Langmuir 1998, 14, 1597-1603
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Characterization of Polystyrene Latex Surfaces by the Adsorption of Rhodamine 6G Ervin Mubarekyan and Maria Santore* Department of Chemical Engineering, Lehigh University, 111 Research Drive, Bethlehem, Pennsylvania 18015 Received August 1, 1997. In Final Form: December 30, 1997 The adsorption of the fluorescent dye rhodamine 6G (R6G) onto polystyrene (PS) latex was investigated as a potential tool for characterizing the surface of PS latex, in particular the density of negative surface charges and their distribution on the surface. The method exploited previous observations that R6G forms nonfluorescent dimers in concentrated solutions and that when R6G is adsorbed to surfaces, R6G dimerization is enhanced. This work examined the influence of ionic strength and particle concentration for variations in R6G adsorption. R6G adsorption was found to be charge-driven in the most dilute regime, and hydrophobically driven in the concentrated regime, with the crossover occurring near the neutralization of negative surface charge by the positive R6G. For negatively charged particles, adsorbed dyes were nearly completely quenched on the surface near the point of charge neutralization, where the dispersions were also unstable. At higher R6G surface loadings, the surface fluorescence was recovered and the dispersions restabilized. No surface quenching could be resolved on cationic latex due to weaker dye adsorption.
Introduction Fluorescence spectroscopy experiments provide insight into a variety of surface chemical properties and interfacial physical phenomena. For instance, it is possible to obtain information on systems of polymeric and inorganic particles, porous structures such as membranes and sieves, cells, and DNA. Of particular interest, nonradiative energy transfer studies have elucidated the fractal dimensions of porous glasses1,2 and copolymer latex morphology3 and provided insight into polymer interdiffusion during the film formation process in coatings.4,5 In studies such as these, the distances between donor and acceptor molecules and the dimensionality of the spatial distribution of acceptors near a donor can be determined from the fluorescence lifetime decay of the donor. Another fluorescence spectroscopy method providing information of molecular conformations or micellization exploits excimer formation by pyrene.6-8 A third fluorescence phenomena which can be exploited to gain interfacial information involves dye molecules which, under some conditions, form nonfluorescent dimers. Rhodamine 6G (R6G) is a positively charged fluorophore which is known to exhibit this characteristic.9 Motivated by Winnik’s studies in preparation for energy transfer studies between R6G and malachite green,10,11 we were optimistic that the dimerization of R6G on latex particles (1) Dozier, W. D.; Drake, J. M.; Klafter, J. Phys. Rev. Lett. 1986, 56, 197-200. (2) Levitz, P.; Drake, J. M. Phys. Rev. Lett. 1987, 58, 686-9. (3) Pekcan, O.; and Winnik, M. A. Phys. Rev. Lett. 1988, 61, 641-4. (4) Pekcan, O.; Winnik, M. A.; Croucher, M. D. Macromolecules 1990, 23, 2673-8. (5) Zhao, C.-L.; Wang., Y.; Zdenek, H.; Winnik, M. A. Macromolecules 1990, 23, 4082-7. (6) Oyama, H. T.; Tang, W. T.; Frank, C. W. Macromolecules 1987, 20, 474-80. (7) Chandar, P.; Somasundaran, P.; Turro, N. J.; and Waterman, K. C. Langmuir 1987, 3, 298-300. (8) Char, K.; Frank, C. W.; and Gast, A. P. Langmuir 1989, 5, 133540. (9) Estevez, J. H. T.; Lopez Arbeloa, F.; Lopez Arbeloa, T.; Lopez Arbeloa, I.; Schoonheydt, R. A. Clay Miner. 1994, 29, 105-13. (10) Charreyre, M. Y.; Zhang, P.; Winnik, M. A.; Pichot, C.; Graillat, C. J. Colloid Interface Sci. 1995, 170, 374-82.
could provide a simple means of determining surface character such as charge density and hydrophobicity and possibly provide some information about the charge distribution on a surface. That is, it might be possible to distinguish between surfaces where charges tend to be clustered as opposed to those where charges are more nearly uniformly distributed. We are also interested in the spectroscopic aspects of rhodamine adsorption onto latex, since much of our group’s work on polymer adsorption involves fluorescently tagged polymers:12-14 Additional information about the polymer conformation and its interaction with a surface might be obtained by the incorporation of fluorescent groups whose affinity for a surface is well understood. R6G is known to form nonfluorescent dimers in concentrated aqueous solutions.9,15 Multimer formation is enhanced by the addition of alkane chains at R6G’s acid functionality.16 When R6G adsorbs to a surface, it is restricted to a nearly two-dimensional space and experiences local concentrations exceeding those in bulk solution. Therefore, to the extent that two molecules can approach each other, the tendency for quenching is increased for adsorbed dyes relative to those in solution. R6G dimer formation and quenching upon adsorption were studied by steady state and fluorescence lifetime methods as a background for energy transfer studies of the surface features of charged vessicles.17 Because quenching is apparent through the steady-state fluorescence intensity,10,17 information about the adsorbed dye and the surface chemistry can be obtained with fairly inexpensive (11) Nakashima, K.; Duhamel, J.; Winnik, M. A. J. Phys. Chem. 1993, 97, 10702-10707. (12) Santore, M. M.; Fu, Z. Macromolecules, in press. (13) Rebar, V. A.; Santore, M. M. J. Colloid Interface Sci. 1996, 178, 29-41. (14) Rebar, V. A.; Santore, M. M. Macromolecules 1996, 29, 627382. (15) Ikonen, M.; Vuorimaa, E.; Moritz, V.; Lemmetyinen, H. Thin Solid Films 1993, 226, 275-81. (16) Nakashima, K.; Fujimoto, Y. Photochem. and Photobiol. 1994, 60, 563-566. (17) Tamai, N.; Yamazaki, T.; Uamazaki, I.; Mizoma, A.; Mataga, N. J. Phys. Chem. 1987, 91, 3503-3508.
S0743-7463(97)00855-X CCC: $15.00 © 1998 American Chemical Society Published on Web 02/11/1998
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Table 1. Characteristics of Latexes polystyrene latex batch
LS 1134-B
av particle size (nm) 30019,20 particle surface charge 3.0 × 10-7 density (mol/m2) area (Å2) per SO4- or 554 NH4+
LS 1102-A
cationic
19019,20 102 1.4 × 10-7 19,20 6.2 × 10-8 1190
2678
instrumentation, avoiding the requirement for picosecond or nanosecond lifetime measurements. Since Winnik’s initial work10,11 which presented some information on R6G adsorption on PS latex and probed the issue of R6G exchange kinetics, a more comprehensive study appeared in the literature,18 probing R6G adsorption onto several latexes of varying charge density, and focusing on adsorption isotherm behavior as a function of pH and ionic strength, mostly for conditions of relatively high R6G concentration. Over the past several years, similar work has been ongoing in our laboratory. Our investigation, however, focuses on dimer formation on the surface of particles of varying charge in addition to the adsorption isotherms. The study presented here corroborates Winnik’s findings and also spans a broader range in R6G solution concentrations. As a result, our findings sample a greater variety in the R6G surface concentration and extent of dimerization of the adsorbed dye. Experimental Section R6G and fluoresceinamine dyes were purchased from Sigma and used as supplied. Two anionic (LS 1134-B and LS 1102-A, gifts from Lehigh’s Emulsion Polymers Institute) and one cationic polystyrene latex, all monodisperse, were studied. The anionic latexes contained negatively charged surface sulfate groups and had been extensively characterized.19 The cationic latex was synthesized inhouse: styrene monomer was cleaned by passing it through an “inhibitor remover” column (Aldrich) to remove hydroquinone. It was reacted with ((2-(methacryloyloxy)ethyl)trimethylammonium chloride (QDM), which provides the cationic component on the latex surface. Into 125 g of DI (deionized) water were added 29.4 g of styrene, 1.81 g of QDM, and 1.56 g of Tween 80, with 0.156 g of 2,2′-azobis-2-amidinopropane-dihydrochloride initiator. The mixture was allowed to react at 65 °C under argon for 6 h. Latexes were purified prior to their use by ion exchange (BioRad AG-1X4 and AG-50WX4 resins) to remove surfactant and contaminant ions. The samples were considered clean when their conductivity was within a few percent of DI water. Latexes were stored in Pyrex containers. Capillary hydrodynamic fractionation was employed to determine latex size and confirm monodispersity. Surface charge densities were measured by conductometric titration for the cationic latex and LS 1134-B. The latter was found in good agreement with previously reported values, so for LS1102-A the previously reported value was used. The latex properties, along with those previously reported for LS1134-B and LS1102-A,19 are summarized in Table 1. R6G adsorption experiments were conducted in phosphate buffer (pH 7.4, containing 0.008 M Na2HPO4 and 0.002 M KH2PO4 ) to maintain a nearly constant pH. We found this was necessary to obtain the most reproducible fluorescence behavior from R6G which is slightly pH sensitive at neutral conditions. Since buffer ions made it impossible to achieve the lowest ionic strengths, the solutions with phosphate buffer were called “moderate” ionic strength. To further test the influence of ionic strength on the interaction between R6G and PS latex surfaces, (18) Nakashima, K.; Lui, Y.; Zhang, P.; Duhamel, J.; Feng. J.; Winnik, M. A. Langmuir 1993, 9, 2825-31. (19) Vanderhoff, J. W.; Van Der Hull, M. H.; Tansk, R. J.; Overbeek, J. T. G. Clean Surfaces: Their Preparation and Characterization for Interfacial Studies, 5; Marcel Dekker: New York 1970. (20) Ahmed, S. M. Preparation and Characterization of Latexes. Ph.D. Thesis, Lehigh University, 1979.
0.145 M NaCl was added to the phosphate buffer giving solutions of “high” ionic strength. In measurements of adsorption isotherms and fluorescence intensity of adsorbed dye, a series of latex-containing R6G solutions were employed, ranging in concentration from 3 × 10-8 to 1 × 10-4 M. These were prepared from a stock solution of R6G with a concentration on the order of 10-4 M. Within each set of data the particle content was fixed. Several data sets were needed to examine different ionic strengths, latex samples, and latex concentrations. It was discovered that dye adsorbed to the container walls, an effect which had significant impact for samples with very low initial R6G concentrations or low latex content (small total available surface area). Depending on the latex content for a particular data set, the wall area was 0.7 to 20% of the total available surface area. To eliminate the error associated with adsorption to the walls, careful kinetic studies were conducted to determine the adsorption rates on the wall relative to those on the latex. It was found that exposure of the dye to the latex for 15 min was sufficient to equilibrate adsorption on the latex particles and avoid much of the adsorption to the containers. (Our findings concur with Winnik’s observation18 that adsorption onto the latex equilibrates on the order of second or minutes, though in some cases, it was necessary to allow more time for adsorption on the particles because of our more dilute conditions.) The data presented here include fluorescence measurements of R6G solutions before and after addition of latex, and measurement of adsorption isotherms. Fluorescence measurements were conducted in a Fluorolog II (Spex Industries) photon counting spectrofluorometer, using excitation at 514 nm, and a right-angle detection geometry. In many instances, the full fluorescence spectrum was measured, and for R6G concentrations up to 10-5 M, the shape of the spectrum was unaltered by changes in concentration or adsorption onto latex. We therefore found it sufficient to report intensity data for emissions at 550 nm. Calibration studies for R6G solutions without latex revealed that fluorescence was linear in concentration for concentrations as high as 5 × 10-5 M for our 10 mm fluorescence cell. For the most dilute cases, it was possible to detect R6G concentrations as low as 10-10 M, as long as care was taken to avoid adsorption onto the walls of the containers. In fact, it was adsorption to the surface of the containers, not a sensitivity cutoff, which tended to reduce the accuracy of our results. With care taken in exposure of solutions to jar surfaces, R6G concentrations as low as 2 × 10-10 M are reported with confidence for adsorption studies. To determine adsorption isotherms, latex-containing R6G solutions of known total R6G content were allowed to equilibrate and then were centrifuged. Supernatant was collected and analyzed for fluorescence to determine the free R6G concentration. For concentrations exceeding 1 × 10-5 M, it was necessary to dilute samples prior to measuring fluorescence spectra to avoid saturation of the photodetector. From the difference of the initial dye concentration in the dispersion, CD and the supernatant, CS, the adsorbed amount, Γ, of dye on the particles was determined:
Γ)
(CD - CS)V A
(1)
Here, V is the sample volume and A is the latex surface area. Each isotherm consisted of 12-18 samples of varied R6G concentration. The isotherms, which represent equilibrium between the free and adsorbed dye, were constructed by plotting the adsorbed amount as a function of the supernatant concentration.
Results and Discussion LS1134-B at Moderate Ionic Strength. Figure 1 presents several aspects of R6G adsorption onto LS1134B, the latex studied most frequently in this work. In Figure 1a, the influence of latex on R6G fluorescence is shown. Figure 1b presents the adsorption isotherm, and Figure 1c combines the results of parts a and b to reveal the influence of the surface concentration on the quenching of the R6G.
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Figure 2. Rhodamine 6G molecule.
Figure 1. (a) Influence of particles on R6G fluorescence in latex dispersions of varying particle content and moderate ionic strength for 0.05 g/L of latex (O), 0.10 g/L of latex (+), and 0.50 g/L of latex (×). (b) Adsorption isotherms for the same dispersions. The dashed lines denote the surface charge density value of LS1134-B and twice that value respectively. The insert shows the same isotherms plotted semilogarithmically. (c) Percent quenching of the adsorbed species as a function of surface coverage for adsorbed R6G.
In Figure 1a, the relative fluorescence, I/Io, of rhodamine in a latex dispersion to that in a solution of the same concentration without latex is presented as a function of R6G concentration for the different amounts of latex. The abscissa represents the total amount of R6G regardless of whether it is adsorbed to particles. I/Io values of unity indicate that the latex does not alter the fluorescence of the solution, which is found to be the case for the most dilute R6G concentrations. On the left side of Figure 1a, over 95% of the total dye is adsorbed to the latex; however, this is not sufficient to saturate the particles. At these dilute conditions, the adsorbed dyes are still spaced sufficiently far apart on the latex surface such that dimerization is negligible. At higher dye concentrations most of the dye still resides on the particles’ surface; but as the surface crowds, nonfluorescent dimers are formed such that the fluorescence falls below that observed without latex. At the highest dye concentrations, the
fluorescence of the latex-containing dispersion again approaches that of the analogous R6G solution. This occurs because at high R6G concentrations the latex surface saturates, leaving most of the R6G free in solution, where it dimerizes less than on the surface. From Figure 1a it is not clear if all the adsorbed dye is quenched when the surface is saturated, on the right side of the figure. In Figure 1a, three data sets for different amounts of latex exhibit minima at different R6G concentrations. The fluorescence minima attain different fluorescence intensities because for runs with different amounts of latex, the relative amounts of free and adsorbed dye are slightly different. For instance, when the dispersion containing 0.05 g/L of latex experiences its minimum fluorescence, there is relatively more free dye (a more fluorescent minima) than when more concentrated latex dispersions are quenched. We repeatedly observe, however, that these fluorescence minima occur roughly at a fixed ratio of total R6G concentration to latex concentration. This suggests that the state of the surface is key in determining the fluorescence, and motivates consideration of the adsorption isotherm, in Figure 1b. In Figure 1b R6G adsorption is presented as a function of free concentration on a log scale, covering six decades in free R6G concentration. Because this representation of adsorption data is unusual, the standard linear abscissa is shown in the inset, which appears roughly to be a highaffinity Langmuir form. On the log scale, however, there are two distinct regions for the adsorption behavior. At low coverage, the adsorption increases sharply with free R6G in solution, and at high concentrations, there is a more gradual rise in adsorbed amount. The crossover between these two adsorption regimes, indicated by the dashed lines, occurs when the molar R6G surface coverage is approximately 1-2 times the surface sulfate group density, when the surface charge is nearly canceled by the adsorbed R6G. Therefore, the steeply rising portion of the adsorption curve is interpreted as being charge driven. In the high concentration regime, additional R6G molecules adhere to the surface despite a net repulsive interaction needed to further increase the positive charge. At the highest concentrations, the surface coverages correspond to 10 R6G's adsorbed per negative sulfate group on the latex surface. The adsorption in this regime is therefore thought to be hydrophobically driven. Figure 2 is a schematic of R6G, showing approximate molecular dimensions. Multiplication of the height and width would give a footprint of 128 Å2, were the dye to adsorb flat. At the kink of the isotherm (4.5 × 10-7 mol/ m2), an average area of 369 Å2 per R6G approaches the 128 Å2 footprint. In contrast, the average area per R6G at the highest isotherm coverage is 17 A2, suggesting that either R6Gs adsorb sideways, or that a multilayer tiling of 7-8 equivalent monolayers occurs. The dispersions in parts a and b of Figure 1 exhibited changes in stability upon R6G addition that were consistent with the isotherms. At the lowest R6G concentra-
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tions, like the pure dispersion, the dye-containing dispersion was stable. As the R6G concentration was increased and as more dye adsorbed to the latex surface, pink flocs appeared. The point at which these flocs were visible (with sufficient time allowed for them to form and settle) corresponded to situations where about 15% of the negative latex surface charge was compensated by the positive charge on the adsorbed R6G, in qualitative agreement with Winnik’s findings.18 As the overall fluorescence in Figure 1a approached a minimum and as the kink in the isotherm was approached, the dispersions became less stable. Ultimately, however, the dispersions restabilized, for conditions corresponding to recovery of the fluorescence, and about three R6G molecules for each surface sulfate group were found. These observations about stability are consistent with the finding that adsorption of R6G in the low coverages regime neutralizes the surface charge. Additional R6G adsorption reverses the surface charge (also consistent with observations of a reversed ζ potential) yielding stable suspensions. Figure 1c addresses the relationship between surface loading and fluorescence quenching. While parts a and b of Figure 1 include the influence of the dye free in solution, the abscissa of Figure 1c counts only the adsorbed dye and the ordinate shows the amount of this dye which is quenched. (The fluorescence from adsorbed R6G was determined by subtracting the supernatant fluorescence from that of the original mixture containing R6G and particles. Since the centrifugation and supernatant analysis were needed to determine the isotherm, no additional experimental steps were needed in the calculation of the surface quenching.) At surface coverages below 3 × 10-8 mol/m2, less than 10% of the adsorbed R6G’s participate in nonfluorescent dimers. The fraction of nonfluorescent adsorbed dyes rises to nearly 100% as the surface crowds. The maximum quenching occurs when there are 1-2 R6G molecules adsorbed per surface sulfate group, corresponding to the kink in the isotherm (indicated by the dashed lines), and is independent of the amount of latex in a particular experiment. At higher R6G coverages, the fraction of adsorbed dyes participating in nonfluorescent dimers is reduced, suggesting that dye which adsorbs beyond the point of surface neutralization (by hydrophobic interactions) has a reduced tendency to dimerize. We examined this possibility by calculating the expected percent quenching if all the dye to adsorb beyond the maximum were not quenched. The result is the curve which lies below the data on the right. The calculations demonstrate that the additional dye which adsorbs beyond the maximum is quenched to some degree, albeit less than the dyes which were previously adsorbed. The results of Figure 1c suggest that the adsorption of R6G near a negatively charged surface group make it easier for a second dye to adsorb and form a nonfluorescent dimer. However, when R6G adsorbs to a surface whose charge is already positive (due to the previously adsorbed rhodamines) the net dimer formation is reduced. In presenting the fluorescence data of parts a and c of Figure 1, we have taken care to ensure that the apparent quenching and fluorescence recovery at high concentrations was not a result of scattering from flocculation or the addition of particles. The observation in Figure 1c that the fluorescence behavior is a function of adsorbed amount and that there are no systematic trends for experiments with different total amounts of latex argues against the possibility of scattering related artifacts. Also, fluorescence measurements were taken at a time allowing dye adsorption to equilibrium coverage but insufficient time for formation and settling of large flocs.
Mubarekyan and Santore
Figure 3. Fluorescence spectra for 1 × 10-7 M Fluorescein sodium salt in phosphate buffer for 0 g/L of latex (solid), 0.1 g/L of latex (dot), 0.5 g/L of latex (dash dot), and 1.0 g/L of latex (dash).
Figure 3 quantitatively addresses the influence of scattering on the total fluorescence. This experiment involved a nonadsorbing negatively charged fluoresceinamine dye solution of 1 × 10-7 M which was excited at 488 nm. At this dye concentration in a 10 mm cuvette, addition of LS 1134-B in amounts as much as 1 g/L caused the fluorescence to increase by as much as 30%. This effect occurs because the 300 nm particles scatter light and increase the effective path length of the cuvette. At dye concentrations exceeding 10-6 M (not shown), increases in path length cause the fluorescence signal to decrease because of the increased probably for reabsorption of the emitted photons, the inner filter effect. This effect is prevalent with dyes exhibiting small Stokes shifts. For R6G concentrations below 10-5 M, scattering did not contribute more than 15% of the total signal and corrections were not needed. The complications associated with scattering argue in favor of experiments at fixed latex concentrations, varying the amount of R6G. Because runs were conducted at fixed latex content, and because several runs with different latex content yielded similar results, the changes in fluorescence in parts a and c of Figure 1 result from dimerization, not scattering artifacts. LS1134-B at High Ionic Strength. The experiments presented in Figure 1 were repeated at elevated ionic strength, employing phosphate buffer with 0.145 M NaCl. Of particular interest was the potential competition between the adsorption of Na+ and positive R6G ions. The results are presented in parts a-c of Figure 4 for the overall fluorescence of the dispersion, adsorption isotherm, and surface quenching, respectively. Figure 4 is similar to Figure 1, suggesting that at the range of ionic strengths tested, there is little effect of a change in ionic strength on R6G adsorption or its fluorescence. The R6G coverage in the hydrophobically driven regime is also nearly independent of ionic strength. Most significant is the finding that changes in ionic strength do not affect the kink in the isotherm which corresponds to the neutralization of surface charge. In Figure 4c, ionic strength had no significant effect on the maximum quenching, though there does appear to be a bit more quenching at the lowest R6G coverages with high ionic strength. Perhaps additional Na+ ions help screen repulsions between neighboring R6G’s facilitating dimer formation. LS1102-A at Moderate Ionic Strength. Experiments with latex 1102-A contrast with those of LS1134-B because the former have approximately half the surface charge density and a smaller particle size resulting in a higher specific surface area by 57%. Parts a-c of Figure 5 present
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Figure 4. (a) fluorescence intensity ratio for LS1134-B in high ionic strength buffer versus total R6G concentration for 0.05 g/L of latex (O), 0.10 g/L of latex (+), and 0.50 g/L of latex (×). (b) The adsorption isotherms for the same dispersions. The dashed lines denote the surface charge density value of LS1134-B and twice that value respectively. (c) Percent quenching vs its surface coverage for adsorbed R6G.
Figure 5. (a) Fluorescence intensity ratio for LS1102-A in moderate ionic strength buffer versus total R6G concentration for 0.10 g/L of latex (+), 0.25 g/L of latex (O), 0.50 g/L of latex (×) and 1.00 g/L of latex (*). (b) Adsorption isotherms for the same dispersions. The dashed lines denote the surface charge density value of LS1102-A and twice that value, respectively. (c) The percent quenching vs its surface coverage for adsorbed R6G.
the dispersion fluorescence, adsorption isotherm, and surface quenching experiments for four different concentrations of this more sparsely charged latex. Figure 5a for LS1102-A is similar to Figure 1a for LS1134-B. Like LS1134-B, LS1102-A exhibits a shift in the fluorescence minimum to higher R6G concentrations for higher latex loadings. Like the results for LS1134-B, the minima for LS1102-A occur roughly at a fixed ratio of R6G to latex, though the value of this ratio should depend on the latex batch. The R6G/latex ratio corresponding to the fluorescence minima is similar for LS1102-A and LS1134-B because the 57% difference in specific surface area between the two latex batches approaches our ability to resolve the fluorescence minima. Though the fluorescence behavior for LS1102-A is qualitatively similar to that of LS1134-B, the adsorption isotherm in Figure 5b makes an interesting comparison with those in Figures 1b and 4b. In Figure 5b, there are still two distinguishable regimes suggesting charge- and hydrophobically driven R6G
adsorption; the crossover between the two regimes corresponds roughly to the neutralization of the negative surface charge by positive R6G dye, with the kink occurring at R6G coverages 1-2 times the surface sulfate density (illustrated by the dashed lines). The charge-driven regime at low concentrations, however, shows higher surface coverages than with the more densely charged latex, contrary to initial expectations. Typically one expects that densely charged surfaces will have a higher affinity for ions of the opposite charge,18 at least in the dilute regime. The coverage in the hydrophobically driven regime is slightly lower for the less densely charged latex, demonstrating the proper trend for the influence of the underlying surface charge and also following the expectation that, in the hydrophobically driven regime, the R6G coverage on various latexes should be more nearly similar than in the charge-driven regime. Figure 5c, illustrating the relationship between surface loading and R6G fluorescence is slightly different than
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Figure 6. Adsorption isotherms for LS1134-B (×) and cationic latex (O) compared. Both latexes are 0.1 g/L and in moderate ionic strength buffer.
Mubarekyan and Santore
latex. The problem with the cationic latex was these high surface coverages occurred at high bulk solution concentrations, making it impossible to resolve changes in the surface signal from the fixed bulk solution signal which was orders of magnitude greater. We therefore could not demonstrate whether the lack of observable quenching was a true surface effect deriving from the influence of the positive surface charge on the adsorbed R6G conformation, or if there was simply a lack of observable quenching due to the low surface signal. Either way, the overall fluorescence of the R6G-containing dispersion provides a clear means of distinguishing between the surfaces of cationic and anionic latexes without having to separately analyze the relative contributions from the free and adsorbed dyes. Summary
seen with the more densely charged LS1134-B. First, it should be noted that because of the greater surface area for LS1102-A, we could better access the regime of low R6G coverage, but could not probe the conditions with the highest amounts of R6G on the surface. In Figure 5c, with the lowest R6G coverages, there appears to be slightly more quenching than with LS1134-B at a similar ionic strength, though the data are more scattered. This may indicate that the distribution of surface charge on LS 1102-A is more clustered, such that dimers form more readily than on LS1134-B. Also of interest for LS1102-A is the observation that the maximum quenching is achieved at the same surface loadings as was found for LS1134-B, independent of the factor of 2 difference in the surface charge of these two latexes. The region of surface neutralization is indicated in Figure 5c, suggesting that quenching and surface neutralization do not always coincide exactly. The observation of the quenching maximum at high R6G coverages is, however, consistent with the observation in Figure 5b of very high affinity adsorption. The fluorescence recovery on the right side of Figure 5a is incomplete because of the inaccessibility of higher surface loadings; however, the data are in agreement with fluorescence levels observed for LS1134B. Cationic Latex at Moderate Ionic Strength. Figure 6 illustrates the adsorption isotherm of R6G on the cationic latex, and is compared with the adsorption of R6G on LS1134-B at the same ionic strength. The coverage on the ordinate is plotted on a log scale to facilitate a comparison between the two latexes: At dilute conditions, the coverage on the cationic latex is orders of magnitude less than on the anionic latex, demonstrating the important role of charge interactions in driving the R6G adsorption. At high R6G solution concentrations, there was significant adsorption of R6G on the cationic latex, demonstrating that other interactions, most likely hydrophobic in nature, ultimately cause adsorption. This high R6G concentration regime, where R6G adsorption is thought to be hydrophobically driven, is similar for both cationic and anionic latexes: the incremental R6G adsorption in both cases occurs despite the electrostatic work to bring positive charges to an interface already bearing positive charge. The fluorescence of the R6G solutions containing cationic latex was always within (5% of solution fluorescence without latex (not shown in a figure), suggesting that no quenching occurred over the full range of R6G concentrations. At high R6G concentrations surface coverages on the order of 4.5 × 10-7 mol/m2 were achieved on the cationic latex, giving the R6G spacing which corresponded to nearly 100% quenching on the anionic
This investigation set out to determine if adsorption of R6G on latex surfaces and the fluorescence of the adsorbed dyes could provide information about the latex surface chemistry in the wet state. The method was of particular interest because it held the potential to provide this information about dispersions from the fluorescence of the entire dispersion, without requiring separation of particles and serum. (Development of the fundamental basis for such a method required, however, the studies presented here which did involve separation of particles and serum.) It was found that on negatively charged particles, two driving forces contribute to R6G adsorption: electrostatic attractions at dilute conditions and hydrophobic interactions at higher R6G concentrations. The crossover between the two adsorption regimes was clear from the shapes of the adsorption isotherms which exhibited a kink at coverages corresponding to 1-2 times the density of negative surface groups. On positively charged latex, only hydrophobic interactions contributed to adsorption, such that in the dilute regime, R6G coverages were orders of magnitude less than on anionic particles. At high R6G solution concentrations, the surface coverages were less influenced by the surface, falling within the same order of magnitude for all the latexes studied. In the moderate to high range of ionic strengths studied here, additional ions had no significant impact on adsorption. For adsorption on negatively charged particles, saturation of the surface was accompanied by the formation of nonfluorescent dimers, which caused a marked and easily observable reduction in the fluorescence from the dispersion, without separate analysis of the serum. On the anionic latex, this dimer formation involved almost all of the adsorbed dye molecules when surface concentrations reached levels near 4.5 × 10-7 mol/m2, or an average adsorption footprint of 369 Å2/R6G dye, near the molecular footprint of 128 Å2. The surface concentration corresponding to this dimerized state appeared to be the same for two anionic latexes differing in surface charge density by a factor of 2. Our inability to distinguish the surface charge density of the two latexes, via quenching alone, may have been confounded by other surface characteristics that we were not able to quantify: These include a surface feature of the sparsely charged latex which gave unexpectedly strong R6G binding at low coverages, and a potentially higher degree of charge clustering on the sparsely charged latex. This work demonstrated that one could make clear distinctions between latexes of different surface character by a simple comparison of the fluorescence from rhodamine-containing dispersions, without a separate analysis
Characterization of Polystyrene Latex Surfaces
of the surface fluorescence. The latter potentially requires disruptive measures (centrifugation or membrane separation) to isolate the surface signal. The distinction between negatively and positively charged surfaces stems primarily from the weaker adsorption of R6G on positive surfaces which is manifest in an R6G fluorescence signal that is virtually uninfluenced by the presence of latex.
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This may be due to a smaller adsorbed fraction of the total dye in the sample or due to surface arrangements of R6G on positive surfaces that tend not to dimerize. Acknowledgment. This work was made possible by NSF support (CTS-9209290 and CTS-9310932). LA970855Y
