Langmuir 1994,10, 2960-2964
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Interactions between Polysoaps and Surfactants in Aqueous Solutions N. Kamenka,?A. Kaplun,*Y. Talmon,$,and R. Zana*>l Laboratoire des Mattriaux et Proctdds Membranaires, Universitt de Montpellier, Place E. Bataillon, 34095 Montpellier Cedex 05, France, Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel, and Institut C. Sadron (C.R.M.), CNRS, 6, rue Boussingault 67000, Strasbourg, France Received February 2, 1994. I n Final Form: June 10, 1994@ Viscosity measurements and cry0 transmission electron microscopy (cryo-TEM)were used to investigate the interaction between the polysoap poly(disodium maleate-co-hexadecylvinyl ether) and two nonionic surfactants, the hexaethylene glycol monodecyl and monododecyl ethers (C10E6 and C12E6). In addition, measurements of self-diffision coefficient of the dodecyl sulfate ion in the polysoap/sodium dodecyl sulfate (SDS)system were performed to determine the amount of surfactant bound to the polysoap. The viscosity of the polysoap solutions decreases and in concentrated solutions the viscoelasticity disappears, upon addition of surfactant. Cryo-TEM shows that the long threadlike micelles, formed by end-to-endlinking of polysoap chains, initially present in the polysoap solutions, progressively break up into the individual polysoap chains upon addition of surfactant. This break up is essentially completed at a molar concentration ratio R = [CloE6Y[polysoap]of about 0.2. In this respect, the nonionic surfactants used appeared to be much more efficient that the anionic surfactant SDS;that requires a ratio R of about 1.3to achieve the same effect on the polysoap micelles. Measurements of the self-diffusioncoefficient of the dodecyl sulfate ion in the presence of polysoap showed that this difference mostly arises from the fact that only a relatively small fraction of the added SDS binds to the polysoap. The binding of the added surfactant to the microdomains at the junctions between polysoap chains explains their dramatic effect on polysoap micelles at relatively low concentration of added surfactant.
Introduction Until recently, it was generally accepted that nonionic and cationic surfactants interact only weakly, if at all, with neutral hydrophilic polymers in aqueous The same appeared to hold for nonionicsurfactanticharged polymer and also for charged surfactants and polymers of like ~ h a r g e s . l - These ~ statements have been recently relaxed, as several studies showed that sfliciently hydrophobic, but still water-soluble, polymers interact with all types of surfactants, irrespective of the nature of the head g r o ~ p . ~ - In l l particular, it has been shown that an anionic surfactant such sodium dodecyl sulfate (SDS) interacts with various types of polyanion~,'~-'~ among Universitb de Montpellier. t Technion-Israel Institute 0 Institut C. Sadron.
of Technology.
Abstract published inAdvance ACSAbstracts, August 15,1994. (1)Saito, S. InNonIonicSurfactants, Physical Chemistry; SchickM. J., Ed.; Marcel Dekker, Inc.: New York, 1987;p 881. (2)Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan,K. P., Eds.; CRC Press: Boca Raton, FL, 1993;Chapter 6. (3)Goddard, E.D.Colloids Surf. 1986,19,255 and 301. (4)Zana, R.; Lang, J.;Lianos, P. Polym. Prep., A m . Chem. SOC.Diu. Polym. Chem. 1982,23,39. (5)Karlsson, G.; Carlsson, A,; Lindman, B. J.Phys. Chem. 1990,94, 5005. (6)Carlsson, A.; Karlstrom, G.;Lindman, B. Colloids Surf. 1990,47, 147. (7)Carlsson, A.; Karlstrom, G.; Lindman, B. J.Phys. Chem. 1989, 93,3673. (8) Carlsson,A.; Lindman, B.; Watanabe, T.; Shirahama, KLangmuir 1989,5, 1250. (9)Winnik, F.; Winnik, M.; Tazuke, S. J.Phys. Chem. 1987,91,594. (10)Winnik, F.; Winnik, M. Polym. J. 1990,22,482. (11)Winnik, F.;Ringsdorf, H.; Venzmer, J . Langmuir 1991,7,905 and 912. (12)Goddard, E.D.; Leung, P. S. Colloids Surf. 1992,65,211. Polymer 1992,33,812. (13)Zhen, 2.; Tung, C.-H. (14)Iliopoulos, I.; Wang, T. K; Audebert, R. Langmuir 1991,7,617. (15)Wang, T. K.;Iliopoulos, I.; Audebert, R. In Water Soluble Polymers; Shalaby, W. S., McCormick,C. L., Butler, G. B., Eds.; American Chemical Society Symposium Series, American Chemical Society: Washington, DC, 1991;p 218. @
them some alternating copolymers of neutralized maleic acid and various hydrophobic c ~ m o n o m e r s , ~ ' -often ~~ referred to as polysoaps.20 In a previous studylg we showed the alternating copolymer poly(disodium maleateco-hexadecylvinyl ether) (PS16)to be a choice material for the study of polymersurfactant interactions for the following reasons: (i)PS16 forms in aqueous solutions long and entangled threadlike micelles that can be directly visualized by cry0 transmission electron microscopy (cryo-TEM),as the radius ofthese micelles is of about 3 nm, a value consistent with the length of a surfactant with a hexadecyl chain and a fairly bulky head-group. Also, the enormous length of the threadlike micelles indicate that they are formed by endto-end linking of a large number of PS16 chains.21 Those threadlike micelles are probably formed as loosely defined helical structures of the polymer backbone,21'22with the long hydrophobic side-chains pointing toward the center, and the hydrophilic heads pointing outward. Successive defects in the helical structure would delineate the hydrophobic microdomains detected by fluorescence probing;23(ii) PS16 solutions of sufficiently high molecular weight polymer are visually viscoelastic at concentration as low as 9 mM.19 One can therefore expect both the microstructure and the rheology of PS16 solutions to be affected by the interaction of the polymer with surfactants. This expectation was borne out by the experiments in the case of the PS16/SDS system.lg The addition of SDS was found to result in the disappearance of the viscoelas(16)Magny, B.; Iliopoulos, I.; Audebert, R. Polym. Commun. 1991, 32,456. (17)McGlade,J. M.; Randall, F. J.; Tcheurekdjian,N. Macromolecules 1987,20,1782. (18)McGlade, J . M.; Olufs, J. L. Macromolecules 1988,21,2346. (19)Zana, R.; Kaplun, A.; Talmon, Y. Langmuir 1993,9, 1948. (20)Strauss, U.P.;Jackson, E. G. J . Polym. Sci. 1951,6 , 649. (21)Cochin, D.;Candau, F.; Zana, R.; Talmon, Y. Macromolecules 1992,25,4220. (22)Turner, M.; Joanny, J.-F. J.Phys. Chem. 1993,97,4825. (23)(a)Binana-Limb&, W.; Zana, R. Macromolecules 1987,20,1331. (b) Macromolecules 1990,23,273.
0743-746319412410-2960$04.50/0 0 1994 American Chemical Society
Interactions between Polysoaps and Surfactants ticity of the PS16 solution at a surfactant concentration well below its critical micelle concentration (cmc). Also, cryo-TEM then showed the progressive break-up of the long threadlike micelles into their components, that is, the individual PS16 chains visualized as short threads, upon addition of SDS. In this paper we report on the interaction between PS16 and two nonionic surfactants. Viscosity and cryo-TEM investigations show that the nonionic surfactants used are more effective than SDS in breaking up the PS16 threadlike micelles. This paper also addresses the problem of the locus of binding of surfactants to PS16. The binding of SDSto PS16 has been investigated by tracer self-diffusion.
Experimental Section Materials. The sample of PS16 was the same as in previous studies.23The degree of polymerization (DP)of the poly(maleic anhydrideqo-hexadecylvinyl ether) from which the PS16 solutions were prepared by alkalinehydrolysis was 4000 f500. The DP of the PS16 is therefore expected t o be significantly lower than this value and to depend on the temperature at which the alkalinehydrolysis was performed and on its duration,because this reaction leadsto partial degradation ofthestartingpolymer.% Unfortunately,the amphiphilic and polyelectrolytecharacter of PS16 made it impossibleto determine its DP by the conventional methods. SDS was obtained from BDH (specially pure grade)and used as received. The nonionic surfactants hexaethylene glycol monodecyl ether and monododecyl ether (ClOE6 and C12E6, respectively) were obtained from Nikko Chemicals Co., Ltd. (Japan), and were used as received. Methods. Aqueous solutions of PS16 were prepared as previously described.238 Viscosities were measured using Fenske capillary viscometers of water flow times of about 15 or 80 s, depending on the polymer concentration. The 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, used only qualitatively. Vitrified specimensfor cryo-TEMwere prepared as previously described, at 25 “C and 100%humidity in a controlled environment vitrification system (CEVS).21s26f26 Thin liquid films ofPS16 solutions, containingthe appropriate amounts of surfactantwere vitrified by ultrafast cooling (aboutlo6Ws)achieved by rapidly plunging the sample into liquid ethane at a temperature close to its solidificationpoint. The samplewas then directly observed at cryogenic temperature in a JEOL JEM 2000FX transmission electron microscope. The self-diffision coefficient of the dodecyl sulfate ion was measured by the “open-endedcapillary tube m e t h ~ d ” . ~ The ~J~ radioactive label used was %-enriched SDS from Amersham Radiochemical Center (Buckinghamshire, UK), with specific activity of 0.113 mCi/mg. The viscosity and self-diffusionmeasurementswere performed at 25 “C. The PS16 concentrationis expressed in moles of repeat unit per liter. Results and Discussion Viscosity. The 9 mM PS16 solution was visually viscoelastic, as seen from the recoil of trapped air bubbles when a swirling motion of the solution was stopped. But the 3 mMPS16 solution was not visually viscoelastic. Upon addition of surfactant, the viscoelasticity ofthe 9 mMPS16 solution was found to either persist or disappear after a (24) Pefferkom, E. These de troisihme cycle, University of Strasbourg,
January 1969.
(25) Clausen, T. M.; Vinson, P. K.; Minter, J.R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992,96, 474. (26) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y .J.Electron Microsc. Technol. lM, 10, 87. (27) Lindman, B.; Kamenka,N.; Kathopoulis, T. M.; Brun, B.; Nilsson, P. G. J. Phys. Chem. 1980, 84, 2485. (28) Lindman, B.; Puyal, M. C.; Kamenka, N.; Rymden, R.; Stilbs, P. J. Phys. Chem. 1984, 88, 5048.
Langmuir, Vol. 10, No. 9, 1994 2961
20
e* t
j
15
P F
5 I
1 1 1 , 1 1 , , 1 , , , 1 1 1 1 1 ~ , 1 ,
0 0
0.4
0.2
0.6
0.8
1
1.2
1.4
[surfactant]/ IPS161
Figure 1. Variation of the specific viscosity of a 9 mM PS16 solution upon addition of SDS (W) and C10E6 (0). 1.6
1.4 1.2 1
E?
0.8
F
0.6
0.4
0.2 0 0
0.2
0.4
0.6
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1
[surfactant]/[PS16]
Figure 2. Variation of the specific viscosity of a 3 mM PS16 solution upon addition of ClOE6 (e)and ClzEs (0). time lag that became shorter with increasing molar concentration ratio R = [surfactantY[polymerl. Thus, an addition of CloE6 corresponding to R = 0.234 resulted in the immediate disappearance of the viscoelasticity. At R = 0.180 and 0.104, the solutions were still viscoelastic immediately after surfactant addition, but the viscoelasticity disappeared after about 15 min at R = 0.180 and overnight at R = 0.104. The viscoelasticity persisted for systems at R 5 0.062. Similar observations had been made upon addition of SDS. The PSlG/surfactant solutions were therefore allowed to equilibrate for 3 days before the viscosity was measured. Figure 1shows that the nonionic CloE6 is much more efficient than the anionic SDS in decreasing the specific viscosity of a 9 mM PS16 solution. The viscoelasticity disappeared already at R values ofbetween 0.07 and 0.10 for C10E6, but only between 0.77 and 0.98 for SDS. The large difference in the effects of C10E6 and SDS most likely reflects the absence, in the case of C & , ,of the repulsive interactions that SDS molecules must overcome to become incorporated into the PS16 microdomains, which bear an electrical charge of the same sign. Figure 2 compares the effects of C10E6 and C1& on the specific viscosity of a 3 mM PS16 solution. The results for the two surfactants fall on the same curve indicating very little dependence on the surfactant chain length. Recall that the cmc values of these two surfactants in water are about 1 and 0.1 mM,29 respectively, that correspond to R values of 0.33 and 0.033. Thus, for C&6 the initial rapid decrease of vSptakes place essentially in (29) Degiorgio, V. In Physics ofAmphiphiles: Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M., Eds.; North-Holland: Amsterdam, 1985; p 303.
2962 Langmuir,
Vol. 10,No.9,1994
Kamenka et al.
Figure 3. Cry0 transmission electron micrographs of vitrified aqueous solutions of 9 mM PS16 in the presence of C ~ O E(a ~ :and b) R = 0.062; (c) R = 0.43; (d) R = 0.36. Bar corresponds to 100 nm.
a concentration range where this surfactant is molecularly dispersed. However, in the case Of C12E6 the decrease of viscosity occurs mostly in the micellar range. The similarity of the results for the two surfactants indicates that they must be nearly completelybound to the polysoap, at least in the concentration range corresponding to the decreasing part of the plot. The comparison of the results for CloE6 in Figures 1and 2 shows that the decrease of specificviscosity takes place in the same range ofR values, at PS16 concentrations of 3 and 9 mM. Figure 2 also shows that the specific viscosity goes through a shallow minimum at R = 0.30, then increases slowly with R . Experiments, repeated in the absence of PS16 and at the same surfactant concentration range, showed almost no change in the viscosity. The slow increase of specific viscosity at large R is thus characteristic of the PS16/nonionic surfactant system. It may be associated with some conformational change of the
individual PS16 chains, resulting from the near complete break-up of the threadlike micelles at R > 0.3, as indicated by the electron micrographs (see below). Cryo-TEM. Figure 3 shows electron micrographs of the PS16/CloEs system at a 9 mM concentration of PS16 and increasing values of the molar concentration ratio R. The micrographs in parts a and b of Figure 3, where R = 0.062, show long threadlike micelles, somewhat oriented, ~~~~~ probably as the result of sample p r e p a r a t i ~ n .The sample is sheared during preparation when it is thinned into a liquid film prior to vitrification. This may cause alignment of elongated structures, such as threadlike micelles. If given sufficient time, a few seconds for low viscosity systems, the shear effect is released. A more detailed discussion of the effect of shear is given in ref 25. Other micrographs (not shown)ofthe same system showed more disordered, entangled threadlike micelles, very similar to those reported for PS16 solutions in the absence
Interactions between Polysoaps and Surfactants of surfactant.21 These micelles are formed by the PS16 macromolecules with the hexadecyl pendent chains constituting the inside of the threads and with the folded polymer backbone and the charged carboxylate groups at the surface of the threads. The micrographs in parts a and b of Figure 3 show no polysoap chain-ends which would have appeared as black dots (see Figure 3c and its comment below). The threadlike micelles span over the whole micrograph and thus have lengths in excess of 1 pm. However, the maximum length of a micelle made of one PS16 macromolecule, based on the PS16 degree of polymerization, 4000, cannot be more than 200 Thus, the threadlike micelles seen in parts a and b of Figure 3 must be made by end to end linking of several macromolecules, as previously discussed.21 Indeed, the hexadecyl chains at the end of a macromoleculeare hydrophobic but cannot organize in microdomains because the neighboring hexadecyl chains, more inside the macromolecule, are already used up for forming microdomains, a situation reminiscent of that in solutions of helix-forming polypeptides. Thus, these free hexadecyl chains associate with other free hexadecyl chains from the end of another macromolecule, via hydrophobic interactions, forming a junction microdomain. At the PS16 concentration used (9 mM) each free end probably associates with another free end giving linear aggregates of macromolecules. Indeed, the viscosity ofthe PS16 solution was low, contrary to what is expected if several free ends associate into a single cross-link, giving rise to a network and a very high viscosity. In Figure 3c, where R = 0.13, many micellar ends are clearly visible as black dots, indicating that the fragmentation of the long plurimolecular micelles has already started. This explains the visual observation of the disappearance of viscoelasticity a t R above 0.07, noted for the 9 mM PS16 solution (see above). Only short elongated micelles and black dots can be seen at R = 0.36 (Figure 3d). These micelles appear to have a length in the 100nm range, a value consistent with the value of the DP of the PS16 sample used. The black dots in micrograph 3d can be ends of PS16 threadlike micelles but also free micelles of C10E6. 'Indeed, at R = 0.36 the surfactant concentration is 3.3 mM, that is, well above its critical micelle concentration. Such is not the case a t R = 0.13. The sequence of micrographs in Figure 3 is very similar to that for the SDSPS16 system.l9 In both cases cryoTEM provides direct visual evidence that, upon addition of an anionic or a nonionic surfactant, the long threadlike micelles of PS16 break-up into their constituting elements, that is, into individual PS16 chains. This disruption can only be caused by the incorpora4ion of the surfactant into the PS16 micelles. The question which then arises is whether the incorporated surfactant distributes in a somewhat uniform manner along the threadlike micelles or accumulates in some specific locations ofthese micelles, causing their break-up. With the nonionic surfactants this break-up starts at very low values of the [surfactantY [PS16] concentration ratio. This suggests that in the near absence of PS16 micelle ends, as is the case in the initial polysoap solution, the nonionic surfactant probably mostly binds at the microdomainsformed at thejunctions between PS16 chains. This leads to break-up of these junctions when enough surfactant is bound. Overall, the behavior of the PS16Isurfactant systems is very similar to that of (30)Assuming that the core ofthe threadlike micelles has a cylindrical shape and a radius equal to the length of the hexadecyl chain (21.5A) and that it is made of hexadecyl chains of volume 460 A3, the length of the cylinder is found to be 140 nm for a degree of polymerization of 4000.
Langmuir, Vol. 10,No.9,1994 2963 Table 1. Self-DifPusionCoefficients and Concentrations of Bounds Dodecyl Sulfate Ion in a 5 mM PS16/SDS System
c (mM) 4.05
1OloDDs(m28-l) 5.35 5.11
6.48
5.07
2.04
cb (mM)
0.26
0.66 1.10
the so-called associating p o l y m e r ~ in ~ ~the j ~presence ~ of surfactants. There, at sufficientlyhigh surfactant content, the surfactant forms micelles around the associating polymer hydrophobic side-chains and destroys the "crosslinks" between polymer chain^.^^^^^ In the SDSPS16 system, the break-up of the threadlike micelles may occur at much higher R values, simply because only part of the added SDS binds to the PS16, due to electrostatic effects. The dodecyl sulfate ion self-diffusion measurements described below were undertaken to check this explanation.
Self-Diffusionof the Dodecyl Sulfate Ion in PS16 Solution. The self-diffusion coefficient of the dodecyl sulfate (DS) ion, DDS,was measured as a function of the concentration, C, of SDS added to a 5 mM PS16 solution. The results are listed in Table 1. The measurements were performed in a restricted range of SDS concentration, below the cmc of this surfactant in water (8.3 mM), to avoid the complexitythat the presence of free SDS micelles would introduce in data interpretation. The measured DDSvalues are all smaller than the self-diffusioncoefficient of free DS measured in a submicellar solution of SDS in m2s-l, and the absence of any additive, D'DS= 6.1 x seem to decrease slightly with increasing C. The difference between DDSand D'DS can arise because part of the dodecyl sulfate ions are bound to the polymer, and thus diffuse a t the same rate as the high molecular weight polymer, that is, very slowly. The measured diffusion coefficient of DS is then a weighted average of the diffusion coefficients of the bound and free DS ions and must be lower than D'Ds. The difference between DDSand D'DS may also result from the obstruction effect of the polymer, which would hinder the surfactant ions in their diffusive motion.33 Measurements of the selfdiffusion coefficient of the chloride ion, Dcl, in the same PS16 solution pointed to the right choice between these two possibilities. If the explanation were correct, Dcl should be the same in the absence and in the presence of PS16, since chloride ions are not expected to bind to polymers having a like electrical charge. If the second explanation holds, DCl should decrease in the presence of PS16 in the same proportion as DDS. The measurement ofDcl was performed in a 5 mM PS16 solution containing 1mM of NaCl, with a small proportion of the chloride ions being 36Cl- radioactive labels. The measured Dcl was found to have the same value as in a dilute aqueous m2 s-l. This solution of NaCl, i.e., (1.86 f 0.05) x result clearly indicates that the difference between the values of DDSin the absence and in the presence of PS16 is due to the binding of some of the dodecyl sulfate ions to the polymer. The concentration of bound DS, Cb,can be evaluated from the equation
c, = cr1 - (DD@"DS>l
(1)
assuming that the contribution of the polymer-bound DS ~
(31)Sau, A. C.;Landoll, L. M. In Polymers in Aqueous Media. Performance Through Association; Glass, J. E., Ed.; Advances in Chemistry Series No. 223;American Chemical Society: Washington, DC, 1989; p 343. (32)Dualeh, A. J.; Steiner, C. A. Macromolecules 1991,24, 112. (33)Johansson, L.; Hedberg, P.; Lofroth, J.-E. J.Phys. Chem. 1993, 97, 747,and references therein.
2964 Langmuir, Vol. 10, No. 9, 1994 to diffusion is negligible, in view of the high molecular weight of PS16. The values of Cb, listed in Table 1, are nearly proportional to C , since DDs decreases only little upon increasing C . The concentration C = 6.48 mM corresponds to R = 1.3,i.e., a system where nearly all of the long threadlike micelles have been fragmented into individual PS16 chains.lg The fraction of bound DS with respect to the polymer repeat units is then only about 20%. The fragmentation is already significant at C = 4.05 mM,which corresponds to R = 0.81, and a fraction of bound DS with respect to PS16 ofonly 13%. We also attemptedto measure the binding of SDS to PS16 using a DS-selectivemembrane e l e ~ t r o d e . The ~ ~ , results ~~ showed the binding to be too small to be measured with any accuracy, in particular because of the small but significant effect of the polysoap, at such low binding ratio, on the membrane potential. Thus, when the partial binding of SDS to PS16 is properly taken into account, the effects of SDS and of and &E6 become quantitatively fairly close. In all instances, in the near absence of polysoap free ends, the surfactant appears to preferentially bind at the junctions between polymer chains and, when bound in sufficient amount, results in their break-up. At R values where the micrographs of the PSlG/surfactant system show only individual PS16 chains, the chain-ends are expected to be coated with bound surfactant, and to resemble micelles of this surfactant. The results suggest a n upper bound estimate of the surfactant aggregation number at a chain-end, NE. At a PS16 concentration of 5 mM, and assuming a polymerization degree DP = 4000, the concentration of polymer ends is calculated to be 2.5 x M. At R = 1.3, where PS16 is under the form of individual polymer chains, the concentration of bound SDS is 1.1 mM (see Table 11, resulting in NE= 440. This value is very large with respect of the aggregation number of free SSDS micelles, about 60. It is so mainly because the DP of PS16 is probably grossly overestimated (see materials section), resulting in too low a value for the concentration of chain-ends. Also, part of the surfactant may incorporate into the microdomains away from the chain-ends. Such a comicellization may not be favored by the relatively large (34) Davidson, C. J.; Meares, P.; Hall, D. G. J . Memb. Sci. 1988,36, 511. ( 3 5 )Wan-Badhi, W.; Palepu, R.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E.J . Phys. Chem. 1991,95,6642;and references therein.
Kamenka et al.
difference in alkyl chain length between added surfactant and PS1636and by the possibly more ordered packing of alkyl chains in these microdomains than in those a t junctions between chain-ends.21,22Nevertheless, in view of the relatively small proportion of microdomains at junctions with respect to the total number of microdomains, this may introduce a significant correction in the calculation of NE. The two reasons for the preferential binding of the added surfactant to microdomains corresponding to PS16 chain junctions are (1)the near absence of polysoap free chainends under the conditions used and (2) these microdomains are probably more disorganized than those away from chain-ends. Indeed, the former involve by necessity two chain-ends from two different polymer chains, while the latter are formed intramolecularly and may correspond t o short helical segments of the polymer backbone, as theoretically and as some electron micrographs appear to show (band pattern).21
Conclusions The long threadlike micelles formed in aqueous solutions of the polysoap PS16, by end-to-end linking of polysoap chains, are broken up into the individual polysoap chains upon addition of the anionic surfactant SDS or of the nonionicsurfactants CloEs or C12E6. This effect takes place a t a relatively low concentration of bound surfactant with respect to the concentration of polysoap repeat units. It suggests that the surfactant binds to the polysoap at the microdomains formed at junctions between polymer chains, resulting in their break-up when surfactant is present in sufficient amount. In this respect, the behavior of PS16 in the presence of surfactant is reminiscent of that of associating polymers. Acknowledgment. We acknowledge the help of Professor E. Wyn-Jones (Salford University, U.K.) in performing the potentiometric measurements on the PS16/ SDS system. We thank Ms. J . Schmidt and Ms. B. Shdemati for their expert help in the course of the work and the preparation of the manuscript. This work was supported in part by grants from the United States-Israel Binational Science Foundation (BSF), Jerusalem, and from the Fund for Promotion of Research at the Technion. (36) Malliaris, A.; Binana-Limb&, W.; Zana, R. J . Colloid Interface Sci. 1986,110, 114.
