Langmuir 1993,9, 2020-2023
2020
Simultaneous Adsorption of Poly(vinylpyrro1idone) and Cationic Surfactant from Their Mixed Solutions on Silica Kunio Esumi' and Michiyo Oyama Department of Applied Chemistry, Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan Received March 5, 1993. In Final Form: May 18,1993 The adsorption of poly(vinylpyrro1idone)(PVP)and dodecyltrimethylammoniumchloride (DTAC) on silica has been investigated as a function of DTAC concentration. The adsorption of PVP decreases gradually with increasing DTAC concentration, whereas the adsorption of DTAC increases, but the magnitude of the increase in the adsorption is smaller than that in the absence of PVP. The colloid stabilityof the silica dispersionis poor at low concentrations of PVP,but is enhanced at higher concentrations of PVP. Electron spin resonance measurementsindicatethat the conformationof adsorbed PVP is relatively flat at low concentrationsof PVP,while the conformation becomes more extended at higher concentrations. The stability of the silica dispersion is thus well correlated with the change in the conformation of PVP adsorbed.
Introduction
It is well known192 that ionic surfactants interact with nonionic polymers, probably due to formation of polymersurfactant complexes similar to polyelectrolytes. These polymer-surfactant interactions have been studied by use of various techniques. The adsorption and orientation of polymers and surfactants have not been extensively studied&' a t the solid/ liquid interface; however, it is important to the understanding of the mechanisms of stabilization and flocculation of dispersions and emulsions. Tadros' reported that poly(vinyl alcohol) adsorption increased significantly in the presence of preadsorbed cationic surfactant on silica a t high pH, while at low pH the cationic surfactant adsorbed to a great extent in the presence of preadsorbed poly(vinyl alcohol). However, adequate information on the conformation of nonionic polymers adsorbed on particles in the presence of surfactants is not yet available, whereas the conformation of nonionic polymers adsorbed a t the solid/liquid interface without surfactants has been studied using many methods. These include small neutron scattering, neutron and X-ray reflectivity, NMR, ellipsometry, internal reflection spectroscopy, electron spin resonance (ESR), and various other technique^.^*^ The object of this paper was to investigate the interaction between dodecyltrimethylammonium chloride (DTAC) and poly(vinylpyrro1idone) (PVP)on silica in aqueous solution. Further, the conformation of PVP adsorbed on silica in the presence of DTAC was also investigated by ESR measurements. Experimental Section Poly(vinylpyrro1idone)(av MW = 40 OOO) was obtained from Wako Chemical Ltd. and used without further purification. The (1)Robb, I. D. In Anionic Surfactants; Lucaseen-Reynders, E. H., Ed.; Surfactant Science Series No. 11; Marcel Dekker: New York, 1981; pp 109-142. (2)Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants; Rubingh, D. N., Holland, P. M., E&.; Surfactant Science Series No. 37;Marcel Dekker: New York, 1991;pp 189-248. (3)Somasundaran, P.J. Colloid Interface Sci. 1969,31,667. (4)Tadmi, Th.F. J. Colloid Interface Sci. 1974,46,628. ( 6 ) Chibowski, S.J. Colloid Interface Sci. 1980,76, 371. (6)Ma, C.; Li, C. J. Colloid Interface Sci. 1989, 131,485. (7)Otauka, H.; & m i , K. Submitted for publication. (8) Takahashi, A.; Kawaguchi, M. Adu. Polym. Sci. 1982,46,1. (9)Cosgrove, T.J. Chem. SOC.,Faraday Trans. 1990,86,1323.
spin-labeled PVP was prepared10 by polymerizing N-vinylpyrrolidoneand allylamine using tert-butyl perbenzoate as initiator and ethanol as solvent, followed by a reaction with 4-ieothiocyanato-2,2,6,6-tetramethylpiperidine-l-oxyl in dichloromethane at 40 O C . The spin-labeledPVP thus obtained was purified with ethylether. The molar ratio of N-vinylpyrrolidone to allylamine in the spin-labeledPVP determined by NMR was about 1W3. The molecular weight of the labeledpolymer determined by static light scattering was about 16 OOO. Dodecyltrimethylammonium chloride was supplied by Tokyo Kasei Co., and purified with acetone. Silicaparticleswere supplied by Nippon Aerosol Co., and their average diameter and specific surface area were 1 pm and 144.5 mZ/g,respectively. The water used in all experiments was purified by a Milli-Q system (NihonMillipore Co.) in which the specificconductivity of water fell below 0.1 p S cm-*. The adsorptionof DTAC and PVP was obtainedby measuring their concentrationin solution before and after adsorption at 25 O C . All suspensions in the presence of 10 mmol dm" NaCl were about pH 6 and were shaken to reach an equilibrium condition for 24 h. The concentrationof PVP was determined by using an UV spectrophotometerand that of DTAC by the dye method." The concentrationof spin-labeledPVP was determinedby ESR. The ESR spectra were recorded on a JEOL JES FE 3-X spectrometer utilizing a 100-kHzfield modulation and X-band microwaves. The slurries for the ESR measurementa were prepared by centrifugation of the adsorption samples. The stability of the silica dispersion was evaluated by measuring the absorbance at 600 nm of the top portion of the aqueous suspension in a sedimentationtube kept for 1 day after the adsorption. A high absorbance indicates high dispersion stability, whereas a low absorbance indicates a flocculated or settled state. The f potential of suspensions was measured with an electroacoustic system (ESA 8o00, Matec Applied Sciences). The electrokineticsonic amplitude (ESA) is the pressure amplitude per unit electricfield generated by the particles. The amplitude of the wave is a functionof the magnitude of the charges displaced per particle or dynamiclhigh-frequency mobility, the particle concentration in the liquid, and the amplitude of the electric field. The measured values of ESA are directly proportional to the electrophoretic mobility. The ESA has been shown to correlate with the { potential.12 Most of the measurementa were carried out at 25 O C . 1 1974,70, 1186.
(11)Few, A. V.; Ottewill, R. H. J. Colloid Sci. 1966,11, 34. (12)OBrien, R. W.J. Fluid Mech. 1988,190, 71.
0743-746319312409-2020$04.OO/0 0 1993 American Chemical Society
Simultaneous Adsorption of PVP and Cationic Surfactant
Langmuir, Vol. 9, No. 8, 1993 2021
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Figure 1. Adsorption isotherms of (a) PVP and (b) DTAC on silica from aqueous solution.
Results and Discussion Figure 1 shows the adsorption isotherms of PVP and DTAC on silica from the respective aqueous solutions. As can be seen, the PVP adsorption exhibits a strong affinity with the silica surface due to a sharp increase in the adsorption at very low PVP concentrations and shows a nearly horizontal plateau in the concentration range of 0.7-1.2 g dm-3. It is knownl3 that at low PVP concentrations the typical conformation of PVP chain molecules at the silica/aqueous solution interface consists of a sequence of many short loops and trains and at higher coverage the bound fraction decreasesdue to the formation of a relatively dense loop layer, with a few long tails protruding into the solution. The interaction between PVP and the silica surface would mainly be governed by hydrogen-bonding. In addition, since PVP molecules are slightlypositivelycharged, an electrostaticattraction force will operate between negatively charged silica and positively charged PVP. In the adsorption of DTAC, twostep adsorption was observed The adsorption gradually increased to reach a first plateau. With a further increase in the DTAC concentration, a gradual increase in adsorption occurred toward a second plateau, which was reached just before the bulk critical micelle concentration of DTAC. This adsorption is similar to the adsorption of dodecyltrimethylammonium bromide onto precipitated silica.14 The adsorption mechanism of DTAC can be deduced from Figure 2. At low concentrationsof DTAC, the DTAC head groups interact with the negative sites on the silica surface, orienting them with their hydrocarbon chains to the solution, resulting in the potential changing from a negative value to zero. As the concentration increases a bilayer is formed by hydrophobic interaction between the (13) Cohen Stuart, M. A.; Fleer, G. J.; Bijeterboech, B. H. J. Colloid Interface Sci. 1982,90,310,321. (14) Bijsterbosch, B. H. J. Colloid Interface Sci. 1974,47, 186.
(16) Schwuger, M. J. J. Colloid Interface Sci. 1975,43,491. (16) Saito, S. J. Colloid Interface Sci. 1967,24, 227.
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Figure 3. Competitive adsorption of PVP and DTAC on silica in the presence of PVP. PVP concentration: (a)0.2; (b) 1.0;(c) 1.8 g dm".
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Figure 4. 5 potential of silica in the PVP-DTAC system. P W concentration: 0.2 ( 0 ) ;1.0 (A);1.8 (0) g dm".
in the {potential. On the other hand, the silica dispersion in the presence of PVP alone except in the concentration region of 0.1-0.4g dm4 PVP was very stable. In the PVPDTAC system, the stability of the silica dispersion decreased with increasing DTAC concentration and reached a minimum, and then increased in the presence of PVP (1.0 and 1.8 g dm-3). However, in the presence of
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Figure 6. Change in the stability of the silica dispersion with (a)DTAC and PVP and with (b) the PVP-DTAC system. PVP concentration: 0.2 (0);1.0 (A);1.8 ( 0 )g dma.
PVP (0.2g dm-9 the silica was flocculated, showing a low dispersion stability over a wide concentration region of DTAC. Thus, the stability of the silica dispersion in the PVP-DTAC system is affected by adsorption of PVP and DTAC. It seems likely that the electrostatic repulsion force is less significantsince silica havingalarger {potential in the presence of PVP (0.2g dm4) showsa lower dispersion stability compared with that having a smaller { potential in the presence of PVP (1.0and 1.8 g dm-3). To elucidate the conformationof PVP adsorbed on silica, the fraction of segments adsorbed in trains @) was estimated from ESR spectra of the adsorbed polymer using spin-labeled PVP. A two-component spectrum subtraction method to estimate a value of p has been used by Robb et al.17 Recently, Sakaiet a1.lSobtained the fraction of segments adsorbed in trains from three different ESR spectra assigned in trains, short loops, and long loops or tails. In the present study, we used two different spectra of spin-labeled PVP having different degrees of motional freedom. These spectra are shown in Figure 6: (a) a free polymer solution of a low viscosity at 10 "C; (b) a solution cooled to the frozen state at -120"C. From the observation of the apparent line shape, the two spectra correspond roughly to the two components of the adsorbed polymer. (17) Robb, I. D.; Smith,R. Polymer 1977,18,500. (18)Sakai, H.;Imamura, Y.Bull. Chem. SOC.Jpn. 1980,53,3457.
Langmuir, Vol. 9, No. 8,1993 2023
Simultaneous Adsorption of PVP and Cationic Surfactant
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Figure 6. ESR spectra of spin-labeledPVP: (a) a free polymer aqueoussolutionof a lowviscosity at 10O C ; (b)an aqueoussolution cooled to the frozen state at -120 "C. These two reference spectra are superimposed upon one another with suitable intensity ratios to reproduce observed spectra. In the polymer chain lying on the solid surface, the sequences of the segments directly bound to the surface should have restricted motion, while the detached segments should be much more mobile. Accordingly, the component in a very strongly immobilized state can be assigned to the train segments. The other component, in a relatively mobile state, is assigned to the free segments, forming a short loop or a long loop. It is noteworthy that the spin labels do not have a stronger affinity for the silica surface than the original vinylpyrrolidone monomer, suggesting that the spin-labeled PVP gives a reasonable representation of the polymer configuration at the silicalsolution interface. We calculated p values from matching observed spectra with the two reference spectra. Figure 7 shows that the p value decreases gradually with increasing DTAC concentration in the presence of higher concentrations of PVP (1.0and 1.8 g dm-9, while the p value shows an almost constant and high value over the whole DTAC concentration region in the presence of a lower concentration of PVP (0.2 g dm-9. Although the adsorbed amount of the labeled PVP on silica is smaller to some extent compared to that of the unlabeled PVP due to their molecular weight difference, it is expected that the conformational change in the adsorbed PVP is not sensitive to the molecular weight of PVP. Actually, it has been reportedl9 that the p value is not significantly affected by the molecular weight of PVP. ~
(19)Robb, I. D.;Smith, R.Eur. Polym. J. 1974, IO, 1006.
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Figure 7. Change in p value for the PVP-DTAC system. PVP 1.0 (A); 1.8 ( 0 )g dm-9. concentration: 0.2 (0);
The above results suggest that, for high concentrations of PVP (1.0and 1.8 g dm-9, PVP adsorbs on silica mainly in trains in the low concentration region of DTAC (2-10 mmol dm-9 whereas the flocculation of silica may be caused by a bridging mechanism. The subsequent high dispersion stability is due to a large steric repulsion force because the p value is relatively low. On the other hand, the large p value in a low concentration of PVP over the whole concentration region of DTAC indicates that some form of bridging of one polymer molecule between two particles may be involved in flocculation mechanisms.l9 Therefore, it is concluded that the dispersion of silica is greatly affected by the conformation of PVP adsorbed for the PVP-DTAC system; the stability of the silica dispersion is enhanced with an increase of the steric repulsion force, Le., the fraction of segments in loops or tails.
Conclusions The simultaneous adsorption of PVP and DTAC on silica is reported. The adsorption of PVP decreased slightly with an increase in the concentration of DTAC, while that of DTAC increased. The silica dispersion showed low stability at a low concentration of PVP, but the stability increased markedly at high concentrations of PVP. Through electrokinetic potential and ESR measurements, we showed that the dispersion stability of silica is significantly affected by the steric repulsion force rather than the electrostatic repulsion force.
