Langmuir 1993,9, 284-287
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Adsorption of Poly(styrenesu1fonate) and Ionic Surfactant from Their Binary Mixtures on Alumina Kunio Esumi,’ Akiko Masuda, and Hidenori Otsuka Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan Received April 7,1992. In Find Form: October 19,1992 Adsorption of poly~styreneedfonate)(PSS), sodium dodecyl sulfate (SDS), and hexadecyltrimethylammonium chloride (HTAC)from singlesolutioneand PSS-SDS and PSS-HTAC blnary mixed solutions on positively charged alumina has been inveatigated. In the PSS-SDS syatem, the adsorbed amount of PSS decreases considerably with increasing concentration of SDS. T h e micropolarity of the PSS-SDS adsorbed layer eeneed by pyrene-l-carboxaldehyde (PCA) in lees polar than that in the supernatant,,while the microviwmity sensed by 2,2,6,~tr~ethylpipe~dinyl-l-oly (TEMPO)is greater than that III the supernatant. In the PSS-HTAC syatem, d r p t i o n of PSS is enhanced due to formation of a complex of PSS-HTAC on alumina. The micropolarity of the PSS-HTAC adsorbed layer in lese polar than that in the supernatant, but the microviecoeity of the PSS-HTAC adsorbed layer is almost the same as that in the supernatant.
Introduction Interactions between polymers and surfactanta in aqueom solution have been intensively investigated by many techniques.1*2 The strength of interactions depends on the kinds of both polymer and surfactant. It has been shown that the binding of ionic aurfactunta to neutral p o l y ” occurs by hydrophobic interactions, while a combination of electrostatic and hydrophobic interactions is involved in the oppositely charged polymers and eurfactanta. Adsorption of polyelectrolytes, on the other hand, has recently received muchattention, both experimentally and theoretically. Papenhuijzen et al.3 reported enhanced adsorption of poly(sty”aulfonate) (PSS) on poly(oxymethylene) single cryetala by increasing the ionic strength and molecular weight. Cosgrove et al.‘ showed that the hydrodynamicthickneeaes of PSS on polystyrene latex were relatively small, compared to the values generally found for adsorbed neutral polymers. On the theoretical side, van der Schee et al.5 have extended the mean field theories for the adsorption of neutral polymers to polyelectrolytes. Very little experimental work, however, bas been done on the interaction between polyelectrolyte and surfactant at the solid/liquid interface. In a previous report6 we showed that the addition of PSS to flocculated sodium dodecyl sulfate (SDS) adsorbed alumina enabled the alumina to redisperse due to the adsorption of PSS on the alumina, where replacement of SDS by PSS occurred to some extent. Further, the microenvironmental properties of the SDS-PSS adsorbed layer on alumina were characterized. In this work, adsorption of SDS/hexadecyltrimethylammonium chloride (HTAC) and PSS from their mixed solutions on a-alumina has been investigated through measurements of the adsorption density, potential, (l)Robb, 1. D. In Anionic Surfactant in Physical Chembtry of Surfactant Action; Lucaeclen-Fbynders,E. H., Ed.; Dekker: New York, 1981; p 109. (2) Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants-Physical Dekker: New York, Ckmirtry; Rubingh, I).N., Holland, P. M., U.; 1991; p 189. (3) Papenhuijzen,J.;Flwr,0.J.;Bijskdmch,B. H. J.Colloidlnterface Sci. 1986,101,630. (4) Corprove, T.; Obey, T. M.; Vincent, B. J. Colloid Interface Sci. 1986,111,409. ( I ) Van der Schee, H. A.; Lyklema, J. J. Phys. Chom. 1984,88,6661. (6) &mi,K.; Yokokawa, M.J. Jpn. SOC.ColourMater. 1992,66,142.
fluorescence spectrophotometry, and electron spin resonance (ESR).
Experimental Section SDS was prepared from l-dodecanol by sulfonation with chlorosulfonic acid, followed by neutralization with sodium hydroxide. The product was recrystallized several times from ethanol and then extracted with diethyl ether. HTAC was obtained from Tokyo Kasei Industries and purified by several recrystallizations with acetone. The purity of the surfactants was ascertained by the absence of a minimum in the surface tension curve. PSS was obtained from Aldrich ChemicalCo. Inc., and purified by dialysis. The molecular weight of PSS estimated from the intrinsic viscosity was about 10 kg/mol. a-Alumina of 99.995% purity was supplied by Showa Denkou K.K., where the particle diameter and the specific surface area were 500 nm and 10.0 m2 gl,respectively. Pyrene-l-carboxaldehyde (PCA) and 2,2,6,Btetramethylpiperidinyl-l-oxy (TEMPO)were obtained from Aldrich and ueed ae received. The water used in all experiments was purified by a Mill-Q system (Nihon Millipore Co.) through which water was paeeed until ita specific conductivity fell below 0.1 pS cm-l. Methods. Alumina (0.3 g) and 30 cm3of PSS and surfactant aqueous solutions were combined. The resulting aqueous suspensions were agitatedfor 24 h to attain equilibrium of adsorption at 25 “C. All the suspensions were adjusted to pH 3.5 by adding a dilute solution of HN03that contained 10 mmol dnr3NaNOa. PCA and TEMPO were used as probes for fluorescenceand ESR measurements. The concentrations of PCA and TEMPO were 1 x 10-6 and 1 x 1W mol dm-3, respectively. The suspensions containing the probes were prepared as follows: A knownvolume (10-3 cm3) of the ethanol solution of a probe was placed in a flask; the solvent was removed under vacuum, and alumina, PSS, and surfactant were added. Measurements. Adsorbed amounts of PSS, SDS, or HTAC on alumina were determined from the differences in concentrations before and after adsorption. The concentration of PSS, which has an absorption band at 265 nm, was determined by spectrophotometry, while thoee of SDS and HTAC were determined by the Abbott method’ and the orange II-chloroform method.* It should be pointed out that the presence of each component in the PSS-surfactant mixed solutione scarcely interfered with the determination of concentration for the other component. Steady-state emission spectra of PCA in the PSSsurfactant adsorbed layer were measured with a Hitachi 650-10s (7) Abbott, D. C. Analyst (London) 1962,87, 286. (8) Few, A. V.; Ottewill, R. H. J. Colloid Sei. 1966,11, 34.
0743-7463/93/2409-02~$~.~/0(B 1993 American Chemical Society
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Adsorption of PoLy(atyrene8uLfonate)
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15t
* PSS
PI
cn
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Equilibriumconcn. of PSS / 1O-’g dm-3
Figure 1. Adsorption isotherm of PSS on alumina.
Adddlve concn. of SDS / mmol d m 3
Figure 2. Adsorption of PSS and SDS on alumina from their mixtures: fixed concentration of PSS,0.4 g dm-3. fluorescence spectrophotometer. Similarly,the emission spectra of the supernatantsolutionobtained from centrifugedsuspensions were ala0recorded. The excitation wavelength was 356 nm. ESR spectra of TEMPO in the suspension and in the supernatant solution were recorded on a JEOL JES FE 3-Xspectrometer utilizing 1 0 0 m field modulation and X-band microwaves. The f potential of the suspensions was measured with an electrophoretic apparatus (Laser %e 500,Pen Kem Inc.). All measurmentewere carried out at a pH of 3.5 and at 25 OC.
Results and Discussion A. PSS-SDS System. Before we examined the adsorption behavior of PSS and SDS,which are both negatively charged in aqueous solutions, on a positively charged alumina surface, we measured the adsorption isotherm of PSS on alumina (Figure 1). The adsorbed amount of PSS increased sharply and then reached a plateau with the PSS concentration, indicating a high affinity between PSS and the alumina surface. This adsorption mainly occurs due to an electrical attraction force between negatively charged PSS and Positively charged alumina surface. A similar isotherm of PSS on a positively charged surfacehas been reported by Cosgrove et al.‘ Further, we studied how the addition of a surfactant affects the adsorption of PSSon alumina. Figure 2 shows the adsorption of SDSand PSSon alumina from the mixed solutions containing a fiied initial concentration of PSS (0.4 g dnrS)as a function of the SDS concentration. The adsorption of SDS reaches a plateau at about the critical
l
l
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o
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Additive concn. of PSS / lO-’g dm-3
Figure 3. Adsorption of PSS and SDS on alumina from their mixtures: fixed concentration of SDS, 5 mmol dm-3.
micelle concentration(cmc)of SDS (additiveconcentration 5.5 mmol dm9. However, the adsorption isotherm of SDS obtained from the solutions containing a constant PSS concentration is obviously different from that o b tained in the absence of PSS: they show a considerable decrease in the amount adsorbed in the region of low SDS concentration, but are almost the same as in the absence of PSS at 5 mmol dm-3 SDS or above. The adsorbed amount of PSS,on the other hand, decreaseslinearly until 5 mmol dm4 SDS and then becomes almost zero. This indicates that the adsorbed PSS is replaced by SDS with an increase of the SDS concentration. Figure 3 showsthe adsorptionof SDS and PSSfrom the mixed solutions containing a fiied concentration of SDS (5 mmol dm-3) as a function of the PSS concentration. It can be seen that the adsorbed amount of PSS i n c r e w , while the adsorbed amount of SDS remains constant with increasing PSS concentration. The adsorbed amount of SDS corresponds to a saturated one, indicating bilayer adsorption of SDS on alumina. Both figures suggest that the adsorption of SDS on alumina is much stronger than that of PSS. It is noteworthy that for both the systems the dispersion stability of alumina is good and the f’ potential of alumina ranges between -50 and -55 mV. These results indicate that adsorption of SDS and PSS occurs through an electric attraction force between their anionic groups and positively charged sites on alumina and their anionic groups oriented to the aqueous solution. Further, it seems likely that a hydrophobic interaction in the hydrocarbon chains between SDS and PSS adsorbed on alumina occurs. This view is also supported by a previous result6 that adsorbed PSS molecules on the preadsorbed SDS on alumina are densely packed through the hydrophobic interaction. The environmental properties of the adsorbed layer of PSS and SDS have been estimated by using fluorescence and ESR techniques. It is well knownsthat the maximum fluorescencewavelength of PCA is very sensitiveto solvent polarity;the maximum fluorescencewavelengthdecrsaeee with decreasing polarity of the solvente. Figure 4 show that in the presence of a fixed concentration of PSS (0.4 g dm-3) the maximum fluorescence wavelength of PCA incorporated in the adsorbed layer shifta gradually from 464 to 464 nm, while that in the supernatant solutions shifts gradually and becomes constant at 7 mmol dms (9)Kdyenasundaram,K.; Thomas, J. K.J. Am. Chem. SOC.1977,81, 2176.
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Enumi et al. ~~
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suspension supernatant
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-A-
supernatant
Figure 4. Changesin maximum fluoremxncewavelength of PCA for the PSS-SDS-alumina system: fmed concentration of PSS, 0.4 g dm-3.
Additive concn. of SDS I mmol dm-'
Figure 5. Changes in rotational correlation time of TEMPO for the PSS-SDS-alumina system: fued concentration of PSS,0.4 g dm-3.
maximum
fluorescence wavelength of PCA in the adsorbed layer is much lower than that in the supernatant solutions, indicating that the environment sensed by PCA in the adsorbed layer of PCA and SDS is less polar than that in the supernatant solutions. A similar result is obtained in the presence of a f i e d concentration of SDS: the maximum fluorescencewavelength of PCA in the adsorbed layer is lower than that in the supernatant solutions and is mainly r e f l d by the SDSadsorbed layer. The change in the maximum fluorescence wavelength of PCA in the adsorbed layer corresponds to the ratio of the adsorbed amounta of SDS and PSS the maximum fluorescence wavelength of PCA decreases with decreasing ratio of PSS to SDS. This result can be easily understood in that SDS is strongly oriented on the positively charged alumina surface toward ita hydrophobic chain to aqueous solution, whereas PSS is leas packed on the alumina due to an intrarepulsion force between negatively charged groups of PSS. This leads to a different compactness between SDS and PSS in the adsorbed layer on alumina. The microviscosity of the adsorbed layer was estimated using TEMPO. It is knownloJ1 that an isotropic threeline spectrum of TEMPO is a f f d markedly with an adsorbed layer and the relative anisotropy in the ESR spectra can be related to the rotational mobility of TEMPO. The rotational mobility is proportional to the microviscosity in which the probe is located. Figure 6 showsthat in the presence of a f i e d concentration of PSS (0.4 g dm-9 the rotational correlation time of TEMPO increases gradually with an increase of the SDS concentration. This indicates that the microviscosity sensed by TEMPO in the adsorbed layer of PSS and SDS increases with the SDS concentration. The fact12 that most of the TEMPO solubilized in the micelles is located at the mide-water interface would suggestthatTEMPO resides in high polarity sites in the adsorbed layer of SDS and PSS on alumina. The feature in the rotational correlation time of TEMPO against the SDS concentration is very similar to that of PCA. This implies that TEMPO moleculesmainly reside at the polar region of the adsorbed layer of SDS and the microviscwity sensed by TEMPO (10) Cbandar, P.;S o ~ ~ u n d a rP.; a~ Turro, , N.J. J. Colloid Interface Sei. 1987, 117, 31.
(11) Eaumi, K.; Otruka, H.; M m , K . hngmuir 1991, 7, 2313. (12) ~ r , R A . ; ~ c ~ & a U , C . ; M u k e r j e J.Phy8.Chem. e,P. 1982, 86,3206.
10
5
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Additive concn. of SDS/mmol dm-3
SDS or above. Further, it is seen that the
~
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HTAC
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Additive concn. of HTAC I mmol d m 3
Figure 6. Adsorption of PSS and HTAC on alumina from HTAC/PSS-HTAC mixtures: fixed concentration of PSS, 0.4 g dm-3.
is significantly a f f d by the adsorbed amount of SDS. Above 6 mmol dm-a SDS the rotational correlation t h e e of TEMPO for the suspension and supernatant still increase. Probably this increase for the suspension is due to a change in the compactness of the SDS bilayer. In a fixed concentration of SDSthe rotational correlationtime of TEMPO is hardly altered (the reeult is not shown in the figure), indicating that this result is also r e f l d by the SDS adsorption. B. PSS-HTAC System. When PSS and W A C were mixed in aqueous solutions, precipitation often occurred due to formation of a complex by hydrophobic as well as electroetaticform between PSS and HTAC. In this study, we performed an adsorption experiment for PSS and HTAC in theconcentration range where the mixed aqueous solutions of PSS and HTAC showed no precipitation. Figure 6 shows the adsorption of PSS and HTAC from the binary mixtures of PSS and HTAC under a f i e d concentration of PSS (0.4g dm-9. The adsorbed amount of PSS increased gradually, while that of HTAC increased above some concentration of HTAC. A similar result has been observed on adsorption of oppositely charged surfactanta on solids.1*16 In the absence of PSS, adsorption (13) Muller, H.;Krempl, E. Teaside 1988,6,335. (14) Schmger, M. J. Kolloid 2.2. Polym. 1971, arS, 129. (16) Huang, 2.; Yan, 2.; Gu,T.Colloids Surf. 1989,36, 363.
Adsorption of Poly(etyrenesu1fonute)
.
475
-0- suspension
Additive concn.of HTAC / mmol d m 3
Figure 7. Changesin maximum fluorescencewavelength of PCA for the PSS-HTAC-aluminasyetm:fiied concentrationof PSS, 0.4 g dm-3.
of HTAC is small on alumina because of an electric repulsion force between positively charged HTA+ and positively charged sites on alumina. On the other hand, since PSS and HTAC can form a complexdue to an electric attraction force between negatively charged PSS and positively charged HTA+ in aqueous solution and a hydrophobic force between PSS and HTAC, it is reasonable to assume that a similar complex of PSS and HTAC forms on the surface of alumina. Accordingly, the adsorption of PSS enhances the adsorption of HTAC due to formation of the complex. In particular, above 1.0 mmol d m - 3 HTAC formation of the complex on the alumina surface occurs remarkably. Abuin and Scaiano16 investigated the aggregation state in PSS-dodecyltrimethylammonium bromide (DTAB) complexes and concluded that the PSS-DTAB aggregates are regarded as an aggregate or minimicelle of aggregates. In this study, the compleaes of PSS and HTAC adsorbed on alumina might consist of some aggregates of PSS and HTAC. Figure 7 shows the maximum fluorescence wavelength of PCA in the PSS and W A C system. The maximum fluorescence wavelength of PCA in the adsorbed layer of (16) Abuin, E.B.;Scaiano, J. C. J. Am. Chem. SOC.1984,106,6274.
Langmuir, Vol. 9, No. 1, 1993 287
PSS and HTAC shiffad from 464 to 446 nm with an increase of the HTAC concentration, while that of PCA in the supernatant also shifted from 470 to 466 nm. This difference in the maximum fluorescence wavelength of PCA is due to the difference between the compactness of the complex of PSS and HTAC in aqueous solution and on a solid/aqueous interface. That is, the Compactnessof the complex between PSS and HTAC on alumina is much greater than that in aqueous solution. This compactness of the complex between PSS and HTAC corresponds to the coiling of PSS chains with HTAC, because formation of the PSS-cationic surfactant complex accompanying coiling of PSS has been confiied from viscosity and fluorescence measurements.16J7 In addition, in the low concentration region of HTAC (0.2-0.6 mmol dm-3) PCA is very sensitive to the environmental change. The microviscosity sensed by TEMPO in the PSS-HTAC adsorbed layer on alumina was also evaluated. Although the result is not given here, the microviscosity in the PSS-HTAC adsorbed layer was almost the same as that in the supernatant solution. A similar feature was observed for the microviscosity in the HTAC adsorbed layer alone. These imply that TEMPO molecules reside at HTAC molecules adsorbed or in solution, in which the microviscosity scarcely changes. Conclusions In the PSS-SDS system, the adsorbed amount of PSS decreases on positively charged alumina with increasing SDSconcentration due to stronger adsorption of SDS. On the other hand, in the PSS-HTAC system, adsorption of PSS is enhanced due to formation of a complex of PSSHTAC on alumina. The probe measurements indicate that the micropolarities of PSS-SDS and PSS-HTAC adsorbed layers estimated by PCA are lese polar thanthoae in the respective supernatants. In addition, the microviscosity of the PSS-SDS adsorbed layer sensed by TEMPO is greater than that in the supernatant, whereas that of the PSS-HTAC adsorbed layer is almost the same as that in the supernatant. (17) Eeumi, K.;Takehana, K.;Nojima, T.;Meguro, K.Colloida Surf. 1992,64,15.
