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Anionic Polyelectrolyte Binding to Mixed Cationic-Zwitterionic Surfactant Micelles: A Molecular Perspective from 2H NMR Spectroscopy Darlene J. Semchyschyn, Mary Anna Carbone, and Peter M. Macdonald* Department of Chemistry and Erindale College, University of Toronto, Toronto, Ontario, Canada M5S 1A2 Received March 28, 1995. In Final Form: July 27, 1995X Complex formation between an anionic polyelectrolyte poly(sodium styrenesulfonate) (PSSS) and mixed zwitterionic-cationic surfactant micelles was monitored using deuterium NMR spectroscopy of the specifically deuterium-labeled surfactants hexadecylphosphocholine (HDPC-γ-d6) and cetyltrimethylammonium bromide (CTAB-γ-d9). The deuterium signal from both surfactants decreased in direct proportion to the amount of added PSSS, reflecting the phase separation of the charge-neutralized PSSS-surfactant complex. The insoluble complex was preferentially enriched with CTAB over HDPC. The PSSS/CTAB charge ratio was the primary determinant of the degree of complex formation. Deuterium NMR T1 and T2 relaxation time measurements indicated that soluble PSSS-surfactant complexes exist prior to their phase separation. Within these soluble complexes motions of the cationic surfactant CTAB, but not the zwitterionic surfactant HDPC, were hindered. Quadrupolar echo deuterium NMR spectra of the phaseseparated complexes showed that the flocculated material is inhomogeneous in composition and that surfactant mobility within the insoluble complex varies according to the CTAB/HDPC ratio.
Introduction Polymer-surfactant systems enjoy widespread use in products of the biochemical, pharmaceutical, paint, cosmetic, and other industries.1 The interaction between polyelectrolytes and oppositely-charged surfactants is particularly strong, since both hydrophobic and electrostatic forces come to bear. The pioneering work from Goddard’s laboratory1-3 demonstrated that phase separation of electrostatic complexes formed by polyelectrolytes and ionic surfactants occurs when the surfactant concentration exceeds the critical micelle concentration (cmc). Kwak and co-workers4-9 showed that even below the cmc flocculation of polyelectrolyte-surfactant aggregates is abundantly evident. It is thought that such polymersurfactant aggregates contain entire surfactant micelles surrounded by neutralizing polymer segments.2,3,10-13 The phase separation is irreversible at concentrations above the surfactant cmc, a difficulty which initially hindered studies of polyelectrolyte-surfactant interactions in this concentration regime. In a series of reports, Dubin and co-workers14-23 demonstrated that diluting the surface * To whom correspondence should be addressed. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, November 1, 1995. (1) Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press; Boca Raton, 1993. (2) Goddard, E. D. Colloids Surf. 1986, 19, 255. (3) Goddard, E. D. Colloids Surf. 1986, 19, 301. (4) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (5) Malovikova, A.; Haykawa, K.; Kwak, J. C. T. In Structure/ Performance Relationships in Surfactants; Rosen, M. J., Ed.; ACS Symposium Series 253; American Chemical Society: Washington, DC, 1984; p 225. (6) Haykawa, K.; Santerre, J. P.; Kwak, J. C. T. Biophys. Chem. 1983, 17, 175. (7) Shimizu, T.; Seki, M.; Kwak, J. C. T. Colloids Surf. 1986, 20, 289. (8) Malovikova, A.; Haykawa, K.; Kwak, J. C. T. J. Phys. Chem. 1984, 88, 1930. (9) Gao, Z.; Kwak, J. C. T.; Wasylishen, R. E. J. Colloid Interface Sci. 1988, 126, 371. (10) Satake, I.; Yang, J. T. Biopolymers 1976, 15, 2263. (11) Abuin, E. B.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 6274. (12) Nagarajan, R. Colloids Surf. 1985, 13, 1. (13) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (14) Li, Y.; Xia, J.; Dubin, P. L. Macromolecules 1994, 27, 7049.
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charge of the surfactant micelles by admixing neutral surfactants permits one to attenuate the strength of the electrostatic interaction between the surfactant micelle and the polyelectrolyte and to avoid irreversible phase separation above the cmc. Employing this innovation these workers identified the ionic strength, the micelle surface charge density, and the polyelectrolyte molecular weight as factors critical to complex formation. Despite this progress, several aspects of complex formation between polyelectrolytes and oppositely-charged surfactants above the surfactant cmc require clarification. Of particular interest is the question of the degree to which the polyelectrolyte-surfactant complex is preferentially enriched with respect to the charged versus the neutral surfactant. Furthermore, little has been firmly established regarding the structure and dynamics of the polyelectrolyte-surfactant complexes at the molecular level, much less how these fundamentals are influenced by surface charge, ionic strength, and other factors. Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for describing structure and dynamics at the molecular level, and examples abound of NMR applications to the study of surfactant, polymer, and surfactant-polymer interactions (e.g., see recent reviews by Chachaty24 and So¨derman and Stilbs25). Recently, we described a novel deuterium NMR technique for monitoring complex formation between polyelectrolytes and ionic surfactant micelles.26 In this approach a zwitterionic (15) Ahmed, L. S.; Xia, J.; Dubin, P. L. Pure Appl. Chem. 1994, A31 (1), 17. (16) Xia, J.; Zhang, H.; Rigsbee, D. R.; Dubin, P. L.; Shaikh, T. Macromolecules 1993, 26, 2759. (17) McQuigg, D. W.; Kaplan, J. I.; Dubin, P. L. J. Phys. Chem. 1992, 96, 1973. (18) Dubin, P. L.; The, S. S.; Gan, L. M.; Chew, C. H. Macromolecules 1990, 23, 2500. (19) Dubin, P. L.; Curran, M. E.; Hua, J. Langmuir 1990, 6, 707. (20) Dubin, P. L.; The, S. S.; McQuigg, D. W.; Chew, C. H.; Gan, L. M. Langmuir 1989, 5, 89. (21) Dubin, P. L.; Cavis, D. D. Macromolecules 1984, 17, 1294. (22) Dubin, P. L.; Rigsbee, D. R.; Gan, L. M.; Fallon, M. A. Macromolecules 1988, 21, 2555. (23) Dubin, P. L.; Oteri, R. J. J. Colloid Interface Sci. 1983, 95, 453. (24) Chachaty, C. Prog. NMR Spectrosc. 1987, 19, 183. (25) So¨derman, O.; Stilbs, P. Prog. NMR Spectrosc. 1994, 26, 445. (26) Macdonald, P. M.; Staring, D.; Yue, Y. Langmuir 1993, 9, 381.
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surfactant, hexadecylphosphocholine (HDPC), is specifically deuterium labeled, and mixed with cationic or anionic surfactants to achieve a desired micellar surface charge dilution according to the principles established by Dubin and co-workers. Complex formation with polyelectrolytes causes an attenuation of the HDPC deuterium NMR signal intensity, which is used to quantify the degree of complexation. We report here deuterium NMR studies of complex formation between the anionic polyelectrolyte poly(sodium styrenesulfonate) (PSSS) and mixed zwitterionic-cationic surfactant micelles. Deuteron labels have been placed on both the zwitterionic surfactant HDPC and the cationic surfactant cetyltrimethylammonium bromide (CTAB). We examine the deuterium NMR signal and perform deuterium NMR T1 and T2 relaxation studies, for both deuteriolabeled surfactants, in both the soluble and insoluble phases present upon addition of PSSS. Our goal is to ascertain whether there is evidence of preferential enrichment of CTAB versus HDPC in the phase-separated electrostatic complexes and to determine whether differences in micellar surface charge result in differences in surfactant dynamics before and after electrostatic complexation. Experimental Section Deuterated Surfactant Synthesis. The syntheses of HDPC and its N-methyl deuterated equivalent, HDPC-γ-d6 have been described previously.27 N-methyl deuterated CTAB (CTAB-γd9) was synthesized by methylation of hexadecylamine with methyl iodide-d3 (both from Aldrich, Milwaukee, WI). Typically, methyl iodide-d3 (35.4 g, 0.24 mol) was added to hexadecylamine (2.00 g, 0.008 mol) in 100 mL of methanol containing 0.08 mol of NaOH. After stirring for 48 h in the dark at room temperature, the excess solvent and reagent were removed under vacuum, and the product was dissolved in a minimal volume of chloroform/ methanol 1:1 (v/v) and chromatographed on Bio-Rad AG 1-X4 anion exchange resin prepared in the bromide form. After reduction of the solvent under vacuum the final product was obtained by precipitation from acetone. The yield was typically greater than 90%. The product chromatographed as a single spot on silica gel G TLC plates with an Rf of 0.52 in the solvent system chloroform/methanol (7:3, v/v), identical to authentic CTAB. The identity of the product was further confirmed by 1H NMR at 200 MHz (Varian Gemini 200) in deuterated chloroform using tetramethylsilane as a chemical shift reference: δ 0.88 ppm, triplet, 3 H, hexadecyl methyl; δ 1.26 ppm, broad singlet, 26 H, hexadecyl methylenes; δ 1.72 ppm, multiplet, 2 H, hexadecyl β-methylene; δ 3.58 ppm, triplet, 2 H, hexadecyl R-methylene. Sample Preparation. Aqueous solutions of CTAB plus HDPC in a particular molar ratio were prepared starting from stock solutions of the pure surfactants in chloroform/methanol. After mixing the desired volumes of the stocks, the organic solvent was removed under a stream of argon gas, followed by high vacuum treatment overnight. The surfactant mixture was then dissolved in double-distilled water to give a final total surfactant concentration of 10 mM. Note that this is well above the cmc reported for either HDPC (32 µM)27 or CTAB (0.9 mM).28 PSSS (MW 70 000, Aldrich) was added as an aliquot from an aqueous stock solution (200 mM). As will be described in the text, it was necessary to perform parallel experiments with mixtures of HDPC-γ-d6/CTAB or HDPC/CTAB-γ-d9. Deuterium NMR Spectroscopy. Deuterium NMR spectra were obtained at 45.98 MHz on a Chemagnetics CMX300 NMR spectrometer using a Doty Scientific Instruments wide line double resonance probe, equipped with a 10-mm-diameter solenoid coil. A single pulse excitation was employed, with full quadrature phase cycling. Particulars regarding the 90° pulse length (6.0 µs), the recycle delay (1 s), the spectral width (10 kHz), the data size (4K) and the number of acquisitions (2000) are those noted (27) Macdonald, P. M.; Rydall, J.; Kuebler, S. C.; Winnik, F. M. Langmuir 1991, 7, 2602. (28) Mandal, A. B.; Nair, B. U. J. Phys. Chem. 1991, 95, 9008.
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Figure 1. Structures of the three chemical species used in these experiments: the zwitterionic surfactant hexadecylphosphocholine (HDPC), the cationic surfactant cetyltrimethylammonium bromide (CTAB), and the anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSSS). in parentheses. In several instances a quadrupolar echo technique was employed,29 and the deuterium NMR spectra were obtained using a Chemagnetics broad line probe with a 5-mmdiameter solenoid coil. The delay between the pair of pulses was typically 40 µs, the 90° pulse length was 2.5 µs, the recycle delay was 500 ms, the spectral width was 100 kHz, the data size was 1K, and full cycling of the pulse pairs was implemented. The deuterium spin-spin relaxation times (T2) were measured via a spin-echo experiment with variable tau values (τ). The deuterium spin-lattice relaxation times (T1) were measured using a standard inversion recovery sequence.
Results and Discussion Electrostatic Complex Formation Involves Both HDPC and CTAB. Figure 1 illustrates the structures of the three chemical species of interest here: HDPC, a zwitterionic surfactant; CTAB, a cationic surfactant; and PSSS, an anionic polyelectrolyte. Electrostatic complex formation between polyelectrolytes and charged surfactants has been investigated with a variety of techniques including dynamic and static light scattering, turbidimetry, viscometry, and electrophoretic light scattering.14-23 In our hands, addition of PSSS to mixed CTAB/HDPC surfactant micelles is accompanied by changes in the turbidity of the solutions that are readily visible to the naked eye. Qualitatively, we observe that the turbidity increases with the amount of added PSSS. At a particular concentration the turbidity reaches a maximum. Further additions of PSSS eventually result in an optically clear solution. It is generally accepted that such changes in turbidity are the result of the formation of electrostatic complexes between the polyelectrolyte and the oppositely-charged mixed surfactant micelles. Soluble neutralized complexes form first, and when their concentration is sufficiently high they aggregate and undergo wholesale phase separation. Further additions of polyelectrolyte can eventually resolubilize the surfactant-polyelectrolyte complexes via electrostatic stabilization. The deuterium NMR results below demonstrate how this technique may be used to gain further insight into the molecular details surrounding these processes. Deuterium NMR spectra of deuterated surfactants in mixtures of CTAB/HDPC (6:4, M/M) in aqueous solution (29) Davis, J. H.; Jeffrey, K. R.; Bloom, M.; Valic, M. I.; Higgs, T. P. Chem. Phys. Lett. 1976, 42, 390.
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Figure 2. Deuterium NMR spectra of mixtures of CTAB/HDPC (6:4, mM/mM) in aqueous solution as a function of the indicated PSSS/CTAB charge ratio. (A) Deuterons located on CTAB-γ-d9. (B) Deuterons located on HDPC-γ-d6. The resonance line at 0 Hz is assigned to the natural abundance deuterium in water (HDO), and the resonance line at -78 Hz is assigned to the deuterated surfactant within the mixed micelle. Any difference between the chemical shifts of CTAB-γ-d9 and HDPC-γ-d6 is smaller than the resonance line width in these measurements so that simultaneous deuterio-labeling of CTAB and HDPC is impractical.
(total surfactant concentration 10 mM) are shown in Figure 2 as a function of added PSSS. For Figure 2A the deuterons were located on the cationic surfactant (CTABγ-d9), while for Figure 2B the deuterons were located on the zwitterionic surfactant (HDPC-γ-d6). (The fact that only six of the nine aminomethyl protons of HDPC are replaced by deuterons, versus the fully-deuterated aminomethyls of CTAB, is strictly a consequence of employing N-methylethanolamine as a precursor in the synthesis of HDPC.) Both types of deuterium NMR spectra show two resonances: one at the reference frequency of 0 Hz (set to the resonance frequency of D2O) and assigned, therefore, to the natural abundance deuterium in water (HDO), and a second at -78 Hz which is assigned to the deuterons located on the surfactant polar headgroups. Calibration experiments demonstrate that, under the conditions in which these spectra were acquired, a strict linear relationship is found between the intensity of the surfactant deuterium signal and the surfactant concentration, over a range up to at least 20 mM. Addition of PSSS to mixed CTAB/HDPC micelles results in two significant changes in the deuterium NMR spectra. First, the integrated deuterium signal intensity from the surfactant decreases, relative to the HDO signal intensity, regardless of whether the deuteron labels are located on CTAB or HDPC. At some particular concentration of PSSS the deuterium NMR signal intensity from both CTAB or HDPC is lost entirely. Upon further addition of PSSS the integrated deuterium NMR signal intensity from both surfactants is eventually fully restored. Second, the widthat-half-height (∆ν1/2) of the remaining visible deuterium signal increases upon adding PSSS. Note that in parts
A and B of Figure 2, all spectra (with the exception of the top spectra in Figure 2A) have been scaled so that the HDO deuterium resonance at 0 Hz is the same height in each. A comparison of the top and bottom spectra in parts A and B of Figure 2 reveals that the signal height (as opposed to the integrated intensity) at high PSSS concentrations never reaches the value observed in the absence of PSSS, although their integrated intensities are virtually identical. This latter effect is more pronounced for CTAB-γ-d9 than for HDPC-γ-d6. The T2 relaxation time results to be discussed below confirm the line width estimates. The PSSS-induced changes in the deuterium NMR spectra of parts A and B of Figure 2 parallel the changes in turbidity of the solutions and are clearly due to the formation of insoluble electrostatic complexes between the oppositely-charged surfactant micelles and the polyelectrolyte. As we demonstrate below, the line widths of the deuterium resonances of the surfactants entrapped in the insoluble electrostatic complexes are so large as to render their signals undetectable under the particular conditions employed in these particular measurements. For the soluble electrostatic complexes, the surfactant’s deuterium NMR spectral line width likewise increases, but to a degree that is only marginal relative to the values observed in the absence of PSSS, so their deuterium NMR signal remains visible. Consequently, the deuterium NMR signal intensity from either the cationic or the zwitterionic surfactant reflects directly the degree to which the particular surfactant bearing the deuterium label is involved in an insoluble electrostatic complex with the polyelectrolyte. The fact that the CTAB and HDPC signal
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Figure 3. The effect of the overall PSSS concentration on the surfactant deuterium NMR signal intensity as a function of the CTAB/HDPC composition. (A) Deuterons located on CTABγ-d9. (B) Deuterons located on HDPC-γ-d6. The CTAB-γ-d9 and HDPC-γ-d6 integrated resonance line intensities were measured relative to that of HDO and normalized with respect to the intensities measured in the absence of PSSS. Symbols (CTAB/HDPC, mM/mM): circles, 10:0; triangles, 8:2; squares, 6:4; upside-down triangles, 4:6; diamonds, 2:8, closed circles, 0:10.
intensities respond to PSSS in similar, if not identical, fashions indicates that the polyelectrolyte interacts with, and entraps, entire surfactant micelles. PSSS/CTAB Charge Ratio Dictates the Degree of Complex Formation. Parts A and B of Figure 3 illustrate the manner in which the CTAB/HDPC composition of the surfactant micelles influences the effect of added PSSS on the integrated deuterium NMR signal intensity from the particular deuteriolabeled surfactant. For the data in Figure 3A the deuteron labels were located on CTAB. For Figure 3B the deuteron labels were located on HDPC. We choose to express the surfactant’s integrated deuterium NMR signal intensity as a ratio relative to that of the HDO resonance line in the spectrum (see Figure 2A or 2B). Since the integrated HDO signal intensity is independent of the PSSS concentration, it performs the role of an internal intensity standard, allowing one to eliminate variations due solely to trivial effects such as differences in NMR probe tuning, sample volume, or other signal acquisition parameters. This
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permits a calculation of absolute concentration from the NMR spectra. Furthermore, we choose to normalize this integrated intensity ratio relative to that observed in the absence of PSSS, to permit ready comparison between data sets for different CTAB/HDPC ratios. Thus the quantity plotted along the ordinate axes in parts A and B of Figure 3 corresponds to [Isi/Iwi]/[Is0/Iw0], where the subscripts s and w indicate the integrated intensity of the surfactant versus the water, while the superscript i refers to a particular PSSS concentration, and the superscript 0 indicates the absence of PSSS. Along the abscissa, the amount of added PSSS is represented in terms of the equivalents of added anionic charge, assuming 100% dissociation. For mixed surfactant micelles containing CTAB-γ-d9/ HDPC, as shown in Figure 3A, addition of PSSS causes the CTAB-γ-d9 integrated deuterium signal intensity to first decrease and then, subsequently, increase again, returning to 100% of its original value in all cases. With increasing mole fraction of CTAB in the mixed surfactant micelles, the PSSS concentration required to achieve the minimal integrated intensity shifts to higher values. Furthermore, only at the higher CTAB ratios is the surfactant completely phase separated, i.e., so that the minimum intensity approaches zero, prior to restabilization of the dispersion. For mixed surfactant micelles containing CTAB/HDPCγ-d6, as shown in Figure 3B, addition of PSSS causes the HDPC-γ-d6 integrated signal intensity to first decrease and subsequently increase, in a fashion paralleling the behavior of the CTAB-γ-d9 signal. If no CTAB is present whatsoever, then the HDPC-γ-d6 integrated signal intensity is unaffected by the addition of PSSS. With increasing mole fraction of CTAB in the mixed surfactant micelles, the PSSS concentration required to achieve the minimal integrated intensity shifts toward higher values, again in a fashion reminiscent of the behavior of CTABγ-d9. Likewise, only at the higher mole fractions of CTAB does the minimal HDPC-γ-d6 integrated intensity actually reach zero. Finally, the HDPC-γ-d6 integrated intensity at the highest PSSS concentrations approaches 100% of its original value in every case. Nevertheless, several differences in the behavior of the CTAB-γ-d9 versus HDPCγ-d6 integrated intensities are evident. For instance, at lower mole fractions of CTAB the decrease in integrated intensity for CTAB-γ-d9 at the minimum is clearly greater than that obtained with HDPC-γ-d6. Also, the PSSS concentration required to achieve the minimum integrated intensity at a given CTAB/HDPC ratio is always larger for HDPC-γ-d6 than for CTAB-γ-d9. These features indicate that, while there is a close linkage between the responses of CTAB and HDPC to the anionic polyelectrolyte flocculant, the behavior of the cationic and the zwitterionic surfactants is by no means identical. The deuterium NMR results in parts A and B of Figure 3 conform to the expectation that, as demonstrated previously by others,14-23 the amount of PSSS required to achieve maximum phase separation of the surfactant micelles is a function of the amount of CTAB initially present. This suggests that the ratio of PSSS anionic charge equivalents to CTAB cationic charge equivalents is the main factor determining the position of the deuterium NMR integrated intensity minimum. Consequently, it should be possible to shift all the curves in parts A and B of Figure 3 onto a universal curve relating the degree of phase separation to the PSSS/CTAB charge ratio. Parts A and B of Figure 4 show that this is indeed the case, in that the integrated NMR signal intensity minima all occur at a PSSS/CTAB charge ratio of approximately 0.3 when the deuteron labels are located
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Figure 5. Relative enrichment of CTAB/HDPC in the PSSS/ CTAB/HDPC electrostatic complexes. The ratio of mole fractions of complexed CTAB/HDPC, [X1bi/X2bi], as function of the PSSS/CTAB charge ratio, for different global CTAB/HDPC ratios. Specific enrichment of CTAB within the phase-separated PSSS/CTAB/HDPC complexes is indicated by a ratio of mole fractions of complexed CTAB/HDPC greater than unity. Symbols (CTAB/HDPC, mM/mM): triangles, 8:2; squares, 6:4; upside-down triangles, 4:6; diamonds, 2:8.
and B of Figure 4 the molar ratio of CTAB/HDPC in the insoluble electrostatic complex formed with PSSS may be determined as follows. The mole fraction of a particular surfactant species remaining soluble, (Xsfi), with subscript s ) 1 for CTAB or s ) 2 for HDPC, at any particular PSSS concentration (superscript i), is determined from the integrated intensity of the deuterium NMR resonance line relative to that of HDO according to
[Isi/Iwi] Figure 4. The effect of the overall PSSS/CTAB charge ratio on the surfactant deuterium NMR integrated signal intensity as a function of the CTAB/HDPC composition. (A) Deuterons located on CTAB-γ-d9. (B) Deuterons located on HDPC-γ-d6. The signal intensity minima all occur at a PSSS/CTAB charge ratio of approximately 0.3 for CTAB-γ-d9 and 0.35 for HDPCγ-d6. Symbols (CTAB/HDPC, mM/mM): circles, 10:0; triangles, 8:2; squares, 6:4; upside-down triangles, 4:6; diamonds, 2:8.
on CTAB, or approximately 0.35 when the deuteron labels are located on HDPC, regardless of the actual molar ratio of CTAB/HDPC in the surfactant micelles themselves. The significance of the PSSS/CTAB charge ratio required to achieve maximum precipitation is not obvious. It may be that counterion condensation at the micelleaqueous interface reduces the effective surface electrostatic charge so that 1:1 PSSS/CTAB is not required for charge neutralization. Indeed a close examination of parts A and B of Figure 4 reveals that the PSSS/CTAB ratio at the minimum in the integrated deuterium NMR intensity shifts toward somewhat lower values with increasing CTAB/HDPC content in the surfactant micelles. Alternately, it may be that complete charge neutralization is not necessary for destabilization and aggregation of the electrostatically-stabilized micellar dispersion. Simply reducing the effective surface charge to the point that the attractive van der Waals potential overbalances the repulsive electrostatic surface potential will result in phase separation. Electrostatic Complexes Are Preferentially Enriched with CTAB. From the data shown in parts A
[Is0/Iw0]
) Xsfi ) (1 - Xsbi)
(1)
where the subscripts f and b differentiate soluble (i.e., micellar or “free”) versus insoluble (i.e., phase-separated or “bound”) surfactant, and the ratio [Is0/Iw0] is the integrated deuterium NMR signal intensity of the surfactant versus HDO in the absence of PSSS. Note that, since one is comparing integrated intensities, the relatively small line width difference between micellar surfactant and surfactant in soluble electrostatic complexes will have only a minimal effect. Consequently, the ratio [X1bi/X2bi] quantifies the relative enrichment of CTAB versus HDPC in the phase-separated complexes, since this should simply equal unity in the absence of enrichment with respect to one surfactant versus the other. Figure 5 shows a plot of [X1bi/X2bi] as a function of the PSSS/CTAB charge ratio for the various CTAB/HDPC surfactant micelle compositions. The analysis reveals that at low PSSS/CTAB charge ratios, when only a fraction of the surfactant micelles are complexed, the phaseseparated electrostatic complexes are enriched with respect to CTAB. At low CTAB/HDPC ratios, when the availability of the cationic surfactant is limited, the extent of enrichment with CTAB is greatest. At high CTAB/ HDPC ratios, when the availability of the cationic surfactant is not limited, the extent of enrichment with CTAB is least. Preferential enrichment of the electrostatic complexes with CTAB arises due to the conjunction of two factors. First, the surfactant micelle must posses a critical surface
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Table 1. Deuterium NMR T1 and T2 Relaxation Times for CTAB-γ-d9 and HDPC-γ-d6 in CTAB/HDPC (6:4, mM/ mM) Micelles as a Function of PSSS/CTAB Charge Ratio PSSS/CTAB charge ratio
T1 relaxation time (ms)
T2 relaxation time (ms)
slow correlation time τs (ns)
0 0.17 1.13
60 50 32
A. CTAB-γ-d9 50 34 22
11 30 46
0 0.17 1.13
100 75 62
B. HDPC-γ-d6 77 60 47
10 11 17
charge density (i.e., mole fraction of CTAB) before polyelectrolyte binding is favorable.23 Secondly, in surfactant mixtures individual micelles will exhibit a distribution of compositions about the bulk average composition.14 Consequently, when PSSS is added the first complexes to be formed contain surfactant micelles with higher CTAB contents. This scenario explains another aspect of the 2H NMR results. At low CTAB/HDPC ratios complex formation never involves 100% of the available surfactant. In terms of a distribution of surfactant compositions, the fraction of micelles remaining free in solution at the maximum of complex formation therefore represents those micelles having insufficient CTAB/HDPC ratios to permit polyelectrolyte binding. Another aspect of such enrichment is that the phase-separated surfactant-polyelectrolyte coacervate should be compositionally heterogeneous, as we demonstrate below. CTAB Mobility Is Preferentially Hindered in PSSS/CTAB/HDPC Soluble Complexes. Deuterium spin-lattice (T1) and spin-spin (T2) relaxation time measurements, reported in sections A and B of Table 1, conform with the observations of Dubin and co-workers14-23 that soluble complexes of polyelectrolyte plus surfactant micelles exist prior to phase separation of the largelyneutralized coacervate. We deduce further that similar soluble complexes likewise exist at high PSSS/CTAB ratios where the dispersion has been electrostatically restabilized. Furthermore, the differences in the relaxation behavior of CTAB versus HDPC suggest that the mobility of CTAB within the complex is preferentially hindered. T1 and T2 relaxation time results are reported here only for a single, representative CTAB/HDPC composition (6: 4) and only for particular, representative PSSS/CTAB ratios. For both CTAB and HDPC deuterons, at all PSSS concentrations tested, monoexponential behavior is observed in both the inversion-recovery experiment for measuring T1 and the spin-echo experiment for measuring T2. It is important to emphasize that in these measurements one is observing the relaxation behavior of the visible deuterium NMR signal only, corresponding to the soluble surfactant fraction. The relaxation behavior of deuterons attached to the phase-separated surfactant fraction cannot be determined under these conditions (but see below). For the surfactant micelles in the absence of PSSS, the CTAB deuterons yield a T1 relaxation time of 60 ms, while the HDPC deuterons yield a T1 of 100 ms. These values fall in the range expected for deuterons located in the polar headgroups of surfactants in free micelles in an aqueous environment.30 Values of T2 are comparable to, or somewhat less than, T1, which is expected for freely tumbling micelles in solution such that the extreme narrowing condition applies (τc2ωo2 , 1), where τc is the effective motional correlation time for the molecular segment to which the deuteron is attached and ω0 is the Larmor frequency of its NMR resonance signal. (30) Kuebler, S. C.; Macdonald, P. M. Langmuir 1992, 8, 397.
When PSSS is added to the extent that soluble and insoluble surfactant coexist in approximately equal amounts (i.e., a PSSS/CTAB charge ratio that reduces the integrated signal intensity by about 50%), the T1 and T2 relaxation times of the remaining visible signal are reduced for both CTAB and HDPC deuterons, relative to the values measured in the absence of PSSS. When further PSSS is added, to the extent that the resolubilized surfactant signal dominates the spectrum (i.e., a PSSS/ CTAB charge ratio exceeding that required to induce maximum phase separation and approaching that yielding a restabilized suspension), there is a further reduction in both T1 and T2 for both the CTAB and HDPC deuterons. Note that these relaxation time effects cannot be attributed merely to an increased solution viscosity resulting from the high polyelectrolyte concentrations, since the integrated deuterium NMR signal intensity and line-widthat-half-height of 100% HDPC surfactant micelles are virtually constant even at very high PSSS levels, while the relaxation delay employed during the acquisition of the various spectra always exceeded 5 times T1. Instead they must be attributed to specific electrostatic interactions between the polyelectrolyte and the surfactant micelles. For surfactants, it is generally accepted that the motions which contribute to the spin relaxation may be separated into fast and slow components.31 The fast motions, occurring on a picosecond time scale, involve local bond fluctuations such as rotatioinal isomerizations. The slow motions involve the entire surfactant molecule and correspond to a combination of rotational tumbling of the entire surfactant micelle plus lateral diffusion of the surfactant over the micellar surface. The correlation time for the slow motions may be predicted for a given physical situation using eq 2
τs-1 ) τt-1 + τd-1 )
[
] [ ]
4πηR3 3kT
-1
+
R2 6D
-1
(2)
where τt is the correlation time for rotational tumbling of the whole micelle, τd is the correlation time for diffusion of the surfactant over the micellar surface, R is the micelle radius (assumed to be 30 Å), η is the viscosity (0.891 cP for water at 25 °C), D is the diffusion coefficient for a surfactant within the micelle (approximately 1.0 × 10-10 m2 s-1 32), and kT is the Boltzmann temperature. Given these parameters one predicts a value of 9.3 × 10-9 s for τs for this particular case. Experimentally, the slow motional correlation time τs may be estimated from the difference in the rate of spinspin versus spin-lattice relaxation, according to eq 3
∆R )
[
]
[
2τs 1 1 9π2 ) (χS)2 2τs + T2 T1 40 1 + ω 2τ 2 0
s
2τs
]
1 + 4ω02τs2
(3)
where χ is the quadrupole coupling constant (167 kHz), and S is an order parameter describing the residual anisotropy due to the local motions.31 From the quadrupolar splitting of HDPC-γ-d6 bound to large latex particles (∼1000 Hz),30 one knows that S is very low for the quaternary methyls of the phosphocholine polar headgroup, so that a proper estimate of τs becomes difficult. (31) Wennerstrom, H.; Lindman, B.; So¨derman, O.; Drakenburg, T.; Rosenholm, J. B. J. Am. Chem. Soc. 1979, 101, 6860. (32) Monduzzi, M.; Ceglie, A.; Lindman, B.; So¨derman, O. J. Colloid Interface Sci. 1990, 136, 113.
Polyelectrolyte Binding to Mixed Micelles
Nevertheless, some insight into the relative mobility of the two surfactants may be obtained by assuming a particular value for the order parameters, recognizing that it is the indicated trends which are of interest, rather than the absolute values themselves. An order parameter of 0.05, and the corresponding values of T1 and T2, yields a slow motion correlation time for the free surfactant micelles in good agreement with the value predicted according to eq 2. Using this order parameter for both HDPC-γ-d6 and CTAB-γ-d9 yields the values of τs listed in Table 1, given the corresponding values of T1 and T2. For mixed CTAB/HDPC micelles, in the absence of PSSS, τs is rather similar for CTAB and HDPC, as one expects. Adding PSSS lengthens τs in the case of both surfactants, but the effect is more profound for CTAB. The apparent PSSS-induced increase in τs could be due to either an increase in the effective radius for micellar tumbling or a decrease in the surfactant diffusion coefficient, assuming that compensating changes in the order parameter S do not occur. The dynamic light scattering results of Dubin and co-workers14 indicate that soluble complexes form between polyelectrolytes and mixed surfactant micelles prior to precipitation and that these complexes are rather compact and not very different in size from the unperturbed surfactant micelles. Thus, one would not anticipate that the effective radius for particle tumbling changes profoundly upon formation of such soluble complexes. At least, if this did occur, one would expect both CTAB and HDPC deuterons to be approximately equally influenced. However, one might anticipate that the lateral diffusion of CTAB-γ-d9 would be much more sensitive to PSSS than would HDPC-γ-d6 due to preferential electrostatic interaction between PSSS and CTAB. Indeed this is the simplest explanation for the differential effects of PSSS on the slow motion correlation times of CTAB-γ-d9 versus HDPC-γ-d6. At high PSSS/CTAB charge ratios, where the dispersion has restabilized, further lengthening of τs is deduced, this time for both CTAB and HDPC. A simple explanation for these effects is that the resolubilized PSSS/CTAB/HDPC complex is large relative to a surfactant micelle. It should be possible to characterize the size of such soluble complexes using any number of techniques, including measurement of their diffusion coefficient using the pulsed gradient spin-echo NMR technique.20 Surfactant Mobility within a Floc Is Determined by the Ratio CTAB/HDPC. It is possible to isolate the phase-separated PSSS/CTAB/HDPC complexes by centrifugation and to observe the deuterium NMR spectrum using the quadrupolar echo technique specific for broad resonances. The most efficient method of obtaining a large quantity of such material is to add the desired amount of PSSS to the surfactant solution in a single addition, as opposed to the titration approach employed in the experiments described above. The amount of PSSS added is chosen to equal, as closely as possible, that required to maximize phase separation. Some typical broad line deuterium spectra which result are shown in Figure 6. They represent, from top to bottom, increasing mole fraction of CTAB in the surfactant mixture. For the mixture CTAB-γ-d9/HDPC (6:4), the deuterium spectrum (top left) of the insoluble material consists of a broad featureless resonance line, having a width-at-half-height of approximately 500 Hz, corresponding to a T2 equal to 600 µs (using ∆ν1/2 (Hz) ) (πT2)-1 s-1). Similar deuterium NMR spectra are observed using insoluble material originating from surfactant mixtures with lower ratios of CTAB-γ-d9/HDPC and are not shown. The T2 of the surfactant in the precipitate is 2 orders of magnitude smaller than the T2 of the corresponding
Langmuir, Vol. 12, No. 2, 1996 259
Figure 6. Broad-line deuterium NMR spectra of phaseseparated PSSS/CTAB/HDPC complexes. These deuterium NMR spectra were acquired using the quadrupolar echo technique, specific for observing broad resonance lines. The spectra on the left were obtained using CTAB-γ-d9 for the three different CTAB/HDPC molar ratios (from top to bottom) 6:4, 8:2, and 10:0. The spectra on the right were obtained using HDPC-γ-d6 for the two different CTAB/HDPC molar ratios (from top to bottom) 6:4 and 8:2.
free surfactant. This is symptomatic of a high degree of immobilization of the surfactant trapped in the flocculated material relative to surfactant micelles free in solution. This difference suffices to explain why the deuterium signal from the phase-separated material apparently disappears from the deuterium spectrum: its intensity is simply spread over such a wide frequency range that it is lost in the background noise on an intensity scale relative to that of the free surfactant micelles. Nevertheless, these deuterium spectra indicate that there remains considerable motional freedom, at least at the local level, even within the precipitate. For the case of 100% CTAB-γ-d9 (bottom left) the spectrum displays a characteristic Pake doublet line shape with a quadrupolar splitting, corresponding to the separation in hertz between the two maxima in the spectrum, of approximately 1600 Hz. Such a line shape is diagnostic of deuterons undergoing anisotropic motional averaging. This indicates an even higher degree of restriction of motional freedom from the CTAB in the flocculated material originating from 100% CTAB surfactant micelles. For the case CTAB-γ-d9/HDPC (8:2) (left middle), the spectrum is clearly a superposition of two spectral components: one a Pake doublet like that observed with 100% CTAB and the other a broad featureless component like that observed with CTAB-γ-d9/HDPC (6:4). Evidently, the insoluble material is not homogeneous in composition but rather is a composite of materials having different proportions of CTAB/HDPC. This agrees with the results in Figure 5 which indicate that the first material to phase separate is enriched with respect to CTAB versus HDPC. In passing, we note that in the spectra of such insoluble material there sometimes appears a narrow central resonance, probably arising from overlapping signals due to residual HDO plus small amounts of free CTAB/HDPC micelles entrapped during the centrifugation step. The size of this narrow resonance is a function of the time and speed of centrifugation of the insoluble material. When the deuterons are located on HDPC-γ-d6, the phase-separated material yields a broad featureless spectrum in all cases, having a width-at-half-height on
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the order of 600 Hz. No evidence could be found in the spectra for compositional inhomogeneities. It seems that the mobility of HDPC, although restricted due to being entrapped within the surfactant-polyelectrolyte complex, is not further influenced by the proportion of CTAB/HDPC. Conclusions and Summary At the molecular level, the electrostatic complexes formed by the interaction between an anionic polyelectrolyte poly(sodium styrenesulfonate) (PSSS) and mixed zwitterionic-cationic surfactant micelles are heterogeneous, in terms of both composition and internal surfactant dynamics. The cation/zwitterion ratio in the surfactant micelles, and by extension the micellar surface electro-
Semchyschyn et al.
static charge, regulates the strength of the polyelectrolyte-micelle interactions. CTAB experiences these interactions most sensitively and manifests their effects most plainly. It is preferentially enriched within the electrostatic complexes and its dynamics within the complex are preferentially hindered. HDPC accompanies the cationic surfactant as it complexes with the anionic polyelectrolyte, but its behavior is more a reflection of the need to associate with other surfactants than any demands of electrostatics. It should prove informative to query the influence of surfactant chain length, polyelectrolyte molecular weight, and ionic strength on these same molecular details. LA950244A
