Langmuir 1993,9, 381-304
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Polyelectrolyte Binding to Ionic Surfactant Micelles Observed Using Deuterium NMR P. M. Macdonald,’ D. Staring, and Y. Yue Department of Chemistry and Erindale College, University of Toronto, Toronto, Ontario, Canada M5S 1A2 Received July 24, 1992. In Final Form: December 1, 1992 The binding of an anionic polyelectrolyte,poly(sodium 4-styrenesulfonate)(PSSS),to mixed cationic/ zwitterionic surfactant micelles is observed via deuterium NMR spectroscopy. The deuteron labels are located in the polar headgroup of the zwitterionic surfactant hexadecylphosphocholine (HDPC-y-de). When mixed with cationic cetyltrimethylammonium bromide (CTAB), HDPC-y-d, yields a narrow deuterium NMR resonance line, the intensity of which decreases, and subsequently increases again, as a function of the concentration of added PSSS, in a manner correlating with the precipitation and resolubilization of the colloidal dispersion by PSSS. Either reducing the micellar surface charge density or increasingthe ionic strength diminishesthis effect of PSSS. The deuterium NMR spectrum of HDPCy d , within the precipitated complexes consists of a broad Pake doublet characteristic of anisotropic motional averaging, indicating that a micellar structure is likely retained by the Surfactants even in the aggregated state.
Introduction In the long historv of studies of Dolvmer-surfactant interactions, is only”recent1ythat poiyel&trolyte binding to ionic surfactant micelles has begun to receive due attention. The strong electrostatic associations formed between oppositely-charged polyelectrolytes and ionic surfactants above their critical micelle concentrations (cmc), and the consequent irreversible phase separation, normally render such complexes intractable to investigation. The pioneering work of Dubin and co-workers demonstrated that reduction of the micellar surfacecharge density by mixing with nonionicor zwitterionicsurfactants attenuates this interaction to the point that complex formation becomes amenable to study.lJ! Subsequent investigations revealed that complex formation may be considered a critical phenomenon, with the critical variables being the micellar surface charge density and the ionic strength, in accordance with an essentially electrostatic mechanism of i n t e r a c t i ~ n .While ~ ~ ~ these polyelectrolytdionic micellar systems are of significance in their own right, they also yield insights into the nature of electrostatic associations in other, analogous situations, such as occur frequently in biology. NMR methods have furnished considerable, and sometimes fundamental, contributions to the understanding of the physical chemistry of surfactants (for a review, see ref 5). The paucity of literature reports concerning the interactions between oppositely-chargedpolyelectrolytes and ionic surfactants above their cmc is paralleled by the fact that, to the best of our knowledge, NMR has never been employed to investigate any feature of these complexes or their formation. In this initial report our goal is to describe the manner in which the interaction between mixed cationiclzwitterionic surfactant micelles and an anionic polyelectrolyte, poly(s0dium 4-styrenesulfonate), (PSSS), is reflected in the deuterium NMR spectrum of a deuterated surfactant, *To whom correspondence should be addressed at Erindale College.
(1) Dubin, P. L.; Otari, R. J. Colloid Interface Sei. 1983,95, 463. ( 2 ) Dubin, P. L.; Davie, D.D. Macromolecules 1984, 17, 1294. (3) Dubin, P. L.; Rigsbee, D. R.; Gan, L. M.; Fallon, M. A. Macromolecules 1988, 21, 2666. (4) Dubin, P. L.; The, S. S.; McQuigg, D. W.; Chew, C. H.; Can, L. M. Langmuir 1989,5, 89. (6)Chachaty, C. Bog. NMR Spectrosc. 1987, 19, 183.
and to define the type of information provided by deuterium NMR concerning complex formation and structure within the complexes. The deuteron labels are located, in this instance, on the quaternary methyls of the zwitterionicsurfactant hexadecylphosphocholine(HDPCyd6). The cationicsurfactant is cetyltrhethylammonium bromide (CTAB). In accordance with the precepta delineated by Dubin and cO-workers,l* we manipulate the surfactant micelle surface charge density by varying the proportion of CTAB to HDPC-+e, and we investigate the consequences of the addition of PSSS. We demonstrate that the deuterium NMR spectrum mirrors the extent of precipitation, and reveals the criticaldependence of complexformationon ionicstrength and micellar surface charge density. Furthermore, we show that deuterium NMR spectroscopy can probe the physical and dynamic properties of the surfactant within the aggregated surfactant/polyelectrolyte complex.
Materials and Methods The synthesis of HDPC has been described previously.6 Deuteron labels were introduced into the methyl groups of the choline quaternary nitrogen by replacing methyl iodide with methyl-&iodide (Aldrich Chemicals,Milwaukee, WI) in the f i reaction step, thereby yielding HDPC-y-&. 2H NMR spectra were obtained at 45.98 MHz on a Chemagnetics CMX300 NMR spectrometer using a Chemagnetice broadline probe with a 5-mm-diameter solenoid coil. A single pulse excitation was employed in most instances,with full cycling of the phase of the pulses and quadraturedetection. Particulars regarding the 90° pulse length (2.0 pa), the recycle delay (500 ms), thespectralwidth(5 kHz),thedatasize (2K),andthenumber of acquisitions (1024) are those noted in parentheses. When a quadrupole echo technique’ was employed, the delay between the pair of pulses equaled 40 ps, and full cycling of the pulse pairs was implemented: while all other acquisition parameters remained as specified above.
Results and Discussion Addition of PSSS (anionic) to mixed CTAB/HDPCyd6 micelles (cationic) results in a concentration-dependent precipitation, which manifests itself in the form of (6)Macdonald, P. M.; Rydall, J. R.; Kuebler, S. C.; Winnik, F. M. Langmuir 1991, 7,2602. (7) Davis,J.; Jeffrey, K. R.;Bloom, M.;Valic, M. I.; Higgs,T. P. Chem. phys. Lett. 1976, 42, 390. ( 8 ) Griffin, R. G. Methods Enzymol. 1981, 72, 108.
0743-7463/93/2409-~381$04.00/0 0 1993 American Chemical Society
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382 Langmuir, Vol. 9, No. 2, 1993
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Figure 1. Deuterium NMR spectra of an aqueous solution of HDPC-y-de(4 mM)plus CTAB (1 mM)in the presenceof various concentrations of PSSS: from the top, 0 mM PSSS, 1.2 mM PSSS, 1.6 mM PSSS, 2.0 mM PSSS, and 2.4 mM PSSS. The PSSS concentration is expressed in terms of the equivalent monomer concentration. The spectra were acquired under conditions normally associated with high-resolution,as opposed to broadline, NMR spectroscopy. The resonance line at 0 Hz is assigned to the natural abundance deuterium in water (HDO), and the resonance line at -79 Hz is assigned to HDPC-yds.
an increaee in the turbidity of the solution visible to the naked eye. The precipitation phenomenon manifests itself, ae well, in the 2HNMR spectrum, as shown in Figure 1. The spectra in Figure 1were acquired with a solution of 4 mM HDPC-r-ds plus 1mM CTAB in water, in the absence of sodium chloride, for various concentrations of added PSSS. At these concentrations both surfactants are far abovetheir respective critical micelle concentrations ( C ~ C ) . ~The ? ~ top spectrum is obtained in the absence of PSSS. It consists of two narrow resonance lines. The resonance at 0 Hz is assigned to the natural abundance deuterium in water (HDO), since the spectrometer frequency was referenced to DzO. The resonance at -79 Hz is assigned to HDPc-r-ds. These resonance lines are narrow because of the rapid tumbling of the molecules and/or micelles in solution. Note that the natural abundance of deuterium in water is a constant 0.015%. Since these spectra were obtained under fully relaxed conditions for both resonance lines,1° the intensity of the HDO resonance line provides an internal concentration standard to which the intensity of the HDPC-r-ds resonance line may be referenced. (9) Attwood, D.; Florence, A. T. Surfoctant S y s t e m ; Chapman and Halk London, 1983. (10) Kuebler, 5.C.; Macdonald, P. M. Langmuir 1992,8, 397.
The addition of PSSS to the mixed CTABIHDPC-r-ds micelles causes the intensity of the HDPC-r-ds resonance line to decrease in a progressive fashion, while leaving the intensity of the HDO resonance line essentiallyunchanged. At the minimum in HDPC-r-ds spectral intensity the turbidity of the solution is maximal. Further additions of PSSS cause the intensity of the HDPC-r-ds resonance line to increase once again. Simultaneously,the turbidity of the solution decreases, eventually becoming optically clear. At high PSSS concentrations the intensity of the HDPC-r-ds resonance line approaches the intensity observed in the absence of PSSS. The loss of intensity in the 2H NMR spectrumof CTAB/ HDPC-r-ds mixed micelles upon addition of PSSS, and the regained intensity at higher PSSS concentrations, is clearly correlated with the precipitation of PSSS/micellar aggregates, and their subsequent resolubilization. Thus, the decrease in signal intensity is likely due to segregation of HDPC-r-ds into the precipitated PSSS/micellar aggregates. Macroscopic turbidity studies and quasi-elastic light scattering measurementsindicate that the complexes formed by the association of polyelectrolytes with ionic surfactant micelles are many times larger than either uncomplexed polyelectrolyte or surfactant micelles.11 Hence, the invisibility of the NMR signal of the complexed HDPC-r-ds may be attributed to the hindered tumblhg of the polyelectrolyte/micellar complexes,which broadens the resonance line of the complexed HDPC-r-ds to the point that, under the inahmental conditions employed in these instances, its contribution becomes lost in the baseline noise. Implicit in this interpretation of events is the supposition that under instrumental conditions optimized for observing broad resonances the NMR signal of the complexed HDPC-y-ds should become visible. Moreover, any exchange of HDPC-yd6 between the micellar and the polyelectrolyte-complexed states must be slow on the 2HNMR time scale, and the two surfactant populations can be considered to be effectivelysegregated and spin-epin from one another. Our spin-lattice (TI) (2'2) relaxation time measurements on the HDPC-r-ds resonance line indicate no change in the values of these parameters as a function of PSSS concentration, a result consistent with the presence of two effectively segregated surfactant populations. Figure 2 illustrates in detail the manner in which the intensity of the HDPC-r-ds resonance line varies with the concentration of PSSS, for micelles containing various proportions of CTAB to HDPC-r-ds. In the experiments shown, mixed micelles consisting of HDPC-r-ds plus CTAB in a predetermined ratio were titrated with PSSS, and the 2HNMR spectrum was acquired after each PSSS addition. The experiment was repeated for different ratios of CTAB to HDPC-y-ds, while keeping the totalsurfactant concentration constant at 5 mM. The intensity of the HDpc-7-d~ resonance line is measured relative to that of the HDO resonance line, and is normalized with respect to its intensity in the absence of PSSS. The PSSS concentration is expressed in terms of the equivalent sodium 4-styrenesulfonate monomer concentration. The presence of CTAB in the mixed surfactant micelles is an absolute requirement for the precipitation by PSSS, since the addition of PSSS has no effect whatsoever on the intensity of the resonance line from 100%HDPC-7ds micelles. Increahg the proportion of CTAB in the mixed CTABIHDPC-r-4 micelles has two effects on the manner in which the HDPC-r-ds intensity varies with (11) Dubin, P. L.; Ripbee, D. R.; McQuigg, D. W. J. ColloidZnterfoce Sci. 1985, 105, 509.
Langmuir, Vol. 9, No.2,1993 383
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Figure 2. Effect of CTAB concentration on the PSSS-dependent
loas of the HDPC-y-de resonance l i e intensity. Various mixed CTABIHDPC-y-de micelles were prepared at a constant total surfactant concentration of 5 mM, and titrated with PSSS.The deuterium NMR spectrum was recorded after each addition of PSSS. The HDPC-y-de resonance line intensity was measured relative to that of HDO, and normalized with respect to the intensity measured in the absence of PSSS. Symbol (CTAB HDPC-y-ds, mM/mM): open circles ( 0 5 ) ; closed circles (1:4); triangles (2:3);squares (32); diamonds (4:l).
added PSSS. First, the depth of the intensity minimum increases with increasing proportion of CTAB. For example, in the case of 1:4 (mM/mM) CTABIHDBC-7d6, the minimal intensity of the HDPC-r-ds resonance line is approximately 20% of its intensity in the absence of PSSS. In the case of 4:l (mM/mM) CTABIHDPC-7de, the HDPC-r-ds resonance disappears completely and fails to reappear until the PSSS concentration exceeds 5 mM. Second, increasing the proportion of CTAB in the mixed surfactant micelles progressively shifts the position of the m i n i " of the HDPC-y-ds resonance line intensity toward higher PSSS concentrations. At nearly all CTAB:HDPC-r-dg ratios, the amount of anionic charge from PSSS necessary to achieve the fullest possible HDPC-r-decomplexation exceeds by far the total CTAB concentration. Only at the highest CTAB concentration does an approximately1:l CTAEkPSSS charge ratio lead to complete complexation of all HDPC-r-d6 originally present. It is likewise evident that greater than stoichiometric amounts of anionic charge from PSSS are required to initiate resolubilization of the precipitate. Dubin et al.1 have suggested that these deviations from the expected stoichiometries arise because there exists a distribution of compositions about the average in any population of mixed micelles and that a polyelectrolyte interacts first with those micelles having the highese ionic surfactant content. If true, then the CTAB:HDPC-r-ds stoichiometry in the precipitate should vary as a function of the degree of complexation. We cannot draw conclusions regarding the CTAB:HDPC-yds stoichiometry in the aggregates from the present data alone. However, parallel deuterium NMR measurements with deuteron labels located on CTAB, or both CTAB and HDPC simultaneoualy,would provide precisely such information, and these are currently in progress. A fundamental property of any such electrostatic association is that it should be attenuated with increasing ionic strength. The data in Figure 3A demonstrate that the addition of sodium chloride eliminates the PSSSdependent loss of HDPC-r-ds spectral intensity in mixed micelles containing CTABIHDPC-r-ds, 23 (mM/mM). With increasing electrolyte concentration the loss of the
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Figure 3. Effect of ionic strength on the PSSS-dependent loss
of the HDPC-y-ds resonance line intensity. (A) Mixed CTAB/ HDPC-y-d6(2:3,mM/mM) micelleswere prepared in the presence of various concentrations of NaC1, and titrated with PSSS. The deuterium NMR spectrum was recorded after each addition of PSSS. The HDPC-y-ds resonance line intensity was measured relative to that of HDO, and normalized with respect to the intensity measured in the absence of PSSS. Symbols: open circles, 0 mM NaCI; closed circles, 30 mM NaCI; squares, 60 mM NaC1; diamonds, 100 mM NaC1. (B)Minimal HDPC-y-de resonance line intensity versus NaCl concentration.
HDPC-r-ds signal intensity is progressively attenuated. At NaCl concentrations above approximately 100 mM, PSSS fails to induce any change in the intensity of the HDPC-r-ds resonance line. In fact, as shown in Figure 3B,there is an apparently linear relationship between the minimum in the PSSS versus intensity curve and the sodium chloride concentration. These results c o n f i i that the loss of the HDPC-r-ds resonance signal may be attributed to the formation of PSSStmicellar aggregates, and that the association of PSSS with mixed CTAB/ HDPC-r-dsmicelles occurs via an electrostatic interaction mechanism. Figure 4shows the deuterium NMR spectrum of HDPCr-ds trapped within precipitated PSSS/micellar aggregates. PSSS was added to mixed CTABIHDPC-7-ds micelles at a PSSS concentration close to that required for maximum precipitation. The spectrum was acquired by using the quadrupole echo technique, which is optimal for observing broad deuterium spectra. The spectrum consists of two narrow resonance lines (attributable to HDO plus free surfactant) superimposed upon a broad Pake doublet spectral component. The latter is indicative of deuterons undergoing anisotropic motional averaging due to the constrictions of the molecular environment.
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384 Langmuir, Vol. 9, No. 2, 1993
axis, is either anisotropic in nature or insufficiently rapid to average to zero the deuterium quadrupolarinteractions. We expect that any differences in the detailed structure and internal dynamics of the complexes, such as might originate with changes in the surface charge density of the mixed surfactant micelles, the linear charge density of the polyelectrolyte, or the ionic strength of the aqueous solution, will become manifest in 2H NMR spectra such as that shown in Figure 4.
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Figure 4. Broadline deuterium NMR spectrum of HDPC-7d$CTAB/PSSS complexes formed upon flocculation of mixed CTABIHDPC-7-de micelles by the addition of PSSS. The spectrum was obtained using the quadrupole echo technique' and other instrumental conditions optimized for observingbroad spectral lines.
The simplest explanation, not necessarily unique, for this spectral line shape is that it arises due to the decreased rate of isotropicrotational diffusion of sphericalsurfactant micelles trapped within the aggregate. The residual motional averaging experienced by the individual surfactant molecules, including lateral diffusion within the micelle and rotations about the surfactant long molecular
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Conclusions The results demonstratethat deuteriumNMR is capable of probing electrostatic complex formation between polyelectrolytesand ionic surfactant micelles, and can provide details concerning structure and dynamics in the aggregated state. In principle, the deuterium NMR spectral line shape will be sensitive to considerations such as the size of the aggregates,the motions of the molecules within the aggregates, and even the surface charge of the immobilized micelles. Hence, it should now be possible to scrutinizesimultaneously the chemical composition and the structure and dynamics of individual species within the electrostatic complexes. '
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Acknowledgment. This work was supported by grants from the National Science and Engineering Research Council (NSERC) of Canada, the Institute for Chemical Science and Technology (ICST), and the University Research Incentive Fund (URIF) of Ontario.
