Langmuir 1995,11, 3211-3217
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Adsorption of Rhodamine 6G on Saponite. A Comparative Study with Other Rhodamine 6G-Smectite Aqueous Suspensions F. L6pez Arbeloa, M. J. Tapia EstBvez, T. L6pez Arbeloa, and I. L6pez Arbeloa" Departamento de Quimica Fisica, Universidad del Pais Vasco - EHU, Apartado 644, 48080 - Bilbao, Spain Received May 15, 1995@ Absorption and fluorescence spectroscopies are applied to study the adsorption of rhodamine 6G on saponite in aqueous suspensions. The interpretation of the experimental results suggests that the dye can be adsorbed as the monomeric and the dimeric forms on both the external and the interlamellar surfaces of the clay. A mechanism for the adsorption is proposed on the basis of the evolution of these species with the stirring time and the relative dydclay concentration. The present results are compared to those previously reported for the adsorption of rhodamine 6G on laponite, montmorillonite of Wyoming, and hectorite in aqueous suspensions, and the effect of the nature of the clay and the particle size is discussed.
Introduction In recent years research on colloidal systems has become more interesting to the scientific community, since colloid chemistry plays a significant role in the progress ofmodern chemistry and engineering.' Most of the applications of colloids are based on the surface properties of the interface between the dispersed phase and the dispersing medium, which affect, for instance, the adsorption andlor the distribution of compounds in both phases. The use of aromatic compounds as molecule probes has allowed the application of electronic spectroscopies to study the properties of microheterogeneous system~,29~ providing significant information on organic organized media such as micelle^,^ protein^,^ and polymers6 and on inorganic systems as silicas7 and clays.8 Clay minerals are aluminosilicates with a bidimensional lamellar structure leading to a high aredvolume ratio. For smectite-type clays, the layers are formed by the condensation of a n octahedrical sheet of A1203 between two tetrahedral Si02 ~heets.~JO Some of the A13+ or Si4+ can be substituted by cations with a lower oxidation number, giving rise to the negative charge of the layers. This charge is compensated by interchangeable inorganic cations, localized a t the lamellar surface. The inorganic cations can cause the stacking of clay layers in parallel planes, leading to the tactoidal structure of smectites with two main different adsorption surfaces, the external and the internal ones. The interest in the aqueous suspensions of organosmectite systems is based on their important applications Abstract published in Advance ACS Abstracts, July 15, 1995. (1)Akhmetov, B.;Novichenko,Y.;Chapurin,V. Physical and Colloid Chemistry; Mir Publushers: Moscow, 1989. (2) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987. (3)Thomas, J. K. The Chemistry of Excitation Chemistry; ACS Monograph 181; American Chemical Society, Washington, DC, 1984. (4) Gehlen, M. H.; De Schryver, F. C. Chem. Rev. 1993, 93, 199. ( 5 ) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983. (6)Winnik, F. M. Chem. Rev. 1993,93, 587. (7) Bauer, R. K.;de Mayo, P.;Ware, W. R.; Wu, K. C. J. Phys. Chem. 1987,91, 5135. Mao, Y.; Thomas, J. K. Langmuir 1992,8, 539. (8)Viane, K.;Crutzen, M.; Kuniyma, R.; Schoonheydt, R. A,; De Schryver, F. C. Prog. Colloid Polym. Sci. 1988, 266, 242. (S)Newmann, A. C. D. Chemistry of Clays and Clay Minerals; Longman Sci. Techn. Min. SOC.:London, 1987. (10)Stoch, L. Miner. Petrog. Acta 1990, X X X I I , 69. @
in several fields: in agriculture as a stoppage for fertilizers" and a photostabilizer of pesticides,12in ~ a t a l y s i s , ~ ~ - ~ ~ in decontamination of wastewater,16 in pharmaceutical and paint industries,17J8and in photochemistry as guide of photoredox reaction^.'^-^' The dispersion of clay particles in aqueous suspensions and the capacity of clays to adsorb organic compounds depend on several factors such as the nature of the negative charge of the layers (octahedral or tetrahedral substitution), the nature of the inorganic cations, the swelling of the interlamellar space, and the particle size.g Thermogravimetric,22 X-ray difiactionlg and IR and NMR s p e c t r o s ~ o p i techniques c~~ have been largely used to study organoclay systems in the solid state, but these methods are very limited to study clays in aqueous suspensions. In these cases, the UV-vis absorption and fluorescence spectroscopies of organocationic dyes adsorbed on clays have provided important information on the dispersion of clay particles in aqueous suspension^.^^-^' The adsorption of dyes on clays can give rise to the metachromatic effect, i.e. the replacement of the main absorption band of the organic compound by others bands (11)Theng,B. K.G. The Chemisty ofClay-OrganicReactions; Adam Hiker: London, 1974. (12) Margulies, L.;Rozen, H.;Cohen,E.Nature 1985,315,658. Clays Clay Mzn. 1988,36, 159. Margulies, L.; Cohen, E.; Rozen, H. Pestic. Sci. 1987. 18. 79. (13) Barre;, R. M. Phil. Trans. R.SOC. London 1984, A311, 333, (14) Dale Ortego, J.;Kowalska, M.; Cocke, D. L. Chemosphere 1991, 22, 769. (15) Laszlo, P. Acc. Chem. Res. 1986, 19, 121. Surf. Sci. Ser. 1991, 38, 437. (16) Srinivasan, K. R.; Fogler, H.S. Clays Clay Min. 1990,38, 277 and 287. London 1984, A311, 391. (17) Odom, I. E. Phil. Trans. R. SOC. (18)Murray, H. H. Appl. Clay Sci. 1991,5, 379. (19) Weiss, R. G.;Ramamurthy,V.; Hammond,G. S.Acc. Chem. Res. 1993,26, 530. (20) Thomas, J. K. Acc. Chem. Res. 1988,21, 275. Kuydenkall, V. G.; Thomas, J. K. Lungmuir 1990,6, 1350. J . Phys. Chem. 1990,94, 4224. (21)Photochemistry andPhotophysics; Rabek, J. F.,Ed.; CRC Press: Boca Raton, FL, 1990; Vols. I and 111. (22) Yariv, S.; Muller-Vonmoos,M.; Kahr, G.;Rub, A. J. Therm.Ana1. 1989.35. 1941. -__-(23) Tennakoon, D. T. B.; Schlogl, R.; Rayment, T.;Klinowski, J.; Joner, W.; Thomas, J. K. Clay Min. 1983, 18, 357. (24) Bergmann, K.; O'Konskii, C. T. J. Phys. Chem. 1963,67,2169. (25)Yariv, S.; Lune, D. Isr. J. Chem. 1971,9,537. Cohen, R.;Yariv, S. J . Chem. SOC.,Faraday Trans 1 1984,80,1705. Grauer, 2.;Grauer, G. L.; Avnir, D.; Yariv, S. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1685. Yariv, S.;Nasser, A.; Bar-on, P. J. Chem. SOC.,Faraday Trans. 1990,86, 1593. ~
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0743-746319512411-3211$09.00/00 1995 American Chemical Society
Lbpez Arbeloa et al.
3212 Langmuir, Vol. 11, No. 8, 1995
tion and fluorescence spectroscopies, characterizing the a t higher energies. This effect has been attributed to a n adsorbed species. In order to evaluate the influence of interaction between the electron lone pair of the oxygen the nature ofthe clay, these results are compared to others atoms of the silicate layer with the n-electron of the previously obtained for the adsorption of rhodamine 6G aromatic system2s and/or to the self-aggregation of the ~~,~~ of Wyoming,36and hecdye when it is adsorbed on the clay s ~ r f a c e . ~ ~ , ~ ~ ! on ~ ~l a ~ o n i t e ,montmorillonite t ~ r i t e The . ~ ~nature of the clay is analyzed in terms of the Rhodamine dyes are adequate molecule probes to type of substitution and of the particle size: saponite and characterize heterogeneous systems since their photomontmorillonite of Wyoming have tetrahedral substituphysical characteristics depend on environmental factors30 tions (saponite is almost a completely tetrahedrally and they have been extensively used in a multitude of substituted clay and montmorillonite is only partially (15systems.31 Moreover these dyes are cationic compounds 50% of the chargeg) substituted in this way), whereas with a high tendency to be adsorbed on the clay surface by cation exchange. Due to steric hindrance of the hectorite and laponite are octahedral clays (laponite is a carboxyphenyl group ofthe dye, the Jt-system rhodaminesynthetic hectorite with a small particle size (30% CEC the adsorption of R6G on Sap takes place on the external surface as monomers and dimers (Figure 4). At this diluted clay range, the clay is assumed to be so dispersed than the adsorption is performed mainly on the external surface. By increasing the dye content for a constant clay concentration (higher %CEC value), the surface available for the adsorption decreases, increasing the coverage of R6G molecules a t the same external surface and, therefore, favoring the formation of the externally adsorbed dimer. Basically, the same evolution is observed in the absorption spectra with the stirring time for very concentrated clay suspension (low loadings, 0.5% CEC, Figure 3) and with the relative dyelclay concentration in diluted clay suspensions (30-100% CEC loadings, Figure 4).For these high loadings (low clay concentrations) R6G molecules are adsorbed on the external surface of well dispersed clay particles (no tactoid formation) instead of the external surface of very compact tactoids obtained for very concentrated clay suspensions (low loadings). Neither situation can be easily distinguished by visible absorption spectroscopy,probably because of the similar environment of R6G monomers adsorbed on a separate clay layer and on the external surface of a clay tactoid.
Comparative Study The adsorption of R6G on several smectites such as laponite B (LapB), montmorillonite of Wyoming (MonW), hectorite (Hec) and Sap presents metachromasy. This phenomenon can be explained by the dimerization of the dye on both the external and the internal surfaces of the clay. All reported aggregates have a sandwich structure, the internal dimer being more constrained than the external one. The monomeric form of R6G can be also adsorbed on the external surface and on the interlamellar space. The spectroscopic characteristics of each of these species are summarized in Table 2. Both the concentration and the spectroscopic characteristics of these species depend on the nature of the clay (particle size, type of substitution) as well as on the relative dye/clay concentration and the stirring time. For the R6G-LapB ~ y s t e m ,a ~monomer ~ ? ~ ~ adsorbed in the interlamellar space has been characterized for relative dyelclay concentrations 12%CEC. The important metachromasy effect observed for the loading range between 1and 15% CEC a t long stirring times was attributed to the dye aggregation on the internal surface. The different metachromasy observed in dilute clay suspensions (high loadings) was assigned to the coexistenceofboth externally and internally adsorbed dimers. Since laponite is a very dispersed clay in aqueous suspensions, the internal species have been attributed to the stacking of clay layers after
the addition of dye molecules when the clay concentration is high enough. For R6G-MonW suspension^,^^ the internally adsorbed monomer is observed only for highly concentrated clay suspensions after long stirring times and is formed in detriment to the externally adsorbed monomer, present at short stirring times. Due to the partially tetrahedral substitution of this clay, the clay layers are stacked to form tactoids before the addition of dye molecules for concentrated clay suspensions. The adsorption of R6G molecules is considered to take place initially as monomers o n the external surface and they migrate to the internal surface on a time scale of days. The metachromasy observed in this system is not so important as in laponite and it is observed immediately after sample preparation for loadings < 10% CEC. This effect has been attributed to the dimerization of the dye on the external surface. The dimer of R6G adsorbed on the internal surface of MonW has not been characterized either because it is not formed owing to the difficulty of a dimer to migrate to the interlamellar space or because under the conditions in which it is formed (for instance, high relative dyelclay concentration a t long stirring time) the samples flocculated. In the R6G-Hec system,38 the externally adsorbed monomer is observed for freshly prepared dyelclay suspensions, and by increasing the stirring time the internal monomer is formed for the 1-10% CEC loading region, which is assumed to be formed by the stacking of clay particles on a time scale of hours. The internally adsorbed dimer has been characterized for loading 30%CEC) andor long periods of stirring. From the above presentation it can be concluded that the concentration of the monomer andor dimer of R6G on the external andor internal surfaces of smectite type clays depends on the nature of the clay, which is now analyzed in terms of the type of substitution and the particle size. One of the most important factors in the R6G-LapB system is the small particle size of the clay ('0.03 pm). This system is characterized by an important concentration of species adsorbed on the internal surface and by the large dye aggregation that occurs even at short stirring times. Although this clay is very dispersed in aqueous solution, the adsorption of cationic molecules on the external surface favors the stacking of the clay layers leading to the observation of the internally adsorbed species even a t short stirring times. Schoonheydt et al.48 also concluded that the internally adsorbed species are more prevalent for clays with low particle size. The internal monomer is a consequence of the stacking of two layers, one of them with a R6G monomer adsorbed on it, while the internally adsorbed dimer is formed by the stacking of two layers, both with monomer adsorbed on them. Clays with tetrahedral substitution have a lower swelling capacity than octahedrally substituted clays because the substitution of Si4+for A13+ favors the hydrogen bonding between water molecules and the oxygens of the clay planes.28 Considering this argument, the swelling capacity of clays would decrease in the order LapB > Hec > MonW > Sap, which can explain the difficulty of obtaining internally adsorbed species by increasing the tetrahedral degree ofthe clay substitution. Indeed, the internally adsorbed species are observed on a time scale of minutes for the R6G-LapB system,35hours (48)Schoonheydt, R. A.; Cenens, J.; De Schrijver, F. C. J . Chem. SOC.,Faraday Trans. 1 1986,82,281.
Langmuir, Vol. 11, No. 8,1995 3217
Adsorption of Rhodamine 6G on Saponite
for the R6G-Hec system,38and days for the R6G-MonW system37while for the R6G-Sap system the predominant species are those externally adsorbed, as is discussed in the previous section. Generally speaking, the species adsorbed on the external surface are observed for low-concentrated clay suspensions (high %CEC), because the clay particles are completely dispersed as layers in the aqueous suspension. These species are also observed for low loading in clay with tetrahedral substitution because of the poor swelling properties of these clays. The internally adsorbed species are obtained a t highly concentrated clay suspensions in dispersed clays (with octahedral substitution) as a consequence of the stacking of clay layers favored by the presence of cationic dyes adsorbed on them. For partially tetrahedrally substituted clays the internal monomer is obtained for low and intermediate loadings due probably to the migration of externally adsorbed monomers on a time scale of days. The absorption spectroscopic characteristics of the different species, summarized in Table 2, can be also correlated with the nature of the clays. For the externally adsorbed monomer the displacement is from about 528 nm (for R6G-LapB and R6G-Hec systems) to 534 nm (for R6G-MonW and RGG-Sap), indicating a spectral shift to lower energies in clay with tetrahedral substitution. This shift is even more important for the case of the R6G monomer adsorbed on the internal surface, with the maximum at 533,545,557, and '550 nm for LapB, Hec, MonW, and Sap, respectively (Table 2). These results suggest a n important variation in the environmental conditions of the internal and external surface with the nature ofthe clay substitution, mainly for the interlamellar water. The physical-chemical characteristics of water at the interlamellar space of LapB are very similar to those of the liquid water,46and a short spectral shift of the absorption maximum for the R6G monomers adsorbed on LapB (528 nm) with respect in water (526 nm) is experimentally observed. The important shift of the absorption maximum of the internal monomer for MonW and Sap to lower energies with respect to that for LapB and Hect (Table 2) should be a consequence of the tetrahedral substitution that affects the general properties of the interlamellar water.24,49,50 As a matter of fact, the dissociation degree of water close to tetrahedrally substituted clay layers can be lo7 times higher than that of the pure water.51 A n increase in the micropolarity of the water as a consequence of this type of substitution could explain the displacement of the absorption maximum as has been extensively discussed in others papers for the internally and externally adsorbed monomer^.^^-^ Moreover, the spectral shift to lower energies of the internal monomer of the RGG-Hec system with respect to the R6G-LapB system (octahedrally substituted clays)would suggest that the physical-chemical properties ofthe water in the interlamellar space of the clay would be more extensively modified in clays with longer particle size.
In relation with the absorption maxima of R6G dimers adsorbed on clay, the exciton predicts that the two absorption bands of the dimer have to be centered a t both sides (lower and higher energies) of the monomeric absorption band when the dipole-dipole interaction between the two monomers in the dimer is much more important that the van der Waals forces. This is the case for the monomer and dimer of R6G in aqueous solution as well as when they are adsorbed on the external surface of the studied clays (Table 2). However, for the case of internally adsorbed species, both absorption maxima of the dimer are situated at higher energies than the monomer absorption maximum (Table 2). Van der Waals interactions would become more important in the interlamellar dimer than in the waterlclay aggregate since the constricted dimensions of the interlamellar space produce a more compact dimer with a lower interchromophore distance. Moreover, higher order approximation and other interaction potentials (;.e.dipole-quadrupole) should be taken into account in the exciton model for smaller distance,
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Conclusions The metachromatic effect observed in the absorption spectra of rhodamine 6G adsorbed on several smectite clay types (laponite B, montmorillonite of Wyoming, hectorite, and saponite) is attributed to the dimerization of the dye when it is adsorbed on both the external and in the interlamellar surfaces of the clay. These dimers have a sandwich type structure with the xanthene planes in a prallel disposition twisted around 80" and a t a n interchromophoric distance between 3.5 and 5.5 A, the shorter separation corresponding to the dimer adsorbed on the internal surface. The monomeric form of the rhodamine 6G can also be adsorbed on the external and the internal surfaces. The concentration of these adsorbed species depends on several factor such as the relative dyelclay concentration, the stirring time, and the nature of the clay. In most of the systems, the higher the relative dyelclay concentration, the higher the dye aggregation. The increase of the stirring time leads to an increase in the adsorption on the interlamellar space orland to a n increase in the dimerization of the dye. Furthermore, the interlamellar species are better observed when the clay particle size is short, whereas it is partially prevented in clays with tetrahedral substitutions. The spectroscopic characteristics of the monomers depend on the nature of the charge on the clay layers, i.e., a shift to smaller energies is reported for tetrahedral with respect to octahedral clays. The monomeric form of R6G adsorbed in the interlamellar space absorbes a t lower energies with respect to the externally adsorbed monomer, but in the case of the dimer the spectral shift is just the opposite. The micropolarity of the internal environment can explain the experimental results for the monomers species whereas the restriction of the interlamellar space would justify the results for the dimer.
(49) Fripiat, J.J.;Letellier, M.;Levitz, P. Phil. Trans.R . SOC. Land. 1984, A311, 287. (50) Lubetkin, S. D.; Middleton, S. R.; Ottewill, R. H. Phil. Trans. R . SOC.Land. 1984,A311,353.
Acknowledgment. We would like to thank Universidad del Pais Vasco - EHU for financial support and Gobierno Vasco for awarding a grant t o M.J.T.E.
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(51)Touillaux, R.; Salvador, P.; Vandermeersche, C.; Fripiat, J. J. Isr. J . Chem. 1968, 6 , 337.
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