Langmuir 1991, 7, 617-619
617
Viscometric Evidence of Interactions between Hydrophobically Modified Poly(sodium acrylate) and Sodium Dodecyl Sulfate I. Iliopoulos,’ T. K. Wang,’ and R. Audebert Laboratoire de Physico-Chimie Macromol&culaire,t E.S.P.C.I., 10, rue Vauquelin, 75231 Paris Cedex 05, France Received November 5, 1990. In Final Form: January 30, 1991 Viscosity measurements have been performed to study the interactions between sodium dodecyl sulfate
(SDS)and hydrophobicallymodified poly(sodium acrylate) (HMPAA). The polymer samples used contain
small amounts (1 or 3 mol 5%) of long alkyl side groups (18 carbon atoms). Addition of SDS in the aqueous solutions of HMPAA results in mixtures of very high viscosity. The viscosity maximum occurs at surfactant concentrations close to or lower than the critical micelle concentration. No viscosity enhancement is observed with the precursor nonmodified poly(sodiumacrylate). These results are interpreted in terms of interactions between SDS and HMPAA. The formation of mixed “micellelike”aggregates containing both SDS molecules and polymer-alkyl side groups can be postulated.
Introduction Interactions between oppositely charged polyelectrolytes and surfactants have received considerable attention in the last decades.’p2 It has been well established that such systems form complexes stabilized by both electrostatic attractions and cooperative hydrophobic effects. On the other hand, the association between polyelectrolytes and surfactants of the same sign can be expected to be feeble or absent mainly because of the unfavorable electrostatic repulsions between these species in aqueous solution. For instance, sodium carboxymethylcellulose3 and partially hydrolyzed polyacrylamide4 do not complex with sodium dodecyl sulfate (SDS). Only anionic polymers with a very pronounced hydrophobic nature, such as poly(1-decene-co-maleic acid) and poly( l-octadecene-co-maleic acid), seem to be able to complex with SDS.5 In a previous work we described the synthesis and the aqueous solution behavior of hydrophobically modified poly(sodium acrylate) (HMPAA) (Figure 1).6-8 These polymers contain small amounts (up to 3 mol %) of alkyl groups (8-22 carbon atoms) and present very interesting properties in aqueous solution. Above a critical polymer concentration, they exhibit viscosities higher than the precursor nonmodified poly(sodium acrylate) (PAA). Further enhancement of their viscosity can be achieved by increasing the ionic strength of the solution. This thickening behavior is believed to be due to the interchain hydrophobic aggregation which is favored by the screening of the electrostatic repulsions.8 Addition of a surfactant to the aqueous solutions of HMPAA may also result to an important viscosity
-(-
CH2- CH -)-(I 100-T
c=o
b-
Na’
CH2- CH -)-
cI = o= I NH - (CH2)n-1 -
CH3
Figure 1. Typical structure of hydrophobically modified poly(sodiumacrylate); r, mol % of modification; n,number of carbon atoms of the alkyl chain.
enhancement. This is especially observed upon addition of nonionic and cationic surfactant^.^ It is obvious that in the latter case, both electrostatic attractions and hydrophobic forces contribute to the stabilization of mixed aggregates which involvesurfactant molecules and polymer alkyl chains8 and act as effective cross-links between HMPAA chains. It must be noted that such a viscosity enhancement is not a general feature of all the associating polymer/surfactant systems. In many cases a decrease in viscosity upon addition of surfactants has been reported.*” In order to obtain more information about the aggregation mechanism in polyelectrolyte/ionic surfactant systems and to estimate the relative importance of electrostatic and hydrophobic interactions, we are studying the behavior of mixtures of hydrophobically modified poly(sodium acrylate) with anionic surfactants. In this letter we give some preliminary results related to the viscosity behavior of HMPAA/SDS mixtures in the semidilute regime.
Experimental Section
+ Present address: Beijing Institute of Chemical Technology, Heping Street, Beijing 100013,China. t Universitb Pierre et Marie Curie. URA CNRS No. 278. (1) Goddard, E. D. Colloids Surf. 1966, 19, 301. (2) Robb, I. D. In Chemistry and Technology of Water-Soluble Polymers; Finch, C. A., Ed.; Plenum Press: New York, 1983; p 193. (3) Schwuger, M. J.; Lange, H. Tenside 1968, 5 , 257. (4) Methemitis, C.; Morcellet, M.; Sabbadin, J.; Franpis, J. Bur. Polym. J . 1986, 22, 619. (5) McGlade, M. J.;Randall, F. J.;Tcheurekdjian, N. Macromolecules
Two samples of poly(acry1ic acid) (PAA-500 and PAA-150) were purchased from Polysciences and their average molecular weight, given by the supplier, was 500 000 and 150 000, respectively. The modification reaction was described elsewhere.6The modified samples were obtained in the sodium salt form and they have the same polymerization degree as the precursor polymer. Furthermore the distribution of the alkyl groups along the PAA chain is expected to be random.8 The sample designation is the following; e.g. PAA-500-1-Cl8is derived from PAA-500 and contains 1 mol 76 of octadecyl groups. Sodium dodecyl sulfate (SDS)was obtained from Kodak Laboratory Chemicals as >99% pure and was used as received. Its critical micelle concentration was estimated by the fluorescence spectroscopy technique using pyrene as a microenvironment sensitive probe12J3 and was found to be (7.7 i 0.3) X
(7) Wang, T. K.; Iliopoulos,I.; Audebert, R. Polym. Prepr. (Am.Chem. SOC.,Diu. Polym. Chem.) 1989, 30 (2), 377. (8) Wang, T. K.; Iliopoulos, I.; Audebert, R. In ACS Symp. Ser., in press.
(9) Landoll, L. M. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 443. (10) Schulz, D. N.; Kaladas, J. J.; Maurer, J. J.; Bock, J.; Pace, S. J.; Schulz, W. W. Polymer 1987,28, 2110. (11) McCormick, C. L.; Nonaka, T.; Johnson, C. B. Polymer 1988,29, 731.
* To whom correspondence should be addressed.
1987,20, 1782. (6) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988,20,577.
0743-7463/91/2407-0617$02.50/0
0 1991 American Chemical Society
Letters
618 Langmuir, Vol. 7, No. 4 , 1991 10
cp=1s
PAA-500-1-Cl8
lo
I
; ;I
,
+
- _ - - - PAA-500
!
1
5 4
id lo-'
1
10
lo2
SDS/cmc
Figure 2. Plot of the viscosity of aqueous solutions of modified and precursor polymers as a function of SDS concentration (expressed in cmc = 7.7 X 10-3mol L-l): (- - -) precursor PAA-
500 and (-) PAA-500-1-C18. Polymer concentrations are given in the figure. Arrow indicates polymer solutions in pure water.
mol L-l. This value is close to those reported in the literature (8 X and 8.3 X mol L-l).12-14 Most of the viscosity measurements were performed with a Contraves LS-30 viscometer at low shear rates (between0.06 and 1.28s-l) corresponding to the Newtonian viscosity of the system. For two very viscous mixtures (7 > l o 5 cP) exhibiting non-Newtonian behavior, a Carri-Med controlled stress rheometer with a cone and plate geometry was used. The temperature was 30 "C controlled to within 0.1 "C. Firstly,concentrated stock solutions of polymer and SDS were prepared under magnetic stirring. Since polymer samples were used in their fully neutralized form (sodium salt form), their solutions were basic (pH = 9.6). Mixtures of the desired composition were prepared by mixing of the stock solutions and, if necessary, by further dilution with water. The mixtures were vigorously shaken and allowed to equilibrate at least for 24 h before measurements were made. The order of mixing did not influence the viscometric behavior of these systems. Results and Discussion Logarithmic plots of the viscosity as a function of SDS concentration (expressed in critical micelle concentration (cmc)) are given in Figures 2 and 3. When the SDS is added in an aqueous solution of the precursor nonmodified poly(sodium acrylate) (PAA-500), a continuous and slight decrease in the viscosity is observed, dashed curves in Figure 2. This is due to the increase in the ionic strength of the solution upon addition of SDS and it is in agreement with results of Methemitis et al.4 related to the behavior of mixtures of anionic surfactants with partially hydrolyzed polyacrylamide. Addition of SDS to aqueous solutions of a modified polymer containing 1mol % of octadecyl groups (PAA-500-1-C18)leads to a viscosity enhancement (Figure 2). The viscosity curve presents a maximum for SDS concentration close to 0.3 cmc. The higher the polymer concentration, the more effective the viscosification of the system. (The molar concentration of polymer-alkyl groups is 3.12 X 10-3 mol L-' for the solution containing 3 3' 4 PAA500-1-C18.) (12) Kalyanasundaram, K.; Thomas, J. K. J.Am. Chem. SOC. 1977,99, 2039. (13) Turro, N.J.; Baretz, B. H.; Kuo, P. L. Macromolceules 1984,17, 1321. (14) Lindman, B.; Wennerstrom, H. In Topics in Current Chemistry; Springer-Verlag: Berlin, 1980;Vol. 87, p 1.
1
l 2 SDS/cmc
Figure 3. Plot of the viscosity of aqueous solutions of modified polymers as a function of SDS concentration (expressedin cmc):
(*) PAA-500-l-Cl8; (+) PAA-500-3-Cl8; (0)PAA-150-3-Cl8. Polymer concentration,C, = 1R . The mixtures corresponding to the two upper points of the figure (curve PAA-500-3-Cl8) present no Newtonian viscosity and the reported values correspond to a shear rate = 0.1 s-l. Arrow indicates polymer solutions in pure water.
The viscosity behavior of the systems HMPAA/SDS depends largely on the alkyl group content of HMPAA. In Figure 3 the viscosity is plotted as a function of SDS concentration for samples modified at 1and 3 mol % The polymer concentration was kept constant, C, = 1 % . The most hydrophobic polymer, PAA-500-3-C18, presents a very important viscosity enhancement and no Newtonian viscosities were measured for SDS concentrations between 0.3 and 1 cmc. (In this case, the molar concentration of polymer-alkyl groups is 2.97 X mol L-I.) Interestingly, this system exhibits higher viscosities than PAA-500-1C18 a t C, = 3% although the two systems have almost the same concentration of polymer-alkyl groups, 2.97 X and 3.12 X mol L-l, respectively (compare Figures 2 and 3). Furthermore, by increasing the alkyl group content, the maximum of the viscosity curve is shifted toward higher SDS concentrations, close to 1cmc (Figure 3). This is also borne out with a polymer of lower molecular weight (PAA-150-3-Cl8 in Figure 3).15 From our viscometric results it is clearly shown that, despite the unfavorable electrostatic repulsions, SDS associates with anionic polyelectrolytes when these polymers contain even very small amounts of very hydrophobic groups. Hydrophobic interaction seems to be the driving force of this type of association. Apart from the electrostatic repulsions, steric constraint due to the polymer backbone is another factor that may influence the association behavior of HMPAA/SDS system. Presumably, this is the reason for the differences observed in the viscosities of PAA-500-3-Cl8 (C, = I % , Figure 3) and PAA-500-1-Cl8 (C, = 396, Figure 2). The increase in viscosity of the modified polymer solutions upon addition of SDS can be ascribed to the aggregation of both surfactant molecules and polymeralkyl groups in mixed hydrophobic clusters. This association reduces the contacts of the polymer-alkyl groups with water and consequently lowers the energy of the
.
(15) It must be noted that in the presence of HMPAA (or PAA) the cmc value of SDS is expected to be lowered because of the increase in the ionic strength of the solution due to the COO-Na+ groups of the polymer.'
Letters system. Presumably, the formation of such clusters may occur a t surfactant concentrations even lower than cmc as usually observed in systems exhibiting surfactant-polymer interactions.lJ3J6J7 Taking into account that the polymer concentrations used range in the semidilute regime,’t8 the mixed hydrophobic clusters may contain polymer-alkyl groups belonging to two or more distinct polymer chains. Such clusters act as effective cross-links and contribute to the viscosity enhancement. The viscosity level and the position of the maximum depend on both the number of polymer-alkyl groups in a mixed hydrophobic cluster and the number of clusters in which a given polymer chain is involved. A t SDS concentrations well below the cmc, few or no mixed clusters are formed and so no important visocisty increase occurs. The increase in SDS concentration induces at first an effective cross-linking of the polymer chains. But, for SDS concentrations well above the cmc, the surfactant micelles are in large excess and generally contain no more than one polymer-alkyl group. In fact, it seems realistic to assume that, at surfactant concentrations well above the cmc, the reduction of the conformational energy of the polymer chains is more effective if the polymer-alkyl groups are distributed in a maximum number of surfactant micelles rather than involved in a (16) Goddard, E. D. Colloids Surf. 1986,19, 255. (17) Ruckenstein, E.;Huber, G.; Hoffmann, H. Langmuir 1987,3,382.
Langmuir, Vol. 7, No. 4, 1991 619 small number of mixed clusters containing more than one polymer-alkyl group. As a consequence, effective crosslinking is prevented and a decrease in viscosity is expected for high surfactant concentrations. Such an association model between hydrophobically modified polymers and surfactants is also proposed for other similar systems.1g20 An attempt a t theoretical modelization of the aggregation behavior of such systems was recently proposed by Balazs and Hua21 Despite the very simplifying assumptions considered, this model seems to be qualitatively in accordance with our findings. Unfortunately, the number of polymer-alkyl groups in a mixed hydrophobic cluster and the number of clusters in which a given polymer chain is involved, which are the most important parameters governing the rheological behavior of these systems, are difficult to estimate, a t least from the viscosity results reported in this letter. Further studies are required on this point.
Acknowledgment. We thank the “SociBtB Francaise Hoechst” for generous support of this work and Drs. A. Blanc and J. Cabestany for helpful discussions and suggestions. (18) Gelman, R. A. Znt. Dissolving Pulps Conf., TAPPZProc. 1987, p 159. (19) Tanaka, R.; Meadows, J.; Phillips, G. 0.; Williams, P. A. Carbohydr. Polym. 1990, 12,443. (20) Winnik, F. M. Langmuir 1990, 6, 522. (21) Balazs, A. C.; Hu, J. Y. Langmuir 1989, 5, 1230 and 1253.
