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Langmuir 2000, 16, 8546-8548
Characterization of Polystyrene Latex Surfaces by Conductometric Titration, Rhodamine 6G Adsorption, and Electrophoresis Measurements Yong-Kuan Gong and Kenichi Nakashima* Department of Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo-machi, Saga 840-8502, Japan Renliang Xu Particle Characterization, Beckman Coulter, Inc., PO Box 169015, Miami, Florida 33116-9015 Received May 3, 2000. In Final Form: July 31, 2000
Introduction Surface charge density is one of the most important characteristics of latex particles. Among several methods for measuring surface charge density, the conventional conductometric titration is the only precise and practical experimental method.1-5 In this method, however, a large amount (usually several grams) of latex at high concentration is needed for obtaining reliable data, and the polar groups which also play an important role in the surface property of latex cannot be detected. It will be useful if an alternative technique for characterizing the surface of latex particles can be developed. Connor and Ottewill reported work on the adsorption of cationic surfactants on polystyrene (PS) latex surfaces.6 14Carbon-labeled surfactants were used to measure the adsorption isotherms. Their work suggested that the adsorption of surface active cations can be used to determine the number of carboxyl groups on the surface. Recently, Winnik et al. and Mubarekyan and Santore used rhodamine 6G (R6G), a cationic dye, to characterize PS latex surfaces.7,8 They concluded that the adsorption of R6G onto PS latex particles depends mostly on electrostatic interactions at low dye concentrations, whereas hydrophobic interactions prevail at high dye concentrations. On the basis of this idea, we expect that a turning point exists in the adsorption isotherm after the negative charges on the particle surfaces are neutralized with the adsorbed R6G dyes. The amount of adsorption of oppositely charged dyes at this turning point should correspond to the amount of surface charges on the latex particles. Thus, it is possible to determine the surface charge density of PS latex particles by measuring the adsorption isotherm. Furthermore, this method enables us to estimate some polar groups which strongly interact with R6G. In this study, we investigated the adsorption of R6G onto three PS latex surfaces by measuring the adsorption isotherms under various conditions. Electrophoretic light scattering (ELS) measurements were also carried out to (1) Labib, M. E.; Robertson, A. A. J. Colloid Interface Sci. 1980, 77, 151. (2) Stone-Masui, J.; Watillon, A. J. Colloid Interface Sci. 1975, 52, 479. (3) Everett, D. H.; Gultepe, M. E.; Wilkinson, M. C. J. Colloid Interface Sci. 1979, 71, 336. (4) Vanderhoff, J. W. Pure Appl. Chem. 1980, 52, 1263. (5) Zwetsloot, J. P. H.; Leyte, J. C. J. Colloid Interface Sci. 1994, 163, 362. (6) Connor, P.; Ottewill, R. H. J. Colloid Interface Sci. 1971, 37, 642. (7) Charreyre, M.-T.; Zhang, P.; Winnik, M. A.; Pichot, C.; Graillat, C. J. Colloid Interface Sci. 1995, 170, 374. (8) Mubarekyan, E.; Santore, M. Langmuir 1998, 14, 1597.
monitor subtle changes in surface charge of the PS latex particles during the adsorption of R6G. In addition to a plateau at the saturated adsorption region, another plateau was observed at the lower concentration side of the adsorption isotherm. On the basis of the amount of absorbed R6G at this plateau, the charge density of the PS latex was estimated. We define this charge density as the “adsorption charge density”, which includes the density of charged groups as well as polar groups that contribute to the adsorption. Experimental Section R6G (99% pure) was purchased from Aldrich Chemical Co. and used as supplied. Water was purified with a Millipore Milli-Q purification system. We synthesized three kinds of PS latexes (L714, L711, and CG17) by standard emulsion polymerization.7,9 All the latexes were purified by repeated dialysis followed by ion-exchange with a mixed-bed resin (Bio-Rad AG 501-X8) until a constant charge density of the latex particles was reached.10,11 The diameter of the latex particles was determined with an Otsuka ELS-800 dynamic light scattering instrument. Adsorption isotherms were determined by ultracentrifugation.12 The concentration of R6G in the supernatant was determined by UVvis absorption spectroscopy. The conductivity of the dispersions was measured with a conductometer (Horiba, model ES-14). Conductometric titration was carried out with a standard solution of 8.10 × 10-3 M NaOH at 25 ( 0.1 °C under a nitrogen atmosphere. To determine hydroxyl groups on the surface, the hydroxyl groups were oxidized to carboxyl groups with potassium persulfate (0.3 g/g latex) under a catalysis of 10-5 M silver nitrate.13,1413,14 The electrophoretic mobility values were obtained using the Otsuka ELS-800 instrument. All electrophoretic mobility measurements for the PS/R6G mixtures were performed under very low ionic strength in order to reduce heat generation.
Results and Discussion A. Conductometric Titration. Figure 1 shows representative titration curves of L714 aqueous dispersions. It is known that the surfaces of PS latex particles have hydroxyl groups in addition to sulfate groups.15-17 Hydroxyl groups can be formed by hydrolysis of the sulfate groups on the latex surface.13,18 To determine hydroxyl groups as well as sulfate groups, the titration was carried out before and after oxidizing the sample with potassium persulfate. The titration before oxidation gives a typical curve for a strong acid. From the value of the slope (0.230.25 S cm-1 M-1), we confirm that there is no evidence for the existence of weak acid moieties on the particle surfaces before oxidization. The existence of weak acid groups often (9) Nakashima, K.; Duhamel, J.; Winnik, M. A. J. Phys. Chem. 1993, 97, 10702. (10) Kawaguchi, S.; Yekta, A.; Winnik, M. A. J. Colloid Interface Sci. 1995, 176, 362. (11) van den Bul, H. J.; Vanderhoff, J. W. J. Colloid Interface Sci. 1968, 28, 336. (12) Gong, Y. K.; Miyamoto, T.; Nakashima, K.; Hashimoto, S. J. Phys. Chem. B 2000, 104, 5772. (13) Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Br. Polym. J. 1973, 5, 347. (14) van den Hul, H. J.; Vanderhoff, J. W. Br. Polym. J. 1970, 2, 121. (15) Yates, D. E.; Ottewill, R. H.; Goodwin, J. W. J. Colloid Interface Sci. 1977, 67, 356. (16) Palit, S. R. Pure Appl. Chem. 1962, 4, 451. (17) Goodwin, J. W.; Ottewill, R. H.; Pelton, R. Colloid Polym. Sci. 1979, 257, 61. (18) Kolthoff, I. M.; Miller, I. K. J. Am. Chem. Soc. 1951, 149, 3055.
10.1021/la000638o CCC: $19.00 © 2000 American Chemical Society Published on Web 09/30/2000
Notes
Langmuir, Vol. 16, No. 22, 2000 8547
Figure 1. Conductometric titration curves of L714 with a 8.10 × 10-3 M NaOH solution at 25 °C: O, before oxidation; b, after oxidation. The volume fraction of latex L714 is 2.0%. Table 1. Characteristics of Latex Particles latex L714
L711
CG17
227 234 1.03
212 216 1.02
94 96 1.02
-4.77
-4.47
-4.67
1.02 3.3 2.3
0.30 0.92 0.62
1.26 2.2 0.9
3.4
2.8
1.6
diametera
DLS Dn (nm) Dw (nm) polydispersity electrophoretic mobility µ0 (10-4 cm2/s‚V) charge densityb σt (10-7 mol/m2) σa (10-7 mol/m2) σa - σt (10-7 mol/m2)c hydroxyl group densityd σOH (10-7 mol/m2) a
Dn and Dw are the number and weight average diameter, respectively. b The symbols σt and σa denote charge densities obtained by the titration and adsorption methods, respectively.c σa - σt is the difference of charge densities of the two methods; it could be a means of evaluation of polar groups which facilitates the adsorption of R6G. d σOH represents the density of hydroxyl groups determined by the oxidation method.
leads to erroneous conclusions, as mentioned by Yamanaka et al.19 The charge density (σt) values are listed in Table 1. The oxidized samples showed two end points in the titration curve, indicating the presence of a weak acid in addition to the strong one. The weak acid is assigned to carboxylic acid, which is formed by oxidization of hydroxyl groups on the particle surfaces.13,14 Therefore, the density of hydroxyl groups, σOH, was determined from that of the carboxylic acid groups (Table 1). The measurements suggest that a large number of hydroxyl groups exist in all three latex samples. The estimated amounts may be higher than the real ones due to the possible hydrolysis of sulfate groups during the oxidation process. However, this error should be small, because the sulfate group density is only slightly changed before and after oxidation. Although the measured hydroxyl group density seems high when compared to the charge density, it is much lower than the density of total functional groups (1-2 × 10-6 mol m-2) estimated by GPC20 or osmometry.14 Therefore, these values seem to be reasonable. B. Adsorption of R6G from Buffered Solution. Figure 2 represents typical adsorption isotherms of R6G (19) Yamanaka, J.; Hayashi, Y.; Ise, N.; Yamaguchi, T. Phys. Rev. 1997, 55, 3028. (20) Rehmet, R.; Killmann, E. Colloids Surf., A 1999, 149, 323.
Figure 2. Adsorption isotherms of R6G onto L714 at 20 °C in buffered (O) and buffer-free (b) dispersions. (a) Entire dye concentration range; (b) expanded view of low dye concentration range. The volume of the dispersion is 10.0 mL. CPS ) 1.00 g/L. The buffered dispersions contain Na2PO4/NaH2PO4 (8 mM/2 mM) at pH 7.4.
onto L714 from buffered and buffer-free solutions. The high R6G concentration range in the adsorption isotherm for the buffered dispersion is similar to that measured by Winnik et al. However, the isotherm at the lower concentration range revealed another plateau. According to Winnik, the change at the curving section of the adsorption isotherm should be due to the change of the main driving force from the electrostatic to hydrophobic interaction after the surface groups are neutralized. Since the electrostatic attraction between the dye and surface charges is much stronger than the hydrophobic interaction between the dye and polymer, the amount of the adsorbed dye should be constant or very slowly increase with the increase of dye concentration. However, this change is not sharp in the buffered solution. The adsorption of oppositely charged surfactants6,21,22 and polymers20 onto a PS latex was shown to be significantly affected by ionic strength. The effect of high ionic strength will screen the charges, resulting in the decrease of the electrostatic interaction and increase of the hydrophobic one. A similar effect seems to exist in the interaction between R6G and a PS latex surface. At high R6G concentration, much of the molecules are adsorbed by hydrophobic interaction. High R6G concentration also causes high ionic strength, which favors the hydrophobic interaction. Considering these, we chose buffer-free water as the medium to reduce the hydrophobic interaction. C. Adsorption from Buffer-Free Solution. The adsorption isotherm in the buffer-free dispersion is also shown in Figure 2. It can be fitted to a Langmuir model. The adsorption isotherm measured in buffer-free water shows a new plateau in contrast to that in the buffered system, which shows a significant increase in the amount of adsorbed R6G after the turning point. The observation of this plateau has never been reported for the adsorption of R6G onto PS latex particles. We describe this plateau as “the first plateau”, since it is located in the very low (21) Zhao, J.; Brown, W. Langmuir 1996, 12, 1141. (22) Brown, W.; Zhao, J. Macromolecules 1993, 26, 2711.
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Figure 3. Particle mobilities versus the equilibrium concentrations of R6G in diluted latex dispersions: L714 (O), L711 (0), and CG17 (]). The inset shows a semilogarithmic plot to present the high dye concentration range.
concentration range of R6G followed by the plateau due to hydrophobic interaction, which is insensitive to the charge density.7 D. Electrophoretic Mobility. The electrophoretic mobilities of PS latex particles at different adsorption stages were monitored (Figure 3). Particle mobilities measured in buffer-free water dispersions (µ0) are almost the same for all three samples despite the fact that the charge densities of the three samples are considerably different (see Table 1). These data suggest that the charge densities of PS latexes cannot be well distinguished simply from the particle mobilities. Figure 3 shows that the electrophoretic mobilities of the R6G-loaded particles (µa) change with the equilibrium concentrations (c) of R6G. The mobility (µa) changes from negative to positive. The positive values of the mobility indicate that the negative charges on the PS latex surface are overcompensated by the adsorption of R6G, and suggest that R6G cations are also adsorbed onto highly polar groups on PS surfaces. The appearance of a plateau in the µa versus c curve corroborates the observation of the first plateau in the adsorption isotherm. E. Charge Density by Adsorption Isotherm. The appearance of the first plateau in Figure 2 suggests that (1) the electrostatic attraction between R6G and the sulfate group together with the ion-dipole interaction between R6G and the hydroxyl group dominates the adsorption of R6G onto the PS latex surface before the plateau and (2) the electrostatic attraction disappears after the turning point. Furthermore, flocculation of the latexes was observed in several experiments when the adsorption reached the first plateau (0.3-5.0 µM supernatant concentration), and it disappeared gradually with a further increase in the R6G concentration. We conclude from these results that the intersection between the fast and slowly increasing parts in the adsorption isotherm corresponds to the point of neutralization of the latex surface charges. Accordingly, the surface charge density can be determined by measuring the adsorption isotherm of the dye. Figure 4 shows the adsorption isotherms of R6G onto the three PS latexes. The charge densities obtained by this method are listed in Table 1 (σa).23 The charge densities obtained by the adsorption of R6G are higher than those obtained by conductometric titration. The difference seems to originate from the different
Notes
Figure 4. Adsorption isotherm of R6G onto L714 (O), L711 (0), and CG17 (]) in a Milli-Q water dispersion at 20 °C. CPS ) 1.00 g/L. The volume of the dispersion is 10.0 mL.
principles in these two methods. The conductometric titration measures only acid groups on the surface, but the adsorption method measures all the functional groups that facilitate the adsorption of the dye, including highly polar groups such as hydroxyl groups. We describe this value as “adsorption charge density” in order to distinguish it from that obtained by conductometric titration. It is known that R6G can form dimers and high-order aggregates in solution24 and on the surfaces of vesicle or latex particles.25 The formation of R6G dimers may increase the charge density obtained by our method. To estimate the aggregation effect, a cationic anthracene derivative12 and cetyltrimethylammonium cations were used to measure the adsorption charge density. The results are almost the same as those obtained with R6G,26 suggesting that the dimer formation of R6G does not significantly affect the determination of the surface charges. Summary This work was oriented to characterize PS latex surfaces by combining R6G adsorption with electrophoresis and conductometric titration. The adsorption isotherms of R6G showed two plateaus. The plateau at the lower concentration range of R6G (the first plateau) was observed for the first time, which corresponds to the point where negative sulfate groups and some of the hydroxyl groups are neutralized by positively charged R6G. The amount of adsorbed R6G at the first plateau could be used to estimate the surface charges of the PS latex. The charge densities obtained by the adsorption of R6G were larger than those obtained by conventional conductometric titration, mainly due to the coexisting hydroxyl groups. By comparing the adsorption charge density with the conductometric one, information on the contribution of hydroxyl groups to the latex surface property can be obtained. LA000638O (23) In this method, the perpendicular axis can be regarded as the extrapolation line in the fast increasing part of the adsorption isotherm. Thus, the adsorbed amount can be simply determined from the interceptor on the perpendicular axis by extrapolating only the slowly increasing part of the isotherm to zero concentration of R6G. (24) (a) Levshin, L. V. Acta Physicochim. U. R. S. S. 1935, 1, 684. (b) Arbeloa, F. L.; Gonzalez, I. L.; Ojieda, P. R.; Arbeloa, I. L. J. Chem. Soc., Faraday Trans. 2 1982, 78, 989. (25) (a) Tamai, N.; Yamazaki, T.; Yamazaki, I.; Mizuma, A.; Mataga, N. J. Phys. Chem. 1987, 91, 3503. (b) Nakashima, K.; Duhamel, J.; Winnik, M. A. J. Phys. Chem. 1993, 97, 10702. (26) The results obtained by these other cationic species will be published elsewhere.
