larger cesium ions accelerates the attainment of the equilibrium (which is reached in less than 2 h), but as this clay does not swell as much as the Na clay,14 a smaller amount of monomers is incorporated. Finally, Ca2+ions will hold the lamellas together, making it more difficult for the MB molecules to enter into those regions, so that equilibrium is reached earlier and with lower concentrations of incorporated MB monomer. The different ions also affect the proportion of the peak at longer wavelengths. This is due to the larger or smaller acidity of the counterions which determine the amount of sites a t which MB can be adsorbed with p r o t o n a t i ~ n . ~ ~ Neutral Red on SWy-1. In Figure 6 are shown the spectra of neutral red in the presence of SWy-1,at different times. An initial band is observed around 545 nm, which may be attributed to dye molecules on the external surface of the clay particles (compared with 535 nm for neutral red in homogeneous aqueous solutionz7). As time passes, a new blue band a t 630 nm begins to appear, and a t the same time, the red band a t 545 nm decreases and is shifted to 555 nm, with a distinct isosbestic point a t 560 nm. The blue band is similar to that found for neutral red in acid solutions,z7confirming the existence of acid sites to which the dye molecules may bind in the interior of the tactoids. The main difference between the behavior of methylene blue and neutral Red in the presence of SWy-1 is that the former is present initially mainly as surface-induced aggregates, whereas the latter is adsorbed on the external part of the tactoids as monomers. The dimerization constant for MB in aqueous solution is around 50OOz8and that for NR must be at least 10 times lower,29which is in accordance with the above postulation. Thus, in this system the processes that occur initially will be
Methylene Blue on Hectorite. The behavior of MB on hectorite is more or less similar to that on SWy-1; i.e. the induced dye aggregates deaggregate and penetrate into the clay, followed by equilibration between the protonated and nonprotonated monomers. Nevertheless, the whole process seems to be much slower, and complete deaggregation (which on SWy-1 is attained after about 3 h) is not reached even after 24 h. At that time, as can be (27) Bartels, P. 2.Phys. Chem. N.F. 1966,9,74. (28) Vitagliano,V. InAggregationProcesses in Solution;Wyn-Jones, E., Gormally, J., Eds.;Elsevier: Amsterdam, 1983; Chapter 11, p 271. (29) Neumann, M. G. Unpublished results.
Langmuir, Vol. 10, No. 10, 1994 3753
Interaction of Basic Dyes with Clays
Conclusions
0.6
8
2640 60 15 1
0.4
m c
2
s
2 0
0.2
0.0 400
500
600
700
Wavelength, nm Figure 7. Spectra of methylene blue (1.73 x M)in the presence of hectorite (0.11 Times expressed in minutes
after mixing.
a).
seen by comparing the spectra in Figure 7, the short wavelength band only looses 20% of its intensity and is shifted by some 20 nm to the red. Also, the amount of dye in the protonated form is much less than on SWy-1. These results reflect the larger ratio between external and ,~~ internal surfaces (or sites) in the case of h e c t ~ r i t eas well as the lower acidity of these sites, which may arrive from the fact that there are less charges on hectorite than on SWy-1, resulting in a lower polarization of the interlamelar water.
When clays are added to aqueous solutions containing dyes, the initial process corresponds to a rapid adsorption on the external surface of the clay particles. Depending on the aggregation easiness of the dyes, they may be adsorbed as induced aggregates (as in the case of methylene blue) or as monomers (asfor neutral red). Depending on the structure, conformation, and counterions of the clays, a slower desorption process takes place, which tends to bring dye molecules inside the clay particles. For MB, this process involves deaggregation. Only monomers are present in the interlamelar region, as dimers would not dissociate to monomers in that region. Furthermore, a t longer times only peaks corresponding to monomers are observed. As proposed by various authors, this region may present higher polarity, or acid sites, with which some of the dye molecules may interact, forming entities similar to the protonated dyes, as evidenced by their absorption bands in the red region. The stiffness and dispersivity characteristics of the clays will determine the amount of dye that may go into the interlayer region.
Acknowledgment. Financial support from FAPESP, FINEP-PADCT, and CNPq is gratefully acknowledged.
