Langmuir 1993,9, 3629-3634
3629
Absorption and Fluorescence Properties of Rhodamine 6G Adsorbed on Aqueous Suspensions of Wyoming Montmorillonite M. J. Tapia EstAvez, F. Lbpez Arbeloa, T. Lbpez Arbeloa, and I. Lbpez Arbeloa' Departamento Qulmica-Fhica, Uniuersidad del Pats Vasco-EHU, Apartado 644, 48080-Bilbao, Spain Received June 21,1993. In Final Form: September 2 0 , 1 9 9 9 The adsorption of Rhodamine 6G (R6G) in aqueous suspensions of Wyoming montmorillonite (WyM) is investigatedby electronicabsorption and emission spectroscopies. The loadingrange studied is between 0.1% and 100% of the cation exchange capacity. The monomer of Rhodamine 6G can be adsorbed on the external and on the internal surfaces of the clay. Experimental results show that the increase of stirring time favors the monomer migration from the external to the internal surface on a time scale of days, mainly for low relative dyelclay concentrations. The metachromatic effect observed for the R6Gl WyM system is attributed to the dye aggregation, which depends not only on the relative dyelclay concentration but also on the dispersion degree of the clay suspension.
Introduction In the last few decades, many monographic studies have been made about clay applications. Currently a great number of works on this subject appear recording the wide field of clay utilization, which is becoming increasingly important, principally owing to the abundance and low cost of the clay materials. Industrial,l environmental,2or catalytiGp4 uses can be pointed out. Many of these applications are based on clay/organic interactions which have also been thoroughly investigated.kT Dyelclay systems are particularly interesting since UVvis absorption and fluorescence spectroscopies are appropriate techniques to study clay/organic systems. Moreover, these techniques can give considerable insight into the research of dilute clay aqueous suspensions when the organic molecules are adsorbed at very small loadings.gl0 In these conditions, traditional techniques such as X-ray diffraction, infrared spectrscopy, or 13C, 27Al,and 'H nuclear magnetic resonancell are very limited. Electronic spectroscopy studies can give information about the orientation of dye molecules at the clay surfaces, the interactionbetween the *-system of the dye and the oxygen lone pair of clay surfaces, the dye aggregation at the clay surface, the internal and external clay surfaces, In this work, the adsorption of Rhodamine 6G in aqueous ~~
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.Abstract published in Advance
ACS Abstracts, November 1, 199s. ( 1 ) Murray, H. H. Appl. Clay Sci. 1991,6, 379. (2) Schomburg, J. Proceedings of the 7th Euroclay Conference; Greifawald: Dresden, 1991. (3)V e d e , F.h e e d i n g 8 of the Sthlnternational Clay Conference; Sci. W L MBm.: Stranbourg, 1990. (4)Barrer, R. M. Philos. Tiann. R. SOC.London 1984, A311,333. (5) Mortland, M. M. Adv. Agron. 1970,22, 75. (6) Theng, B. K. G. The Chemistry of Clay-Organic Reactionn, let ed.; Adam Hilge~London, 1974; Chapter 5. ('7) Lagaly, G. Philos. Trans. R. Soc. London 1984, A311,315. (8) Viene, K.: Caiaui, J.: Schoonheydt, R. A.; De Schryver, F. C. Lor&uir 1987,3, 10% (9) &ne8111), J.; Schoonheydt, R. A. c&yS Clay Miner. 1988,36,214. (10) &nema, J.; Schoonheydt, R. A.; De Schryver, F. C. Spectroscopic chtaracterizatwn of Minerale and Their Surfacee: American Chemical Society: w a s h g k , DC,1990, Chapter 19.. (ll)TennaLoon, D. T. B.; Schl6g1, R.; Flay", T.; Klinoweki, J.; Jones,W.; Thouma,J. M. Clay Miner. 1983,18,357. (12)Findo,T.;Nakada,N.;Sato,T.;Shimada,M. J. Phys. Chem. Solids
----. (13)Endo, T.;Nakada,N.;Sato,T.;Shimada,M. J. Phys. Chem. Solids 1988.49.1423. -~---I
1989,M), 133. (14)Yariv, 5.Znt. J. !hop. Agric. 1988, VI, 1.
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Figure 1. Molecular structure of Rhodamine 6G.
suspensions of Wyoming montmorillonite is studied. This clay is dioctahedral, showing not only substitutions in octahedral sites of A13+ for Mg2+ but also tetrahedral substitutions, which produce 15-50% of the total layer charge.17 The metachromatic effect (replacement of the principal absorption band by a band at higher energies) observed in the absorption spectrumof some dyes adsorbed on clays with tetrahedral substitution has been attributed to an interaction between the electron mystem of the dye and the electron lone pair of the oxygen atom at the clay surface.14 However, this interaction does not seem to be produced for Rhodamine dyes, since these dyes have an o-carboxyphenyl group nearly perpendicular to the xanthene plane skeleton (Figure 1)which sterically preventa this interaction.l4J*Jg The metachromasy observed in the Rhodamine 6GILaponite B system has therefore been attributed to the dye aggregation.16J6 (15)Tapia EetBvez,M. J.; U p e z Arbeloa, F.;Upez Arbeloa, T.;Upez Arbeloa, I.; Schoonheydt, R. A. Clay Miner., in prem. (16) Tapia EetBvez,M. J.; Upez Arbeloa,F.; Upez Arbeloa,T.;L6pez Arbeloa, 1. J. Colloid Interface Sei., in prese. (17) Newman, A. C. D. Chemistry of Clays and Clay Minerale, let ed.; Longman ScienWic & Technical Mineralcgical Society: London, 1987; Chapter 1. (18) Yariv, 5.;Nasser, A,; Bar-on,P. J. Chem. Soc., Faraday Trone. 1990,86, 1593. (19)Grauer, Z.; A&, D.; Yariv, S. Can. J. Chem. 1984,62, 1889.
Q743-7463/93/24Q9-3629$Q4.oO/QQ 1993 American Chemical Society
Tapia Estkvez et al.
3630 Langmuir, Vol. 9, No. 12, 1993 The goal of this work is to investigate the adsorption of Rhodamine 6G in Wyoming montmorillonite, analyzing the external and internal surfaces, studying the effect of the sample preparation, and discussing the different dye/ clay rearrangements with the loading and the probable influence of the tetrahedral substitution of the clay.
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Experimental Section Sodium montmorillonite of Wyoming (WyM) was supplied by Clay Minerals Society (U.S.A.) and was saturated in Na+ with NaC1. The 30% CEC in nonfiltered samples coincides with the flocculation of the sample which is observed with the naked eye in this percent CEC range. Influence of Sample Stirring Time. Since the stirring time of the samples has a strong influence on the spectroscopic characteristics of dye/clay suspensions, as was clearlyestablished in a previousstudy of R6G adsorbed on Laponite B,16the influence of this factor on the R6G/ WyM system is now treated. In order to clarify this discussion, the results obtained for 0.4 !% , 7% ! ,20%, and 50% CEC loadings are considered. For these suspensions the dye concentration is kept constant and a t 1o-S M and the clay concentration is that necessary to obtain the desired percent CEC, Le., 3.27 X lpl, 1.87 X 1k2, 6.54 X 103, and 2.62 X 103 g/L, respectively. Figure 2 shows the absorption spectra of the 0.4 % CEC
Rhodamine 6G Adsorbed on Wyoming Montmorillonite
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Langmuir, Vol. 9, No. 12,1993 3631
2), a molecular migration of the adsorbed R6G monomer from the external to the internal surface of the clay for loadings of
