A Fluorescence Study of Divalent and Monovalent Cationic

(cac) was about the same for the two surfactants in solutions of a given polyion. ... arranged in order of increasing cac: dextran sulfate < polyvinyl...
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Langmuir 2001, 17, 4161-4166

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A Fluorescence Study of Divalent and Monovalent Cationic Surfactants Interacting with Anionic Polyelectrolytes† Per Hansson* Department of Physical Chemistry, Uppsala University, PO Box 532, S-75121 Uppsala, Sweden Received April 10, 2000. In Final Form: April 5, 2001

Self-assemblies of the divalent surfactant dodecyl-1,3-propylenepentamethylbis(ammonium chloride) (DoPPDAC) and the monovalent dodecyltrimethylammonium bromide (DoTAB) were investigated in dilute solutions of anionic polyelectrolytes. Pyrene probing showed that the critical aggregation concentration (cac) was about the same for the two surfactants in solutions of a given polyion. The polyions could be arranged in order of increasing cac: dextran sulfate < polyvinyl sulfate < polyacrylate < poly(styrenesulfonate) < carboxymethyl cellulose. The surfactant aggregation number (N) for DoPPDAC, obtained from time-resolved fluorescence quenching, was 2-3 times smaller than that for DoTAB in the presence of all polyions. With poly(styrenesulfonate) taken out of consideration, it was found that a large N correlated with a low cac, and vice versa. For both surfactants, pyrene lifetime measurements indicated an insignificant binding of negatively charged quenchers to the micelles, showing that the micelle charges were neutralized mainly by the polyions. The presence of polyion reduced the quenching by oxygen dissolved in the water. The effect was larger for DoPPDAC than for DoTAB, suggesting that the micelles of the former are surrounded by a more dense layer of polyion.

Introduction Aqueous mixtures of ionic surfactants and oppositely charged polyions have been studied in great detail recently.1-4 One reason for this interest is the desire to control the stability, rheology, and water content of technical dispersions containing both components. It is generally believed that flexible polyions can neutralize charged surfactant micelles by folding around them.5 From the point of view of surfactant self-assembly this is very favorable as the entropic penalty of binding simple counterions is removed.6 Thus, the critical aggregation concentration7 (cac) in a polyelectrolyte solution is much lower than the critical micelle concentration (cmc) of the pure surfactant.8 The neutralization of the micelles by a “layer” of polyion has important consequences also for the phase behavior.9 Noticably, the absence of long-range repulsive forces between the micelles strongly diminishes the stability range of the micellar solution phase found in related binary ionic surfactant/water systems. Instead a * E-mail: [email protected]. † This work was initiated when the author was working at the Division for Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University. (1) Goddard, E. D. Colloids Surf. 1986, 19, 301. (2) Hayakawa, K.; Kwak, J. C. T. Interactions Between Polymers and Cationic Surfactants. In Cationic Surfactants: Physical Chemistry; Rubingh, D., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; Vol. 37. (3) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604. (4) Polymer-surfactant systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998; Vol. 77. (5) Linse, P.; Piculell, L.; Hansson, P. Models of Polymer-Surfactant Complexation. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998; Vol. 77. (6) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1990, 94, 4289. (7) Chu, D.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270. (8) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (9) Piculell, L.; Lindman, B.; Karlstro¨m, G. Phase behavior of polymer/ surfactant systems. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998; Vol. 77.

phase separation takes place,6,10-13 resembling the “complex coacervation” of oppositely charged polyions. Furthermore, the possibility of the micelles to interact with either polyion chains or simple ions (or both), with different effects on curvature and micelle-micelle interactions, gives rise to a rich phase behavior.14-16 In our lab we have investigated17-22 the influence of polyion on self-assemblies of alkyltrimethylammonium ions in dilute dispersions close to the cac. Under these conditions the presence of the polyion was found to completely rule out the binding of simple counterions to the micelles. Thus it was possible to directly relate variations of surfactant aggregation number (N) and cac to certain properties of the polyions. For instance, both N and cac were found to depend on the polyion linear charge density, backbone stiffness, and nature of the charged group. N was obtained from time-resolved fluorescence quenching (TRFQ), and the cac from surfactant binding studies, the latter complemented with earlier data from Kwak and co-workers.8,23 Interestingly, a correlation between cac and N was found17-21 for dodecyltrimethylammonium bromide (DoTAB), resembling the effect of different monovalent counterions on cmc and N in polymer(10) Goddard, E. D.; Hannan, R. B. J. Colloid Interface Sci. 1976, 55, 73. (11) Thalberg, K.; Lindman, B.; Bergfeldt, K. Langmuir 1991, 7, 2893. (12) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1991, 95, 6004. (13) Thalberg, K.; Lindman, B. Langmuir 1991, 7, 277. (14) Carnali, J. O. Langmuir 1993, 9, 2933. (15) Ilekti, P.; Piculell, L.; Tournilhac, F.; Cabane, B. J. Phys. Chem. B 1998, 102, 344. (16) Ilekti, P.; Martin, T.; Cabane, B.; Piculell, L. J. Phys. Chem. B 1999, 103, 9831. (17) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. (18) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115. (19) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 99, 16694. (20) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 99, 16684. (21) Hansson, P.; Almgren, M. J. Phys. Chem. 1996, 100, 9038. (22) Hansson, P.; Almgren, M. J. Phys. Chem. B 2000, 104, 1137. (23) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642.

10.1021/la000539a CCC: $20.00 © 2001 American Chemical Society Published on Web 06/06/2001

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free solutions of the surfactant.24 The observations in the latter systems were explained by Tanford25 from considerations of attractive and repulsive interactions in the interfacial region of the micelles. Very recently we used similar arguments in a thermodynamic model of polyion/ surfactant interactions.26 In the present paper we investigate the importance of the surfactant-surfactant repulsions by increasing the charge of the headgroup. For this purpose we employ dodecyl-1,3-propylenepentamethylbis(ammonium chloride) (DoPPDAC), a divalent surfactant known to form smaller micelles (smaller N) than its monovalent analogue DoTAB, both in solutions close to the cmc and at the charged silica/water interface.27-29 N and cac are determined in solutions of the following anionic polyelectrolytes: sodium dextran sulfate (DxS), potassium poly(vinyl sulfate) (PVS), sodium poly(styrenesulfonate) (PSS), sodium polyacrylate (PA), sodium carboxymethylcellulose (CMC). The microstructure of the complexes is further probed by using water-soluble quenchers. To facilitate comparisons all experiments are carried out with DoTAB as well. It can be mentioned that divalent surfactants are used in many applications, e.g., as softerners, as disinfectants, and as stabilizers for asphalt emulsions. Their solution properties and phase behavior were recently investigated by Hagsla¨tt et al.30-32 Furthermore, Brackman et al.33,34 have investigated the interaction of anionic divalent surfactants with neutral polymers. Experimental Section Chemicals. DoTAB from Serva (analytic grade), DoPPDAC (CH3-(CH2)11-N+(CH2)2-(CH2)3-N+(CH3)3 + 2Cl-) from Synthelec AB, Lund, Sweden, N-cetylpyridinium chloride (CPC) from Merck, pyrene from Jansen (>99%), potassium iodide from Merck (pro analysis), DxS (Mw ≈ 106) from Serva (pure), PVS (Mw ) 245 000) from Serva (pure), and poly(acrylic acid) (Mw ) 450 000) from Aldrich were all used as received. PA was obtained from poly(acrylic acid) by titration with NaOH (pH ) 9). PSS35 (Mw ) 1.7 × 106) and CMC36 were kind gifts from Professor Hans Vink, Uppsala. All samples were prepared as described elsewhere21 using Millipore water. Steady-State Fluorescence Measurements. Pyrene fluorescence spectra were recorded on a SPEX Fluorolog 1680. The excitation wavelength was 320 nm. The pyrene concentration was kept low (10-7 M) to minimize possible effects on the surfactant self-assembly. To improve the signal-to-noise ratio, the slit width on the emission side was rather large (0.6 mm). This resulted in broad bandwidths and a somewhat elevated ratio between the third and the first vibronic peaks in the pyrene emission spectrum. Time-Resolved Fluorescence Measurements. Pyrene decay curves were recorded with the single-photon counting technique. The experimental setup has been described else(24) Anacker, E. W.; Ghose, H. M. J. Phys. Chem. 1963, 67, 1713. (25) Tanford, C. J. Phys. Chem. 1974, 78, 2469. (26) Hansson, P. Langmuir 2001, 17, 4167. (27) Stro¨m, C.; Jo¨nsson, B.; So¨derman, O.; Hansson, P. Colloids Surf., A 1999, 159, 109. (28) Hansson, P.; Jo¨nsson, B.; Stro¨m, C.; So¨derman, O. J. Phys. Chem. B 2000, 104, 3496. (29) Stro¨m, C.; Hansson, P.; Jo¨nsson, B.; So¨derman, O. Langmuir 2000, 16, 2469. (30) Hagsla¨tt, H.; So¨derman, O.; Jo¨nsson, B.; Johansson, L. B.-A° . J. Phys. Chem. 1991, 95, 1703. (31) Hagsla¨tt, H.; So¨derman, O.; Jo¨nsson, B. Liq. Cryst. 1992, 12, 667. (32) Hagsla¨tt, H.; So¨derman, O.; Jo¨nsson, B. Langmuir 1994, 10, 2177. (33) Brackman, J. C.; Engberts, J. B. F. N. J. Am. Chem. Soc. 1990, 112, 872. (34) Brackman, J. C.; Engberts, J. B. F. N. Langmuir 1991, 7, 46. (35) Vink, H. Makromol. Chem. 1981, 182, 279. (36) Vink, H. J. Chem. Soc., Faraday Trans. 1986, 1, 2353.

Hansson where.17 All measurements were performed at 20 °C. The excitation wavelength was 325 nm, and the pyrene emission was selected using colored glass filters. Analysis of TRFQ Data. All quenched decays were analyzed by fitting the following function37 to the data

F(t) ) A1 exp(-A2t + A3[exp{-A4t} - 1])

(1)

where A1 ) F(0). In the case of stationary probes and Poissonian distributed quenchers free to migrate between small uniform micelles38

A2 )

1 kqk-〈n〉 + τ0 kq + k-

A3 )

kq2〈n〉 (kq + k-)2

A4 ) kq + k-

(1a)

(1b)

(1c)

where τ0 is the probe lifetime, 〈n〉 is the average number of quenchers per micelle, kq is the intramicellar quenching rate constant, and k- is the quencher escape frequency from micelles. With some precaution, eq 1 can be used also in systems with a general exchange of probes and quenchers,39 in polydisperse systems,40,41 and when there are minor deviations from a Poisson distribution due to interaction between quenchers in the same micelle.28,42 In the determination of surfactant aggregation numbers, the choice of quencher is often critical. For DoTAB micelles we have demonstrated20,22 the convenience of using dodecylpyridinium ions, which are themselves surfactants. The close to ideal mixing of the two surfactants in the micelles ensures a random distribution of the quencher among the micelles28 and facilitates the estimation of XQ, the average mole fraction of quencher in the micelles, from which the average aggregation number (N) is calculated:20

N)

〈n〉 XQ

(2)

By taking advantage of the mixing properties, we were able to accurately determine N in solutions containing as little as 0.25 mM of aggregated DoTAB.19-21 Unfortunately, there is no divalent analogue to the dodecylpyridinium ions available to us. Therefore, to have control over the quencher partitioning between the aqueous and micellar subphases, we use in the present study CPC which will be distributed exclusively in the micelles at the surfactant concentrations used here.22 It was shown recently 28 that an accurate value of XQ can be calculated from the relationship

XQ )

CQ CQ + CS + Cf,S(R - 1)

(3)

where CQ and CS are, the total concentration of quencher and surfactant in the system, respectively, Cf,S is the free concentration of the latter, and R is a constant. R < 1 means that the quencher has a stronger preference for the micelles than the surfactant. Thermodynamic model calculations show that R is on the order of 10-3 for CPB/DoTAB and even smaller for (37) Infelta, P. P.; Gra¨tzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78, 190. (38) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (39) Almgren, M.; Lo¨froth, J.-E.; van Stam, J. J. Phys. Chem. 1986, 90, 4431. (40) Almgren, M.; Lo¨froth, J.-E. J. Chem. Phys. 1982, 76, 2734. (41) Warr, G. G.; Grieser, F. G. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1813. (42) Almgren, M.; Hansson, P.; Wang, K. Langmuir 1996, 12, 3855.

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Figure 1. Pyrene 3/1 ratios as a function of log CS for DoPPDAC (filled) and DoTAB (open) in 5 mM polyelectrolyte solutions. The lines are just guides to the eye. CPC/DoPPDAC.28 Thus, as long as Cf,S ) cac is a good approximation,XQ can be calculated using the relationship

CQ XQ ) CQ + CS - cac

Table 1. Aggregation Numbers (N) and cac for DoPPDAC and DoTAB in 5 mM Polyelectrolyte Solutions at 20 °C

(4)

Results and Discussion Critical Aggregation Concentration. It is common to construct surfactant binding isotherms from activity data. In this respect surfactant-sensitive electrodes are useful.2 Unfortunately, the type of electrode used earlier20 in our lab to measure the DoTAB activity turned out to be insensitive to DoPPDAC. Instead, we used the pyrene method to probe the onset of micelle formation, where it is utilized that the ratio of the third to the first vibronic peaks (3/1) in the fluorescence spectra is sensitive to the polarity of the environment.43 Before the results are presented, it should be mentioned that pyrene can stabilize surfactant aggregates formed below the cac. However, since the cac values obtained here for DoTAB/PSS and DoTAB/PA (at 5 mM polyelectrolyte) are in close agreement with the values obtained earlier18,20 from surfactant electrode measurements, the effect appears to be negligibly small. The agreement justifies also our choice of the cac as the concentration where the 3/1 ratio starts to increase (see below). Figure 1 shows the 3/1 ratio as a function of log(CS/M) in 5 mM polyelectrolyte solutions, where CS is the molar concentration of surfactant. With all polyions a sharp (43) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.

system

N

cac/M

PA/DoTAB PA/DoPPDAC CMC/DoTAB CMC/DoPPDAC DxS/DoTAB DxS/DoPPDAC PVS/DoTAB PVS/DoPPDAC PSS/DoTAB PSS/DoPPDAC DoTAB DoPPDACc

58 29 65 30 107a 40-60 120a,b 52 23 8 60c ≈20c

6 × 10-5 6 × 10-5 2 × 10-4 2 × 10-4 1.2 × 10-5 1.2 × 10-5 2.2 × 10-5 2.2 × 10-5 1 × 10-4 1 × 10-4 1.5 × 10-2 d 4.8 × 10-2 d

a 0.5 mM polyelectrolyte, 0.25 mM DoTAB. Taken from ref 19. Temperature: 50 °C c Measured close to the cmc. Taken from ref 28. d cmc for the surfactant in water.

b

increase is observed within a narrow range of surfactant concentrations. This occurs at approximately the same concentration for DoPPDAC and DoTAB, revealing that they have about the same cac (see Table 1). The final 3/1 ratios (i.e., when all pyrene molecules are solubilized in micelles) are always largest for DoTAB, indicating that the probe experiences the least polar environment in the micelles formed by this surfactant. Furthermore, the increase of the 3/1 ratio with surfactant concentration is larger for DoTAB than for DoPPDAC. This may simply be a consequence of the generally larger ratios for DoTAB but may also reflect the lower N for DoPPDAC (see next section), as suggested by a previously found21 correlation between steep binding isotherms and large N.

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Figure 2. Pyrene fluorescence from a mixture of 5 mM PA and 1 mM DoPPDAC at 20 °C. The upper and the lower curves were obtained in the absence and in the presence of quencher (CPC), respectively.

The polyions can be listed in order of increasing stability of the micelles: CMC < PSS < PA < PVS < DxS. Here, a high “stability” means a favorable surfactant selfassembly, i.e., a low cac. The same order is observed with DoPPDAC and DoTAB. It is important to keep in mind, however, when comparing cac values that the activity of the divalent surfactant is expected to be considerably lower than that for the monovalent (at a given concentration of unimers in equilibrium with the micelles) due to the strong interaction of the divalent unimers with the polyion chains. Aggregation Numbers. The aggregation numbers for DoPPDAC and DoTAB were determined using TRFQ as described in the Experimental Section. The concentrations of surfactant and polyelectrolyte were 1 and 5 mM, respectively, a composition at which the free concentration of the added quencher (CPC) could be neglected. Our previous studies17-21 were carried out at 10 times lower concentrations to avoid working close to phase boundaries. However, as explained in the Experimental Section, this was not possible with DoPPDAC due to the lack of a suitable quencher. At the present composition (but not at surfactant concentrations closer to the cac) the PVS samples were slightly turbid. It should be pointed out, however, that TRFQ can be used to determine N also under those conditions. Figure 2 shows the pyrene fluorescence decay in a DoPPDAC/PA mixture with and without quencher present. The initially rapid decay of the quenched curve reflects the short average distance between the probe and the quencher in the micelles. At long times the decay rate is the same as in the absence of quencher (upper curve). This behavior, typical44 for quenching in discrete micelles using probes and quenchers stationary during the time scale of the experiment (see eq 1), was observed for all the systems investigated, except for DoPPDAC/DxS, where only a rough estimate of N could be made, and for DoTAB/ DxS and DoTAB/PVS, where N was available, however, from a previous study.19 Aggregation numbers obtained from fits of eq 1 to the quenched decays are given in Table 1. It can be seen that N for DoPPDAC is 2-3 times smaller than for DoTAB, both in the presence and in the absence of polyelectrolyte. (The values for the pure surfactants in Table 1 were obtained earlier.28) The small N for DoPPDAC in the (44) Almgren, M. Kinetics of Excited-State Processes in Micellar Media. In Kinetics and Catalysis in Microheterogeneous Systems; Gra¨tzel, M., Kalyanasundaram, K., Eds.; Marcel Dekker: New York, 1991.

Hansson

absence of polyion has been attributed to the strong repulsion between the headgroups.30 The present result shows that the situation is not altered when the micelle charges are screened by a layer of polyion. It should be noted that, despite the difference in N, the average charge density may not differ much between DoPPDAC and DoTAB micelles. For example, if each alkyl chain contributes 351 Å3 to the volume of a (spherical) micelle, the core radius is 16.9 and 13.4 Å for DoTAB/PA (N ) 58) and DoPPDAC/PA (N ) 29), respectively (see Table 1). However, by considering the length of the propylene spacer between the headgroup charges in DoPPDAC, the radius of the charged interface in a DoPPDAC micelle may be located, on the average, 2-3 Å further out from the core than in a DoTAB micelle, compensating for the difference in core radius. Thus, from an electrostatic point of view the complexes are rather similar. The DoPPDAC/PA system was further investigated at different surfactant binding ratios. For this purpose surfactant concentrations between 0.24 and 0.73 mM were studied at a fixed PA concentration of 2 mM. (The highest DoPPDAC concentration corresponds to a maximum binding ratio of 0.36 surfactant molecules per polyion charge.) N was found to be 30 for all samples, showing that N does not depend on the binding ratio. The same observation was made earlier20 in our lab for DoTAB/PA and by Kiefer et al.45 for tetradecyltrimethylammonium bromide/PA and by Anthony and Zana46 for DoTAC binding to a copolymer of maleic acid and methyl vinyl ether. In a previous study19 on DoTAB/PVS we observed quenched decays resembling the ones recorded here for this system, i.e., the decays at long times after the excitation event were not single exponential. However, in that study, a well-developed steady state with the same decay rate as in quencher-free samples was observed at an elevated temperature (50 °C). Thus, it was concluded that the intramicellar quenching was too slow at room temperature for a single-exponential tail to be observed during the probe lifetime. The long average distance between the probe and the quencher in the micelles, due to the large N, contributed to the effect.44 In Table 1 is given the value of N determined at 50 °C, which we believe is a reasonable estimate also at room temperature. For instance, it was possible to conclude from the decays recorded at 25 °C that the micelles were large,19 and surfactant binding isotherms recorded at 25 and 50 °C were nearly identical.19 We emphasize that for DoPPDAC/ PVS these problems were not encountered, probably due to the small N. As in the case of DoTAB/PVS the quenched curves recorded in the presence of DxS never reached the same decay rate as the unquenched curves. For DoTAB/DxS the effect is due to migration of the quencher between micelles bound to the same polyion chain, as was demonstrated earlier.19 In that study, a steady state, proving the discreteness of the micelles, was observed after extending the pyrene lifetime by bubbling the solutions with nitrogen. The resulting value of N is given in Table 1. Most likely, a migration of the quencher can explain the shape of the curves observed in the DoPPDAC/ DxS system as well. The interpretation would be a nonuniform distribution of the micelles among the polyelectrolyte chains, as has been suggested earlier by others (45) Kiefer, J. J.; Somasundaran, P.; Ananthapadmanabhan, K. P. In Polymer Solutions, Blends, and Interfaces; Noda, I., Rubingh, D. N., Eds.; Elsevier: Amsterdam, 1992; Vol. 11, p 423. (46) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967.

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Figure 3. Pseudo-first-order quenching rate constant, k+(O2), for quenching of pyrene by oxygen in 1 mM surfactant/5 mM polyelectrolyte solutions at 20 °C.

for a cationic surfactant interacting with DxS.47 The estimate of N in Table 1 results from an analysis based on the assumption of a migration of the quencher between discrete micelles (eqs 1a-c). Quenching with Water-Soluble Quenchers. To investigate the accessibility of aqueous solutes to the probe solubilized in the micelles, we employed a neutral quencher (O2) and two ionic quenchers (I- and Br-). Quenching with Oxygen. O2 is an efficient quencher of pyrene fluorescence. It has no particular affinity for micelles, so no rapid quenching of the type discussed above is observed. However, from the fluorescence lifetimes measured in deareated solutions and in solutions in equilibrium with air, the apparent pseudo-first-order quenching rate constant, k+, was obtained from the relationship17

k+[O2] )

1 1 τair τN2

Figure 5. Pseudo-first-order quenching rate constant, k+(I-), for quenching of pyrene by iodide ions in 1 mM surfactant/5 mM polyelectrolyte solutions at 20 °C.

(5)

where τair and τN2 are the pyrene lifetimes in solutions before and after bubbling with nitrogen and [O2] is the O2 concentration in water. The result presented in Figure 3 shows that the quenching is more efficient in DoTAB than in DoPPDAC micelles. However, even in the former micelles k+ is significantly smaller than in a polymer-free micellar solution of the surfactant, where k+ ) 5.1 × 109 M-1 s-1 was obtained earlier.17 The theoretical encounter frequency for free diffusion is given by the relationship48

k+ ) 4πR0DNA

Figure 4. Pyrene lifetimes in deareated 1 mM surfactant/5 mM polyelectrolyte solutions at 20 °C.

(6)

where D is the diffusion constant for the quencher (2.1 × 10-9 m2/s for O2 in water)48 and NA is Avogadro’s constant. By taking the encounter radius (R0) of the couple pyrene/ oxygen to be 4 Å, we obtain k+ ) 6 × 109 M-1 s-1, suggesting that O2 has essentially free access to the probe in regular DoTAB micelles. The significantly lower values in the micelle/polyion complexes indicate that the layer of polyion surrounding the micelles is more dense than the layer of bromide ions surrounding a DoTAB micelle. Likewise, the difference between DoPPDAC and DoTAB may be interpreted as a denser polyion layer surrounding the former. Quenching with Bromide. From the lifetimes in deareated solutions, it is possible to extract information about (47) Shirahama, K.; Kameyama, K.; Takagi, T. J. Phys. Chem. 1992, 96, 6817. (48) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1980, 102, 3188.

counterion binding. Here one takes advantage of the fact that Br- is an efficient quencher, but Cl- is not. Figure 4 shows that the lifetimes in nitrogen-bubbled solutions are about the same for DoTAB and DoPPDAC in the presence of all polyelectrolytes. Furthermore, the lifetimes in PA, PSS, and DxS solutions are the same as in (nitrogenbubbled) pure CTAC (340 ns)17 but considerably longer than those in a DoTAB micellar solution (τN2 ) 180 ns).17 The conclusion is that there is no significant binding of the surfactant counterions to the DoTA+ micelles in the presence of the polyion, in agreement with earlier observations.17,49 A comparison with CTAC micelles suggests that there is no pronounced effect on the lifetime (in deareated solutions) by replacing the chloride ions surrounding a DoPPDAC micelle with a layer of polyion, except with PVS. The comparatively short lifetime in the PVS systems may indicate that the probe is distributed closer to the micelle/water interface than in the other systems. Quenching with Iodide. This is a very efficient quencher in pure DoTAB solutions, giving rise to a rapid quenching similar to that of CPC, but with an additional contribution from the migration between micelles during the lifetime of the probe.50 In the present solutions no rapid quenching was observed, and the effect on the lifetime was small. Figure 5 shows the pseudo-first-order quenching rate constant calculated in the same way as for the O2 (49) Abuin, E. B.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 6274. (50) Almgren, M.; Linse, P.; Van der Auweraer, M.; De Schryver, F. C.; Gelade´, E.; Croonen, Y. J. Phys. Chem. 1984, 88, 289.

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quenching. With all polyions, I- was less efficient than O2, except with CMC where the two quenchers had about the same effect. The result shows that negative ions are not bound to DoPPDA+ (or DoTA+) micelles in the presence of the polyions. In the case of PSS, DxS, and PVS there may even be an electrostatic expulsion of negative ions. Relation between N and cac. It can be seen in Table 1 that N for DoPPDAC and DoTAB depends on the polyion used. For both surfactants the polyions can be arranged in order of increasing N: PSS < CMC ≈ PA < DxS ≈ PVS, i.e., the same order as obtained for the stability of the micelles. (Here we have left PSS out of consideration, since in this case the particularly small N is due to a welldocumented8,17,18,51,52 hydrophobic interaction between the polymer backbone and the micelles.) As mentioned in the Introduction, a similar relation between N and cac was observed earlier21 for DoTAB at 0.25 mM surfactant and 0.5 mM polyelectrolyte. The result can be rationalized from considerations of repulsive and attractive interactions between the surfactant molecules in the headgroup region of the micelle.25 The attraction, due to the strive to minimize the hydrocarbon/water contacts, leads to a reduction of the area per headgroup, consistent with an increase of N. The growth of the micelle is restricted, however, by the electrostatic repulsion between the headgroups, favoring small N. At the optimal N the attractive and repulsive interactions exactly balance. Thus, an effective screening of the headgroup repulsion allows for micelles with large N to form. Now, since this means a small area per molecule, and therefore also a low “hydrophobic” energy, the micelles start to form at a lower surfactant concentration. The differences observed between PA and PVS must be attributed to the nature of the charged groups of these (51) Gao, Z.; Kwak, J. C. T.; Wasylishen, R. E. J. Colloid Interface Sci. 1988, 126, 371. (52) Gao, Z.; Wasylishen, R. E.; Kwak, J. C. T. J. Phys. Chem. 1990, 94, 773.

Hansson

polymers as the backbones and the distance between the substituents are identical. By considering the size of the two groups, the sulfate group is expected to be more polarizable than the carboxylate, which makes chargedipole interactions stronger. Of course, there may be other effects due to hydration, ion-ion correlation, etc. In any case, a stronger attraction between the polyion and micelles leads to a more effective screening of the surfactant headgroups, consistent with a larger N and a lower cac. Conclusions DoPPDAC interacts with oppositely charged polyions to form small micelles. The surfactant charges are neutralized by the polyion chains decorating the micelle surfaces. N is distinctly smaller than for DoTAB with a given polyion, but varies significantly between different polyions. For both surfactants the polyions can be ordered according to increasing N: PSS < PA ≈ CMC < DxS ≈ PVS. The same order is observed for the stability of the micelles. The correlation between N and micelle stability resembles the one observed for ionic micelles in the presence of different monovalent counterions,24,25 which has been accounted for by phenomenological models of micelles.25,53 As shown eleswhere,26 by extending these models to polyion/surfactant mixtures, it is possible account for the fact that DoPPDAC and DoTAB have the same cac with a given polyion. Acknowledgment. The author is grateful to Mats Almgren for valuable comments. This work has been financially supported by the Swedish Board for Industrial and Technical Development (TFR) and by the Center for Amphiphilic Polymers (CAP), Lund University. LA000539A (53) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1 1976, 72, 1525.