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Simultaneous Adsorption of Poly(1-vinylpyrrolidone-co-acrylic acid) and Sodium Dodecyl Sulfate at Alumina/Water Interface Kentaro Sakagami, Tomokazu Yoshimura, and Kunio Esumi* Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received March 12, 2002. In Final Form: May 31, 2002 The simultaneous adsorption of poly(1-vinylpyrrolidone-co-acrylic acid) (PVcA) and sodium dodecyl sulfate (SDS) on positively charged alumina particles is carried out at pH 5.2 by measuring the amount of PVcA and SDS adsorbed, ζ potential, and sedimentation rate of alumina suspensions. In addition, conformation of PVcA and SDS adsorbed using their corresponding spin-labeled ones is estimated. The amount of PVcA adsorbed increases sharply and becomes a plateau with an increase of PVcA concentration, while a typical adsorption isotherm of SDS is obtained. In the simultaneous adsorption of PVcA and SDS under fixed initial concentrations of PVcA, the amount of PVcA adsorbed increases and reaches a maximum, and then decreases with SDS concentration. The enhancement in the adsorption is due to the interaction between SDS and PVcA as confirmed by surface tensiometry. The dispersion stability of alumina suspensions by the simultaneous adsorption of PVcA and SDS is considerably high compared with that by SDS adsorption alone. The high dispersion stability is due to the extended conformation of the adsorbed PVcA and the electrostatic repulsion force. Further, the mobility of SDS molecules in the PVcA-SDS adsorbed layer is found to be more restricted than that in the SDS adsorbed layer alone.
Introduction The adsorption characteristics of surfactant-polymer binary mixtures are an important criterion to control the dispersion of suspensions.1 The knowledge base has been used in the development of various industrial products, including cosmetics, detergents, and pharmaceuticals. Surfactants are generally used to control the dispersion, flocculation, and wetting properties of suspensions, while water-soluble polymers serve to meet rheological requirements. The coadsorption behavior of surfactants and polymers at the solid/liquid interface depends on their interactions in bulk solution. Certain surfactant-polymer combinations exhibit a very weak interaction, while others interact strongly, in more specific manner. In the former case, the surfactant and polymer may compete for adsorption. For example, despite its high affinity to R-alumina, the anionic polymer poly(acrylic acid) (PAA) adsorbed on positively charged R-alumina is replaced by sodium dodecyl sulfate (SDS) with increasing SDS concentration.2 By contrast, in the latter case, the simultaneous adsorption of the anionic surfactant and neutral polymer will be favored, as in the familiar case of poly(vinylpyrrolidone) (PVP) and SDS mixtures. The driving force for the complexation of SDS-PVP in bulk solution is due to either hydrophobic interaction between the SDS hydrocarbon chain and the PVP polyethylene backbone3-5 or electrostatic attraction between the SDS headgroup and the PVP polarizable pyrrolidone side group (pearl-necklace model6-11). (1) Otsuka, H.; Esumi, K. Structure-Performance Relationships in Surfactants; Esumi, K., Ueno, M., Eds.; Marcel Dekker: New York, 1997; Chapter 12. (2) Esumi, K.; Sakagami, K.; Torigoe, K. J. Jpn. Soc. Colour Mater. 2001, 74, 444. (3) Gilanyi, T.; Wolfram, E. Colloids Surf. 1981, 3, 181. (4) Sesta, B.; Segre, A. L.; D’Aprano, A.; Proietti, N. J. Phys. Chem. B 1997, 101, 198. (5) Wang, G.; Olofsson, G. J. Phys. Chem. B 1998, 102, 9276. (6) Chari, K.; Lenhart, W. C. J. Colloid Interface Sci. 1990, 137, 204. (7) Chari, K. J. Colloid Interface Sci. 1992, 151, 294.
Adsorption of PVP from PVP-SDS binary solutions on positively charged titanium dioxide,12,13 iron oxide,14,15 and R-alumina16,17 is heavily dependent on SDS concentration. PVP itself does not readily adsorb onto these surfaces. However, the amount of adsorbed PVP increases rapidly with SDS concentration, particularly at low SDS concentration, up to maximum and thereafter decreasing sharply. This behavior has been attributed to the formation of surface complexes between SDS and PVP. At low SDS concentration, PVP binds via a hydrophobic interaction to the adsorbed SDS, the hydrocarbon tail of which extends into the solution. As the SDS concentration increases, SDS and PVP form polyelectrolyte-like complexes in bulk solution concomitantly with the formation of SDS bilayers on solid surfaces. The adsorption of PVP is inhibited by the resultant electrostatic repulsion between the solution complexes and bilayers. This implies that, in the presence of excess SDS, the chemical potential of SDS-PVP complexes formed on the solid surface rises above that of the complexes in solution.18 As already described, PVP itself does not adsorb onto R-alumina, whereas PAA adsorbs strongly onto it. Accordingly it is basically very interesting to investigate adsorption behavior of a copolymer of vinylpyrrolidone and acrylic acid onto R-alumina. Further, it is substantially (8) Nikas, Y. J.; Blankschtein, D. Langmuir 1994, 10, 3512. (9) Norwood, D. P.; Minatti, E.; Reed, W. F. Macromolecules 1998, 31, 2957. (10) Sukul, D.; Pal, S. K.; Mandal, D.; Sen, S.; Bhattacharyya, K. J. Phys. Chem. B 2000, 104, 6128. (11) Li, Y.; Xu, R.; Blor, D. M.; Penfold, J.; Holzwarth, J. F.; WynJones, E. Langmuir 2000, 16, 8677. (12) Ma, C. Colloids Surf. 1985, 16, 185. (13) Esumi, K.; Sakai, K.; Torigoe, K.; Suhara, T.; Fukui, H. Colloids Surf. A 1999, 155, 413. (14) Ma, C.; Li. C. J. Colloid Interface Sci. 1989, 131, 485. (15) Ma, C.; Li, C. Colloids Surf. 1990, 47, 117. (16) Esumi, K.; Mizuno, K.; Yamanaka, Y. Langmuir 1995, 11, 1571. (17) Esumi, K.; Iitaka, M.; Torigoe, K. J. Colloid Interface Sci. 2000, 232, 71. (18) Shimabayashi, S.; Uno, T.; Nakagaki, M. Colloids Surf. A 1997, 123, 283.
10.1021/la0257236 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/12/2002
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Scheme 1. Chemical Structures of PVcA, Spin-labeled Polymer, and SDS
Figure 1. Adsorption isotherms of SDS and PVcA on alumina.
important to characterize a simultaneous adsorption of SDS and the copolymer. In the present study, a simultaneous adsorption of SDS and poly(1-vinylpyrrolidone-co-acrylic acid) onto R-alumina was investigated by measuring the amount of SDS and the polymer adsorbed, ζ potential, and sedimentation rate of alumina suspensions. Conformation of SDS and the polymer adsorbed was also estimated using the elcetron spin resonance (ESR) technique. Experimental Section Materials. R-Alumina of 99.995% purity was kindly supplied by Showa Denkou K. K., and its specific surface area and average diameter were 8.9 m2 g-1 and 2.51 µm, respectively. The polymer used was a copolymer of vinylpyrrolidone and acrylic acid (PVcA) MW ) 96 000 and was obtained from Aldrich Chemical Co. The ratio of vinylpyrrolidone to acrylic acid was 75:25 wt %, and the polymer was used without purification. SDS was obtained from Tokyo Kasei Co. and was purified by recrystallization from ethanol several times. Spin-labeled PVcA and SDS were synthesized according to the literature.19,20 The molar ratio of acrylic acid to the spinlabel determined from ESR measurements was about 200:1. The chemical structures of PVcA and spin-labeled compounds are given in Scheme 1.The water used was purified through a Milli-Q Plus system. The other chemicals were of analytical grade. Methods and Measurements. The amounts of SDS and PVcA adsorbed were determined by a depletion method. The pH value adjusted after mixtures of PVcA and SDS were added to alumina suspensions was about 5.2. All suspensions (0.1 g of alumina/10 cm3 H2O) in screw-vialed sample tubes were equilibrated for 24 h by agitation in a water bath at 25 °C. Subsequently, the suspensions were centrifuged at 15 000 rpm using a Beckman Avanti 30 centrifuge for 15 min and filtered through a 0.2 µm PTFE membrane filter. The clear supernatant solution was analyzed using a high-performance liquid chromatograph (Shiseido Nanospace); the residual concentration of PVcA was determined with a UV-vis detector 2002 at 210 nm using a TSKGEL column (Super SW 3000) and that of SDS using a Shodex RI 74 RI detector and a CAPCELL PAK C18 (UG 120) column. Water was used as a mobile phase for PVcA and a mixture of (19) Cafe´, M. C.; Robb, I. D. Polymer 1979, 20, 513. (20) Yamaguchi, T.; Yamaguchi, A.; Kimoto, E.; Kimizuka, H. Bull. Chem. Soc. Jpn. 1980, 53, 372.
methanol and water (78:22 in volume) containing 0.04 mol dm-3 NaCl for SDS. The ζ potential of alumina suspensions was measured using an electrophoretic apparatus (Pen Kem 500); ζ potentials were converted from electrophoretic mobilities using the Smoluchowski equation, µ ) ζE/4πη, where µ is the electrophoretic mobility, is the dielectric constant, E is electric field, and η is the viscosity. The dispersion stability of alumina suspensions was evaluated by measuring the sedimentation rate using a Turbiscan MA 2000 (Formulaction). The suspension equilibrated was added to a test tube and placed in a Turbican, and the sedimentation rate was measured from transmittance change of the test tube with elapsed time. The ESR spectra were measured using a JEOL JES-FA200 spectrometer operating at 100 kHz field modulation and X-band microwaves. After the adsorption equilibria, the suspensions were centrifuged at 2000 rpm for 5 min; the solid slurries obtained were used for ESR measurements. The conformation of the adsorbed polymer molecules was established from the shape of the ESR spectra as detailed previously.21,22 The static surface tension of aqueous surfactant/polymer solutions was measured using a Kru¨ss K122 tensiometer by the Wilhelmy plate technique. All measurements were carried out at 25 °C.
Results and Discussion Adsorption isotherms of PVcA and SDS on the alumina at pH 5.2 are given in Figure 1. As the ζ potential of alumina at this pH is positive, it is expected that a strong interaction between SDS and positively charged alumina particles occurs. From the adsorption isotherm of SDS, the occupied area of SDS is calculated as about 0.2 nm2 at the plateau where a SDS bilayer is formed by the hydrophobic interaction between the first and second layer of SDS on the alumina. In the case of PVcA, the adsorption of PVcA increased sharply and reached a plateau with the PVcA concentration. The main driving forces for the adsorption are probably due to electrostatic attraction forces between negatively charged carboxyl groups of PVcA and positively charged alumina sites. These electrostatic interactions are confirmed from the ζ potential measurements (Figure 2): for both systems the positive ζ potentials decreased to zero and reached some negative ζ potentials with the SDS and PVcA concentration. In addition, the change in the ζ potential is in good agreement with the change in the adsorption. The results of simultaneous adsorption of PVcA/SDS are shown in Figure 3, where the initial concentrations of PVcA are fixed at 0.05 and 0.3 g dm-3, respectively. In both cases, one can see that the amount of PVcA adsorbed increases at low SDS concentration, up to a maximum, and thereafter decreases sharply. On the other hand, the
Simultaneous Adsorption of PVcA and SDS on Alumina
Figure 2. Change in ζ potential of alumina suspensions by adsorption of SDS and PVcA.
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Figure 4. Surface tensions of SDS and SDS-PVcA aqueous solutions. The closed triangle and square show the surface tensions of PVcA alone (0.05 and 0.3 g dm-3.
Figure 5. Change in sedimentation rate of alumina suspensions by adsorption of SDS and SDS-PVcA.
Figure 3. Simultaneous adsorption of SDS and PVcA on alumina at fixed concentration of PVcA: (a) 0.05 g dm-3; (b) 0.3 g dm-3.
adsorption of SDS in the presence of PVcA was slightly smaller than that in the absence of PVcA. The enhancement in the adsorption of PVcA is more remarkable in the system of 0.3 g dm-3 PVcA than in 0.05 g dm-3. Since aqueous solution properties of SDS and PVcA will influence their adsorption behavior at the solid/liquid interface, the surface tension of their mixtures was measured. Figure 4 shows that PVcA solution itself is considerably surface active, and in the PVcA/SDS mixed system, the surface tensions are quite low in comparison with SDS alone. This low surface tension at a region between 1 and 6 mmol dm-3 SDS is attributed to the formation of some complexes
between vinylpyrrolidone groups of PVcA and SDS. At higher SDS concentrations the surface tensions increase and reach the value of SDS alone in which the complexes behave like a hydrophilic polyelectrolyte. Based on the results of adsorption isotherm and surface tension measurements, we propose the following coadsorption model: at very low SDS concentrations SDS monomers and PVcA molecules adsorb on the alumina and then PVcA molecules adsorb to a certain extent on the SDS-coated layer through hydrophobic interactions. With increasing SDS concentration the SDS-PVcA complexes in bulk adsorb electrostatically, anchoring by a negatively charged segment. However, when a SDS bilayer is formed, PVcA adsorption is inhibited due to electrostatic repulsion between the SDS-PVcA complexes and SDS bilayer. The dispersion stability of alumina suspensions is influenced by adsorption of PVcA and SDS. From the sedimentation rate shown in Figure 5, it is seen that the sedimentation rate in the SDS system alone increases, up to maximum, and thereafter decreases with the SDS concentration, while in the PVcA/SDS system the sedimentation rates are extremely small in the whole SDS concentration region for both fixed concentrations of PVcA. It is found that in the mixed system of PVcA/ SDS an extremely stable suspension is obtained. This behavior is different from that in the mixed system of PVP/SDS,21 which is similar to the dispersion behavior of SDS system alone. In addition, the ζ potential of alumina suspensions (21) Otsuka, H.; Esumi, K. Langmuir 1994, 10, 45.
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Figure 7. Changes in P values by adsorption of SL-PVcA alone and of SDS-SL-PVcA.
Figure 6. ESR spectra of spin-labeled PVcA (SL-PVcA) adsorbed on alumina: (a) from SL-PVcA alone; (b) from SDSSL-PVcA (initial concentration of SL-PVcA 0.3 g dm-3).
decreased from -4 to -30 mV with SDS concentration for the mixed system of PVcA (0.05 g dm-3) and SDS, while it decreased from -15 to -30 mV for the mixed system of PVcA (0.3 g dm-3) and SDS. This high stability may be induced by electrostatic repulsion force as well as a steric hindrance caused by PVcA adsorbed. To estimate the conformation of PVcA adsorbed on the alumina, the ESR spectra of the adsorbed polymer were measured using a spin-labeled PVcA. When a spin-labeled polymer adsorbs on a solid, if the label is attached to segments extending away from the surface into solution in the form of loops and tails, a motionally narrowed spectrum is obtained. On the other hand, a broad anisotropic spectrum is obtained when the label is attached to trains of segments in contact with the surface. Figure 6 shows the ESR spectra of spin-labeled PVcA adsorbed on the alumina for the PVcA and PVcA/SDS systems at pH 5.2. It is important to note here that adsorption of spin-labeled PVcA is almost the same as that without spin-label. The procedure for analyzing the ESR spectra is principally the same as those described previously.21,22 The ESR spectra of the nitroxide spin-label are very sensitive to the mobility of the given segment. When the motion is relatively fast (rotational correlation time 3 × 10-9s-3 × 10-11 s), the spectrum consists of three wellresolved derivatives of Lorentzian lines which can be calculated completely theoretically using the very simple theory of Kivelson.23 When the motion is slower, the shape of the spectrum is influenced by the anisotropic part of the spin Hamiltonian and new characteristic features appear which can be explained through the theory of (22) Sakai, H.; Imamura, Y. Bull. Chem. Soc. Jpn. 1987, 60, 1261. (23) Kivelson, D. J. Chem. Phys. 1960, 33, 1107.
Schneider and Freed.24 The parameters of the spin Hamiltonian, the gyromagnetic tensor, and the hyperfine tensor have been determined using the solution spectrum and the rigid limit powder spectrum at the temperature of liquid nitrogen. At high temperature the motion is rather fast and the first kind of spectrum is the most important. At lower temperature, the motion becomes slow and the second kind of spectrum is detected. In an intermediate range, the spectrum appears as a superposition of the two precedent types and has been interpreted with a twostate model, analyzing both contributions and evaluating their respective weights with computer simulation. These two characteristic environments have been attributed to trains in close contact with the surface and rather immobilized and to loops and tails extending into a solution with rather fast motion. The ratio of the two populations corresponds to the ratio of the integrals of the absorption spectra that are characteristic for each environment.25 On the basis of development of this two-state model, Sakai et al.22 suggested that any composite ESR spectra can be separated into three components which may be assigned to signals from segments adsorbed in trains, short loops, and long loops, or tails. We obtained three different spectra of spin-labeled polymer which have different degrees of motional freedom: (a) unadsorbed in water at 25 °C, (b) at high temperature of 140 °C, and (c) in frozen solutions at -120 °C. Apparently, the chain mobility decreased in the order of (a), (c), and (b). From the observation of the apparent line shape, the three spectra were found to correspond roughly to the three components of the adsorbed polymer, i.e., trains, loops, and tails. These three model spectra were superimposed upon one another with suitable intensity ratios to reproduce the observed spectrum. The amplitude of each model spectrum was determined by a least-squares method involving multiple regression to fit a summation of the three spectra with the observed spectrum. In a three-component analysis, a close curve fitting is found in every case, so that the ESR spectra of PVcA adsorbed on alumina are composed of three portions which have different local-chain mobilities. Consequently, the component in a very strongly immobilized state can be assigned to the train segments, where the fraction of train segments is expressed as P. In (24) Schneider, D. J.; Freed, J. In Bimaterial Magnetic Resonance; Berliner, L. J., Reuben, J., Eds.; Plenum: New York, 1989; Vol. 8. (25) Hommel, H.; Legrand, A. P.; Ben Ouada, H.; Bouchriha, H.; Balard, H.; Papirer, E. Polymer 1992, 33, 181.
Simultaneous Adsorption of PVcA and SDS on Alumina
Figure 7, both systems show relative low P values over a whole concentration region, suggesting that the PVcA adsorbed takes mainly loop or tail conformation from adsorption of PVcA alone or of mixed solutions of PVcA/ SDS. It is interesting to note that the P values obtained from spin-label PAA alone range between 0.8 and 0.9, indicating that PAA adsorbs predominantly as train segments on alumina.26 To confirm the mobility of SDS molecules in the adsorbed layer on the alumina, the ESR measurements using a spin-labeled SDS were also carried out. From the rotational correlation time for the system of spin-labeled SDS alone, faster mobility for SDS alone with respect to PVcA when adsorbed onto alumina has been observed. On the other hand, lower mobility for PVcA-SDS coadsorbed onto alumina at low SDS concentration has been obtained due to the interactions between SDS and vinylpyrrolidone groups of PVcA. At high SDS concentration, as found in the absence of PVcA, the mobility has increased since the adsorption of PVcA is not significant. Such differing behavior in the SDS mobility for the single and mixed systems has also been observed in the measurements of interaction forces using colloidal probe atomic force microscopy.27 (26) Ishizuki, K.; Esumi, K. Langmuir 1997, 13, 1587. (27) Sakai, K.; Yoshimura, T.; Esumi, K. Langmuir 2002, 18, 3993.
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Conclusions Simultaneous adsorption from binary solutions of PVcA/ SDS on R-alumina has been elucidated through adsorption isotherm, ζ potential, sedimentation rate, and ESR measurements. In the case of PVcA adsorption alone, PVcA adsorbs appreciably on the alumina due to electrostatic attraction force between the acrylic acid group of PVcA and positively charged sites of alumina. In the case of simultaneous adsorption, the amount of adsorbed PVcA adsorbed increases with SDS concentration, particularly at low SDS concentrations, up to maximum and thereafter decreases. The enhancement in the adsorption of PVcA at low SDS concentrations is attributed to the adsorption of PVcA-SDS complexes as well as formation of surface aggregates between PVcA and SDS. In addition, the system of binary solutions of PVcA/SDS exhibits a very stable dispersion of alumina over a whole SDS concentration region, where the adsorbed PVcA chains at low SDS concentrations causes steric hindrance. It is also found that the mobility of SDS adsorbed in the case of PVcA/ SDS system is more restricted than that in the case of SDS system alone. LA0257236
