4566
Langmuir 1998, 14, 4566-4573
The Hydrophobic Effect on the Adsorption of Rhodamines in Aqueous Suspensions of Smectites. The Rhodamine 3B/Laponite B System F. Lo´pez Arbeloa,* J. M. Herra´n Martı´nez, T. Lo´pez Arbeloa, and I. Lo´pez Arbeloa Departamento Quı´mica Fı´sica, Universidad Paı´s Vasco EHU, Apartado 644, 48080-Bilbao, Spain Received December 9, 1997. In Final Form: May 4, 1998 Electronic absorption and fluorescence spectroscopies are applied to study the adsorption of rhodamine 3B dye on Laponite particles in aqueous suspensions. The adsorption on the clay favors the aggregation of the dye. The monomer and aggregates of rhodamine 3B can be adsorbed on both the external and the internal surfaces of Laponite, and their presence depends on the loading of the dye on the clay and the aging of the samples. The aggregation in the interlamellar space leads to the dimer and the trimer (or higher aggregates) of rhodamine 3B with a head-to-tail geometry. The comparison of the present results with those previously reported for the rhodamine 6G/Laponite system demonstrates the effect of the hydrophobicity on the adsorption of dyes in Laponite which affect not only the tendency of the dye to self-associate but also the geometric structure of the aggregates.
Introduction Clay minerals have a lamellar structure with important colloidal properties.1 Most of their applications are based on the possibility of adsorbing a multitude of organic compounds.2 Thus, clays play a fundamental role in agriculture (retention of humus), decontamination (treatment of wastewaters), and catalysis (e.g. photoredox reactions).3 The adsorption is mainly performed by a cation exchange mechanism, but the incorporation of amphiphibic molecules improves the hydrophobic character of clay surfaces, enhancing the adsorption of apolar molecules.4 Electronic UV-vis absorption and fluorescence techniques can be applied to study the colloidal properties of microheterogeneous systems in general5,6 and of clays particles in aqueous suspensions in particular,7,8 using aromatic compounds as probe molecules. For instance, from changes in the photophysical properties of dyes adsorbed on clays, two different adsorption environments have been spectroscopically characterized in intercalated clay systems such as smectites. These two adsorption surfaces are identified with the water/clay interface (external surface) and the interlamellar space (internal surface) of the tactoidal structure of smectites,9 which is formed as a consequence of the stacking of clay layers. In * To whom correspondence should be addressed. Fax: +34-4464 85 00. E-mail:
[email protected]. (1) Van Olphen, H. An Introduction to Clay Colloid Chemistry, Wiley: New York, 1977. (2) Murray, H. H. Appl. Clay Sci. 1991, 5, 379. (3) Laszlo, P. Acc. Chem. Res. 1986, 19, 121. (4) Lyklema, J. Prog. Colloid Polym. Sci. 1994, 95, 91. (5) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987. (6) Thomas, J. K. The Chemistry of Excitation Chemistry, ACS Monograph 181; American Chemical Society: Washington, DC, 1984. (7) Cenens, J.; Schoonheydt, R. A.; De Schryver, F. C. Spectroscopic Characterization of Minerals and Their Surfaces; Coyne, L. M., Mckeever, S. W., Blake, C. F., Eds.; American Chemical Society: Washington, DC, 1990; Chapter 19. (8) Lo´pez Arbeloa, F.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I. Trends Chem. Phys. 1996, 4, 191. (9) Newmann, A. C. D. Chemistry of Clays and Clay Minerals: Longman Sci. Techn. Min. Soc.: London, 1987.
a complementary way, the photophysical and photochemical properties of aromatic compounds in intercalated systems can be modified in such a way as to have useful applications in optical molecular devices.10 On the other hand, the adsorption of organic dyes on clays can cause the substitution of the principal absorption band of the dye by other new bands placed at higher energies,11 the so-called metachromasy effect. This metachromasy has been ascribed to two different processes: (i) the aggregation of the dye molecules when they are adsorbed on the clay surfaces;8,12,13 and (ii) an interaction between the π-electron of the aromatic dye and the electron lone-pairs of the oxygen atoms of the clay surfaces.14 This last interaction does not occur for rhodamine dyes,15 probably due to the geometric restriction of the carboxyphenyl group of rhodamines which is nearly perpendicular to the xanthene ring (Figure 1). The metachromasy of dyes is also observed in homogeneous solutions by increasing the dye concentration and has been attributed to the aggregation of the dye molecules.16 Rhodamine dyes are considered good probe molecules to study clay particles in aqueous suspension for several reasons: (i) they are cationic dyes and can be adsorbed on clays by a cation exchange mechanism; (ii) they have strong absorption and fluorescence bands in the visible spectral region; (iii) their photophysics depend on environmental factors;17,18 and (iv) the aggregation in homogeneous solutions is a well-documented phenomenon.19-21 In previous papers, the adsorption of rhodamine 6G (R6G) (10) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399. (11) Bergman, K.; O’Konski, C. T. J. Phys. Chem. 1963, 67, 2169. (12) Cenens, J.; Schoonheydt, R. A., Clay Clay Miner. 1988, 36, 214. (13) Gessner, F.; Schmitt, C. C.; Neumann, M. G. Langmuir 1994, 10, 3749. (14) Cohen, R.; Yariv, S. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1705. (15) Grauer, Z.; Malter, A. B.; Yariv, S. Colloid Surf. 1987, 25, 41. (16) Rohatgi-Mukkerjee, K. K. Ind. J. Chem. 1992, 31A, 500. (17) Lo´pez Arbeloa, F.; Lo´pez Arbeloa, T.; Tapia Este´vez, M. J.; Lo´pez Arbeloa, I. J. Phys. Chem. 1991, 95, 2203. (18) Lo´pez Arbeloa, F.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I. Trends Photochem Photobiol. 1994, 3, 145. (19) Ruiz Ojeda, P.; Katime, I. A.; Ochoa, J. R.; Lo´pez Arbeloa, I. J. Chem. Soc., Faraday Trans.2 1988, 84, 1.
S0743-7463(97)01350-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/03/1998
The Rhodamine 3B/Laponite B System
Figure 1. Molecular structure of rhodamine 3B.
on clays has been studied in a series of smectites:22-26 Laponite, hectorite, montmorillonite, and saponite. Generally, the adsorption leads to the dimerization of the dye and both the R6G monomer and the R6G dimer can be adsorbed in the interlamellar space and at the water/clay interface. The predominance of these species depends on the dispersion of clay particles in the aqueous suspensions. Factors such as the nature of the clay (affecting the accessibility of the interlamellar space of the tactoids), the particle size, the clay concentration, the loading of dye and the clay surface, and the aging of the samples, affect the degree of dispersion of the clay particles in water.8 The dimer of R6G on smectites (in both the external and internal surfaces) is reported to be a sandwich-type aggregate in which the xanthene rings are disposed in parallel planes. In this paper the adsorption of rhodamine 3B (R3B) on Laponite (Lap) is reported. This dye, with diethylamino groups in its molecular structure (Figure 1), is a more hydrophobic dye than R6G (with monoethylamino groups), and the comparison of the results between R3B/Lap and R6G/Lap systems would demonstrate the effect of the hydrophobicity on the adsorption of dyes on clays. Laponite clay is chosen because this smectite is characterized by a high degree of dispersion in water giving very stable colloidal suspensions;27 it is a very pure clay with a very small particle size, and its content in iron, which is a potential fluorescence quencher of many aromatic compounds,28 is very low. Experimental Section Laponite B (Lap) was supplied by Laporte Ltd (England). This synthetic hectorite, characterized by a high grade of purity and a small particle size ( 2.27 g/L) in order to minimize the reabsorption and reemission effects on the fluorescence spectra.30 The dye/clay suspensions were aged by magnetic stirring, and the time from sample preparation to sample registration was taken into account in all spectral measurements. The stirring time does not affect the photophysics of diluted solutions of R3B in water. To compare the present results with those previously reported for the adsorption of rhodamine 6G (R6G) on Laponite,22,23 the effect of loading had been also studied for a fixed stirring time of 11/2 hours, the same aging as that used in the R6G/Lap system. The adsorption of R3B on Lap surfaces in the concentration range of this study (up to 100% CEC) is complete, as has been confirmed by different techniques.8 Absorption and fluorescence spectra were registered in a Cary4E spectrophotometer and a Shimadzu RF-5000 spectrofluorimeter, respectively, with 1-cm optical pathway polystyrene cuvettes. The absorption spectra were recorded using the same clay suspension but without dye in the reference beam. The fluorescence emission was registered by exciting the samples at 530 nm and the fluorescence quantum yield was evaluated using a diluted R3B aqueous solution as reference (φr ) 0.26 at 20 °C). The scattering of the excitation light by the clay particles was also recorded.
Results and Discussion The adsorption of R3B on Lap leads to changes in the absorption and fluorescence spectra of the dye in the whole loading range (from 0.06 up to 100% CEC) and stirring range (up to 1 week). For very low loading of R3B on Lap, the absorption and fluorescence bands of R3B shift to lower energies and are slightly more intense than those in a homogeneous water solution, Figure 2. These changes are observed immediately after sample preparation and the adsorption is considered to take place in a very short time scale (less than seconds) by a cation exchange mechanism although other forces (e.g. hydrophobic interactions) can participate in the adsorption process, as is discussed below. The shape of the absorption spectra of R3B on Lap changes with the relative dye/clay concentration and with the stirring time of the samples (Figure 3). These changes consist of a substitution of the main absorption band centered at 567 nm by a new absorption band at lower wavelengths. Although the diminution in the 567 nm band could be due to a photochemical or a photomechanical decomposition of the dye, the metachromasy effect observed in a multitude of dye/clay systems has been attributed to the dye aggregation12,13 or an interaction between the π-system of the dye and the electron lone pairs of the clay surface.14 This last interaction can be ruled out in the rhodamine/Laponite system since it takes (30) Lo´pez Arbeloa, I. J. Photochem. 1980, 14, 97.
4568 Langmuir, Vol. 14, No. 16, 1998
Lo´ pez Arbeloa et al.
Figure 2. Absorption and normalized fluorescence spectra of rhodamine 3B (1 × 10-6 M) in water (dashed curves) and in 2.2 g/L Laponite aqueous suspension (0.06% CEC) recorded just after sample preparation (solid curves). Figure 4. Evolution of the wavelength of the fluorescence maximum (b) and the fluorescence quantum yield (2) of rhodamine 3B on Laponite with the relative dye/clay concentration (logarithm scale) for samples stirred during 11/2 h.
Figure 3. Absorption spectra of rhodamine 3B (2 × 10-6 M) on Laponite for different relative dye/clay concentrations and stirring times: (a) loading ) 0.1% CEC and aging ) 90 min, (b) loading ) 5% CEC and aging ) 1 day; (c) loading ) 70% CEC and aging ) 1 day; (d) loading ) 70% CEC and aging ) 1 week.
place only in clay with tetrahedral substitution31 of Si4+ by Al3+, which is not the case for Laponite, and because it does not occur in rhodamines.15 (The o-carboxyphenyl group, which is nearly perpendicular to the xanthene plane, could prevent such an interaction). So, the metachromasy in Figure 3 is assigned to the selfassociation of R3B molecules when they are adsorbed on the clay surface. The high tendency of dyes to selfassociate in the presence of clays (observed for dye concentrations as low as 10-6 M) is attributed to an increase of the local dye concentration on the clay surface and/or to a reduction of the thermal motion of the aggregate adsorbed on the clay which decreases the dissociation tendency of the aggregate. The aggregation of R3B when it is adsorbed on Lap is confirmed by fluorescence measurements. Figure 4 shows the evolution of the emission wavelength and quantum yield with the relative dye/clay concentration for a common stirring time of 11/2 hours. The shape and the position of the fluorescent band do not change with the dye/clay concentration for loading 20% CEC) of Laponite R3B/Lap λab (nm) λfl (nm)
R3B/water
< 12% CEC
>20% CEC
558 578
567 584
572
R3B in water can be attributed to changes in the polarity and acidity of the environment,8,31 although changes in the specific dye/surrounding interactions can also contribute to the spectral shifts.18 The shape of the absorption spectrum of R3B monomer in Lap is not identical to that in water. The adsorption of the dye molecules to the clay surface probably causes modifications in the vibrational structure of the electronic transition, leading to changes in the shape of the absorption band. The fluorescence spectrum of R3B monomer in Lap is broader with respect to that in water (Figure 2). The experimental conditions in which the monomer of rhodamines is the only adsorbed species are more restrictives in the R3B/Lap system (loading < 0.2% CEC and stirred < 11/2 min) than in the rhodamine 6G/Laponite system (loading < 0.5% CEC and aging < 90 min22). The hydrophobicity of the diethylamino groups of R3B with respect to that of the monoethylamino groups of R6G should be responsible for the higher tendency of R3B than R6G to aggregate. Thus, the R3B molecules self-associate in order to minimize their contact with water molecules. Similar conclusions were derived from the aggregation of rhodamines in homogeneous solution,20,21 where the dye aggregation decreases drastically in a “good solvent” such as ethanol (the aggregation is observed for dye concentrations > 10-2 M) with respect to that in water (where the dimer is reported for samples with rhodamine concentrations as low as 10-4 M). The adsorption of rhodamine monomers on smectites is better studied by the fluorescence spectroscopy than by the absorption technique,8 since the aggregates of rhodamines do not emit. As Figure 4 shows, the emission maximum of R3B in Lap does not change in the 0.06-12% CEC loading range, suggesting that the monomer above characterized by absorption spectroscopy in the loading region 20% CEC the emission band does not further shift with loading (Figure 4). These results indicate the presence of a second clay environment for the adsorption of R3B monomer, whose presence cannot be confirmed by absorption spectroscopy because of the metachromasy effect. Due to the fact that smectites can form tactoids,9 these results suggest that the adsorption of the R3B monomer can take place at the interlamellar space (internal surface) and in the clay/water interface (external surface) of Laponite. Taking into account that the formation of tactoids is favored in concentrated clay suspensions, the monomer characterized at low loading (i.e. 0.019 g/L) would correspond to the R3B monomer adsorbed at the interlamellar space, whereas that observed for high loading (i.e. >20% CEC, [Lap] < 0.011 g/L) could be ascribed to the R3B monomer adsorbed in the water/Lap interface. Table 1 lists the absorption and fluorescence maxima of the R3B monomer adsorbed in the internal and external surfaces of Lap and in water, for comparison. The
absorption and fluorescence bands of R3B in the internal surface of Lap are shifted about 6-9 nm to lower energies with respect to those values in water, whereas the fluorescence band of the R3B monomer at the external surface shows an opposite shift of 6 nm. These results are qualitatively similar to those observed in the R6G/ Lap system with respect to R6G in water,22,23 and confirm the different polarity/acidity of both adsorption surfaces of Lap. In fact, Yariv et al.31,33 have proposed the use of spectroscopic data of dye molecules adsorbed on clay to determine the polarity and the acidity of clay surfaces, properties which depend mainly on the type and extension of the isomorphic substitution, as has been proven by the spectral shifts of R6G adsorbed on a series of smectites.8,34 ii. Aggregation of R3B on Lap for Low Loading (1.14 g/L, loading < 0.2% CEC) by increasing the aging of the samples consists of a diminution of the main monomeric absorption band at 567 nm and the slight increase in the absorbance at 530 nm leading to an isosbestic point around 545 nm (Figure 5A). Given the low relative dye/clay concentration, one can assume that this metachromasy is due, in the first stage, to the dimerization of the dye. From the evolution of the ratio between the absorbance at 567 and 530 nm, the absorption spectrum and the equilibrium constant (KD) of the dimer can be evaluated, following the method described previously.35 The calculated absorption spectra for the aggregate obtained for the 0.06, 0.10, and 0.15% CEC samples have two absorption bands, and their shapes are nearly independent of the relative dye/clay concentration and of the aging, indicating the viability of the method and that the dimer is the only aggregate formed for loading 20% CEC)
λab (nm)
U (cm-1)
R (Å)
θ (deg)
530/581 531/582
-828 -825
9.2 9.2
82 78
a D , dimer characterized in the interlamellar space of laponite 1 tactoids (loading < 0.2% CE); D2, dimer adsorbed at the water/ laponite interface (loading > 20% CEC).
The geometric parameters for the R3B/Lap dimer obtained from the spectrum of Figure 5B are summarized in Table 2. The results suggest a dimer in which the two monomers are disposed with the xanthene rings in the same plane, at an intermolecular distance of about 9.2 Å and an angle between the dipole moment of the chromophores of around 82°. This dimer is considered to be adsorbed at the interlamellar space of Lap since it is observed in concentrated clay suspensions and because it is in equilibrium with the monomer adsorbed in the internal surface. The head-to-tail dimer of R3B adsorbed in the interlamellar space of Lap contrasts with the sandwich-type aggregate reported previously for the R6G dimer in the interlamellar space of Lap.22 This different dimer geometry could be due, once again, to the different hydrophobicity of both dyes. Thus, to avoid the water molecules, the disposition of the R3B molecules adsorbed on Lap surface could be such that the dye-clay contact is as high as possible (the highest coverage of the clay surface by R3B molecules), in other words, with the xanthene ring parallel to the clay surface, as has been previously
proposed for rhodamines adsorbed on clay.39 With this arrangement of the R3B molecules, the head-to-tail dimer is favored for the R3B/Lap system. Similar hydrophobic effects are observed for the aggregation of rhodamines in homogeneous solutions.19-21 The aggregates of rhodamines in water have a sandwich structure (a compact geometry in which the contact with solvent molecules is reduced), while head-to-tail type aggregates are formed in ethanol (a “good solvent” for rhodamines). A metachromasy different from that shown in Figure 5A is observed by increasing the relative dye/clay concentration or by increasing the stirring time in the 0.212% CEC loading range (clay concentration from 1.14 to 0.019 g/L). As Figure 6A shows, the diminution in the intensity of the monomer absorption band and the increase in the absorbance at about 505 nm by increasing the loading for a constant stirring time leads to a new isosbestic point at 515 nm. Since at this loading range (20% CEC). For samples with loading >20% CEC (clay suspensions < 0.011 g/L), the R3B molecules are adsorbed at the water/Lap interface, as fluorescence results suggest (Figure 4), since the formation of tactoids in Lap is negligible.27 The metachromasy observed in the absorption spectrum for samples with loading >20% CEC (i.e. for the 100% CEC with the stirring time, Figure 7A) indicates the aggregation of R3B adsorbed in the external surface of Lap. The absence of a clear isosbestic point could suggest the existence of more than one aggregate species. The aggregation for loading >20% CEC cannot be adequately studied because the absorption spectrum of the corresponding monomer is not experimentally observed due to the metachromasy. However, to obtain an approximate absorption spectrum of the aggregates at the water/Lap interface, one could consider the absorption spectrum of the R3B monomer adsorbed on the external surface to be the same as that observed for the R3B monomer in the interlamellar space but shifted to higher energies. Taking into account that the absorption and emission bands of the R6G monomer on the external surface are placed 5 and 10 nm at lower wavelength, respectively, with respect to those of R6G monomer in the interlamellar space of Lap22,23 and that the R3B monomer at the water/ Lap interface emits 12 nm at lower wavelength than that on the internal surface, the absorption band of the R3B monomer on the external surface is considered to be centered at 561 nm (6 nm at lower wavelength than the internally adsorbed monomer). This shift seems to be
reasonable since it is close to the experimental absorption maximum for samples with loading > 20% CEC. The calculated absorption spectra for the 100% CEC sample using this shifted absorption band for the monomer are shown in Figure 7B. For aging the sample not longer than 1 h, the calculated absorption spectra (Figure 7B(a,b)) have two absorption bands centered at around 531 and 582 nm. This absorption spectrum points out the dimerization of R3B molecules at the water/Lap interface. However, the increase in the stirring time above 1 h causes a diminution in the absorption bands of the dimer and a new absorption band appears at higher energies (Figure 7B(c)), indicating the formation of higher aggregates of R3B on the external surface of Lap. The existence of an isosbestic point in the calculated absorption spectra would suggest that the higher aggregate formed at high aging is in equilibrium with the dimer. The calculated absorption spectrum of the R3B aggregates on the external surface of Lap have to be considered as illustrative since the absorption spectrum of the monomer is not exactly known. The shape of absorption spectrum of the dimer of Figure 7B(a), with the most intense band at lower energies, would suggest a head-to-tail structure with geometric parameters derived from eqs 1-3 which are listed in Table 2: an intermonomeric distance of around 9.2 Å and an angle between the monomeric dipole moments of around 78°. This dimer seems to be very similar to that obtained in the interlamellar space (Table 2), probably because the limitation of the interlamellar space would not drastically affect on the absorption and geometric parameters of an “aligned” dimer, as is the case of R3B dimers. On the contrary, this is not the case for a sandwich-type dimer (R6G/Lap system) where the restriction of the interlamellar space leads to a more constricted dimer of R6G in the interlamellar space than that at the water/Lap interface,22,23 affecting not only the geometric parameters but also the absorption maxima of R6G dimer. The aggregation of the dye on the external surface of Lap increases with the relative dye/clay concentration and the stirring time. A decreasing of the clay concentration (to increase the loading for a constant dye concentration)
The Rhodamine 3B/Laponite B System
reduces the area of adsorption and increases the coverage of R3B molecules on the Lap surfaces, favoring the dye aggregation. The aging also favors the dye aggregation, probably due to a dye-dye interaction, to which hydrophobic forces can also contribute. Conclusions Absorption and fluorescence spectroscopies using R3B as a probe molecule are adequate techniques to observe the behavior and the characteristics of both the clay suspension and the dye molecules. The adsorption of R3B molecules on Laponite particles in aqueous suspensions leads to the aggregation (dimer and higher aggregates) of the dye in the interlamellar space and at the water/clay interface. Internally adsorbed species are observed for concentrated clay suspensions ([Lap] > 0.019 g/L, loading < 12% CEC) while in diluted clay concentrations ([Lap]
Langmuir, Vol. 14, No. 16, 1998 4573
< 0.011 g/L) the R3B species are adsorbed on the external surface. In both environments, the aggregation of the dye is enhanced by increasing the relative dye/clay concentration and the aging of the samples. The hydrophobicity of the dye plays an important role not only in the extension of the self-association (increase in the tendency to aggregate and in the formation of trimer and/ or higher aggregates) but also on the type of structure of the aggregates. Thus, the formation of head-to-tail aggregates is favored in the R3B/Lap system in detriment to the sandwich-type dimer characterized in the R6G/Lap system. Acknowledgment. The authors want to thank Universidad del Paı´s Vasco (EHU) for financial support (Project 039.310-EA053/96). LA971350A
