Langmuir 1994,10, 45-50
45
Simultaneous Adsorption of Poly(vinylpyrro1idone) and Anionic Hydrocarbon/Fluorocarbon Surfactant from Their Binary Mixtures on Alumina Hidenori Otsuka and Kunio Esumi' Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku,Tokyo 162, Japan Received March 10,1993. In Final Form: June 25, 199P The adsorption of lithium dodecyl sulfate (LiDS)/lithiumperfluorooctanesulfonate (LiFOS)and poly(vinylpyrrolidone) (PVP)from PVP-LIDS and PVP-LiFOS binary mixed solutions on positively charged alumina has been investigated. The conformation of PVP adsorbed on alumina is also estimated by using ESR. In the PVP-LiDS system, the adsorption of LiDS is shown to be two distinct breakpoints in the presence of PVP, while the adsorption of PVP increases remarkably due to the presence of LiDS at low LiDS concentration,followed by a decrease at high LiDS concentration. It is suggested that this remarkable increase in the adsorption is due to complex formation between PVP and LiDS on the alumina surface. A similar trend is observed in the PVP-LiFOS system. In both PVP-LiDS and PVP-LiFOS systems, the fractions of train segments in the adsorbed PVP steeply increase with the surfactant concentration, and they show a maximum, followed by remaining constant with their segments in trains in spite of the remarkable decrease in the adsorption of PVP. Also, the fraction of train segments in the PVP-LiFOS system is greater than that in the PVP-LiDS system over the whole surfactant concentration region. The results obtained from both dispersion stability and {potential correspond to the changes in the adsorption isotherm and values of the fraction of train segments.
Introduction There have been many studies13 on interactionsbetween nonionic water-soluble polymers and Surfactants in aqueous solutions. When surfactants are mixed with polymers in aqueous solution, complex formation takes place by the binding of the surfactants onto the polymers. Factors influencing polymer-surfactant complexation, such as temperature, salt, surfactant chain length, polymer structure, hydrophobicity, and a variety of surfactants have been studied.'P2 It is especially interesting that nonionic water-soluble polymers have a stronger interaction or complex more favorably with anionic surfactants in aqueous solutions than with cationic or nonionic surfactant~.'-~Although various hydrocarbon surfactants have been used in studies of interactions between polymers and Surfactants, there are only a few reports4p6 about interactions between polymers and perfluorinated surfactants. Complexation of a fluorocarbon surfactant with polymer can be expected to be different from that of a hydrocarbon surfactant, because the fluorocarbon chain has both hydrophobic and lipophobic properties.@ On the other hand, very little experimental work has been reportedg-13 on interactions between polymers and 0 Abstract published in Advance ACS Abstracts, December 15, 1993. (1)Robb, I. D. In Anionic Surfactant in Physical Chemistry of Surfactant Action; Lucassen-Reynders, E. H., Ed.; Dekker: New York, 1981;p 109. (2)Goddard, E. D. Colloids Surf. 1986,19,255. (3)Hayakawa, K.;Kwak, J. C. T. In Cationic Surfactants-Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Dekker: New York, 1991;p 189. (4)Nojima, T.; Esumi, K.; Meguro, K. J. Am. Oil Chem. SOC.1992,69,
64.
(5)Eeumi, K.; Takehana, K.; Nojima, T.; Meguro, K. Colloids Surf. 1992,64,15. (6)Meguro, K.; Ueno, M.; Suzuki, T. J. Jpn. Oil Chem. SOC.1982,31,
909.
(7)Muto,Y.;Aeada,M.;Takasawa,K.;Esumi,K.;Meguro,K. J.Colloid Interface Scr. 1988,124,632. (8)Meguro, K.;Muto, Y.; Sakurai, F.; Esumi, K. Phenomena in Mixed Surfactant Systems; Scamehom, J. F., Ed.;ACS Symposium Series, No. 31; American Chemical Society: Washington, DC, 1986; p 61.
surfactants at the solid/liquid interface, although it is important for many industrial fields such as stabilization and flocculation of dispersions and emulsions. Recently Ma et al.14have reported that the adsorption of sodium dodecyl sulfate (SDS) and poly(vinylpyrro1idone)(PVP) from their mixed solutions onto the surface of titanium dioxide showed synergistic effects at low SDS concentrations but antagonistic effects at high SDS concentrations. Such a synergistic effect is mainly governed by a complex formation of PVP and SDS. Conformation of polymer adsorbed on particles seems important to understand stability of the particle dispersion. Adsorbed polymer layers have also been characterized by several parameters such as adsorbed amount, average thickness of adsorbed layer, and fraction of bound segments. Reliable information on the fraction of bound segments of adsorbed polymers can be obtained from various techniques including NMR16 and ESR.16JT In this work, we will discuss the interaction of lithium dodecyl sulfate (LiDS)/lithium perfluorooctanesulfonate (LiFOS)and PVP from their mixed solutions on a-alumina by measuring adsorbed amount, [potential, and turbidity. The conformation of PVP adsorbed was also estimated by using ESR.
Experimental Section Materials. The polymer was prepared by polymerizing N-vinylpyrrolidone (0.5 mol) and allylamine (0.11 mol), both obtained from Tokyo Kasei Co. and distilled just before use, using tert-butylperbenzoateas initiator and ethanol as solvent. (9) Esumi, K.;Yokokawa,M.J . Jpn. SOC.Colour Mater. 1992,65,142.
(10)Esumi, K.; Masuda, A.; Otsuka, H. Langmuir 199'3,9,284. (11)Tadros, Th. F. J. Colloid Interface Sci. 1974,46,528. (12)Ma, C. M.; Li, C. J . Colloid Interface Sci. 1989,131,485. (13)Kilau, H.W.; Voltz, J. 1. Colloids Surf. 1991,57,17. (14)Ma, C. M. Colloids Surf. 1986,16,185. (15)Cosgrove, T.;Griffiths, P. C. Adv. Colloid Interface Sci. 1992,42, 175. (16)Fox, B. K. K.; Robb, I. D.; Smith, R. J. Chem. SOC.,Faraday Tram. 1, 1974,70, 1186. (17)Sakai, H.; Asakura, T.; Suzuki, K.; Horie, K. Bull. Chem. SOC. Jpn. 1981,54,2180.
0 1994 American Chemical Society 0743-7463/94/2410-0045~04.50~0
Otsuka and Esumi
46 Langmuir, Vol. 10, No. 1, 1994
High-resolution NMR analysis indicated that the allylamine content was about 3 mol % Themolecularweightof the polymer was about 16OOO, as determined by static light scattering. The polymer was reprecipitated 3 times and then reacted with 4-isothiocyanate2,2,6,6-tetramethylpiperidinooxyl (used as supplied by Aldrich ChemicalCo. Inc.). The polymer,labeled at the NH1group,lewas reprecipitated to remove unreacted spin label. The spin-labeledpolymer was purified by repeated precipitations; the ethanol solution was poured into ether. Lithium dodecyl sulfate (LiDS)and lithium perfluorooctanesulfonate (LiFOS) were synthesized.'* LiDS was purified by repeated crystallizations from ethanol and a mixture of hexane and 2-propanol and LiFOS from dioxane after extraction with ethanol. The purity of these sampleswas confirmedby elemental analysis and by the lack of a minimum in the surface tension. The critical micelle concentrations (cmc's) of LiDS and LiFOS, estimated from fluorescencespectra of pyrene-3-carboxaldehyde (PyCHO)which were obtained from Aldrich ChemicalCo., Inc., in the presence of 10 mmol dm3 LiN03 were 4.1 and 4.0 mmol dm" at 25 "C, respectively.1s a-Alumina of 99.995% purity was supplied by Showa Denko K.K. The specific surface area, determined from the nitrogen gasadsorption at -196 OC,was 10.1m2gland the averageparticle diameter was 500 nm, as determined by means of a particle analyzer based on photocorrelation spectroscopy (Autosizer Model 700, Malvern Co., Ltd.). The water used in this study was purified by passingit through a Milli-Q System (Nihon Millipore Co.) where ita specific conductivity fell below 0.1 p S cm-1. Methods and Measurements. Ten milliliters of spin-labeled PVP and surfactant aqueous solutions was added to 0.3 g of alumina in a test tube, followed by stirring for 24 h to attain equilibrium of adsorption at 25 OC before centrifuging at loo00 rev min-1. All the suspensions were adjusted to pH 3.5 by adding a dilute solution of HNO:,in the presenceof 10mmol dm4LiNOB. The adsorbed amounts of spin-labeled PVP, LiDS, or LiFOS on alumina were determined from the differences in concentrations before and after adsorption. The concentration of spinlabeled PVP was determined by ESR, while those of LiDS and LiFOS were determined by the Abott method.20 It should be pointed out that the presence of each component in the spinlabeled PVP-surfactant mixed solutions scarcelyinterfered with the determination of concentration for the other component. The ESR spectra were recorded on a JEOL JES FE 3-X spectrometer utilizing 100-kHz field modulation and X-band microwaves. The slurries for the ESR measurements were prepared by centrifugation of the adsorption samples. In order to estimate dispersion stability, the transmittance of aqueous suspension taken from the top portion of the sedimentation tube kept for 1day after the adsorption was measured at 600 nm with an Hitachi 220A double-beam UV spectrophotometer. Alow transmittance indicates high dispersionstability, whereas a high transmittance indicates flocculated or settled state. The f potential of suspension was measured with an electroacoustic system (ESA 8000,Matec Applied Sciences).21
.
Results a n d Discussion Figures 1and 2 show the adsorption of anionic surfactants and PVP on alumina from PVP-LiDS/PVP-LiFOS mixed aqueous solutions containing fixed initial concentrations of PVP (0.2 and 0.5 g dm-9 as a function of surfactant. In the PVP-LIDS system, the amount of PVP adsorbed increased remarkably with the LiDS concentration and showed a maximum and then decreased for both the fixed concentrations of PVP. The adsorption of LiDS also increased with the LiDS concentration and reached a plateau in the absence of PVP, whereas in the (18) Suzuki, T.;Esumi, K.; Meguro, K. J.Colloid Interface Sci. 1983, 93,205. (19) Esumi, K.; Otauka, H.; Meguro, K. J.Colloid Interface Sci. 1990, 136, 224. (20)Abott, D. C. Analyet (London) 1962,87, 286. (21) OBrien, R. W. J. Fluid Mech. 1988, 190, 171.
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presence of PVP the adsorbed amount of LiDS was less compared with that of LiDS alone. It is noteworthy that there are two distinct breakpoints in the adsorption of LiDS. These breakpoints have been observed in the mixed aqueous solution^:^*^^ At the first breakpoint (TI), association of PVP and LiDS in the bulk phase begins. The second one (T2) corresponds to a concentration in which PVP is saturated with LiDS for complex formation. At concentrations greater than T2, the regular micelles of LiDS and PVP-LiDS complex coexist in the bulk phase. As can be seen in Figure 1, TIwas almost independent of the fixed concentration of PVP,while the value of T2 increased as the fixed concentration of PVP increased. A similar trend was observed for the PVP-LiFOS system shown in Figure 2. The remarkable increase in the adsorption of PVP occurred quite close to the first breakpoint (7'1) for both systems. This result can be explained by a view that PVP-surfactant complexes are formed on alumina. Since it is known4922 that PVP and an anionic surfactant can form a polyelectrolyte-like complex by hydrophobic interaction in aqueous solution, it is reasonable to assume12J4that the same interaction ~
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(22) Murata, M.; Arai, H. J. Colloid Interface Sci. 1973, 44, 475.
Interactions between Polymers and Surfactants may take place and a type of surface complex of PVP and surfactant forms a t the solid/liquid interface. The anionic surfactant (below the cmc) adsorbs much faster on the alumina surface than PVP by the electrostatic attraction force between negative charged hydrophilic groups of the surfactant and positive charged sites on alumina with its hydrophobicchain extendedtoward the water phase. Then, PVP attaches to the hydrophobic chain to the inner adsorbed layer of the anionic surfactant. This model also applies to surfactant concentrations greater than the cmc, but instead of a bilayer of PVP-surfactant attachments, there might also be patches of hemimicelles to which PVP attaches. As a result, PVP and the surfactant adsorbed on alumina are able to combine with each other by hydrophobic binding, so that more PVP and surfactant are adsorbed. Because the adsorption of surfactant onto alumina is much stronger than that of PVP, the enhancement on PVP adsorption in the presence of surfactant is much more obvious (the maximum adsorption of PVP, 2.92 X 103g g', that of LiDS, 7.49 X 106 mol g-l). The fact that this enhancement of PVP adsorbed on alumina in the presence of surfactant becomes rather distinct at the surfactant concentration far below the 21' in the bulk phase implies that the formation of PVP-surfactant complex at the alumina/solution interface begins at a much lower concentration than that in the bulk solution. With increasing concentration of the anionic surfactant, the adsorption of PVP rapidly decreases because PVP-anionic surfactant complexes formed in the bulk are relatively surface inactive and scarcely adsorb at the alumina/ solution interface. In addition, the formation of surfactant bilayer on alumina may also prevent the formationof PVPanionic surfactant complex on alumina with increasing concentration of the anionic surfactant. It is interesting to note that at the maximum adsorption the amounts of LiDS bound to the PVP are 1.85 and 1.56 mol of LiDS/ mol of monomer of PVP in the presence of 0.2 and 0.5 g dm-3 PVP, while they are 1.67 and 1.30mol of LiFOS/mol of monomer of PVP. These values are similar to those in aqueous solution.4 Further, the decrement in the adsorption of PVP after the maximum adsorption with increasing surfactant concentration is more remarkable for the PVP-LiFOS system than that for the PVP-LiDS system. This implies that the PVP-surfactant complex is a more favorable energy state for LiFOS than for LiDS in bulk. T o elucidate the change of the conformation of PVP adsorbed on alumina with increasing surfactant concentration, the ESR spectra of spin-labeled PVP on alumina were measured (Figures3 and 4). The line shapes indicate the diverse motion of the adsorbed chain. The ESRsignals from the nitroxide labels attached to the polymer chain reflect sensitively the changesin themotion of the adsorbed chain segments arising from the differences in the conformational states, whether in the loop or the train.l6l23 For free polymer in solution undergoing Brownian motion, movement is relatively unrestricted and motionally narrowed three-line ESR spectra are obtained as a result of isotropic tumbling. If, however, the movement of the polymer segments is restricted, for example, by precipitating the polymer or increasing the solvent viscosity, then the shape of the spectrum changes due to anisotropic broadening (Figures 3 and 4). For polymers adsorbed on to surfaces, a motionally narrowed spectrum is obtained if the label is attached to segments extending away from the surface into solution in the form of loops and tails. On (23)Kobayashi, K.; Yajima, H.; Imamura, Y.; Endo, R. Bull. Chem. SOC.Jpn. 1990,63,1813.
Langmuir, Vol. 10, No. 1, 1994 47 I
20G c . (
Figure 3. Change in ESR spectra of the spin-labeledP W (0.5 g dm4) adsorbed on alumina with various LiDS concentrations.
V
20G
Figure 4. Change in ESR spectra of the spin-labeled PVP (0.6 g dma) adsorbed on aluminawith various LiFOS concentrations.
the other hand, a broad anisotropic spectrum is obtained if the label is attached to trains of segments in contact with, or in close proximity to, the surface. In Figures 3 and 4, the ESR spectra are gradually broadening with increasing LiDS/LiFOS concentrations. The spectral intensity ratio of the wings (marked with A in Figures 3 and 4) to the inner parta (B)increased with an increase of the surface coverage. This shows an increase in the content of the immobile segments. That is, as the surface coveragewas increased, very broad immobile-type spectra were obtained, indicating that the PVP molecules adsorb with the majority of their segments close to the surface in trains. Glass reportedU that water-soluble polymers at low concentration adsorb at the interface with a high proportion of their segments as trains, but as the con-
Oteuka and Esumi
48 Langmuir, Vol. 10,No.1, 1994 a
PVP ; 0.2gdm-' D PVP ; 0.59 dm"
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Figure 5. ESR spectra used for simulation of spin-labeledPVP adsorbed on alumina: (top) (a) unadsorbed in water at 10 "C, (b) in the adsorbed state at -120 O C , (c) in the molten state at 140 O C ; (center) (d) adsorbed on a-alumin in water at 25 O C ; (bottom) (e) reproduced by superposition of the above three model spectra. centration is increased, fewer segments are located in the interface and most segments are extended normal to the interface as loops and as tail fragments. Gebhardt and Fuerstenau mentioned26that adsorption of a nonionic polymer would lead to a coiled adsorbed polymer conformation with a small number of polymer segments in actual contact with the surface. However, in this study, it is noteworthy that in spite of the higher surface coverage the broad ESR spectra are obtained, indicating that the PVP conformation is flattened on the alumina surface. This result may be explained by a type of surface complex of PVP and LiDS/LiFOS forms at the alumina/aqueous solution interface as mentioned previously. A similar trend was observed for both the PVP-LiDS system and PVPLiFOS system. However, there is a striking difference between two systems: The anionic surfactant (0.05 mmol dm-3) in the PVP-LiFOS system at which the spectra begin to broaden is much lower than that (0.5 mmol dm-9 in the PVP-LiDS system. In an attempt to gain quantitative as well as qualitative data on the conformation of adsorbed polymer from composite spectra, Sakai et aZ.17have suggested that any composite spectra can be separated into three components which may be assigned to signals from segments adsorbed in trains, short loops, and long loop or trails. We obtained three different spectra of PVP which have different degrees of motional freedom. These spectra are shown in Figure 5a-c: (a) a free PVP solution of a low viscosity a t 10 "C; (b) an adsorbed state on alumina at -120 "C; ( c ) a highly viscous PVP in the molten state at 140 "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 PVP. These three reference spectra were superimposed upon one another with suitable intensity ratios to reproduce the observed spectrum (e). The reproduced spectrum (d) closely fits in (24) Glass, J. E. Water-Soluble Polymers, Beauty uith Performance; American Chemical Society: Washington, DC, 1986, p 86. (25)Gebhardt, J. E.; Fuerstenau, D. W. StructurelPerformance Relationships in Surfactants; Rosen, M. J., Ed.; American Chemical Society: Washington, DC, 19W p 291.
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the observed line shape. In the PVP chain lying on the alumina surface, the sequences of the segments directly bound to the surface should have restricted motion, while the detached segments should be much more mobile. Therefore, the component in a very strongly immobilized state was assigned to the train segments directly bound to the surface. The other two components, in relatively mobile states, were assigned to the free segments having a short loop or a long loop. Our experimental evidence that the spin labels do not have a stronger affinity for the alumina surface than the original vinylpprolidone monomer demonstrates that specific adsorption through the nitroxide label does not proceed during the PVP-adsorption process. The fraction of train segments in the adsorbed polymer, P, was estimated from the integrated intensities of the signal components divided into three parts. The valuesof P for the spin-labeled PVP on alumina from PVP-LiDS/PVP-LiFOS mixed aqueous solutions are plotted against the anionic surfactant concentration in Figures 6 and 7. The values of Psteeply increased with increasing anionic surfactant concentration, and they showed a maximum a t the concentration where the adsorption of P W approached a maximum. This is probably because the PVP-surfactant complex formed by the interaction between PVP and hydrophobic chain of anionic surfactant on alumina becomes more rigid. Then, though the amount of PVP adsorbed was remarkably decreased, the values of P remained constant with their segments in trains for both PVP-LiDS and PVP-LiFOS
Interactions between Polymers and Surfactants
Langmuir, Vol. 10, No. 1, 1994 49
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Figure 10. {potential of alumina suspension with equilibrium concentration of surfactant for LiDS/LiFOS solutions.
Figure 8. Stability of alumina dispersion with equilibrium concentration of surfactant for LiDS/LiFOS solutions.
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Figure 9. Stability of alumina dispersion with equilibrium
concentrationof surfactantfor PVP-LiDS/PVP-LiFOS systems. systems. It seems likely that PVP attaches to patches of hemimicelles in spite of the anionic surfactant concentration more than the cmc. In the PVP-LiDS system, the values of P were larger in 0.2 g dm" than in 0.5 g d m 3 of PVP at the maximum PVP adsorption. This difference in the P is correlated with the bound ratio of between PVP and LiDS adsorbed The bound ratio of LiDS to PVP in the presence of 0.2 g dmd PVP is greater than that in the presence of 0.5 g dm" PVP. Higher values of P in the PVP-LiFOS system than those in the PVP-LiDS system over the whole surfactant concentration region can probably be attributed to a rigidity of the fluorocarbon chain of LiFOS.' The stability of the alumina dispersion in surfactant and PVP-surfactant system is shown in Figures 8 and 9. In the LiDS system, the transmittance of the suspension increased rapidly less than 1 mmol dm3 and then decreased, followed by becoming almost zero more than 5 mmol dma. A similar change as in the LiDS system was observed in the LiFOS system (Figure 8). These results show that in both LiDS and LiFOS systems a sequence of dispersion-flocculation-redispersionB*27of alumina occurs with the concentration of anionic surfactant. The (26) Meguro, K.; Tomioka, 9.; Kaweehima, N.;Eeumi, K. Prog. Colloid Polym. Sci. 1988,68, 97. (27) Esumi, K.; Ono, Y.;Iahizuka, M.; Meguro, K. Colloids Surf. 1983, 32,139.
concentration of surfactantfor PVP-LiDS/PVP-LiFOS systems.
transmittance in the PVP-surfactant mixed suspensions rapidly increased at much lower surfactant concentrations than that in the surfactant alone. Further, the flocculation in the PVP-LiFOS system occurred at a lower concentration of the surfactant than that in the PVP-LiDS system. Generally, as anionic surfactants, LiDS and LiFOS stabilize the dispersion of alumina mainly through the { potential and, hence, the electrostatic repulsion between alumina particles. On the other hand, as a neutral polymer,PVP stabilizes the dispersion of alumina by steric stabilization by an adsorbed layer. Figures 10and 11show that the { potential of alumina decreases from positive to zero and then converts to negative with the surfactant concentration for LiDS, LiFOS, PVP-LiDS system and PVP-LiFOS system. This change in the {potential agrees with a sequence of dispersion-flocculation-redispersion of alumina. The {potential in the PVP-surfactant mixed suspensions became zero at much lower concentrations than that in the surfactant alone. In the flocculation region for the PVP-surfactant system, it seems that the reduction of the electrostatic repulsion force and of steric Stabilization by PVP contributes to the flocculation of alumina. Actually, the PVP molecules had a flat structure with a large fraction of train segments and the alumina showed very small {potentials. In the redispersion region, the { potential showed large negative values, while the values of P were still high. This indicates that the redispersion of alumina occurs mainly due to the increment of electrostatic repulsion force. However, it has to be considered that neutral polymer adsorbed layer affects the {potential, both by shift in the shear plane and by shielding of charge
50 Langmuir, Vol. 10,No.1, 1994 to the surface. In fact, in the redispersion region, the P potential in the PVP-urfactant mixed suspensions showed smaller negative values than that in the surfactant alone.
Conclusion The interaction of a nonionic water-soluble polymer with anionic surfactants at the alumina/liquid interface is investigated. For the PVP-LiDS system, the adsorption of PVP and LiDS shows synergistic enhancement at low LiDS concentration, due to the formation of a PVP-LiDS complex at the alumina/solution interface, and antago-
Otsuka and Esumi nistic decrease at high LiDS concentration, due to formation of a PVP-LiDS complex in solution. A similar adsorption is observed in the PVP-LiFOS system. The values of P in the PVP-LiFOS system steeply increase at a much lower concentration of the surfactant than in the PVP-LiDS system. In both systems, the values of P remain constant with PVP segments in trains, though the adsorption of PVP remarkablydecreases at high surfactant concentrations. The stability of alumina dispersion is significantly influenced by the nature of the adsorption layer and { potential.
