Effect of Laponite and a Nonionic Polymer on the Absorption

J. Bernhard Wehr , F. Pax C. Blamey and Neal W. Menzies. Journal of Agricultural and Food Chemistry 2010 58 (8), 4554-4559. Abstract | Full Text HTML ...
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Langmuir 2005, 21, 5825-5830

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Effect of Laponite and a Nonionic Polymer on the Absorption Character of Cationic Dye Solutions Koray Yurekli, Emily Conley, and Ramanan Krishnamoorti* Department of Chemical Engineering, University of Houston, Houston, Texas 77204-4004 Received October 5, 2004. In Final Form: February 7, 2005 The effect of Laponite and an amphiphilic triblock copolymer of poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO)-poly(ethylene oxide) (PEO) (PEO99-PPO65-PEO99, and labeled F127) on the absorption character of two cationic dyes, methylene blue and toluidine blue O, was studied and interpreted in terms of the changing state of aggregation of the dye molecules. The combined effect of the polymer and Laponite on dye absorption was significantly different from their individual influences. Specifically, the presence of Laponite resulted in an increase in monomer population by dispersing the dye on the silicate surface. The presence of F127 also resulted in an increase in the dye monomer population, although to a smaller extent. The combined effect of the polymer and Laponite was an increase in the dimer or aggregate populations attributed to the competition of F127 with the dye molecules for the silicate surface.

Introduction Layered silicates, such as Laponite, montmorillonite, saponite, and hectorite, are an active field of study due to their unique colloidal and rheological properties as aqueous dispersions1 as well as the useful properties they impart to polymers when blended.2 Layered silicate polymer nanocomposites have been found to exhibit improved chemical, mechanical, barrier, and thermal properties.2 The extent of the improvement in properties for the nanocomposites and rheological properties of aqueous solutions of layered silicates are strongly dependent on the dispersion of the silicates.3,4 As a result, a simple and effective way of determining the state of dispersion is an invaluable tool for predicting properties and evaluating processing methods. A potential method to probe layered silicate dispersions in solution and in polymer nanocomposites is the use of cationic dyes. Maupin et al.5 have recently demonstrated a laser-induced fluorescence spectroscopy method where the concentration quenching of fluorescence of Nile blue A can be used to distinguish between unmixed, intercalated, and exfoliated states of polymer-layered silicate composites. Numerous studies have also been reported on probing aqueous dispersions of layered silicates by metachromatic changes in the absorption spectra of dyes such as methylene blue, rhodamine 6G, thionine, acridine orange, and neutral red.6-12 Metachromasy in aqueous solutions of cationic dyes such as methylene blue (MB) and toluidine blue O (TBO), have * Corresponding author: e-mail [email protected]. (1) Fossum, J. O.; Gudding, E.; Fonseca, D. D. M.; Meheust, Y.; DiMasi, E.; Gog, T.; Venkataraman, C. Energy 2005, 30, 873-883. (2) (a) Giannelis, E. P.; Krishnamoorti, R.; Manias, E. Adv. Polym. Sci. 1999, 138, 107-147. (b) Krishnamoorti, R.; Vaia, R. A.; Giannelis, E. P. Chem. Mater. 1996, 8, 1728. (c) Ren, J.; Silva, A. S.; Krishnamoorti, R. Macromolecules 2000, 33, 3739-3746. (d) Krishnamoorti, R.; Yurekli, K. Cur. Op. Coll. Interf. Sci. 2001, 6, 464-470. (3) Ramsay, J. D. F.; Lindner, P. J. Chem. Soc., Faraday Trans. 1993, 89, 4207-4214. (4) Schmidt, G.; Nakatani, A. I.; Butler, P. D.; Karim, A.; Han, C. C. Macromolecules 2000, 33, 7219-7222. (5) Maupin, P. H.; Gilman, J. W.; Harris, R. H., Jr.; Bellayer, S.; Bur, A. J.; Roth, S. C.; Murariu, M.; Morgan, A. B.; Harris, J. D. Macromol. Rapid Commun. 2004, 25, 788-792. (6) Bergmann, K.; O’Konski, C. T. J. Phys. Chem. 1963, 67, 21692177. (7) Bujdak, J.; Iyi, N.; Fujita, T. Clay Miner. 2002, 37, 121-133. (8) Bujdak, J.; Komadel, P. J. Phys. Chem. 1997, 101, 9065-9068.

been reported as early as the 1940s.13 The metachromatic effect is characterized by a departure from Beer’s law with a decrease in intensity of the dye monomer (R) absorption band and concomitant appearance of one or more new bands as the dye concentration is increased.13 The new bands are attributed to the formation of dye dimers and trimers/higher aggregates (β and µ bands, respectively), and their location in the absorption spectrum depends on the nature of the aggregation and the resulting interaction of the π electron systems of the cations. Face-to-face aggregation (H-aggregates) results in electrostatic repulsion of dipoles and an increase in the excitation energy.6,8,14,15 The absorption, therefore, occurs at lower wavelength and with a lower probability of occurrence possesses a lower intensity.7,14 Head-to-tail aggregation (J-aggregates) has the opposite effect, with dipole attraction resulting in decreased excitation energies and a shift of the absorption maximum to higher wavelength.7,15,16 It has been documented that metachromasy occurs at considerably lower concentrations of the dye when anionic polyelectrolytes,14,17 zeolites,18 and layered silicates7-11 are present in the dye solution. (9) (a) Cenens, J.; Schoonheydt, R. A. Clays Clay Miner. 1988, 36, 214-224. (b) Cione, A. P. P.; Neumann, M. G.; Gessner, F. J. Colloid Interface Sci. 1998, 198, 106-112. (c) Jacobs, K. Y.; Schoonheydt, R. A. J. Colloid Interface Sci. 1999, 220, 103-111. (d) Neumann, M. G.; Schmitt, C. C.; Gessner, F. J. Colloid Interface Sci. 1996, 177, 495-501. (e) Ogawa, M.; Tsuijimura, M.; Kuroda, K. Langmuir 2000, 16, 42024206. (f) Ogawa, M.; Inagaki, M.; Kodama, N.; Kuroda, K.; Kato, C. J. Phys. Chem. 1993, 97, 3819-3823. (g) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399-438. (h) Ogawa, M.; Kawai, R.; Kuroda, K. J. Phys. Chem. 1996, 100, 16218-16221. (10) Gessner, F.; Schmitt, C. C.; Neumann, M. G. Langmuir 1994, 10, 3749-3753. (11) Jacobs, K. Y.; Schoonheydt, R. A. Langmuir 2001, 17, 51505155. (12) (a) Thomas, J. K. Acc. Chem. Res. 1988, 21, 275-280. (b) Usami, H.; Takagi, K.; Sawaki, Y. Bull. Chem. Soc. Jpn. 1991, 64, 3395-3401. (c) Bujdak, J.; Iyi, N.; Sasai, R. J. Phys. Chem. B 2004, 108, 4470-4477. (13) (a) Braswell, E. J. Phys. Chem. 1968, 72, 2477-2483. (b) Rabinowitch, E.; Epstein, L. F. J. Am. Chem. Soc. 1941, 63, 69-78. (14) Schubert, M.; Levine, A. J. Am. Chem. Soc. 1955, 77, 41974201. (15) Sunwar, C. B.; Bose, H. J. Colloid Interface Sci. 1990, 136, 5460. (16) (a) Higgins, D. A.; Reid, P. J.; Barbara, P. F. J. Phys. Chem. 1996, 100, 1174-1180. (b) Gvishi, R.; Narang, U.; Ruland, G.; Kumar, D. N.; Prasad, P. N. Appl. Organomet. Chem. 1997, 11, 107-127. (17) Pal, M. K.; Schubert, M. J. Phys. Chem. 1963, 67, 1821-1827. (18) (a) Calzaferri, G.; Gfeller, N. J. Phys. Chem. 1992, 96, 34283435. (b) Ehrl, M.; Kindervater, H. W.; Deeg, F. W.; Brauchle, C.; Hoppe, R. J. Phys. Chem. 1994, 98, 11756-11763.

10.1021/la047540k CCC: $30.25 © 2005 American Chemical Society Published on Web 05/27/2005

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These layered silicates, which are the subject of this study, are minerals belonging to the class of 2:1 smectites. Each layer consists of an octahedral aluminum or magnesium hydroxide sheet sandwiched by two tetrahedral silica sheets.19 The roughly circular disklike layers have diameters ranging from tens of nanometers to 10 µm and a thickness of ∼1 nm. These layers are stacked into tactoids with interlayer galleries of a few nanometers, which contain Li+, Na+, or Ca2+ cations that balance the negative charge resulting from isomorphous substitution in the octahedral or tetrahedral sheets. It is generally accepted that aggregation and arrangement of cationic dyes near the silicate surface are responsible for the enhanced metachromasy. Further, the dye cations rearrange over time, resulting in temporal changes to the absorption spectra.7,10,11 The mobility of the dye cations on the silicate surface depends on the relative strength of the dye-silicate versus dye-dye and silicate-silicate interactions.11 In the case of clays substituted in the octahedral layers, the negative charges are not localized (i.e., somewhat diffuse) on the surface of the silicate and the dye-surface interactions are relatively weak. This results in high mobility of the dye cations on the surface and evolution of the spectra over long periods of time. Laponite, despite being substituted in the octahedral layers, is an exception to this rule due to its smaller lateral diameter and superior dispersibility, allowing for a fast equilibrium of dye dispersion.11 The fact that the spectroscopic properties of dyes such as MB are highly dependent on layered silicate properties and dispersion makes them excellent potential probes. Equally intriguing is the ability to control the absorption and emission spectra of dyes by using small amounts of additives such as layered silicates or polymers. In this paper, we report the effects of Laponite and an amphiphilic triblock copolymer of poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO)-poly(ethylene oxide) (PEO99PPO65-PEO99-labeled F127) on the UV-vis absorption spectra of two cationic dyes, methylene blue (MB) and toluidine blue O (TBO). We examined this particular system because F127 has been used as a carrier for drug delivery systems and these dyes have been used as markers in F127 to quantify the release profile and erosion kinetics of F127.20 Laponite was the layered silicate of choice for this study because the fast kinetics allow us to probe equilibrium structures and not metastable states and their slow evolution to equilibrium as afforded by studies on other layered silicate-based systems. We find that the combined effect of the two additives is significantly different from their individual influences on dye absorption. Experimental Section F127 (BASF Corp.) is a symmetric PEO-PPO-PEO triblock copolymer with 25 mol % PPO and a Mw of 12 700. The critical micelle concentration (cmc) for the block copolymer in water is ∼10 g/dm3 at 25 °C.21 Poly(ethylene glycol) (Sigma-Aldrich Chemical Co.), used to understand the influence of micellization on the observed spectroscopic results, has a Mn of ∼8000. Laponite RD (Southern Clay Products) is a synthetic 2:1 smectite with a layer diameter of ∼30 nm. The isomorphous substitutions (Li+ for Mg2+) are in the octahedral layer and the interlayer counterions are Na+. The cationic dyes methylene blue (MB) and (19) Brindley, G. W.; Brown, G. Crystal Structure of Clay Minerals and their X-ray Identification; Mineralogical Society: London, 1980. (20) (a) Liu, V. A.; Jastromb, W. E.; Bhatia, S. N. J. Biomed. Mater. Res. 2002, 60, 126-134. (b) Anderson, B. C.; Pandit, N. K.; Mallapragada, S. K. J. Controlled Release 2001, 70, 157-167. (c) Moore, T.; Croy, S.; Mallapragada, S.; Pandit, N. J. Controlled Release 2000, 67, 191-202. (21) Lopes, J. R.; Loh, W. Langmuir 1998, 14, 750-756.

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Figure 1. Molecular structures of the two cationic dyes used in the study: (a) methylene Blue (MB); (b) toluidine Blue O (TBO). toluidine blue O (TBO) were obtained from Sigma-Aldrich Chemical Co. and their molecular structures are given in Figure 1. All materials were used as received, with no further purification. Master batches of dye solutions were prepared at various concentrations by dissolving MB and TBO in deionized H2O and storing in a refrigerator away from light until used. Appropriate amounts of the dye solution, Laponite, and F127 (or PEG) were mixed to achieve the desired concentrations of each in solution and allowed to stir overnight prior to measurement. UV-Vis spectroscopy was performed on the resulting dye solutions on a Jasco V-570 UV-vis-NIR spectrometer over a wavelength range of 200-1000 nm at 25 °C. The spectra were fit to a mixed Gaussian/Lorentzian model with a linear background by use of eXPFit v 1.10 for Microsoft Excel 97.22

Results and Discussion Figure 2 shows the concentration dependence of the UV-vis absorbance spectra for aqueous solutions of MB (4.3 × 10-6 to 1.7 × 10-4 mol/dm3) and TBO (5.1 × 10-6 to 1.7 × 10-4 mol/dm3). For MB (Figure 2a), the peak locations are assigned according to reported values6,8 for dye monomers, dimers, and higher aggregates. The monomer (R) and dimer (β) peaks are at 670 and 610 nm, respectively, while the trimers/higher aggregates (µ) result in a broad absorption maximum at ∼500 nm. The peak assignments for the TBO absorption are based on the MB peaks and are similar to those for rhodamine 6G-Laponite dispersions;23 they occur at 650 (R), 580 (β), and 490 nm (µ) for monomers, dimers, and higher aggregates, respectively (Figure 2b). The concentration where dimerization begins to occur has been reported to be 2.5 × 10-6 mol/dm3 for MB.7 With the available equipment, it was not possible to probe dye concentrations at or below this concentration. However, decreasing the dye concentration for both dyes results, as expected, in a decrease of the µ and β bands with respect to the R band, as well as an overall decrease in absorbance. The effect of the layered silicate and the polymer on the dye properties will be characterized on the basis of the changes to the intensities of these three bands and their wavelength shifts. It is noted that dimerization appears to be more prevalent for TBO than MB at each comparable concentration. The same trend is also observed for the experiments with dye solutions at all concentrations of Laponite and F127 discussed below. This difference in propensity to aggregate between TBO and MB is presumably attributable to the differences in the chemical structure of the two dyes and their interactions with the silicate. In the case of the aqueous dispersions of the dyes, we observe that TBO is capable of hydrogen bonding to itself by organizing in a head-to-tail manner and forming J-aggregates, while MB cannot organize in this manner and typically forms H-aggregates. The effect of the addition of various amounts of Laponite, ranging from 0.13 to 2.0 g/dm3, on the MB absorbance (MB concentration 1.7 × 10-4 mol/dm3, with largest (22) Nix, R. http://www.chem.qmul.ac.uk/software/eXPFit.htm. (23) (a) Estevez, M. J. T.; Arbeloa, F. L.; Arbeloa, T. L.; Arbeloa, I. L.; Schoonheydt, R. A. Clay Miner. 1994, 29, 105-113. (b) Martinez, V. M.; Arbeloa, F. L.; Prieto, J. B.; Lopez, T. A.; Arbeloa, I. L. J. Phys. Chem. B 2004, 108, 20030-20037.

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Figure 3. Dependence of the absorption spectra of aqueous (a) MB and (b) TBO solutions on the concentration of added Laponite (1, 2.0 g/dm3; 2, 1.0 g/dm3; 3, 0.5 g/dm3; 4, 0.25 g/dm3; 5, 0.1 g/dm3; 6, 0 g/dm3). For both dyes, higher concentrations of Laponite result in an increase in the number of monomers (R) while low Laponite concentrations result in significant amount of dimers (β) and/or higher dye aggregates (µ). The concentration of the dye is constant at 1.7 × 10-4 mol/dm3 for both MB and TBO solutions. Figure 2. Concentration dependence of the absorption spectra of aqueous dispersions of (a) MB (1, 1.7 × 10-4 mol/dm3; 2, 8.6 × 10-5 mol/dm3; 3, 4.0 × 10-5 mol/dm3; 4, 2.0 × 10-5 mol/dm3; 5, 1.1 × 10-5 mol/dm3; 6, 4 × 10-6 mol/dm3) and (b) TBO (1, 1.7 × 10-4 mol/dm3; 2, 8.7 × 10-5 mol/dm3; 3, 4.4 × 10-5 mol/dm3; 4, 2.2 × 10-5 mol/dm3; 5, 1.1 × 10-5 mol/dm3; 6, 5 × 10-6 mol/ dm3). Note that as the concentration decreases, the dimer (β) peak becomes smaller relative to the monomer (R) peak.

fraction of aggregated species) is shown in Figure 3a. For the highest concentration of Laponite, there is a significant increase in the dye monomer population (R-band) at the expense of the dimers (β-band). This disappearance of dimers is in agreement with the reported interactions of MB with layered silicate surfaces.10,11 The dye molecules are electrostatically attracted to the negatively charged silicate surface and thereby displace intergallery cations. At the highest concentration of Laponite, the surface charge density of the silicate (∼ 1 × 10-3 mol/dm3, based on the charge exchange capacity of Laponite and concentration of Laponite sheets as individual layers) is significantly greater than the MB concentration (1.7 × 10-4 mol/dm3). Under these conditions and due to the high mobility of the dye molecules, the dye cations spread out on the silicate sheets and form a large fraction of monomers that interact favorably (through ionic interactions and possibly hydrogen bonding) with the silicate. At lower concentrations of Laponite (i.e., lesser available surface area and hence charge density of Laponite per dye molecule), the dye coverage on the silicate layer becomes large enough that the dye molecules cannot individually and completely spread out on the sheets and enable monomer formation. In fact, at the lowest con-

centration of Laponite, the surface charge density of the silicate (∼7.0 × 10-5 mol/dm3) is approximately half the concentration of MB in solution (1.7 × 10-4 mol/dm3). As a result, for the lower concentrations of Laponite, there is a significant aggregate absorbance peak (µ ∼ 550 nm). For a TBO solution at the same concentration (Figure 3b), Laponite has similar qualitative effects on the absorption, with a large monomer (R) peak at high concentrations of Laponite and a growing µ peak indicating aggregation as the Laponite concentration decreases. Furthermore, the dimer peak decreases dramatically upon addition of Laponite and for all concentrations remains extremely small. We note that the TBO exhibits a higher propensity to aggregate into large aggregates as compared to MB in aqueous solutions, and this tendency is somewhat exaggerated by the addition of Laponite. These observations are corroborated quantitatively by monitoring the R, β, and µ band peak intensities as a function of the Laponite concentration (Figure 4). We note that examination of the peak area as opposed to peak intensity demonstrates the same qualitative trends (Supporting Information). For both MB and TBO solutions, the highest value for the µ peak intensity occurs for the solutions with the lowest nonzero dispersed Laponite concentration. The R peak on the other hand has its highest value for the solutions with the highest Laponite concentration. These data provide a quantitative basis for the analysis of available adsorption sites as a function of dye concentration detailed above. Additionally, the aggregation is more pronounced in the TBO solutions, presumably due to the propensity of TBO to self-aggregate,

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Figure 4. Evolution of the monomer (R), dimer (β), and higher aggregate (µ) peak areas as a function of the Laponite concentration for (a) MB and(b) TBO, showing the increase in monomers as the layered silicate concentration increases. In the case of MB solutions, the increase in the µ band is strongest for the lowest nonzero Laponite concentration. Note that since the extinction coefficients of the monomers, dimers, and higher aggregates are not the same, the peak intensity ratio does not equal to the number ratio of cations in different states of aggregation. The MB and TBO concentrations are 1.7 × 10-4 mol/dm3.

the formation of J-aggregates as opposed to H-aggregates for MB, and possibly due to differences in interactions with the silicate surface as compared to MB. The adsorption of the dye molecules on the layered silicate surface also affects the location of the absorption maximum. There is a small blue shift of the R-band (6-7 nm) in the high Laponite concentration MB solutions with respect to its location in the aqueous environment that contain no Laponite. Such blue shifts are typically observed in face-to-face aggregates (H-aggregates) resulting from electrostatic repulsion of the dipoles. Further, this blue shift indicates a more hydrophobic environment for the dye monomer and has been documented for layered silicate-MB systems where the layered silicate, like Laponite, has isomorphous substitutions in the octahedral layer and the resulting negative charges are diffuse at the exposed surface. The TBO-Laponite solutions, on the other hand, display a small (∼10 nm) red shift for the monomer absorption wavelength, indicative of a perturbation to the head-to-tail aggregation (J-aggregates) and the presence of a strong polar environment. On the basis

Figure 5. Dependence of the absorption spectra of aqueous (a) MB and (b) TBO solutions on the concentration of added F127 (1, 40 g/dm3; 2, 20 g/dm3; 3, 10 g/dm3; 4, 5 g/dm3). For both dyes, higher concentrations of F127 result in an increase in the number of monomers (R) and a concomitant decrease in the dimer (β) band. (c) Comparison of the effect of F127 and the homopolymer PEO on the absorbance spectra of TBO and MB (1, 40 g/dm3 F127 with TBO; 2, 40 g/dm3 PEO with TBO; 3, 40 g/dm3 F127 with MB; 4, 40 g/dm3 PEO with MB). There is virtually no difference in the absorption characteristics between the PEO and the F127 samples, suggesting that effects observed do not result from the micellization (and the consequent sequestration of the dyes) of the block copolymer.

of the differences in chemical structure of the two dyes, we anticipate that the interaction of the TBO molecule

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Figure 6. (a) Comparison of the two three-component mixtures with the highest (1, 2.0 g/dm3 Laponite and 40 g/dm3 F127) and lowest (2, 0.1 g/dm3 Laponite and 40 g/dm3 F127) Laponite concentrations with TBO (3), F127/TBO (4, no Laponite and 40 g/dm3 F127), and Laponite/TBO samples (5, 2.0 g/dm3 Laponite and no F127) for TBO solutions at 1.7 × 10-4 mol/dm3. (b) Dependence of the absorption spectra of aqueous TBO in the presence of Laponite and F127 (40 g/dm3) on Laponite concentration (1, 2.0 g/dm3; 2, 1.0 g/dm3; 3, 0.5 g/dm3; 4, 0.25 g/dm3; 5, 0.1 g/dm3). Note the presence of a significant higher aggregate (µ) peak for all concentrations of Laponite. The TBO concentration is maintained at 1.7 × 10-4 mol/dm3.

Figure 7. (a) Comparison of the two three-component mixtures with the highest (1, 2.0 g/dm3 Laponite and 40 g/dm3 F127) and lowest (2, 0.1 g/dm3 Laponite and 40 g/dm3 F127) Laponite concentrations with MB (3), F127/MB (4, no Laponite and 40 g/dm3 F127), and Laponite/MB samples (5, 2.0 g/dm3 Laponite and no F127) for MB solutions at 1.7 × 10-4 mol/dm3. (b) Dependence of the absorption spectra of aqueous MB in the presence of Laponite and F127 (40 g/dm3) on Laponite concentration (1, 2.0 g/dm3; 2, 1.0 g/dm3; 3, 0.5 g/dm3; 4, 0.25 g/dm3; 5, 0.1 g/dm3). Note the presence of a significant dimer peak (β) for all concentrations of Laponite. The MB concentration is maintained at 1.7 × 10-4 mol/dm3 for all samples.

with the silicate surface is significantly different from that of the MB molecule with the surface and results in some differences in the structures reported in Figures 3 and 4 and the differences in the direction of wavelength shift in the presence of Laponite. The differences in the self-interaction of the dyes, their ability to hydrogen-bond in the presence of the silicate surface, and the significant differences in the ionic interactions between the negatively charged basal plane of the silicate are potential causes for the observed differences. Additionally, the presence of resonance structure in the case of MB and the lack of such resonance structures in TBO could lead to significant differences in the H-bonding of the dyes with the silicate and also significant differences in ionic (specific) interactions between the dyes and the silicate sheets. The addition of the triblock copolymer F127 (cmc of F127 at 25 °C ∼ 10 g/dm3) to the dye solutions results in an increase in the number of monomers at the expense of dimers for both dye systems (Figure 5). Above the cmc, F127 forms spherical micelles, with the PPO segments forming a dehydrated core and the PEO groups remaining hydrated in the corona.21 Possible reasons for deaggre-

gation of the dyes in the presence of F127 are participation of the dye cations in the F127 micelles (likely in the corona) or the stabilization of dye monomers due to hydrogen bonding with the ether on the polymer.24 The fact that a similar effect is observed upon use of the homopolymer PEO (Figure 5c), which does not micellize, suggests that the preferential hydrogen bonding between PEO and the dye is the more probable mechanism for dye disassociation. The addition of F127 to a Laponite-TBO solution results in a large fraction of the dye molecules appearing as dye aggregates (Figure 6). On the other hand, for the solutions of F127 with Laponite and MB, the addition of F127 results in an increase in the dimer fraction (Figure 7). These effects are quantified in Figure 8, where for the case of the TBO solutions, at low concentrations of the polymer there is a significant increase in the intensity of the µ band with respect to the R and β peaks. However, with increasing concentration of the polymer (i.e., the highest polymer: (24) (a) Hammouda, B.; Ho, D.; Kline, S. Macromolecules 2002, 35, 8578-8585. (b) Ho, D. L.; Hammouda, B.; Kline, S. R. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 135-138.

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ably the F127 takes up enough of the silicate surface that significant amounts of the TBO are excluded and remain as monomers and dimers in solution. Alternatively, the F127 could also cause the layers of Laponite to no longer be well dispersed and collapse into a stack and therefore present a much smaller external surface area to the dyes. Recent neutron scattering measurements (not reported here) have demonstrated that the Laponite does not undergo any change in dispersion state with the addition of the F127, similar to previous results of Schmidt and co-workers4,27 for Laponite-PEO dispersions. For the case of MB-Laponite-F127 solutions, while the µ band is never significant, the evolution of dimer concentration in comparison with monomer concentration mirrors the behavior of TBO higher aggregates and monomers discussed above. On the basis of the poorer aggregation of MB in aqueous dispersions, these results are consistent and similar arguments for the lack of availability of silicate surface appear to be compelling. Such competing surface interactions and the resulting changes in the state of aggregation are responsible for the trends for the ternary (Laponite + F127 + dye) systems to be significantly different from either of the binary (Laponite + dye and F127 + dye) systems.

Figure 8. Evolution of the monomer (R), dimer (β), and higher aggregate (µ) peak intensities as a function of the F127:Laponite ratio for TBO solutions (a) and for MB solutions (b). Notations made on the right-hand y-axis correspond to peak intensities for the R, β, and µ peaks for the polymer-dye solutions. For the TBO solutions, the µ band is dominant for most concentrations while the R and β peaks become significant as the polymer: Laponite ratio increases. On the other hand, for the MB solutions, the β peak is dominant for most concentrations while the R peak becomes significant as the polymer:Laponite ratio increases. For the MB-based solutions, the µ peak is much smaller than that observed for the TBO system. The TBO and MB concentrations are maintained at 1.7 × 10-4 mol/dm3.

Laponite ratio25), the R and β peaks increase in intensity while the peak corresponding to the higher aggregates decreases in intensity. These results suggest a structure where the F127 molecules compete for interaction sites on the layered silicate surface with TBO, effectively limiting the ability of the dye aggregates on the layered surface to disperse into monomers. It is expected that the PEO units in F127 will participate in crown ether formation with Na+ (the intergallery counterion) and form bulky structures near the layered silicate surface and prevent the dye molecules from accessing the silicate surface.26 At the lowest Laponite concentration, presum(25) In these experiments, the amount of F127 in the solution was kept constant while the Laponite concentration was systematically varied.

Concluding Remarks It was found that the absorption spectra in the visible region of both MB and TBO were significantly influenced by the presence of both Laponite and F127. At concentrations below the stoichiometric equivalence point, Laponite caused the dye molecules to form aggregates or dimers on the silicate surface. At higher concentrations of Laponite, the opposite effect was observed, with dye monomers increasing in ratio to the aggregates when compared with the pure dye solution. F127, in solutions of the same dye concentration, had a similar but smaller deaggregating influence. When F127 and Laponite were both present in solution, the result was completely different, with dimers or aggregates of dye increasing overwhelmingly in relation to the monomer population. This aggregation of the dye cations is attributed to the competition of the F127 chains with dye cations for the negative charges on the silicate surfaces and edges. Acknowledgment. We thank Ms. Kimberly Kowalski and Mr. Jeremy Strauch for help with identification of the changes in absorption character and for performing the preliminary measurements. We thank the National Science Foundation (DMR 9875321) for funding this research. We also thank the reviewers for their helpful comments and comparison of the present results to the rhodamine-Laponite systems. Supporting Information Available: Supplementary background, experimental evidence, and data interpretation in terms of peak areas and tabulation of fit parameters. This material is available free of charge via the Internet at http://pubs.acs.org. LA047540K (26) Belcheva, N.; Tsvetanov, C.; Smid, J. J. Polym. Sci., Part A: Polym. Chem. 1996, 35, 1819-1824. (27) (a) Malwitz, M. M.; Butler, P. D.; Porcar, L.; Angelette, D. P.; Schmidt, G. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 3102-3112. (b) Schmidt, G.; Nakatani, A. I.; Butler, P. D.; Han, C. C. Macromolecules 2002, 35, 4725-4732.