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Microstructural Aspects of Polysoap/Sodium Dodecyl Sulfate Interactions R. Zana,* A. Kaplun, and Y. Talmon Department of Chemical Engineering, Technion, Israel Institute of Technology, Haif a 32000,Isra e1 Received January 19,1993. I n Final Form: April 18, 1993 Viscosity and cry0 transmission electron microscopy (cryo-TEM) have been used to show that the anionic surfactant sodium dodecyl sulfate interacts with the anionic polysoap poly(disodium maleateco-hexadecyl vinyl ether) (PS16). The interaction results in the loss of the viscoelasticity of the solution owing to the breakdown of the giant threadlike micelles formed by end to end linking of the PS16 polymer chains into isolated chains. In 1992,Goddard and Leungl stated "until very recently it was considered a truism that a charged surfactant would not interact with a polyelectrolyte of like charge". However, five studies,14 reported since 1987, have suggested that this situation can change if the charged polymer is either sufficiently hydrophobic or made so by chemical modification. The study of Iliopoulos et is particularly convincing. The enormous changes of viscosity they reported leave no doubt that the surfactant (sodium dodecyl sulfate, SDS) is indeed bound to the hydrophobically modified sodium polyacrylate, and, in being so, favors extensive network formation in the system. The evidence presented in the other studies is less straightforward. The effects observed in the system cationic celluloselalkyltrimethylammonium bromides' may arise from an incompatibility between the polycation and the positively charged micelles: and not from the binding of the latter to the polyelectrolyte. The study of the interaction between SDS and carboxymethylamylose (CMA)5 is open to criticism. Indeed, the result showing an increase of the critical micelle concentration (cmc) of SDS in the presence of CMA is contrary to the predictions of theoretical calculation^.^ In the study of the interaction of SDS with poly(a1kene-co-maleicacid) at pH 8,394surface tension did show adsorption of SDS at hydrophobic sites at the polyelectrolyte-water interface for SDS concentrations less than the cmc (critical micelle concentration) but the fluorescence data were ambiguous. Thus, fluorescence anisotropy measurements using labeled copolymers showed surprisingly little dependence on the length of the pendent alkyl chain.3 This brief review reveals that the evidence for the binding of ionic surfactants to polyions of like charge is scarce. Nevertheless, such evidence can be obtained by a proper choice of the system and of the methods of investigation. We report in this letter direct observation of the interaction (binding) between SDS and a strongly hydrophobic polyelectrolyte, poly(disodium maleate-cohexadecyl vinyl ether). This copolymer, referred to as
* To whom correspondence should be addressed at the Institut Charles Sadron (CNRS), 6 rue Bowingault, 67083 Strasbourg Cedex, France. (1) Goddard, E. D.; Leung, P. S. Colloida Surf. 1992, 65,211. (2) McGlade, J. M.; Randall, F. J.;Tcheurekdjian, N. Macromolecules 1987,20,1782. (3) McGlade, J. M.; Olufs, J. L. Macromolecules 1988, 21, 2346. (4) Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991, 7, 617. (5) Zhen, 2.;Tung, C.-H. Polymer 1992, 33, 812. (6) Lindman, B.; Thalberg, K. In Polymer-Surfactant Interactions;
Goddard, E. D., Ananthapadmanathan, K. P., Eds.; CRC Press: Boca Raton, FL, Chapter 6, in press. Piculell, L.; Lindman, B. Adu. Colloid Interface Sci. 1992,41, 149. (7) Nagarajan, R. J. Chem. Phys. 1989,90,1980; Colloids Surf. 1985, 13, 1.
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Figure 1. Variation of the specific viscosity of 9 mM (top) and 3 mM (bottom)PS16 aqueous solutions with the molar concentration ratio R = [SDS]/[PSlG] at 25 'C. The arrow in the top panel indicates the cmc of SDS in water. PS16, is a typical polysoap,s whose behavior in aqueous solution has been well characteri~ed.~ Due to the strong hydrophobic interactions between the hexadecyl side chains, PS16 adopts in aqueous solution a very compact conformation,g which reflects the essentially intramolecular formation of hydrophobic microdomains.'& The successive microdomains on a given PS16 chain appear to be almost in contact.lob The conformation of the polymer backbone has not been experimentally investigated, but theoretical calculations predict it to be helical, with the (8) Straws, U. P. In Polymers In Aqueous Media; Glaee, E. J., Ed.; American Chemical Society: Washington, DC, 1989;p 317 and references therein. (9) Pefferkorn, E.; Schmitt, A.; Varoqui,R. C. R. Acad. Sci., Ser. 2 1968,267,349. Varoqui,R.;Pefferkom, E. P. Polym.Prepr. (Am. Chem. Soc., Diu. Polym. Chem.) 1982,23, 9 and references therein. (IO) Binana-LimbelB,W.; h a , R. (a) Macromolecules 1987,20,1331; (b) Macromolecules 1990, 23, 2731.
0743-7463/93/2409-1948$04.00/00 1993 American Chemical Society
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r . .
Figure 2. Cry0 transmission electron micrographs of PS16 solutions ([PS16] = 9 mM in all micrographs, except (F),where [PS16] = 4 mM) with increasing amounta of SDS: (A, B) R = 0.57, note long, entangled threadlike micelles, disordered in (A), somewhat oriented in (B),as a result of sample preparation; (C, F) representative micrographs atR = 0.79, threadlike micelles are still predominant, but some shorter ones can be seen, too;(D, E) R = 1.0 and 1.14, respectively, only short threadlike micelles coexisting with spheroidal micelles observed. The bar equals 100 nm all micrographs.
alkyl chains in the interior of the helix." In practice, defects in the helical conformation may delineate the microdomains of finite aggregation number detected by time-resolved fluorescence quenching.lob
The study of PS16 aqueous solutions by cry0 transmission electron microscopy (cryo-TEM)l2 has revealed long, entangled, threadlike micelles, not unlike those visualized in systems of giant micelles (aqueous solutions
1950 Langmuir, Vol. 9,No. 8,1993
of cetyltrimethylammoniumbromide + sodium salicylate, for instance).l3 The threadlike micelles of PS16 can be visualized by cryo-TEM because their diameter is about 6 nm, a value consistent with the dimension of the PS16 repeat unit. These threadlike micelles represent in fact the envelope of the successive microdomains. A very important feature of the micrographs was the enormous length of the micelles, well above 1pm.12 In view of the molecular weight of the PS16 used, it was concluded that the threadlike micelles were formed by end-to-end linking of a large number of PS16 chains. Indeed, the ends of these chains are not organized in microdomains and have a tendency to connect one to another, to prevent unfavorable contacts between water and hexadecylside chains. Another interesting property of the PS16 aqueoussolutions is their viscoelasticity which can be observed visually in sufficiently concentrated systems, by the recoil motion of air bubbles trapped in the solution after stopping the rotation of the flask containing it. The viscoelasticity of the PS16 solutions is of course due to the length of the PS16 micelles. The above-described features of PS16 make it a choice material for studying its interaction with a surfactant of like charge. Indeed the binding of the surfactant to PS16 micelles may induce changes in their microstructure and in the rheology of the solutions. This letter specifically reports the effect of SDS on these two aspects of PS16 solutions. SDSwas chosen for the present study, because it is probably the best characterized surfactant. Aqueous solutions of PS16 were prepared as previously described,'O using the same sample of PS16 of polymerization degree 4000 f 500. The viscosities were measured at 25 OC, using Fenske capillary viscometers of water flow times of 15or 80 s, depending on the PS16 concentration. The solution flow times were used to calculate the specific viscosity qsp = (t-to)/towhere t and to are the flow times of the solution and of water, respectively. No corrections were applied to the data, which in this paper are used only qualitatively. The samples for cryo-TEM studies wee prepared as previously described, at 25 "C and 100% humidity in a controlled environment vitrification system (CEVS).12-14 Thin liquid films of PS16 solutions, containing the appropriate amounts of SDS, were vitrified by ultrafast cooling (about 105K/s), and directly observed at cryogenic temperature using a JEOL JEM 2000FX electron microscope. Figure 1 shows the variation of the specific viscosity, qBP,with the molar concentrationratio, R = [SDSI/[PS161, with the PS16 concentration expressed in moles of repeat unit per liter. At [PS161 = 9 mM the solutions were viscoelastic up to R = 0.76, but the viscoelasticity disappeared at R 1 0.9. The solutions at [PS16] = 3 mM were not viscoelastic, even in the absence of SDS. The decrease of vsp upon increasing R takes place in both instances at concentrationsof SDSbelow its cmc (8.2 mM), and in the same range of R values. These results strongly suggest mixed micellization between PS16 and 8DS. It may be argued that the decrease of qsp occurs because PS16 is a polyelectrolyte, and that the viscosity of polyelectrolyte solutions decreases upon addition of salts, such as SDS or NaCl. This point was checked by measuring the viscosity of systemswhere SDSwas replaced (11) Turner, M.; Joanny, J.-F. J. Phys. Chen. 1993, 97, 4825. (12) Cochin, D.; Candau, F.; Zana, R.; Talmon, Y. Macromolecules
1992 25.4220. -, -, --- - .
(13) Clausen,T.M.; Vinson, P. K.; Minter, J. R.; Davis,H. T.;Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992,96, 474. (14) Bellare, J. R.; Davis, H. T.; Scriven,L. E.; Talmon, Y. J. Electron Microsc. Technol. 1988, 10, 87.
Letters by NaC1. At [PS161 = 3 mM, the specific viscosity was found to decrease by only 25% upon NaCl addition corresponding to R = 2, whereas the equivalent addition of SDSgave a specificviscosity decreaee of 99 ?6 (seeFigure l)!These results clearly show that the observed decreases of qw are the result of SDS binding to the polymer. Other measurements, still in progress, have shown extremely rapid decreases of viscosity, upon addition of nonionic surfactants, such as Clo(EO)s and Cn(EO)e, to PS16 solutions, at R C 0.2. Figure 2 shows electron micrographs of 9 mM PS16 solutions with increasing amounts of SDS. Parta A and B of Figure 2 refer to R = 0.57. Very long, entangled threadlike micelles are seen, disordered in Figure 2A, somewhat oriented in Figure 2B, as a result of sample preparation.13 The micrograph of Figure 2A is similar to that previously reported for the PS16 solution in the absenceof SDS.12Figure 2C is a representative micrograph of the R = 0.79 system. Threadlike micelles are still predominant, and the system is visually viscoelastic. However, one can already distinguish among the very long micelles some shorter ones. A micrograph at the same R value, but at [PS161=4 mM (Figure 2F), reveals the same features. Parta D and E of Figure 2 are typical of systems at R = 1.0 and 1.14, respectively. These systems are not viscoelastic and show only short threadlike micelles coexisting with spheroidal micelles. The former are probably mixed PS16/SDS micelles and the latter pure SDS micelles. Conductivity measurements, to be presented in full in a future publication, indicate the presence of free SDS micelles in the PSlG/SDS system at [SDSI > 10 mM. The threadlike micelles in Figure 2D,E have a minimum length of 100-200 nm, which is consistent with the expected value for the PS16 polymer chains on the basis of the degree of polymerization of the sample used. Thus, cryo-TEM provides us direct visual evidence of the disruption of long threadlike micelles of PS16 into their constitutingelements, i.e., individual polymer chains, upon addition of SDS. This disruption is clearly the result of the incorporation of SDS into the PS16 microdomains, and also into the junctions between polymer chains. The situation is similar to that in solutions of associating polymers upon addition of surfactant.3JEJ6 An excess of surfactant destroysthe three-dimensionalnetwork formed upon association of the hydrophobic side chains with the help of the added surfactant, as each side chain becomes the locus of a surfactant micelle bound to the polymer. In PS16 solutions, each polymer chain end becomes incorporated into an SDS micelle and the ends lose their capacity to self-associate into aggregates resembling threadlike micelles. In conclusion,the results presented here constitute the first direct, visual evidence of the interaction between a polyelectrolyte and asurfactant of like charge. In addition, this letter demonstrates the wealth of information that the coupling of cryo-TEM and other techniques can provide on complex systems such as the one described here.
Acknowledgment. We thank Me.Judith Schmidt and Ms. Berta Shdemati for their help in the experimental work and the preparation of the paper. We also thank Drs. R. Varoqui and E. Pefferkorn (ICs, Strasbourg) for the PS16 sample. We acknowledge with thanks partial financial support from the United States-IsraelBinational Science Foundation (BSF), Jerusalem. (15) Magny, B. Thesis, University of Paris 6, 1992. (16) Tanaka, R.; Meadows, J.; Williams, P. A:; Phillips, G. 0. MaCron~OkCUk81992,26, 1302 and references therem.