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Absorption of Polyelectrolytes on Colloidal Surfaces as Studied by Electrophoretic and Dynamic Light-Scattering Techniques
Journal of Colloid and Interface Science 213, 565–571 (1999) Article ID jcis.1999.6171, available online at http://www.idealibrary.com on
Absorption of Polyelectrolytes on Colloidal Surfaces as Studied by Electrophoretic and Dynamic Light-Scattering Techniques Tsuneo Okubo 1 and Mitsuhiro Suda Department of Applied Chemistry, Gifu University, Gifu 501-1193, Japan Received November 10, 1998; accepted February 17, 1999
ers have clarified the electrostatic and nonelectrostatic interactions of colloidal spheres with polyelectrolytes (11–22). In our previous work (18), the abrupt sign reversal in the electrophoretic mobility and the effective valency of a colloidal sphere has been observed by the addition of the various kinds of oppositely charged macro-ions and their low valent simple ions such as calcium and lanthanum ions. This supports strongly that the excess amount of the macro-ions against the amount of the charges on the colloidal surfaces is absorbed in the manner of the so-called avalanche-type synchronous absorption mechanism, i.e., the absorption of macro-ions occurs abruptly on the colloidal surfaces above the critical concentration of macro-ions added. Below the critical concentration, absorption does not occur so significantly. However, in our previous work this important absorption mechanism has not been discussed at all, since size information of the adsorbed spheres has not been available. In this paper, z-potential and the effective diameters of colloidal spheres absorbed with the macro-ions have been studied systematically as possible in order to clarify the synchronous nature in the absorption.
z-Potential and the effective diameter of the colloidal spheres absorbed with the macro-cations and macro-anions are studied by the electrophoretic light-scattering and dynamic light-scattering measurements. Colloidal spheres used are monodispersed polystyrene (220 nm in diameter) and colloidal silica spheres (110 nm). Macro-ions used are sodium polyacrylate, sodium polymethylacrylate, sodium poly(styrene sulfonate), and poly-4-vinyl pyridines quaternized with ethyl bromide, n-butyl bromide, benzyl chloride, and 5% hexadecyl bromide and 95% benzyl chloride. Reversal of colloidal surface charges from negative to positive occurs abruptly above the critical concentration of macroions by the excess absorption of the macro-cations onto the anionic colloidal spheres, i.e., avalanche-type absorption. The effective diameter of colloidal spheres including the absorbed layers increases substantially by four- to tenfold. In the presence of large amount of macro-cations aggregation of colloidal spheres mediated by the layers of absorbed macro-cations may occur. Absorption also occurs on the anionic colloidal spheres in the presence of an excess amount of macro-anions by the dipole– dipole-type attractive interactions. © 1999 Academic Press Key Words: avalanche-type absorption; macro-ions; polystyrene spheres; colloidal silica spheres; electrophoretic light-scattering; dynamic light-scattering.
2. EXPERIMENTAL 1. INTRODUCTION
2.1. Materials Monodispersed polystyrene D1A19 spheres were purchased from Dow Chemical Co. The diameter (d o ) and standard deviation (d) from the mean diameter and polydispersity index ( d /d o ) were 220 nm, 6.5 nm, and 0.030, respectively. Colloidal silica spheres of CS91 were a gift from Catalyst & Chemicals Ind. Co. (Tokyo). The values of d o , d, and d /d o were 110 nm, 4.5 nm, and 0.041, respectively. These size parameters were determined from an electron microscope. Charge densities of strongly acidic groups were 1.32 and 0.48 mC/cm 2 for D1A19 and CS91 spheres, respectively. Sodium polyacrylate (NaPAA, degree of polymerization (DP) 5 640) was a gift from Toa Gosei Chemicals Co.
Study on the complexation (or absorption) of polyelectrolytes with colloidal particles is very important to understanding the physico-chemical properties of natural and synthetic substances such as flocculants. The main driving forces for the complexation are the attraction between colloidal particles and polyelectrolytes by the electrostatic, hydrophobic, and dipole– dipole interactions. The electrostatic and hydrophobic interactions of linear-type macro-ions with other macro-ions and/or small molecules have been studied in detail by many researchers including our group, for example (1–10). Several research1
To whom correspondence should be addressed.
565
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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FIG. 1. z-potential of D1A19 spheres in the presence of NaPAA (E), NaPMA (3), NaPSS (‚), PB (h), C2PVP (F), C4PVP (Œ), BzPVP (■), and C16BzPVP (w) at 25°C. f 5 1 3 10 25 in volume fraction, ELS method.
(Nagoya). Sodium polymethylacrylate (NaPMA, DP 5 200) was purchased from Polyscience Inc. (Warrington, PA). Sodium poly(styrene sulfonate) (NaPSS, molecular weight 5 18,000, polydispersity index; M W /M n 5 1.14) was obtained from Pressure Chemicals (Pittsburgh, PA). Polybrene (1,5-dimethyl-1,5-diazaundecamethylene polymethobromide) is an ionen-type cationic polymer purchased from the Aldrich Chemical Co. (Milwaukee, Wi). The details on the preparation and purification of poly-4-vinyl-N ethylpyridinium bromide (C2PVP), poly-4-vinyl-N-nbutylpyridinium bromide (C4PVP), poly-4-vinyl-N benzylpyridinium chloride (BzPVP), and copolymer of 4-vinyl-N-benzylpyridinium chloride (95%) and 4-vinyl-Nn-hexadecylpyridinium bromide (5%) (C16BzPVP) were described in previous papers (2, 23). The degrees of quaternization were 0.96, 0.95, 0.92, and 0.97, respectively. The degree of polymerization of the parent polymer, poly-4vinylpyridine, was 3800 by viscometry. Water used for the purification and suspension preparation
FIG. 2. z-potential of CS91 spheres in the presence of NaPAA (E), NaPMA (3), NaPSS (‚), PB (h), C2PVP (F), C4PVP (Œ), BzPVP (■), and C16BzPVP (w) at 25°C. f 5 7 3 10 25, ELS method.
FIG. 3. Effective diameter of D1A19 spheres in the presence of NaPAA (E), NaPMA (3), NaPSS (‚), PB (h), C2PVP (F), C4PVP (Œ), BzPVP (■), and C16BzPVP (w) at 25°C. f 5 1 3 10 25, ELS method.
was purified by a Milli-Q reagent grade system (Milli-RO5 plus and Milli-Q plus, Millipore Co., Bedford, MA). 2.2. Electrophoretic Light-Scattering Measurements Electrophoretic light-scattering (ELS) measurements were made on a Leza-600 ELS Zeta-meter (Otsuka Electronics, Osaka) at 25 6 0.02°C. 2.3. Dynamic Light-Scattering Measurements The dynamic light-scattering (DLS) measurements were performed on a DLS spectrophotometer (DLS-7000, Otsuka Electronics, Osaka) at 25 6 0.02°C in a cylindrical vat containing silicone oil (24). The sample of 5 mL was prepared in a Pyrex cuvette cell (12 mm outside diameter and 130 mm long). Data analysis was made with the cumulant analyses. Histogram methods including the nonnegative least square (NNLS) and the Marquadt analyses were also made for discussing the size distribution in the
FIG. 4. Effective diameter of CS91 spheres in the presence of NaPAA (E), NaPMA (3), NaPSS (‚), PB (h), C2PVP (F), C4PVP (Œ), BzPVP (■), and C16BzPVP (w) at 25°C. f 5 7 3 10 25, ELS method.
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exist for the z-potentials, it is clear that the z-potentials are larger for the more strongly hydrophobic macro-ions. The hydrophobicity of linear-type macro-ions increases in the order (2, 23) PB , C2PVP , C4PVP , BzPVP , C16BzPVP.
[1]
The driving force for the excess absorption is, therefore, ascribed to the electrostatic and hydrophobic synchronous attraction between strongly hydrophobic and anionic colloidal spheres and hydrophobic macro-cations. We should note here that the equivalent concentration of the negative charges dissociated in suspension is estimated to be 3.7 3 10 28 equiv/L, which is one-third to one-tenth of the critical concentration of cationic macro-ions, i.e., 1 3 10 27 equiv/L, where the charge reversal occur abruptly as is shown in the figure. Thus, the excess absorption of macroions takes place suddenly when the excess charge amount of macro-ions exist compared with that of spheres in suspension. This sharp charge reversal above the critical macroion concentration may support the existence of the socalled avalanche-type synchronous absorption mechanism
FIG. 5. Time dependencies of z-potential (E) and effective diameter (F) of D1A19 (a) and CS91 (b) spheres in the presence of NaPSS at 25°C. f 5 (a) 1 3 10 25, (b) 7 3 10 25, [NaPSS] 5 3 3 10 25 monoM, ELS method.
effective size of colloidal spheres including the absorbed layers of the linear-type macroions. 2.4. pH Measurements pH values of sample suspensions in test tubes were measured on a Beckman Model f34 pH meter with a glass electrode (Model 6378-10D, Horiba, Kyoto). 3. RESULTS AND DISCUSSION
3.1. Change in z-Potential of Colloidal Spheres in the Presence of Cationic or Anionic Polyelectrolytes Figure 1 shows the z-potentials of polystyrene spheres, D1A19 in the presence of various kinds of macro-ions. By the addition of a tiny amount of cationic macro-ions above 10 27 monoM z-potential increased sharply and turned to the positive values. This reversal of the signs in the z-potential from negative to positive supports strongly that the excess absorption of the cationic macro-ions compared with the amount of the anionic charges of the colloidal surfaces takes place. Though the rather large experimental errors
FIG. 6. Time dependencies of z-potential (E) and effective diameter (F) of D1A19 (a) and CS91 (b) spheres in the presence of C4PVP at 25°C. f 5 (a) 1 3 10 25 , (b) 7 3 10 25 , [C4PVP] 5 2.61 3 10 25 monoM, ELS method.
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silica spheres in the presence of cationic and anionic polyelectrolytes. The z-potential increased by the addition of the macro-cations above ca. 10 26 monoM, tenfold higher compared with the value for polystyrene spheres. However, magnitude of the increase for silica spheres was not so large compared with those for polystyrene spheres. The hydrophobicity of CS91 spheres is quite low, since the silanol moeities of the sphere surfaces are highly hydrophilic. Thus, the driving force is mainly electrostatic ones between anionic colloidal spheres and macro-cations. The z-potential vs polyelectrolyte curves were quite similar irrespective to the kind of the macro-cations, which also supports strongly that the hydrophobic interaction is not important between the silica spheres and macro-cations. Sign reversal in the z-potential caused by the excess absorption of the macro-cations on the surface of CS91 spheres is clear again. It should be further mentioned here that the avalanche-type absorption mechanism is also supported for the colloidal silica spheres. Note here that the zeta-potential decreased slightly in the presence of macro-anions, which supports the weak absorption occurs between anionic spheres and the macro-anions by the dipole– dipole type attractive forces. FIG. 7. Effective diameter of D1A19 (a) and CS91 (b) spheres in the presence of C4PVP at 25°C: 3 h (E), 1 day (3), and 2 days (‚) after sample preparation. f 5 7 3 10 25, ELS method.
between the colloidal spheres and oppositely charged macro-ions; below the critical concentration of macro-ions the attractive forces between colloidal spheres and the macro-cations are not strong enough to be bound. It should be mentioned here that no flocculation took place in all the experiments in this work. Though most suspensions of this work have no foreign salts, the effect of the electrical double layers formed around colloidal spheres has been neglected on the z-potential and the effective size from the electrophoretic light-scattering and dynamic lightscattering measurements. However, this effect is significant only for the highly deionized suspension as has been discussed previously from the dynamic light-scattering measurements (24). Addition of anionic macro-ions in the colloidal suspension does not affect the z -potential of spheres so much, which supports no or weak absorption of anionic macro-ions takes place on the colloidal spheres. Figure 1 shows that the NaPSS, strongly hydrophobic macro-anions (2, 23), decreases the z-potential, and the weak absorption is supported. Figure 2 shows the change in z-potential of colloidal
FIG. 8. Effective diameter of D1A19 spheres in the presence of NaPSS, 3 h (a), 1 day (b), and 2 days (c) after sample preparation at 25°C. f 5 1 3 10 25: (E) DLS method; (3) ELS.
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spheres. The critical concentration ranged between 10 27 and 10 26 monoM. It is highly plausible that the association of the colloidal spheres mediated with the macro-cations occurs for the complexation between the colloidal spheres and the macrocations. Studies of the cryogenic electron microscopic observation are now in progress in our laboratory in order to clarify the association. We have now a few experimental evidences to support no association of colloidal spheres; the alternative multilayered absorption upto ten layers of macro-cations and macro-anions on the colloidal spheres have been observed without any association of each colloidal spheres (25). However, final conclusion will be obtained after the cryogenic electron microscope observation of the suspension. 3.3. Kinetic Aspects of the Complexation of Colloidal Spheres with Macroions Figure 5 shows the time dependencies of z-potential (open circles) and the effective diameter (solid circles) of polystyrene (a) and silica spheres (b), respectively after mixing of the spheres with NaPSS macro-ions. The z-potential increased with time. The effective diameter also seems to FIG. 9. z-Potential (E, ‚) and effective diameter (F, Œ) of D1A19 (a) and CS91 (b) spheres in the presence of sodium chloride at 25°C. f 5 (a) 3 3 10 25, (b) 7 3 10 25: (E, F) [NaPSS] 5 3 3 10 25 monoM; (‚, Œ) [BzPVP] 5 3 3 10 25 monoM, ELS method.
3.2. Change in the Effective Diameter of Colloidal Spheres Including the Absorbed Layers in the Presence of Cationic or Anionic Polyelectrolytes Figure 3 shows the effective diameter (d*) of colloidal spheres of D1A19 including the adsorbed layers of the macro-ions. Though the d* values contain rather large experimental errors, they increased as concentration of the macro-cations and also macro-anions increased above the critical macro-ion concentration around 10 26 monoM. The d* values seem to increase in the order for the macrocations: PB , C2PVP , C4PVP , BzPVP , C16BzPVP.
[2]
It is interesting to note that the maximum value of d* with C16BzPVP was fivefold larger than the D1A19 spheres themselves without the macro-cations. On the other hand, the effective diameters of CS91 spheres increased abruptly by tenfold in maximum by the addition of cationic macro-ions as is clear in Fig. 4, which supports the cationic macro-ions being bound on the silica surfaces more loosely compared with the absorption on the polystyrene
FIG. 10. Effective diameter of D1A19 (a) and CS91 (b) spheres in the presence of C4PVP at 25°C: (E) f 5 (a) 2 3 10 27, (b) 2 3 10 26; (‚) f 5 (a) 1 3 10 26, (b) 1 3 10 25; (h) f 5 (a) 1 3 10 25, (b) 7 3 10 25, ELS method.
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3.4. Influence of Foreign Salt and Sphere Concentration on the Complexation
FIG. 11. Effective diameter of D1A19 (a) and CS91 (b) spheres in the presence of NaPSS at 25°C: (E) f 5 (a) 2 3 10 27, (b) 2 3 10 26; (‚) f 5 (a) 1 3 10 26, (b) 1 3 10 25; (h) f 5 (a) 1 3 10 25, (b) 7 3 10 25, ELS method.
increase with time, though the experimental errors are rather large. It seems to take more than 15 h before the binding is completed. Figure 5 supports that the binding forces between likely charged colloidal spheres and macros-ions are so weak that it takes a long time for completion of the stable complexation. Figure 6 shows changes in the z-potential and the effective diameter with time when the macro-cations (C4PVP) are mixed with polystyrene and silica spheres. Clearly, both increased and reached the equilibrium values within 5 h. Figure 7 shows the change in the effective diameters of D1A19 and CS91 spheres determined by ELS measurements 3 h, 1 day, and 2 days after mixing of colloidal spheres with macro-cations. Clearly, the complexation was completed within 3 h when the experimental error was taken into account. Figure 8 shows the results for the addition of the macroanions, NaPSS. In this case, absorption occurred only when the excess amount of the macro-anions was added. DLS and ELS measurements gave the similar data of the effective diameter. Furthermore, d* values 3 h, 1 day, and 2 days after the mixing were quite similar to each other, which strongly supports complexation being completed within 3 h.
Figure 9 shows changes of z-potentials and d* values by the addition of sodium chloride for the NaPSS1D1A19 (a), BzPVP1D1A19 (a), NaPSS1CS91 (b), and BzPVP1CS91 (b) complexes. Interestingly, z-potential values were insensitive to or approched to zero when sodium chloride was added. Effective diameter decreased but very slightly as the concentration of NaCl increased. The latter results will be explained with the shrinking of the absorbed macro-ion layers by the shielding effect of the salt on the intra- and/or inter-macro-ion electrostatic repulsion (26, 27). Figure 10 shows the d* vs [C4PVP] plots for C4PVP1D1A19 (a) and C4PVP1CS91 complexes (b), where the sphere concentrations changed from 2 3 10 27 to 1 3 10 25 in volume fraction for D1A19 spheres and from 2 3 10 26 to 7 3 10 25 for CS91 spheres, respectively. Surprisingly, sharp increase in the effective diameter was observed only when the sphere concentration is high. This may support the idea that the synchronous adsorption accompanied with the association of several spheres mediated with the macro-cations occurs favorably when the sphere concentration is high. Studies on the association phenomena of the colloidal spheres are now in progress using the technique of the cryogenic electron microscope as described above. Figure 11 shows the effective diameters of the complexes between colloidal spheres and NaPSS. When a large amount of the macro-anions was added, the d* increased especially for polystyrene spheres, which means that the hydrophobic attraction between phenyl groups of the macro-anions and the spheres surpasses the electrostatic repulsion between them. ACKNOWLEDGMENTS This work was supported by a grant-in-aid from the Ministry of Education and Culture, Japan. Drs. M. Komatsu and M. Hirai of Catalysts & Chemicals Ind. Co. (Tokyo and Kitakyusyu) are deeply thanked for providing the silica samples. Professor Akira Tsuchida of Gifu University is acknowledged for his valuable comments.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
Okubo, T., and Ise, N., J. Phys. Chem. 73, 1488 (1969). Okubo, T., and Ise, N., J. Am. Chem. Soc. 95, 2293, 4031 (1973). Okubo, T., and Ise, N., J. Biol. Chem. 249, 3563 (1974). Kitano, H., Tanaka, M., and Okubo, T., J. Chem. Soc., Perkin Trans. 2 1074 (1976). Mita, K., Okubo, T., and Ise, N., J. Chem. Soc., Faraday Trans. 1 72, 504 (1976). Maruno, T., Okubo, T., and Ise, N., Ber. Bunsenges. Phys. Chem. 85, 667 (1981). Turro, N. J., and Okubo, T., J. Phys. Chem. 86, 1535 (1982). Okubo, T., J. Colloid Interface Sci. 125, 386 (1988). Okubo, T., and Mohri, N., Macromolecules 21, 2744 (1988).
ABSORPTION OF MACRO-IONS ON COLLOIDS 10. Okubo, T., and Kobayashi, K., J. Colloid Interface Sci. 205, 433 (1998). 11. Stromberg, R. R., Tutas, D. J., and Possaglia, E., J. Phys. Chem. 69, 3955 (1965). 12. Ottewill, R. H., and Walker, T., Kolloid Z. Z. Polym. 227, 108 (1968). 13. Killmann, E., and Eckart, R., Makromol. Chem. 144, 65 (1971). 14. Fleer, G. J., Koopal, L. K., and Lyklema, J., Kolloid Z. Z. Polym. 250, 689 (1972). 15. Varoqui, R., and Dejardin, P., J. Chem. Phys. 66, 4395 (1977). 16. Napper, D. H., “Polymeric Stabilization of Colloidal Dispersions.” Academic Press, London, 1983. 17. Hunter, R. J., “Foundations of Colloidal Science,” Vols. I and II. Oxford University Press, Oxford, 1989. 18. Okubo, T., Polym. Bull. 23, 211 (1990).
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19. de L. Costello, B. A., Luckham, P. F., and Tadros, Th. F., Langmuir 8, 464 (1992). 20. Fleer, G. J., Scheutjens, J. H. M. H., Cohen-Stuart, M. A., Cosgrove, T., and Vincent, B., “Polymers at Interfaces.” Chapman and Hall, London, 1993. 21. Tadros, Th. F., Colloids Surfaces A 91, 39 (1994). 22. Grover, G. S., and Bike, S. G., Langmuir 11, 1807 (1995). 23. Ishiwatari, T., Okubo, T., and Ise, N., J. Polym. Sci. 18, 1807 (1980). 24. Okubo, T., Kiriyama, K., Nemoto, N., and Hashimoto, H., Colloid Polym. Sci. 274, 93 (1996). 25. Okubo, T., and Suda, M., Colloid Polym. Sci., in press. 26. Rice, S. A., and Nagasawa, M., “Polyelectrolyte Solutions.” Academic Press, London, 1961. 27. Morawetz, H., “Macromolecules in Solution.” Wiley, New York, 1965.
Absorption of Polyelectrolytes on Colloidal Surfaces as Studied by Electrophoretic and Dynamic Light-Scattering Techniques Tsuneo Okubo 1 and Mitsuhiro Suda Department of Applied Chemistry, Gifu University, Gifu 501-1193, Japan Received November 10, 1998; accepted February 17, 1999
ers have clarified the electrostatic and nonelectrostatic interactions of colloidal spheres with polyelectrolytes (11–22). In our previous work (18), the abrupt sign reversal in the electrophoretic mobility and the effective valency of a colloidal sphere has been observed by the addition of the various kinds of oppositely charged macro-ions and their low valent simple ions such as calcium and lanthanum ions. This supports strongly that the excess amount of the macro-ions against the amount of the charges on the colloidal surfaces is absorbed in the manner of the so-called avalanche-type synchronous absorption mechanism, i.e., the absorption of macro-ions occurs abruptly on the colloidal surfaces above the critical concentration of macro-ions added. Below the critical concentration, absorption does not occur so significantly. However, in our previous work this important absorption mechanism has not been discussed at all, since size information of the adsorbed spheres has not been available. In this paper, z-potential and the effective diameters of colloidal spheres absorbed with the macro-ions have been studied systematically as possible in order to clarify the synchronous nature in the absorption.
z-Potential and the effective diameter of the colloidal spheres absorbed with the macro-cations and macro-anions are studied by the electrophoretic light-scattering and dynamic light-scattering measurements. Colloidal spheres used are monodispersed polystyrene (220 nm in diameter) and colloidal silica spheres (110 nm). Macro-ions used are sodium polyacrylate, sodium polymethylacrylate, sodium poly(styrene sulfonate), and poly-4-vinyl pyridines quaternized with ethyl bromide, n-butyl bromide, benzyl chloride, and 5% hexadecyl bromide and 95% benzyl chloride. Reversal of colloidal surface charges from negative to positive occurs abruptly above the critical concentration of macroions by the excess absorption of the macro-cations onto the anionic colloidal spheres, i.e., avalanche-type absorption. The effective diameter of colloidal spheres including the absorbed layers increases substantially by four- to tenfold. In the presence of large amount of macro-cations aggregation of colloidal spheres mediated by the layers of absorbed macro-cations may occur. Absorption also occurs on the anionic colloidal spheres in the presence of an excess amount of macro-anions by the dipole– dipole-type attractive interactions. © 1999 Academic Press Key Words: avalanche-type absorption; macro-ions; polystyrene spheres; colloidal silica spheres; electrophoretic light-scattering; dynamic light-scattering.
2. EXPERIMENTAL 1. INTRODUCTION
2.1. Materials Monodispersed polystyrene D1A19 spheres were purchased from Dow Chemical Co. The diameter (d o ) and standard deviation (d) from the mean diameter and polydispersity index ( d /d o ) were 220 nm, 6.5 nm, and 0.030, respectively. Colloidal silica spheres of CS91 were a gift from Catalyst & Chemicals Ind. Co. (Tokyo). The values of d o , d, and d /d o were 110 nm, 4.5 nm, and 0.041, respectively. These size parameters were determined from an electron microscope. Charge densities of strongly acidic groups were 1.32 and 0.48 mC/cm 2 for D1A19 and CS91 spheres, respectively. Sodium polyacrylate (NaPAA, degree of polymerization (DP) 5 640) was a gift from Toa Gosei Chemicals Co.
Study on the complexation (or absorption) of polyelectrolytes with colloidal particles is very important to understanding the physico-chemical properties of natural and synthetic substances such as flocculants. The main driving forces for the complexation are the attraction between colloidal particles and polyelectrolytes by the electrostatic, hydrophobic, and dipole– dipole interactions. The electrostatic and hydrophobic interactions of linear-type macro-ions with other macro-ions and/or small molecules have been studied in detail by many researchers including our group, for example (1–10). Several research1
To whom correspondence should be addressed.
565
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
566
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FIG. 1. z-potential of D1A19 spheres in the presence of NaPAA (E), NaPMA (3), NaPSS (‚), PB (h), C2PVP (F), C4PVP (Œ), BzPVP (■), and C16BzPVP (w) at 25°C. f 5 1 3 10 25 in volume fraction, ELS method.
(Nagoya). Sodium polymethylacrylate (NaPMA, DP 5 200) was purchased from Polyscience Inc. (Warrington, PA). Sodium poly(styrene sulfonate) (NaPSS, molecular weight 5 18,000, polydispersity index; M W /M n 5 1.14) was obtained from Pressure Chemicals (Pittsburgh, PA). Polybrene (1,5-dimethyl-1,5-diazaundecamethylene polymethobromide) is an ionen-type cationic polymer purchased from the Aldrich Chemical Co. (Milwaukee, Wi). The details on the preparation and purification of poly-4-vinyl-N ethylpyridinium bromide (C2PVP), poly-4-vinyl-N-nbutylpyridinium bromide (C4PVP), poly-4-vinyl-N benzylpyridinium chloride (BzPVP), and copolymer of 4-vinyl-N-benzylpyridinium chloride (95%) and 4-vinyl-Nn-hexadecylpyridinium bromide (5%) (C16BzPVP) were described in previous papers (2, 23). The degrees of quaternization were 0.96, 0.95, 0.92, and 0.97, respectively. The degree of polymerization of the parent polymer, poly-4vinylpyridine, was 3800 by viscometry. Water used for the purification and suspension preparation
FIG. 2. z-potential of CS91 spheres in the presence of NaPAA (E), NaPMA (3), NaPSS (‚), PB (h), C2PVP (F), C4PVP (Œ), BzPVP (■), and C16BzPVP (w) at 25°C. f 5 7 3 10 25, ELS method.
FIG. 3. Effective diameter of D1A19 spheres in the presence of NaPAA (E), NaPMA (3), NaPSS (‚), PB (h), C2PVP (F), C4PVP (Œ), BzPVP (■), and C16BzPVP (w) at 25°C. f 5 1 3 10 25, ELS method.
was purified by a Milli-Q reagent grade system (Milli-RO5 plus and Milli-Q plus, Millipore Co., Bedford, MA). 2.2. Electrophoretic Light-Scattering Measurements Electrophoretic light-scattering (ELS) measurements were made on a Leza-600 ELS Zeta-meter (Otsuka Electronics, Osaka) at 25 6 0.02°C. 2.3. Dynamic Light-Scattering Measurements The dynamic light-scattering (DLS) measurements were performed on a DLS spectrophotometer (DLS-7000, Otsuka Electronics, Osaka) at 25 6 0.02°C in a cylindrical vat containing silicone oil (24). The sample of 5 mL was prepared in a Pyrex cuvette cell (12 mm outside diameter and 130 mm long). Data analysis was made with the cumulant analyses. Histogram methods including the nonnegative least square (NNLS) and the Marquadt analyses were also made for discussing the size distribution in the
FIG. 4. Effective diameter of CS91 spheres in the presence of NaPAA (E), NaPMA (3), NaPSS (‚), PB (h), C2PVP (F), C4PVP (Œ), BzPVP (■), and C16BzPVP (w) at 25°C. f 5 7 3 10 25, ELS method.
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exist for the z-potentials, it is clear that the z-potentials are larger for the more strongly hydrophobic macro-ions. The hydrophobicity of linear-type macro-ions increases in the order (2, 23) PB , C2PVP , C4PVP , BzPVP , C16BzPVP.
[1]
The driving force for the excess absorption is, therefore, ascribed to the electrostatic and hydrophobic synchronous attraction between strongly hydrophobic and anionic colloidal spheres and hydrophobic macro-cations. We should note here that the equivalent concentration of the negative charges dissociated in suspension is estimated to be 3.7 3 10 28 equiv/L, which is one-third to one-tenth of the critical concentration of cationic macro-ions, i.e., 1 3 10 27 equiv/L, where the charge reversal occur abruptly as is shown in the figure. Thus, the excess absorption of macroions takes place suddenly when the excess charge amount of macro-ions exist compared with that of spheres in suspension. This sharp charge reversal above the critical macroion concentration may support the existence of the socalled avalanche-type synchronous absorption mechanism
FIG. 5. Time dependencies of z-potential (E) and effective diameter (F) of D1A19 (a) and CS91 (b) spheres in the presence of NaPSS at 25°C. f 5 (a) 1 3 10 25, (b) 7 3 10 25, [NaPSS] 5 3 3 10 25 monoM, ELS method.
effective size of colloidal spheres including the absorbed layers of the linear-type macroions. 2.4. pH Measurements pH values of sample suspensions in test tubes were measured on a Beckman Model f34 pH meter with a glass electrode (Model 6378-10D, Horiba, Kyoto). 3. RESULTS AND DISCUSSION
3.1. Change in z-Potential of Colloidal Spheres in the Presence of Cationic or Anionic Polyelectrolytes Figure 1 shows the z-potentials of polystyrene spheres, D1A19 in the presence of various kinds of macro-ions. By the addition of a tiny amount of cationic macro-ions above 10 27 monoM z-potential increased sharply and turned to the positive values. This reversal of the signs in the z-potential from negative to positive supports strongly that the excess absorption of the cationic macro-ions compared with the amount of the anionic charges of the colloidal surfaces takes place. Though the rather large experimental errors
FIG. 6. Time dependencies of z-potential (E) and effective diameter (F) of D1A19 (a) and CS91 (b) spheres in the presence of C4PVP at 25°C. f 5 (a) 1 3 10 25 , (b) 7 3 10 25 , [C4PVP] 5 2.61 3 10 25 monoM, ELS method.
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silica spheres in the presence of cationic and anionic polyelectrolytes. The z-potential increased by the addition of the macro-cations above ca. 10 26 monoM, tenfold higher compared with the value for polystyrene spheres. However, magnitude of the increase for silica spheres was not so large compared with those for polystyrene spheres. The hydrophobicity of CS91 spheres is quite low, since the silanol moeities of the sphere surfaces are highly hydrophilic. Thus, the driving force is mainly electrostatic ones between anionic colloidal spheres and macro-cations. The z-potential vs polyelectrolyte curves were quite similar irrespective to the kind of the macro-cations, which also supports strongly that the hydrophobic interaction is not important between the silica spheres and macro-cations. Sign reversal in the z-potential caused by the excess absorption of the macro-cations on the surface of CS91 spheres is clear again. It should be further mentioned here that the avalanche-type absorption mechanism is also supported for the colloidal silica spheres. Note here that the zeta-potential decreased slightly in the presence of macro-anions, which supports the weak absorption occurs between anionic spheres and the macro-anions by the dipole– dipole type attractive forces. FIG. 7. Effective diameter of D1A19 (a) and CS91 (b) spheres in the presence of C4PVP at 25°C: 3 h (E), 1 day (3), and 2 days (‚) after sample preparation. f 5 7 3 10 25, ELS method.
between the colloidal spheres and oppositely charged macro-ions; below the critical concentration of macro-ions the attractive forces between colloidal spheres and the macro-cations are not strong enough to be bound. It should be mentioned here that no flocculation took place in all the experiments in this work. Though most suspensions of this work have no foreign salts, the effect of the electrical double layers formed around colloidal spheres has been neglected on the z-potential and the effective size from the electrophoretic light-scattering and dynamic lightscattering measurements. However, this effect is significant only for the highly deionized suspension as has been discussed previously from the dynamic light-scattering measurements (24). Addition of anionic macro-ions in the colloidal suspension does not affect the z -potential of spheres so much, which supports no or weak absorption of anionic macro-ions takes place on the colloidal spheres. Figure 1 shows that the NaPSS, strongly hydrophobic macro-anions (2, 23), decreases the z-potential, and the weak absorption is supported. Figure 2 shows the change in z-potential of colloidal
FIG. 8. Effective diameter of D1A19 spheres in the presence of NaPSS, 3 h (a), 1 day (b), and 2 days (c) after sample preparation at 25°C. f 5 1 3 10 25: (E) DLS method; (3) ELS.
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spheres. The critical concentration ranged between 10 27 and 10 26 monoM. It is highly plausible that the association of the colloidal spheres mediated with the macro-cations occurs for the complexation between the colloidal spheres and the macrocations. Studies of the cryogenic electron microscopic observation are now in progress in our laboratory in order to clarify the association. We have now a few experimental evidences to support no association of colloidal spheres; the alternative multilayered absorption upto ten layers of macro-cations and macro-anions on the colloidal spheres have been observed without any association of each colloidal spheres (25). However, final conclusion will be obtained after the cryogenic electron microscope observation of the suspension. 3.3. Kinetic Aspects of the Complexation of Colloidal Spheres with Macroions Figure 5 shows the time dependencies of z-potential (open circles) and the effective diameter (solid circles) of polystyrene (a) and silica spheres (b), respectively after mixing of the spheres with NaPSS macro-ions. The z-potential increased with time. The effective diameter also seems to FIG. 9. z-Potential (E, ‚) and effective diameter (F, Œ) of D1A19 (a) and CS91 (b) spheres in the presence of sodium chloride at 25°C. f 5 (a) 3 3 10 25, (b) 7 3 10 25: (E, F) [NaPSS] 5 3 3 10 25 monoM; (‚, Œ) [BzPVP] 5 3 3 10 25 monoM, ELS method.
3.2. Change in the Effective Diameter of Colloidal Spheres Including the Absorbed Layers in the Presence of Cationic or Anionic Polyelectrolytes Figure 3 shows the effective diameter (d*) of colloidal spheres of D1A19 including the adsorbed layers of the macro-ions. Though the d* values contain rather large experimental errors, they increased as concentration of the macro-cations and also macro-anions increased above the critical macro-ion concentration around 10 26 monoM. The d* values seem to increase in the order for the macrocations: PB , C2PVP , C4PVP , BzPVP , C16BzPVP.
[2]
It is interesting to note that the maximum value of d* with C16BzPVP was fivefold larger than the D1A19 spheres themselves without the macro-cations. On the other hand, the effective diameters of CS91 spheres increased abruptly by tenfold in maximum by the addition of cationic macro-ions as is clear in Fig. 4, which supports the cationic macro-ions being bound on the silica surfaces more loosely compared with the absorption on the polystyrene
FIG. 10. Effective diameter of D1A19 (a) and CS91 (b) spheres in the presence of C4PVP at 25°C: (E) f 5 (a) 2 3 10 27, (b) 2 3 10 26; (‚) f 5 (a) 1 3 10 26, (b) 1 3 10 25; (h) f 5 (a) 1 3 10 25, (b) 7 3 10 25, ELS method.
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3.4. Influence of Foreign Salt and Sphere Concentration on the Complexation
FIG. 11. Effective diameter of D1A19 (a) and CS91 (b) spheres in the presence of NaPSS at 25°C: (E) f 5 (a) 2 3 10 27, (b) 2 3 10 26; (‚) f 5 (a) 1 3 10 26, (b) 1 3 10 25; (h) f 5 (a) 1 3 10 25, (b) 7 3 10 25, ELS method.
increase with time, though the experimental errors are rather large. It seems to take more than 15 h before the binding is completed. Figure 5 supports that the binding forces between likely charged colloidal spheres and macros-ions are so weak that it takes a long time for completion of the stable complexation. Figure 6 shows changes in the z-potential and the effective diameter with time when the macro-cations (C4PVP) are mixed with polystyrene and silica spheres. Clearly, both increased and reached the equilibrium values within 5 h. Figure 7 shows the change in the effective diameters of D1A19 and CS91 spheres determined by ELS measurements 3 h, 1 day, and 2 days after mixing of colloidal spheres with macro-cations. Clearly, the complexation was completed within 3 h when the experimental error was taken into account. Figure 8 shows the results for the addition of the macroanions, NaPSS. In this case, absorption occurred only when the excess amount of the macro-anions was added. DLS and ELS measurements gave the similar data of the effective diameter. Furthermore, d* values 3 h, 1 day, and 2 days after the mixing were quite similar to each other, which strongly supports complexation being completed within 3 h.
Figure 9 shows changes of z-potentials and d* values by the addition of sodium chloride for the NaPSS1D1A19 (a), BzPVP1D1A19 (a), NaPSS1CS91 (b), and BzPVP1CS91 (b) complexes. Interestingly, z-potential values were insensitive to or approched to zero when sodium chloride was added. Effective diameter decreased but very slightly as the concentration of NaCl increased. The latter results will be explained with the shrinking of the absorbed macro-ion layers by the shielding effect of the salt on the intra- and/or inter-macro-ion electrostatic repulsion (26, 27). Figure 10 shows the d* vs [C4PVP] plots for C4PVP1D1A19 (a) and C4PVP1CS91 complexes (b), where the sphere concentrations changed from 2 3 10 27 to 1 3 10 25 in volume fraction for D1A19 spheres and from 2 3 10 26 to 7 3 10 25 for CS91 spheres, respectively. Surprisingly, sharp increase in the effective diameter was observed only when the sphere concentration is high. This may support the idea that the synchronous adsorption accompanied with the association of several spheres mediated with the macro-cations occurs favorably when the sphere concentration is high. Studies on the association phenomena of the colloidal spheres are now in progress using the technique of the cryogenic electron microscope as described above. Figure 11 shows the effective diameters of the complexes between colloidal spheres and NaPSS. When a large amount of the macro-anions was added, the d* increased especially for polystyrene spheres, which means that the hydrophobic attraction between phenyl groups of the macro-anions and the spheres surpasses the electrostatic repulsion between them. ACKNOWLEDGMENTS This work was supported by a grant-in-aid from the Ministry of Education and Culture, Japan. Drs. M. Komatsu and M. Hirai of Catalysts & Chemicals Ind. Co. (Tokyo and Kitakyusyu) are deeply thanked for providing the silica samples. Professor Akira Tsuchida of Gifu University is acknowledged for his valuable comments.
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