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The effect of cations on the interaction between dodecylsulfate micelles and poly(ethyleneoxide)
The Effect of Cations on the Interaction between Dodecylsulfate Micelles and Poly(Ethyleneoxide) PAUL L. DUBIN, ~ JAMES H. GRUBER, JIULIN XIA, AND HUIWEN ZHANG 2 Department o f Chemistry, lndiana University-Purdue University at Indianapolis, Indianapolis, Indiana 46205 Received March 12, 1991; accepted May 30, 1991 The effect of the micelle counterion on the interaction of poly(ethyleneoxide) with dodecylsulfate micelles has been studied, using dye solubilization to determine the critical micelle concentration and dynamic light scattering to measure the relative a m o u n t of uncomplexed rnicelles. In the presence of 0 . 1 0 - M Na +, Li +, and NH~-, the CMC-lowering effect of the polymer is strongly dependent on the nature of the cation. A parallel influence of the cation is seen in the distribution of scattering intensities between well-defined modes corresponding to free micelle and complex. These results are taken as evidence for a direct role of the cation in the stabilization of the complex, in which the cation interacts simultaneously with the micelle (through electrostatic forces) and with the polymer (via coordination complexation). This type of association m a y occur simultaneously with other interaction forces. © 1992AcademicPress,Inc.
Duplessix ( 11 ) concluded from neutron scattering that the polymer "is in water, weakly Complexes between micelles of sodium doadsorbed at the (hydrocarbon/water) interdecylsulfate (SDS) and nonionic hydrophilic face" ( 11 ), and also stated elsewhere that the polymers such as poly(vinylpyrrolidone) or "hydrated part of the SDS molecule is bound poly ( ethyleneoxide ) [ poly (oxyethylene) ] to the polymer" and that there is "an electrowere identified over 30 years ago and have static interaction of PEO with the polar groups been the subject of continued investigation (of SDS)" ( 5 ). Contrariwise, Nagarajan (12) ( 1 ). The interaction between SDS micelles and and Ruckenstein et al. (13) suggested that inpoly (ethyleneoxide) (PEO) has been studied teractions between polymer and surfactant by many techniques, including equilibrium headgroups are unfavorable to complex fordialysis (2), surface tension (32, dye solubimation, for which the driving force is the relization (4), NMR (5), neutron scattering (62, duction in interfacial energy between the hyviscometry (7), specific ion electrodes (8), drocarbon core and the local solvent medium conductimetry (9), and fluorescent probes (which may include PEO segments). This is (10), leaving little doubt that these micelles in agreement with Francois et aL's statement indeed bind to PEO, but without definitive that "the -CH2- groups of PEO tend to be identification of the mechanism of interaction. bound within the aliphatic part of the miA variety of explanations of the force undercelles" (7), but contradicts earlier conclusions lying the binding appears in the literature. For by Moroi et al. to the effect that "the complex example, Schwuger suggested that a positive comes mainly from the interaction between charge on PEO could arise from partial prothe ionic head group and the ethylene oxide tonation of ether oxygens (3). Cabane and group of the polymer and is hardly affected by the hydrophobic interaction between the surfactant tail and the ethylene oxide group" (14). Brackman and Engberts ( 15 ) noted that 1 To w h o m correspondence should be addressed. head group hydration inhibits the binding of z Present address: R.I.D.C.L, Taiyuan, Shanxi, PRC. INTRODUCTION
35
Journal of Colhfidand Intofiwe Science, Vol. 148,No. 1, January 1992
0021-9797/92 $3.00 Copyright© 1992 by AcademicPress,Inc. All rightsof reproductionin any fbrm reserved.
36
DUBIN
ET
AL.
PEO to micelles. Specific interactions between rate of sodium ion even when further polysulfonate and PEO were s~2_ggested by mer-micelle complexation is unlikely. Most Prud'homme (16) who noted effects of Congo recently, the affinity of PEO with Na + in water red (a disulfonated dye) and poly(styrene- was conclusively demonstrated by Abuin and sulfonic acid) on the viscosity of PEO. Most co-workers using ion selective electrode and recently, Gao et al. (17) concluded from N M R ultrafiltration measurements (20). The finding that PEO binds Na + suggests paramagnetic relaxation that PEO units are that we consider this p h e n o m e n o n as a conpredominantly solubilized within the micelle, tributory mechanism in the P E O - S D S interand noted the discrepancy between this conaction. The concentration o f N a + in a solution clusion and that of Cabane ( 11 ). of SDS above the critical micelle concentration It has thus been variously proposed that the is largest in the vicinity o f the micelle, as a PEO-SDS complex is stabilized by hydrophoconsequence of the influence o f the colloidal bic interactions between the methylene units particle on its ion atmosphere. A favorable inof the polymer and those of the surfactant alkyl teraction between Na + and PEO would then groups, by interactions between polymeric ether groups and sulfate groups, or by effects result in a tendency for the polymer chains to more closely related to the nature of the mi- reside in the micelles' vicinity. This effect celle-water interface. Much of the experimen- could be described a s "binding." Put differtal results obtained illuminate the long-range ently, if, as suggested by Abuin et al., the asstructure of the complexes, while shedding less sociation of PEO with alkali-metal cation crelight on the specific nature of the local ener- ates a positive potential on the polymer chain getics. In any event, none of the foregoing which attracts anions (20), one would also exproposals has been clearly substantiated, and pect anionic micelles to be attracted to this it must be recognized that the driving force "quasi-polycation." A consequence of the above hypothesis for the binding of SDS micelles to PEO is open would be sensitivity of complex formation to to debate. We suggest here as an alternative hypothesis the nature of the counterion. In this comthat the binding of PEO to SDS is mediated munication we report the results of limited in part by the cation. It is well known that experiments intended to explore the role of cyclic ethers bind Na + ions with sufficient in- the cation in the PEO-SDS interaction. We tensity to transfer them (and their accompa- compare the effect of PEO on dodecylsulfate nying counterions) from water to organic sol- with Na +, Li +, and NH~- cations, which are vent (18). Noncyclic ethers, sometimes called known to differ strongly with regard to their "podands," can display similar effects, al- binding to polyethers ( 18, 19, 20). As a quant h o u g h - l a c k i n g a preformed binding site-- titative measure of the polymer-micelle inthey exhibit smaller binding constants. Thus, teraction we use the CMC-lowering effect of the binding constant of monovalent cations the polymer, which may be related to the free to PEO oligomers has been measured in energy o f stabilization of the micelle by commethanol by electrochemical methods, with plexation with the polymer (21 ), which may the result for the relative binding: K + > Cs + be simply measured by dye solubilization (the > Na + (19). Early evidence for the binding perturbing effect of the dye on the micelle of Na + to PEO in water appears in Cabane's structure seems to be small, since CMC values measurements of the relaxation rate r of 23Na + obtained by this technique are in good agreein solutions o f PEO and SDS (5): r increases ment with those from other methods, such as continuously with addition of PEO well be- surface tension). Since this technique only yond the region designated as "excess PEO," measures the properties of the solution in the i.e., the addition of PEO affects the relaxation range of conditions near incipient micelle forh)urttal ~fC'olloid and Inle([bce Science, Vo[. 148, No, I, January 1992
37
MICELLES-PEO INTERACTIONS
marion, we also carry out quasi-elastic light scattering (QELS) measurements of solutions with relatively larger surfactant and polymer concentrations, and compare the effect of these three ions on the size distributions obtained by QELS for mixtures o f PEO and SDS well above the CMC. EXPERIMENTAL
SDS was from Fluka, 99.5%. Solvent Orange 2 oil-soluble dye3 (1-(o-tolylazo)-2naphthol, FW 262.31 ) was from Dupont. PEO ( " P E O X 200K," Mw = 204,000, Mn = 47,000) was from American Polymer Standards (Mentor, OH). All salts were reagent grade, from Fisher. CMC values were determined by dye solubilization. In initial work, erratic solubilization data were obtained. Two sources of error were identified: incomplete dissolution in samples with aggregated dye particles, and loss ofsolubilized dye by adsorption on membrane filters which were used to remove solid dye particles. The following procedure was then employed: 1.5-2.0 mg of dye were combined with 10.0 m L of surfactant solution containing the desired a m o u n t of SDS in 0.1M NaCI, LiC1, or NH4CI. The solutions were sonicated in 15-mL Kimax glass tubes for 2 min to disperse the dye, and then tumbled for 48-72 h at 23 + 0.5°C. Unsolubilized dye was removed by centrifugation at 3500 rpm for 60 min, and the absorbance of the supernatant at 496 n m was read with an HP8450 diode array UV-vis spectrophotometer. For CMC determinations in the presence of PEO, the surfactant solutions also contained 0.40 g / L polymer. This concentration should represent an excess of polymer for surfactant concentrations on the order of 1 m M (5).
For QELS, solutions containing 5.0-mM SDS in 0.4-M NaC1, LiC1, or NH4C1 (with and without 2.5 g / L PEO) were filtered through 0.20-t~m filters into an optically clean, dust-free sample cell. Measurements were made at 90 ° scattering angle with a Brookhaven system equipped with a 72 channel digital correlator (BI-2030 AT) and using a 15m W H e - N e laser (Jodon). We obtain the homodyne intensity-intensity correlation function G(q, t) with q, the amplitude of the scattering vector, given by q = (4rrtiD,)(sin(0/ 2), where t~ is the refactive index of the medium, ~ is the wavelength of the excitation light in a vacuum, and 0 is the scattering angle. G(q, t) is related to the time correlation function of concentration fluctuations g(q, t) by
G(q, t) = A(1 + bg(q, t)2),
where A is the experimental baseline and b is the fraction of the scattered intensity arising from concentration fluctuations. The quality of the measurements were verified by determining that the difference between the measured value of A and the calculated one was less than 1%. The distribution of diffusivities and hence the size distribution function was obtained from g(q, t) by inverse Laplace transformation using the program CONTIN (25). RESULTS AND DISCUSSION
Dye Solubilization The results of dye solubilization measurements are presented in Fig. 1 and in Table I. The thermodynamic stability of the micelle is proportional to - R T I n CMC (21 ) so it has been suggested (15) that the effect of the polymer on the micelle stability is
6 = RTln(CMC/CMCp), 3 This compound is sometimes referred to as Orange OT, but the Colour Index lists Orange OT as Pigment Orange 13, which is formed by diazotization of 3,3-dichlorbenzidine, followed by coupling with 3-methyl-lphenyl-5-pyrazolone (22). We therefore refer to the compound l-(o-tolylazo)-2-naphthol as Solvent Orange 2.
[11
[2]
where CMCe is measured in the presence of the polymer. Since the solubilization curves with and without polymer are so similar, only shifted with respect to CMC, it seems quite Journal of Colloid and lnteffhce Science
Vol. 148,No. 1, January 1992
38
DUBIN 0.50
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[SDS], mM FIG. 1. Absorbance of solubilized Solvent Orange 2 in SDS (open symbols) and SDS in the presence of 0.40 g/L PEO (filled symbols), in various 0.10-M electrolyte solutions. Results of duplicate experiments are shown.
reasonabIe to consider CMCp as the point of formation of the polymer-stabilized micelles, and not some unrelated phenomenon. From repeated experiments, the reliability of the CMC measurements is taken as +5% relative, leading to an expected uncertainty in 6 on the order of + 10%. As seen from Table I, the differences for the various counterions are significant, namely 6 {i > 6 ~a > b ~H4. It is known that the binding of N H + to cryptands or podands is particularly weak, and this observation m a y be related to the small effect of the polymer in the presence of this ion. hmrnal tffCol]oid and lnle([hce S¢qence, Vol. 148, No. I, January 1992
The effect of the counterion on the polymerfree micelle needs to be noted. The binding of N H ~ to dodecylsulfate is stronger than the binding o f Na +, with a selectivity exchange constant of K ( N H ~ / Na + ) of 2 (23). This observation is in agreement with the effects of these two ions on the CMC seen here, inasm u c h as the stronger interaction o f NH~- with the SDS headgroups should reduce the electrostatic intramicellar repulsive force and thus stabilize the micelle. (The lowering of the free energy of micellization when the degree of dissociation of the counterion is diminished
39
MICELLES-PEO INTERACTIONS TABLE I CMC ~ for SDS and SDS/PEO in 0.10-M Univalent Electrolytes Salt
CMC (raM)
CMCp t'
t~ (kJ/mol)
LiCI NaCI NH4C1
1.66 1.27 1.03
0.98 0.83 0.76
1.29 1.04 0.74
"Determined from the abcissa intercepts of Fig. 1, by linear regression analysis of combined duplicat~ data sets. b In the presence of 0.40 g/L PEO.
is also consistent with classical thermodynamic considerations (24). However, despite the fact that N H ~ appears to be the more tightly bound to the micelle than the other counterions studied here, it has the least influence on polymer-micelle binding. This suggests that the weak interaction o f N H g with PEO is more important than its strong binding to the micelle in determining its influence on polymermicelle complexation.
the complex is larger in Na + solution than in the presence o f N H ft. Since the total scattering intensity from all three solutions was very similar, the relative areas in each of the three distributions may be compared. The fraction of the (correlated) scattered light due to unbound miceUes for Li ÷, Na ÷, and NH~ is, respectively, 3, 6, and 11%, indicating a binding intensity Li + > Na + > N H 2 , as also deduced above from dye solubilization. It is evident that the counterion also affects the properties of the free micelle so that to some extent the effect on the interaction with the polymer may be indirect. In particular, Li + raises the CMC ofdodecylsulfate and increases the micelle size, compared to Na +. The stron-
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QELS QELS results are reported in Fig. 2 for PEO / SDS in the presence of 0 . 4 0 - M NaC1, LiC1, and NH4C1. (It was necessary to employ a higher ionic strength than that used for solubilization studies in order to reduce intermicellar interactions which would obscure the interpretation of the measured decay constants in terms of particle size). The measured autocorrelation function was well-resolved with two decay modes, using the method of CONT I N (25). The fast-diffusion modes correspond to diffusivities identical to those obtained in polymer-free micelle solutions (apparent Stokes radii Rg °p 2-3 nm) and could be identified with unbound micelles. The slow diffusion modes exhibit R~ °p larger than those of the pure polymer by 160-360%, and must be the complex (the scattering of the pure polymer is very weak). It is evident from Fig. 2 that the relative magnitude of scattering from
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0.02 o. 0.01 0 3 4 5 18 37 95 0.015! 0.01 0.005 0 5 6 7 23 65 d
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(nm)
FIG. 2. Apparent (intensity-weighted) distribution of Stokes diameters from QELS for SDS ( 5.0 raM) and PEO (2.5 g/L) in (from top to bottom) 0.40-M NH4C1, 0.40M NaC1, and 0.40-M LiCI.
Journal ~,fColloid and lnte~[iwe Science, Vol. 148, No. l, January 1992
40
DUBIN ET AL.
ger binding of P E O to SDS in LiC1 might partly be a consequence of these changes in micelle structure and stability. In this regard, comparison of Na + and NH~ is more conclusive, inasmuch as these two counterions have a relatively similar effects on the polymer-free micelle, but significantly different effects on 6 and on the QELS distributions. As the concentration of the counterion is increased, the difference between its concentration near the micelle and in the bulk solution must diminish (put differently, the double layer shrinks). According to our hypothesis, the effect of PEO should then vary inversely with counterion concentration. To test this prediction, we measured CMC values in 0.01-MNaCI and obtained for SDS and for PEO/SDS, 5.4 m M a n d 2.8 mM, respectively. This corresponds to a 50% increase in 6, relative to the result in 0.1-M NaC1, in the predicted direction, i.e., that the effect of PEO is larger at lower ionic strength. Surprisingly, 6 for LiDS in 0.01-M LiC1 was essentially unchanged from the value for NaDS in 0.1-M LiCI. In the absence of commercially available NH4DS, no low ionic strength measurements were made for this surfactant. The present studies yield no insight into the location of PEO segments in the complex. As noted above, NMR studies (5) suggest penetration of the PEO structural units into the micelle beyond the Stern layer. It must be noted that such studies give the average locus of any particular set of NMR-active nuclei, all of which may not be in the same environment; i.e., even if some polymer units were in the hydrocarbon "core," many units must pass through the domain of the sulfate groups and their ion atmosphere. For example, in 0.1-M NaC1, the volume within the hydrocarbon "core" of the micelle cannot be more than onehalf of the volume containing the sulfate head groups and their ion atmosphere (with a thickness of one Debye length). Interactions between PEO and the micellar counterions could occur anywhere in the latter region. Ex-
Jmtrnal o[Colloid and h~IeiJbce Science,, Vol. 148, No, 1, January 1992
actly where the strongest interaction of PEO with the metal ion takes please is conjectural. CONCLUSIONS
The interaction between dodecylsulfate micelles and poly(ethyleneoxide) was studied in the presence of Na +, Li ÷, and NH~- ions. Both dye solubilization measurements of the CMC and quasi-elastic light scattering measurements at higher surfactant concentrations indicate that the cation plays a role in this phenomenon, with the polymer-micelle interaction strongest in the presence of Li ÷ and weakest in NH~. These findings suggest that the simultaneous (electrostatic) affinity of the cations for the micelle and (coordination) association with the polymer could energetically contribute--along with other effects--to the binding force between PEO and dodecylsulfate micelles. ACKNOWLEDGMENTS This work was supported by NSF grant DMR-9014945. HZ acknowledges the support of a United Nations Development Fund Fellowship. Helpful comments by N. Muller are acknowledged. REFERENCES 1. For review, see Goddard, E. D., Colloids Surf. 19, 255 (1986). 2. Shirahama, K., ColloidPolym. Sci. 252, 978 (1974). 3. Schwuger, J. Colloid Interface Sci. 43, 491 (1973). 4. Jones, M. N., J. ColloidlnterfaceSci. 26, 532 (1968). 5. Cabane, B., J. Phys. Chem. 81, 1639 (1977). 6. Cabane, B., and Duplessix, R., J. Phys. 43, 1529 (1982). 7. Francois, J., Dayantis, J., and Sabbadin, J. Eur. Polym. J. 21, 165 (1985). 8. Gilanyi, T., and Wolfram, E., Colloids Surf. 3, 181 (1981). 9. Jones, M. N., J. Colloid lnterface Sci. 23, 36 (1967). 10. Zana, R., Lang, J., and Lianos, P,, in "Microdomains in Polymer Solutions" (P. Dubin, Ed.), Plenum Press, New York, 1985. 11. Cabane, B., and Duplessix, R., J. Phys. 43, 1529 (1982). 12. Nagarajan, R., Colloids Surf. 13, 1 (1985). 13. Ruckenstein, E., Huber, G., and Hoffmann, H., Langmuir 3, 382 (1987).
MICELLES-PEO INTERACTIONS 14. Moroi, Y., Akisada, H., Saito, M., Matuura, R., J. Colloid Interface Sci. 61, 233 (1976). 15. Brackman, J. C., and Engberts, J. B. F. N., J. Colloid Interface Sci. 132, 250 (1989). 16. Prud'homme, R. K., paper no. 10675, presented at 1982 SPE/DOE Third Joint Symposium on Enhanced Oil Recovery, Tulsa, OK, April 1982. 17. Gao, Z., Wasylishen, R. E., and Kwak, J. C. T., aT. Phys. Chem. 95, 462 ( 1991 ). 18. See for example, Vrgtle, F., Ed., "Host Guest Complex Chemistry No. I, Topics in Current Chemistry, Vol. 98. Springer-Veflag, Berlin, 1981; Vrgtle, F., and Boschke, F. I., Eds., "Host Guest Complex Chemistry No. II," Topics in Current Chemistry, Vol. 101. Springer-Verlag, Berlin, 1982; Vrgtle, F., et aL, Eds., "Host Guest Complex Chemistry No. III," Topics in Current Chemistry, Vol. 121. SpringerVerlag, Berlin, 1983.
41
19. Chaput, G., Jeminet, G., and Juillard, J., Can. J. Chem. 53, 2240 (1975). 20. Sartori, R., Sepulveda, L., Quina, F., Lissi, E., and Abuin, E. Macromolecules 23, 3878 (1990). 21. Shirahama, K., and Ide, N., J. Colloid Interface Sci. 262, 450 (1976). 22. "The Colour Index," 2nd ed., Supplement p. $986, The Society of Dyers and Colourists, Yorkshire, England, and the American Association of Textile Chemists and Colourists, Lowell, MA, 1963~ 23. Bonilha, J. B. S., Georgetto, R. M. Z., Abuin, E., Lissi, E., and Quina, F. J. Colloid lnterfaceSci. 135, 238 (1990). 24. Attwood, D., and Florence, A. T., "Surfaetant Systems," Chap. 3. Chapman &Hall, New York, 1982. 25. For a general discussion of size distribution analysis via QELS, see, for example, Phillies, G. J. D., Anal. Chem. 62, 1049A (1990), and references therein.
Journal of Colloidand lnter[o.ceScience, Vol.148,No, 1,January1992
Duplessix ( 11 ) concluded from neutron scattering that the polymer "is in water, weakly Complexes between micelles of sodium doadsorbed at the (hydrocarbon/water) interdecylsulfate (SDS) and nonionic hydrophilic face" ( 11 ), and also stated elsewhere that the polymers such as poly(vinylpyrrolidone) or "hydrated part of the SDS molecule is bound poly ( ethyleneoxide ) [ poly (oxyethylene) ] to the polymer" and that there is "an electrowere identified over 30 years ago and have static interaction of PEO with the polar groups been the subject of continued investigation (of SDS)" ( 5 ). Contrariwise, Nagarajan (12) ( 1 ). The interaction between SDS micelles and and Ruckenstein et al. (13) suggested that inpoly (ethyleneoxide) (PEO) has been studied teractions between polymer and surfactant by many techniques, including equilibrium headgroups are unfavorable to complex fordialysis (2), surface tension (32, dye solubimation, for which the driving force is the relization (4), NMR (5), neutron scattering (62, duction in interfacial energy between the hyviscometry (7), specific ion electrodes (8), drocarbon core and the local solvent medium conductimetry (9), and fluorescent probes (which may include PEO segments). This is (10), leaving little doubt that these micelles in agreement with Francois et aL's statement indeed bind to PEO, but without definitive that "the -CH2- groups of PEO tend to be identification of the mechanism of interaction. bound within the aliphatic part of the miA variety of explanations of the force undercelles" (7), but contradicts earlier conclusions lying the binding appears in the literature. For by Moroi et al. to the effect that "the complex example, Schwuger suggested that a positive comes mainly from the interaction between charge on PEO could arise from partial prothe ionic head group and the ethylene oxide tonation of ether oxygens (3). Cabane and group of the polymer and is hardly affected by the hydrophobic interaction between the surfactant tail and the ethylene oxide group" (14). Brackman and Engberts ( 15 ) noted that 1 To w h o m correspondence should be addressed. head group hydration inhibits the binding of z Present address: R.I.D.C.L, Taiyuan, Shanxi, PRC. INTRODUCTION
35
Journal of Colhfidand Intofiwe Science, Vol. 148,No. 1, January 1992
0021-9797/92 $3.00 Copyright© 1992 by AcademicPress,Inc. All rightsof reproductionin any fbrm reserved.
36
DUBIN
ET
AL.
PEO to micelles. Specific interactions between rate of sodium ion even when further polysulfonate and PEO were s~2_ggested by mer-micelle complexation is unlikely. Most Prud'homme (16) who noted effects of Congo recently, the affinity of PEO with Na + in water red (a disulfonated dye) and poly(styrene- was conclusively demonstrated by Abuin and sulfonic acid) on the viscosity of PEO. Most co-workers using ion selective electrode and recently, Gao et al. (17) concluded from N M R ultrafiltration measurements (20). The finding that PEO binds Na + suggests paramagnetic relaxation that PEO units are that we consider this p h e n o m e n o n as a conpredominantly solubilized within the micelle, tributory mechanism in the P E O - S D S interand noted the discrepancy between this conaction. The concentration o f N a + in a solution clusion and that of Cabane ( 11 ). of SDS above the critical micelle concentration It has thus been variously proposed that the is largest in the vicinity o f the micelle, as a PEO-SDS complex is stabilized by hydrophoconsequence of the influence o f the colloidal bic interactions between the methylene units particle on its ion atmosphere. A favorable inof the polymer and those of the surfactant alkyl teraction between Na + and PEO would then groups, by interactions between polymeric ether groups and sulfate groups, or by effects result in a tendency for the polymer chains to more closely related to the nature of the mi- reside in the micelles' vicinity. This effect celle-water interface. Much of the experimen- could be described a s "binding." Put differtal results obtained illuminate the long-range ently, if, as suggested by Abuin et al., the asstructure of the complexes, while shedding less sociation of PEO with alkali-metal cation crelight on the specific nature of the local ener- ates a positive potential on the polymer chain getics. In any event, none of the foregoing which attracts anions (20), one would also exproposals has been clearly substantiated, and pect anionic micelles to be attracted to this it must be recognized that the driving force "quasi-polycation." A consequence of the above hypothesis for the binding of SDS micelles to PEO is open would be sensitivity of complex formation to to debate. We suggest here as an alternative hypothesis the nature of the counterion. In this comthat the binding of PEO to SDS is mediated munication we report the results of limited in part by the cation. It is well known that experiments intended to explore the role of cyclic ethers bind Na + ions with sufficient in- the cation in the PEO-SDS interaction. We tensity to transfer them (and their accompa- compare the effect of PEO on dodecylsulfate nying counterions) from water to organic sol- with Na +, Li +, and NH~- cations, which are vent (18). Noncyclic ethers, sometimes called known to differ strongly with regard to their "podands," can display similar effects, al- binding to polyethers ( 18, 19, 20). As a quant h o u g h - l a c k i n g a preformed binding site-- titative measure of the polymer-micelle inthey exhibit smaller binding constants. Thus, teraction we use the CMC-lowering effect of the binding constant of monovalent cations the polymer, which may be related to the free to PEO oligomers has been measured in energy o f stabilization of the micelle by commethanol by electrochemical methods, with plexation with the polymer (21 ), which may the result for the relative binding: K + > Cs + be simply measured by dye solubilization (the > Na + (19). Early evidence for the binding perturbing effect of the dye on the micelle of Na + to PEO in water appears in Cabane's structure seems to be small, since CMC values measurements of the relaxation rate r of 23Na + obtained by this technique are in good agreein solutions o f PEO and SDS (5): r increases ment with those from other methods, such as continuously with addition of PEO well be- surface tension). Since this technique only yond the region designated as "excess PEO," measures the properties of the solution in the i.e., the addition of PEO affects the relaxation range of conditions near incipient micelle forh)urttal ~fC'olloid and Inle([bce Science, Vo[. 148, No, I, January 1992
37
MICELLES-PEO INTERACTIONS
marion, we also carry out quasi-elastic light scattering (QELS) measurements of solutions with relatively larger surfactant and polymer concentrations, and compare the effect of these three ions on the size distributions obtained by QELS for mixtures o f PEO and SDS well above the CMC. EXPERIMENTAL
SDS was from Fluka, 99.5%. Solvent Orange 2 oil-soluble dye3 (1-(o-tolylazo)-2naphthol, FW 262.31 ) was from Dupont. PEO ( " P E O X 200K," Mw = 204,000, Mn = 47,000) was from American Polymer Standards (Mentor, OH). All salts were reagent grade, from Fisher. CMC values were determined by dye solubilization. In initial work, erratic solubilization data were obtained. Two sources of error were identified: incomplete dissolution in samples with aggregated dye particles, and loss ofsolubilized dye by adsorption on membrane filters which were used to remove solid dye particles. The following procedure was then employed: 1.5-2.0 mg of dye were combined with 10.0 m L of surfactant solution containing the desired a m o u n t of SDS in 0.1M NaCI, LiC1, or NH4CI. The solutions were sonicated in 15-mL Kimax glass tubes for 2 min to disperse the dye, and then tumbled for 48-72 h at 23 + 0.5°C. Unsolubilized dye was removed by centrifugation at 3500 rpm for 60 min, and the absorbance of the supernatant at 496 n m was read with an HP8450 diode array UV-vis spectrophotometer. For CMC determinations in the presence of PEO, the surfactant solutions also contained 0.40 g / L polymer. This concentration should represent an excess of polymer for surfactant concentrations on the order of 1 m M (5).
For QELS, solutions containing 5.0-mM SDS in 0.4-M NaC1, LiC1, or NH4C1 (with and without 2.5 g / L PEO) were filtered through 0.20-t~m filters into an optically clean, dust-free sample cell. Measurements were made at 90 ° scattering angle with a Brookhaven system equipped with a 72 channel digital correlator (BI-2030 AT) and using a 15m W H e - N e laser (Jodon). We obtain the homodyne intensity-intensity correlation function G(q, t) with q, the amplitude of the scattering vector, given by q = (4rrtiD,)(sin(0/ 2), where t~ is the refactive index of the medium, ~ is the wavelength of the excitation light in a vacuum, and 0 is the scattering angle. G(q, t) is related to the time correlation function of concentration fluctuations g(q, t) by
G(q, t) = A(1 + bg(q, t)2),
where A is the experimental baseline and b is the fraction of the scattered intensity arising from concentration fluctuations. The quality of the measurements were verified by determining that the difference between the measured value of A and the calculated one was less than 1%. The distribution of diffusivities and hence the size distribution function was obtained from g(q, t) by inverse Laplace transformation using the program CONTIN (25). RESULTS AND DISCUSSION
Dye Solubilization The results of dye solubilization measurements are presented in Fig. 1 and in Table I. The thermodynamic stability of the micelle is proportional to - R T I n CMC (21 ) so it has been suggested (15) that the effect of the polymer on the micelle stability is
6 = RTln(CMC/CMCp), 3 This compound is sometimes referred to as Orange OT, but the Colour Index lists Orange OT as Pigment Orange 13, which is formed by diazotization of 3,3-dichlorbenzidine, followed by coupling with 3-methyl-lphenyl-5-pyrazolone (22). We therefore refer to the compound l-(o-tolylazo)-2-naphthol as Solvent Orange 2.
[11
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where CMCe is measured in the presence of the polymer. Since the solubilization curves with and without polymer are so similar, only shifted with respect to CMC, it seems quite Journal of Colloid and lnteffhce Science
Vol. 148,No. 1, January 1992
38
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[SDS], mM FIG. 1. Absorbance of solubilized Solvent Orange 2 in SDS (open symbols) and SDS in the presence of 0.40 g/L PEO (filled symbols), in various 0.10-M electrolyte solutions. Results of duplicate experiments are shown.
reasonabIe to consider CMCp as the point of formation of the polymer-stabilized micelles, and not some unrelated phenomenon. From repeated experiments, the reliability of the CMC measurements is taken as +5% relative, leading to an expected uncertainty in 6 on the order of + 10%. As seen from Table I, the differences for the various counterions are significant, namely 6 {i > 6 ~a > b ~H4. It is known that the binding of N H + to cryptands or podands is particularly weak, and this observation m a y be related to the small effect of the polymer in the presence of this ion. hmrnal tffCol]oid and lnle([hce S¢qence, Vol. 148, No. I, January 1992
The effect of the counterion on the polymerfree micelle needs to be noted. The binding of N H ~ to dodecylsulfate is stronger than the binding o f Na +, with a selectivity exchange constant of K ( N H ~ / Na + ) of 2 (23). This observation is in agreement with the effects of these two ions on the CMC seen here, inasm u c h as the stronger interaction o f NH~- with the SDS headgroups should reduce the electrostatic intramicellar repulsive force and thus stabilize the micelle. (The lowering of the free energy of micellization when the degree of dissociation of the counterion is diminished
39
MICELLES-PEO INTERACTIONS TABLE I CMC ~ for SDS and SDS/PEO in 0.10-M Univalent Electrolytes Salt
CMC (raM)
CMCp t'
t~ (kJ/mol)
LiCI NaCI NH4C1
1.66 1.27 1.03
0.98 0.83 0.76
1.29 1.04 0.74
"Determined from the abcissa intercepts of Fig. 1, by linear regression analysis of combined duplicat~ data sets. b In the presence of 0.40 g/L PEO.
is also consistent with classical thermodynamic considerations (24). However, despite the fact that N H ~ appears to be the more tightly bound to the micelle than the other counterions studied here, it has the least influence on polymer-micelle binding. This suggests that the weak interaction o f N H g with PEO is more important than its strong binding to the micelle in determining its influence on polymermicelle complexation.
the complex is larger in Na + solution than in the presence o f N H ft. Since the total scattering intensity from all three solutions was very similar, the relative areas in each of the three distributions may be compared. The fraction of the (correlated) scattered light due to unbound miceUes for Li ÷, Na ÷, and NH~ is, respectively, 3, 6, and 11%, indicating a binding intensity Li + > Na + > N H 2 , as also deduced above from dye solubilization. It is evident that the counterion also affects the properties of the free micelle so that to some extent the effect on the interaction with the polymer may be indirect. In particular, Li + raises the CMC ofdodecylsulfate and increases the micelle size, compared to Na +. The stron-
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QELS QELS results are reported in Fig. 2 for PEO / SDS in the presence of 0 . 4 0 - M NaC1, LiC1, and NH4C1. (It was necessary to employ a higher ionic strength than that used for solubilization studies in order to reduce intermicellar interactions which would obscure the interpretation of the measured decay constants in terms of particle size). The measured autocorrelation function was well-resolved with two decay modes, using the method of CONT I N (25). The fast-diffusion modes correspond to diffusivities identical to those obtained in polymer-free micelle solutions (apparent Stokes radii Rg °p 2-3 nm) and could be identified with unbound micelles. The slow diffusion modes exhibit R~ °p larger than those of the pure polymer by 160-360%, and must be the complex (the scattering of the pure polymer is very weak). It is evident from Fig. 2 that the relative magnitude of scattering from
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FIG. 2. Apparent (intensity-weighted) distribution of Stokes diameters from QELS for SDS ( 5.0 raM) and PEO (2.5 g/L) in (from top to bottom) 0.40-M NH4C1, 0.40M NaC1, and 0.40-M LiCI.
Journal ~,fColloid and lnte~[iwe Science, Vol. 148, No. l, January 1992
40
DUBIN ET AL.
ger binding of P E O to SDS in LiC1 might partly be a consequence of these changes in micelle structure and stability. In this regard, comparison of Na + and NH~ is more conclusive, inasmuch as these two counterions have a relatively similar effects on the polymer-free micelle, but significantly different effects on 6 and on the QELS distributions. As the concentration of the counterion is increased, the difference between its concentration near the micelle and in the bulk solution must diminish (put differently, the double layer shrinks). According to our hypothesis, the effect of PEO should then vary inversely with counterion concentration. To test this prediction, we measured CMC values in 0.01-MNaCI and obtained for SDS and for PEO/SDS, 5.4 m M a n d 2.8 mM, respectively. This corresponds to a 50% increase in 6, relative to the result in 0.1-M NaC1, in the predicted direction, i.e., that the effect of PEO is larger at lower ionic strength. Surprisingly, 6 for LiDS in 0.01-M LiC1 was essentially unchanged from the value for NaDS in 0.1-M LiCI. In the absence of commercially available NH4DS, no low ionic strength measurements were made for this surfactant. The present studies yield no insight into the location of PEO segments in the complex. As noted above, NMR studies (5) suggest penetration of the PEO structural units into the micelle beyond the Stern layer. It must be noted that such studies give the average locus of any particular set of NMR-active nuclei, all of which may not be in the same environment; i.e., even if some polymer units were in the hydrocarbon "core," many units must pass through the domain of the sulfate groups and their ion atmosphere. For example, in 0.1-M NaC1, the volume within the hydrocarbon "core" of the micelle cannot be more than onehalf of the volume containing the sulfate head groups and their ion atmosphere (with a thickness of one Debye length). Interactions between PEO and the micellar counterions could occur anywhere in the latter region. Ex-
Jmtrnal o[Colloid and h~IeiJbce Science,, Vol. 148, No, 1, January 1992
actly where the strongest interaction of PEO with the metal ion takes please is conjectural. CONCLUSIONS
The interaction between dodecylsulfate micelles and poly(ethyleneoxide) was studied in the presence of Na +, Li ÷, and NH~- ions. Both dye solubilization measurements of the CMC and quasi-elastic light scattering measurements at higher surfactant concentrations indicate that the cation plays a role in this phenomenon, with the polymer-micelle interaction strongest in the presence of Li ÷ and weakest in NH~. These findings suggest that the simultaneous (electrostatic) affinity of the cations for the micelle and (coordination) association with the polymer could energetically contribute--along with other effects--to the binding force between PEO and dodecylsulfate micelles. ACKNOWLEDGMENTS This work was supported by NSF grant DMR-9014945. HZ acknowledges the support of a United Nations Development Fund Fellowship. Helpful comments by N. Muller are acknowledged. REFERENCES 1. For review, see Goddard, E. D., Colloids Surf. 19, 255 (1986). 2. Shirahama, K., ColloidPolym. Sci. 252, 978 (1974). 3. Schwuger, J. Colloid Interface Sci. 43, 491 (1973). 4. Jones, M. N., J. ColloidlnterfaceSci. 26, 532 (1968). 5. Cabane, B., J. Phys. Chem. 81, 1639 (1977). 6. Cabane, B., and Duplessix, R., J. Phys. 43, 1529 (1982). 7. Francois, J., Dayantis, J., and Sabbadin, J. Eur. Polym. J. 21, 165 (1985). 8. Gilanyi, T., and Wolfram, E., Colloids Surf. 3, 181 (1981). 9. Jones, M. N., J. Colloid lnterface Sci. 23, 36 (1967). 10. Zana, R., Lang, J., and Lianos, P,, in "Microdomains in Polymer Solutions" (P. Dubin, Ed.), Plenum Press, New York, 1985. 11. Cabane, B., and Duplessix, R., J. Phys. 43, 1529 (1982). 12. Nagarajan, R., Colloids Surf. 13, 1 (1985). 13. Ruckenstein, E., Huber, G., and Hoffmann, H., Langmuir 3, 382 (1987).
MICELLES-PEO INTERACTIONS 14. Moroi, Y., Akisada, H., Saito, M., Matuura, R., J. Colloid Interface Sci. 61, 233 (1976). 15. Brackman, J. C., and Engberts, J. B. F. N., J. Colloid Interface Sci. 132, 250 (1989). 16. Prud'homme, R. K., paper no. 10675, presented at 1982 SPE/DOE Third Joint Symposium on Enhanced Oil Recovery, Tulsa, OK, April 1982. 17. Gao, Z., Wasylishen, R. E., and Kwak, J. C. T., aT. Phys. Chem. 95, 462 ( 1991 ). 18. See for example, Vrgtle, F., Ed., "Host Guest Complex Chemistry No. I, Topics in Current Chemistry, Vol. 98. Springer-Veflag, Berlin, 1981; Vrgtle, F., and Boschke, F. I., Eds., "Host Guest Complex Chemistry No. II," Topics in Current Chemistry, Vol. 101. Springer-Verlag, Berlin, 1982; Vrgtle, F., et aL, Eds., "Host Guest Complex Chemistry No. III," Topics in Current Chemistry, Vol. 121. SpringerVerlag, Berlin, 1983.
41
19. Chaput, G., Jeminet, G., and Juillard, J., Can. J. Chem. 53, 2240 (1975). 20. Sartori, R., Sepulveda, L., Quina, F., Lissi, E., and Abuin, E. Macromolecules 23, 3878 (1990). 21. Shirahama, K., and Ide, N., J. Colloid Interface Sci. 262, 450 (1976). 22. "The Colour Index," 2nd ed., Supplement p. $986, The Society of Dyers and Colourists, Yorkshire, England, and the American Association of Textile Chemists and Colourists, Lowell, MA, 1963~ 23. Bonilha, J. B. S., Georgetto, R. M. Z., Abuin, E., Lissi, E., and Quina, F. J. Colloid lnterfaceSci. 135, 238 (1990). 24. Attwood, D., and Florence, A. T., "Surfaetant Systems," Chap. 3. Chapman &Hall, New York, 1982. 25. For a general discussion of size distribution analysis via QELS, see, for example, Phillies, G. J. D., Anal. Chem. 62, 1049A (1990), and references therein.
Journal of Colloidand lnter[o.ceScience, Vol.148,No, 1,January1992