Structure and Properties of Fluids Composed of Polyelectrolyte and

Effect of Water-Soluble Polymers on the Morphology of Aerosol OT Vesicles. José I. Briz , M.Mercedes Velázquez. Journal of Colloid and Interface Sci...
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Langmuir 1998, 14, 4737-4743

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Structure and Properties of Fluids Composed of Polyelectrolyte and Ionic Surfactant in Organic Phase: Poly(acrylic acid) and Didodecyldimethylammonium Bromide Akihisa Shioi,* Makoto Harada, Motoharu Obika, and Motonari Adachi Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611, Japan Received November 24, 1997. In Final Form: June 2, 1998 The structure and properties of the oil-rich microemulsion (L2 phase) containing anionic polyelectrolyte, poly(acrylic acid) (PAA-H, Mw ) 450 000) and its sodium salt PAA-Na (Mw ) 30 000), were investigated. The cationic surfactant, didodecyldimethylammonium bromide (DDAB), which generates the rodlike aggregates in cyclohexane, was selected. Small-angle X-ray scattering, dynamic light scattering, electric conductivity, and viscosity measurements were performed. The rodlike structure was unchanged upon dissolving the polymers. PAA-Na is a strong polyelectrolyte, which takes a rather extended conformation in the bulk aqueous solution. However, PAA-Na adsorbed DDAB molecules by electrostatic interaction in the microemulsions. Then, the extended conformation was loosened in the microemulsion and was dissolved within the water pool of the DDAB aggregates. The characteristic size of the PAA-Na dissolved within the water pool of the DDAB aggregates was similar to that of the original DDAB aggregates. High-molecular-weight polymer PAA-H was also dissolved within the water pool, and the polymer chain connected the rodlike aggregates. However, the diameter of the rodlike constituent for the connected structure is almost the same as that of the original DDAB aggregates.

Introduction Polymer and surfactant complexes are important in many industries and future advanced materials, e.g., enzymatic microreactor, catalysis, photoelectric device,1 and controlled nanomesosized materials.2,3 Thus, a large number of physicochemical investigations have been performed especially in aqueous solutions where polymers interact with surfactant molecules through electrostatic and the hydrophobic interactions.4,5 The interactions induce a cooperative self-assembling, often resulting in pearl necklace structures.6,7 Considerable attention has recently been paid to organic medium containing water-soluble polymers and surfactants to develop new types of structure and materials such as organogels and enzyme-containing reversed micelles.8-19 (1) Pileni, M. P., Ed. Structure and Reactivity in Reverse Micelles; Elsevier: Amsterdam, 1989. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (3) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (4) Lucassen-Reynders, E. H., Ed. Anionic Surfactants: Physical Chemistry of Surfactant Action; Marcel Dekker: New York, 1981. (5) Goddard, E. D., Ananthapadmanabhan, K. P., Eds. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (6) Nagarajan, R. J. Chem. Phys. 1989, 90, 3. (7) Loyen, K.; Iliopoulos, I.; Audebert, R.; Olsson, U. Langmuir 1995, 11, 1053. (8) Luisi, P. L.; Scartazzini, R.; Haering, G.; Schurtenberger, P. Colloid Polym. Sci. 1990, 268, 356. (9) Radiman, S.; Fountain, L. E.; Toprakcioglu, C.; de Vallera, A.; Chieux, P. Prog. Colloid Polym. Sci. 1990, 81, 54. (10) Cherian, A.; Rakshit, A. K. J. Colloid Interface Sci. 1993, 156, 202. (11) Lianos, P.; Modes, S.; Staikos, G.; Brown, W. Langmuir 1992, 8, 1054. (12) Caldarau, H.; Caragheorgheopol, A.; Dimoine, M.; Donescu, D.; Dragutan, I.; Marinescu, N. J. Phys. Chem. 1992, 96, 7109. (13) Suarez, M.-J.; Levy, H.; Lang, J. J. Phys. Chem. 1993, 97, 9808. (14) Veggeland, K.; Nilson, S. Langmuir 1995, 11, 1885.

In most of them, water-in-oil microemulsions composed of spherical droplets of nanometer size are selected as a surfactant system, and the effects of polymers on the structure of the surfactant assemblies are studied. In such cases, the hydrophilic polymers are confined within the nanodroplets. When a polymer includes both hydrophilic and hydrophobic chains, the former chain of the polymer is confined within the water pool of the aggregates and the latter is present in the organic continuum surrounding the droplets.17-19 The droplet structure is thermodynamically stable despite the presence of polymers. Then, the water-in-oil microemulsions are suitable solvents to dissolve the polymers comprising hydrophilic and hydrophobic chains, which are responsible for the function of composite materials such as an immobilized enzyme and an optically transparent catalysis composed of semiconductor ultrafine particles.1 The investigations reported so far demonstrate that the geometries of water-in-oil aggregates are not changed despite the presence of water-soluble polymers9-20 although a counterview has been presented.21 The geometry of the aggregates in a water-in-oil microemulsion is strongly influenced by the structure of the surfactant molecules, added-salts species and their concentrations, coexisting alcohol, temperature, and many other parameters.22-24 Thus, it is surprising that the droplet structure (15) Meier, W. Langmuir 1996, 12, 1188. (16) Eicke, H.-F.; Gauthier, M.; Hilfiker, R.; Struis, R. P. W.; Xu, G. J. Phys. Chem. 1992, 96, 5175. (17) Quellet, C.; Eicke, H.-F.; Sager, W. J. Phys. Chem. 1991, 95, 5642. (18) Atkinson, P. J.; Robinson, B. H.; Howe, A. M.; Heenan, R. K. J. Chem. Soc., Faraday Trans. 1991, 87, 3389. (19) Petit, C.; Zemb, Th.; Pileni, M. P. Langmuir 1991, 7, 223. (20) Schubel, D.; Ilgenfritz, G. Langmuir 1997, 13, 4246. (21) Schlicht, L.; Spilgies, J.-H.; Lipgens, S.; Boye, S.; Schubel, D.; Ilgenfritz, G. Biophys. Chem. 1996, 58, 39. (22) Safran, S. A., Clark, N. A., Eds. Physics of Complex and Supermolecular Fluids, An Exxon Monograph; Wiley-Interscience: New York, 1987. (23) Karlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 3881.

S0743-7463(97)01283-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/17/1998

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is unchanged by the presence of polymers. Most of the systems studied so far contain the nonionic species as the surfactants and/or the polymers.9-15,20 This may be the reason the droplet structure is kept despite polymer addition, because the electrostatic interaction between them is not so violent due to the presence of the nonionic species. Aqueous solutions containing polyelectrolyte and oppositely charged ionic surfactants provide unique properties in the phase diagram and the structure. For example, their phase diagrams are similar to those of the polycation/ polyanion mixed systems,25,26 and sometimes they generate highly ordered structure in the gel state.27-29 These characteristics probably arise from the strongly attractive interaction between the polymers and the surfactants. The strong interactions influence the self-assembling of surfactant molecules which may result in the formation of new types of structures. Such a cooperative selfassembling is interesting from physicochemical and practical points of view. However, the structure and properties of ionic surfactant assemblies containing the oppositely charged polyelectrolyte in organic solvents have not been well studied. Due to the strong electrostatic interaction between the surfactants and the polymers, the resulting structure may be different from that of the systems reported previously. In this work, we selected the cationic surfactant didodecyldimethylammonium bromide (DDAB) and anionic polyelectrolytes poly(acrylic acid) PAA-H and its sodium salt PAA-Na. The structure of DDAB aggregates in n-alkane and cyclohexane is well understood and is cylindrical when the water to surfactant mole ratio W0 is sufficiently low.24,30-33 The aggregates structure composed of DDAB and polymers were investigated by small-angle X-ray scattering, dynamic light scattering, viscosity and electric conductivity. Experiments Poly(acrylic acid) (PAA-H; Product No. 18128-5) Mw ) 450 000, poly(acrylic acid) sodium salt (PAA-Na, Product No. 41604-5) Mw ) 30000, and acrylic acid sodium salt (AA-Na; Product No. 40822-0) were purchased from Aldrich and used without further purification. Surfactant, didodecyldimethylammonium bromide (DDAB), was provided by Tokyo Kasei and used as supplied. Cyclohexane of spectroscopic grade purchased from Nacalai Tesque and distilled water were used. DDAB was dissolved in cyclohexane. Aqueous solution of PAA-H or PAA-Na was injected into the organic solution and stirred over several hours. The solutions were equilibrated in a thermobath. The duration time in the thermobath was over 3 days, because the patterns of the small-angle X-ray scattering and the viscosity were (24) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1986, 90, 2817. (25) Thalberg, K.; Lindman, B.; Karlstrom, G. J. Phys. Chem. 1991, 95, 6004. (26) Lindman, B.; Khan, A.; Marques, E.; Miguel, M. da G.; Piculell, L.; Thalberg, K. Pure Appl. Chem. 1993, 65, 953. (27) Antonietti, M.; Conrad, J.; Thunenmann, A. Macromolecules 1994, 27, 6007. (28) Sokolov, E. L.; Yeh, F.; Khokhlov, A.; Chu, B. Langmuir 1996, 12, 6229. (29) Meier, W. Langmuir 1996, 12, 6341. (30) Zemb, T. N.; Hyde, S. T.; Derian, P.-J.; Barnes, I. S.; Ninham, B. W. J. Phys. Chem. 1987, 91, 3820. (31) Barnes, I. S.; Hyde, S. T.; Ninham, B. W.; Derian, P.-J.; Drifford, M.; Zemb, T. N. J. Phys. Chem. 1988, 92, 2286. (32) Hyde, S. T.; Ninham, B. W.; Zemb, T. J. Phys. Chem. 1989, 93, 1464. (33) Eastoe, J.; Heenan, R. K. J. Chem. Soc., Faraday Trans. 1994, 93, 487.

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Figure 1. Examples of SAXS pattern of DDAB microemulsions: O, polymer-free DDAB microemulsion; 4, PAA-Na/DDAB microemulsion; ], PAA-H/DDAB microemulsion. Cm/CDDAB ) 0.036 in the polymer-containing DDAB microemulsion. CDDAB ) 0.2 M and W0 ) 4. The solid curve is the theoretical pattern for the polydispersed rod mentioned in the text. The diameter Dd and average length L for the solid curve are shown in the figure.

independent of the duration time over 3 days. After the equilibrium was attained, small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) experiments were performed. The viscosity and the electric conductivity of the solution were also measured. Experimental methods for SAXS and DLS were the same as reported in previous papers.34-36 Viscosity and electric conductivity were measured by the rheometer MR-300 (Rheology Engineering Co. Ltd.) with a cone-plate type cell and electric conductivity meter CM-115 (Kyoto Densi) (measurement frequency 1.2 kHz), respectively. The ratio W0 was set to be 4 where W0 was calculated from φaqFwater/ (MwaterCDDAB). Here, φaq, the range of which is from 0.0014 to 0.036, is the volume fraction of aqueous solution injected. Fwater and Mwater are the density and molecular weight of water, respectively. CDDAB is the molar concentration of DDAB. W0 is the water-to-DDAB mole ratio in the absence of the polymer. The experiments were carried out at 25 °C. Results and Discussion An example of the SAXS patterns of the DDAB microemulsion without polymers is shown in Figure 1. Here, CDDAB ) 0.2 M. It has been found that DDAB molecules form cylindrical water-in-oil aggregates24,30-33 with polydispersed length30-32 when W0 is low. The solid curve is calculated from the theoretical pattern for a rod37 with monodispersed diameter Dd and polydispersed length lrod. The method of calculation was essentially the same as that described in the previous paper35 except for a minor difference in the distribution function of the lengths. The distribution function of the rod length was assumed to be proportional to exp(-lrod/L), where L stands for the characteristic length. The integral for the length distribution function is taken from Dd to infinity. The Dd and L values are 1.6 and 10 nm, respectively, that reproduces the experimental results. Although we did not use the nonlinear least-squares method, the agreement between experimental and theoretical patterns is good enough. The diameter of the water pool of DDAB aggregates in cyclohexane is reported to be 0.84-1 nm at W0 ) 4 33 and 1-1.2 nm at W0 ) 2.8.32 Since the Dd determined by SAXS (34) Shioi, A.; Harada, M.; Tanabe, M. J. Phys. Chem. 1993, 97, 8281. (35) Shioi, A.; Harada, M.; Matsumoto, K. J. Phys. Chem. 1991, 95, 7495. (36) Kurumada, K.; Shioi, A.; Harada, M. J. Phys. Chem. 1995, 99, 16982. (37) Guinier, A.; Fournet, G. (translated by Walker, C. B.) Small Angle Scattering of X-rays; John Wiley & Sons: New York, 1955.

Polyelectrolyte and Ionic Surfactant Fluids

Figure 2. Diameter and average length of rodlike structure of DDAB microemulsions: CDDAB ) 0.2 M and W0 ) 4. Open and closed keys represent Dd and L, respectively. O, b, polymerfree DDAB microemulsion; 4, 2, PAA-Na/DDAB microemulsion; 3, 1, PAA-H/DDAB microemulsion; 0, 9, AA-Na/DDAB microemulsion. The keys, 2 and 1, for L are superimposed on each other at Cm/CDDAB ) 0.036 and 0.072. The keys, 3 and 0, for D are superimposed on each other at Cm/CDDAB ) 0.036. The keys, 4, 3, and 0, for D are superimposed on each other at Cm/CDDAB ) 0.072. The lines are guides to the eye.

includes the thickness of the layer composed of the hydrophilic groups, the obtained Dd values almost agree with the literature values. The comparison of the obtained L values with the literature ones is difficult because the lengths of the cylindrical aggregates reported so far depend in a complicated fashion on the experimental conditions.30-33 Examples of the SAXS patterns of DDAB/polymer microemulsions are also shown in Figure 1 where Cm/ CDDAB is 0.036. Here, Cm represents the concentrations of the monomer units of polymers. All the SAXS patterns for DDAB, DDAB/PAA-H, and DDAB/PAA-Na systems agree with each other and can be interpreted in terms of the polydispersed rods. We have also investigated the SAXS patterns of AA-Na-containing microemulsions. The patterns were also interpreted in terms of the polydispersed rods. Figure 2 shows the Dd and L against Cm/CDDAB. In the AA-Na system, L significantly decreases with increasing Cm/CDDAB although the Dd is almost unchanged. The length of the rodlike water-in-oil aggregates composed of sodium bis(2-ethylhexyl) phosphate (NaDEHP) decreases with an increase of the salt concentration in the water pool.34 The enriched salt stabilizes the spherical end caps attached to the aggregates due to the electrostatic screening effect, forming a large number of short rodlike aggregates.34 The AA-Na molecules in the present system play the same role as that of the salt in the NaDEHP system. The Dd and L values of the PAA-Na and PAA-H systems are not influenced by the presence of the polymer although the same concentration of AA-Na as the Cm values of the polymers strongly affects the L value. The unchanged structure suggests that PAA-Na and PAA-H are confined within the water pool of the DDAB aggregates without changing the geometry. The hydrophilic group of PAANa may play the same role as AA-Na in the influence on the cylindrical length. However, the experimental results do not indicate the structural change. The PAA-Na chain within the cylinder may stretch the cylinder against the effect of the unstable spherical caps. (The length of the PAA-Na confined within the cylinder will be discussed later.) However, it is difficult to ascertain the confinement from the SAXS experiments alone because the SAXS

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Figure 3. Specific viscosity against Cm: CDDAB ) 0.2 M. 2, PAA-Na aqueous solution; 1, PAA-H aqueous solution; 4, PAANa/DDAB microemulsion; 3, PAA-H/DDAB microemulsion. 0, AA-Na-containing microemulsion. In microemulsions, specific viscosity is defined as in eq 1a. Equation 1b expresses it in aqueous solution. The curves are guides to the eye.

Figure 4. Dependency of specific viscosity on the volume fraction of aggregates in the DDAB microemulsion. Specific viscosity is defined as in eq 1b: O, polymer-free DDAB microemulsion; 4, PAA-Na/DDAB microemulsion; 3, PAA-H/ DDAB microemulsion; 0, AA-Na/DDAB microemulsion. Cm/ CDDAB ) 0.072 in the microemulsions and W0 ) 4. The curves are guides to the eye.

experiments only provide geometrical information of the structure whose size is below several tens of nanometers. Figures 3 and 4 show the effects of polymers on the specific viscosity. Surfactant concentration CDDAB is 0.2 M. The “specific viscosity” of the microemulsion ηsp,m, which represents the effect of polymers alone on the viscosity, is defined by

ηsp,m ) (η(Cm) - η(0))/η(0)

(1a)

Here, η(Cm) represents the viscosity of the DDAB microemulsion at Cm, and η(0) the viscosity of the microemulsion in the absence of the polymer. The usual specific viscosity ηsp is defined by

ηsp ) (η - ηsolvent)/ηsolvent

(1b)

Here, η and ηsolvent denote the viscosities of the solution and the solvent, respectively. Figure 4 shows the dependency of ηsp (not ηsp,m) on φ. Here φ represents the sum of the volume fractions of DDAB and the aqueous solution injected. In this experiment, Cm/CDDAB is fixed to be 0.072. As evident from the SAXS experiments, AA-Na molecules make the aggregate length shorter due to the electrostatic interaction with the headgroup of DDAB.

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Figure 5. Electric conductivity shown as a function of the volume fraction of the aggregates: O, polymer-free DDAB microemulsion; 4, PAA-Na/DDAB microemulsion; 3, PAA-H/ DDAB microemulsion; 0, AA-Na/DDAB microemulsion. Cm/ CDDAB ) 0.072 in microemulsions containing the polymer. W0 is fixed to be 4. The curves are guides to the eye.

This causes the negative dependency of both ηsp,m and ηsp on the Cm (see Figures 3 and 4), while PAA-Na does not strongly affect the ηsp,m and ηsp. Thus, PAA-Na seems to cause no structural change in the large scale that affects the viscosity. The addition of PAA-H into the microemulsion increases the ηsp,m and ηsp as observed for PAA-H aqueous solution. This implies that the spatial extent of PAA-H in the DDAB microemulsion takes a size similar to that in the aqueous solution in the experimental Cm range. Although the geometry of DDAB aggregates below the size of several tens of nanometers is not influenced by the presence of PAA-H (see Figure 2), the PAA-H introduces a drastic change in the aggregates structure in larger scale than that detected by the SAXS. Figure 5 shows the effect of φ on the electric conductivity κ of the microemulsions. Cm/CDDAB is 0.072. The percolation transition in κ is observed at φ ∼ 0.03 in the polymerfree DDAB microemulsion. Such a low percolation threshold is attributed to the formation of rodlike aggregates.24,35 The infinite percolation cluster composed of the rods is formed above the threshold φ value. The decrease in κ with the addition of AA-Na reflects the shortening of the aggregate length as observed in the SAXS experiments. It has been reported that shortening of the rodlike aggregates drastically lowers the electric conductivity of DDAB microemulsion.24 The viscosity measurements also support shortening of the rods by AA-Na. PAANa does not influence the electric conductivity. No structural change upon PAA-Na addition is deduced from the SAXS, viscosity, and electric conductivity measurements. The presence of PAA-H causes some structural change in larger scale than that observable in SAXS, which is suggested by the viscosity measurements. The PAA-H increases the viscosity, which implies the formation of a larger scale structure than the polymer-free DDAB aggregates. If this structural change participates in the formation of a longer water channel than that formed in the polymer-free system, the electric conductivity should increase with adding PAA-H. However, the κ value does not increase upon the PAA-H addition. This will be discussed later. Parts a and b of Figure 6 show examples of the photon correlation function g1(t) in DDAB/PAA-Na and DDAB/ PAA-H systems, respectively. In the DDAB/PAA-Na system, g1(t) is approximately expressed as g1(t) ) APAA-Na exp(-ΓPAA-Nat) where APAA-Na and ΓPAA-Na, respectively, denote the amplitude and the decay constant in the photon

Figure 6. Results of dynamic light scattering: Cm ) 0.2 M and W0 ) 4. Cm/CDDAB ) 0.216 in PAA-Na/DDAB microemulsion, and 0.072 in PAA-H/DDAB microemulsion. (a) An example of photon correlation function of PAA-Na/DDAB microemulsion at scattering angle 90°. (b) An example of photon correlation function of PAA-H/DDAB microemulsion at scattering angle 90°. (c) Dependency of the decay constants on q2. 4 and dashed line, ΓPAA-Na; 3 and solid line, ΓPAA-H,f. (d) ΓPAA-H,s shown against q2. All the lines are the least-squares fits for the corresponding data.

correlation function. In the DDAB/PAA-H system, g1(t) is approximately expressed by

g1(t) ) APAA-H,f exp(-ΓPAA-H,ft) + APAA-H,s exp(-ΓPAA-H,st) (2) Here, ΓPAA-H,f, and ΓPAA-H,s are the decay constants for the fast and the slow decay modes, respectively. APAA-H,f and APAA-H,s are the corresponding amplitudes. The examples of the dependency of ΓPAA-Na, ΓPAA-H,f, and ΓPAA-H,s on q2 are shown in Figure 6c and d, where q denotes the scattering wavenumber defined as q ) (4πnS/λ) sin(2θ/2). Here, λ represents the wavelength of the incident light, 632.8 nm, and nS denotes the refractive index of cyclohexane. 2θ stands for the scattering angle. Since Γi(i ) PAA-Na, PAA-H,f, or PAA-H,s) is proportional to q2 as shown in Figure 6c and d, the relaxation modes are diffusive. Then, the apparent diffusion constants, DPAA-Na, DPAA-H,f, and DPAA-H,s, are obtained from Di ) Γi/q2. We introduce the characteristic lengths ξi by ξi ) kBT/ (6πηsolventDi). Here, kB and T represent the Boltzmann constant and the temperature. Figure 7 shows the dependency of ξi on Cm/CDDAB. The characteristic lengths from the initial decay of g1(t) in DDAB microemulsion without polymer ξDDAB, aqueous solution of PAA-H ξPAA-H,aq, and aqueous solution of PAA-Na ξPAA-Na,aq are also shown. ξPAA-H,f and ξPAA-Na are a little smaller than the ξDDAB. ξPAA-H,s is close to the ξPAA-H,aq. There is no clear dependency of the characteristic lengths on Cm/CDDAB. In polymer-free DDAB microemulsion, the ξDDAB is ∼40 nm, which is larger than the L value (∼10 nm). When CDDAB ) 0.2 M, the φ value is ∼0.11. As shown in Figures 4 and 5, the rodlike aggregates form the percolation clusters at this φ value. Taking into account the analogy

Polyelectrolyte and Ionic Surfactant Fluids

Figure 7. Characteristic lengths obtained from DLS: 4, PAANa/DDAB microemulsion; 3, PAA-H/DDAB microemulsion; 2, PAA-Na aqueous solution ξPAA-Na,aq; 1, PAA-H aqueous solution ξPAA-H,aq. CDDAB ) 0.2 M and W0 ) 4 in microemulsions. Cm ) 0.0144 M in aqueous solution, and the data are plotted at 0.0144/ 0.2 of the abscissa. The horizontal solid line is the characteristic length of polymer-free DDAB microemulsion ξDDAB. The lines are connecting the corresponding data.

Figure 8. Schematic view for the structure of PAA-Na/DDAB (top) and PAA-H/DDAB (bottom) microemulsions.

to polymer solutions,38 the ξDDAB evaluated from the DLS experiments is considered to be the characteristic size of the “mesh” of the network composed of the percolation clusters.38 The network structure is not dense because the φ value is not so high. The larger mesh size is responsible for the rather large ξDDAB value. The size 40 nm is too large to be measured by our SAXS. Thus, many short aggregates, which are not major constituents of the percolation clusters, are mainly observed by SAXS, resulting in L < ξDDAB. The fast mode alone is observed in the PAA-Nacontaining microemulsion. The two major constituents of the aggregates should contribute to the ξPAA-Na. One is the DDAB aggregate containing PAA-Na and another is the DDAB aggregate free from PAA-Na. The two characteristic sizes take similar values, as discussed later, resulting in the unimodal decay of g1(t). From all the experiments performed in this work, there is no sign of structural changes in the DDAB microemulsion upon PAANa addition. Thus, PAA-Na is considered to be dissolvable within the water pool of the cylindrical aggregates as shown in Figure 8. The slow mode expressed by the second term of the right-hand side of eq 2 appears in the presence of PAA-H. Figure 9 shows the APAA-H,s/APAA-H,f value at 2θ ) 90° in (38) de Gennes, P.-G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979.

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Figure 9. Ratio of the scattering amplitude between the slowand the fast-decay modes in PAA-H/DDAB microemulsions. Scattering angle is 90°. The curves are guides to the eye and extrapolated to (0, 0) because the slow decay mode does not appear in the absence of PAA-H.

the PAA-H/DDAB system. APAA-H,s/APAA-H,f becomes larger with increasing Cm/CDDAB. Thus, the slow mode of this system arises from the PAA-H in the microemulsion. The ξPAA-H,s is much larger than ξDDAB and nearly equal to ξPAA-H,aq. This indicates that the spatial extent of PAA-H in the microemulsions takes a size similar to that in the aqueous solution, which is also supported by the viscosity measurements (see Figure 3). Taking into account that the SAXS patterns are not influenced by the presence of PAA-H, the PAA-H probably connects the rodlike aggregates without changing their essential geometry, as shown in Figure 8. The connecting structure usually raises the electric conductivity. However, the polymer dissolved in the water pool of the aggregates significantly increases the viscosity of the internal aqueous phase, which makes the ionic species in the internal aqueous phase less mobile. This probably prevents the electric conductivity from rising. The ξPAA-H,f locates between ξDDAB and L. The ξPAA-H,f may reflect the characteristic size of DDAB aggregates free from PAA-H. The difference in structure between PAA-Na and PAA-H systems is probably due to the difference in the molecular weights of the polymers as discussed in the next section. Further Discussion The scaling theory of uncharged polymer chain gives the critical concentration Cm,c from the dilute to the semidilute regimes by N/(bNν)3 38 where b and N represent the segment length and the degree of polymerization, respectively. In the uncharged polymer with b ) 0.15 nm and N ) 330 corresponding to the “uncharged” PAA-Na, Cm,c is 4.8 M when using ν ) 0.6. Since this concentration is much larger than the present experimental range, the present concentration range is in the dilute regime if the PAA-Na chain behaves like an uncharged one. Thus, the large ξPAA-Na,aq value shown in Figure 7 cannot be explained by the scaling theory of uncharged polymers and is a characteristic of polyelectrolytes. The pH value of PAA-Na aqueous solutions was measured to be 9.2 at Cm ) 0.1 M, from which the degree of electrolytic dissociation R is almost unity for PAA-Na. Thus, the sodium ions of PAA-Na are dissociated in the aqueous solution. This supports the idea that the large ξPAA-Na,aq value reflects a characteristic of polyelectrolytes. The PAA-Na chain may take a rather extended rodlike conformation,39-42 which probably causes the large (39) Khokhlov, A. R. J. Phys. A 1980, 13, 979. (40) Raphael, E.; Joanny, J.-F.; Europhys. Lett. 1990, 13, 623. (41) Higgs, P. G.; Raphael, E. J. Phys. I 1991, 1, 1.

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ξPAA-Na,aq value, although the details are not clear: The present ξPAA-Na,aq may correspond to the “slow mode” observed in polyelectrolyte aqueous solution43-48 which has not been well understood. The long-time decay corresponding to ξPAA-Na,aq disappears for the PAA-Na in the DDAB microemulsions. This implies that the characteristic as polyelectrolytes is not observed in the presence of DDAB. Since polyacrylic anion PAA- and the didodecyldimethylammonium cation DDA+ attractively interact through electrostatic interaction, PAA- anions probably adsorb the surfactant cation DDA+ in the DDAB/PAA-Na microemulsion. Thus, the electrostatic repulsion between the anion segments in PAAis weakened by DDA+. The PAA-Na adsorbing DDA+ acts like an uncharged polymer, and the degree of the spatial extent becomes smaller than that in the bulk aqueous solution. The viscosity increases with increasing Cm in PAA-Na aqueous solution as shown in Figure 5, which is attributed to a rather extended conformation of PAA-.39-42 In DDAB microemulsion, however, the viscosity does not increase with PAA-Na addition. This is also due to the difference in the spatial extent of PAA-Na in both systems. Thus, we may discuss the PAA- chain adsorbing DDA+, i.e., PAA-Na in DDAB microemulsions, in terms of the theory for uncharged polymers. According to the scaling theory for an uncharged polymer,38 the length R| of the polymer confined within the cylinder whose diameter is Dd is calculated from

R| ) Nb(b/Dd)2/3

(3)

The length of the polymer with N ) 330 (molecular weight 30 000) is 10 nm (Dd ) 1.6 nm). This value is almost the same as the L value of DDAB aggregates. Thus, it is geometrically possible to confine PAA-Na within the aggregates. PAA-Na adsorbing DDA+ is confined within the aggregates as shown in Figure 8. In the aqueous solution of PAA-Na, a large number of the counterion Na+ is present, and they are possibly adsorbed to PAA- as DDA+’s are in the microemulsions. However, the experimental result of ξPAA-Na,aq . ξPAA-Na indicates that the DDA+ in the oil phase is adsorbed to PAA- more strongly than Na+ in the aqueous phase. The DDA+ molecules adsorbed to PAA- interact with each other by hydrophobic interaction, which may induce further adsorption of DDA+. If PAA-H in the microemulsion changes the rodlike structure of DDAB aggregates, the electric conductivity must significantly decrease compared with the polymerfree microemulsion.24 The electric conductivity of the PAAH/DDAB system is a little lower than the polymer-free microemulsion as shown in Figure 5. However, the difference is very small and the percolation thresholds for the electric conductivity of the PAA-H/DDAB and the polymer-free DDAB systems almost agree with each other. The SAXS patterns of DDAB/PAA-H also agree with that of the polymer-free DDAB as shown in Figure 1. If the rodlike structure changes to other smaller structures such as spheres, a change in the SAXS pattern in low scattering angle should be observed. Accordingly, the original DDAB structure is not changed significantly. PAA-H is concluded (42) Dobrynin, A.; Rubinstein, M.; Obukhov, S. P. Macromolecules 1996, 29, 2974. (43) Schmidt, M. Makromol. Chem., Rapid Commun. 1989, 10, 89. (44) Sedlak, M. Macromolecules 1993, 26, 1158. (45) Sedlak, M. J. Chem. Phys. 1984, 101, 10140. (46) Reed, W. F. Macromolecules 1994, 27, 873. (47) Sedlak, M. Macromolecules 1995, 28, 793. (48) Topp, A.; Belkoura, L.; Woermann, D. Macromolecules 1996, 29, 5392.

to be dissolvable in the microemulsion without altering the rodlike structure, and the PAA-H chain connects the rodlike aggregates, which are illustrated in Figure 8. The pH values of PAA-H aqueous solution was measured to be 2.7 at Cm ) 0.1 M, and then R ) 0.02. PAA-H is not a strong polyelectrolyte. Thus, the PAA-H in the microemulsions does not strongly adsorb DDA+. The spatial extent of the PAA-H in the microemulsions is similar to that in the aqueous solution, which is confirmed by the DLS results. The length of the PAA-H chain (b ) 0.15 nm and N ) 6300) in the cylindrical water pool with Dd ) 1.6 nm is calculated from eq 3 to be 200 nm. This is much longer than L of DDAB aggregates, and the PAA-H chain connects the DDAB aggregates. The difference in the molecular weight between PAA-Na and PAA-H plays the dominant role in the difference in the viscosity and the DLS results between both systems. However, we have neglected the effects of the polydispersity of the molecular weights and the aggregates lengths on the geometrical considerations. This may affect the detailed quantitative discussion for the sizes evaluated from the various kinds of experimental techniques. The energy A needed to confine an uncharged polymer within a cylinder of the diameter Dd is given by the scaling theory:38

A/kBT ) N(b/Dd)5/3

(4)

The average number of DDAB molecules to confine a polymer is πDdR|/Σ, where Σ represents the interfacial area occupied by a DDAB headgroup. Using eq 3, the energy for the confinement per DDAB molecule, Ec()A/[πDdR|/Σ]) is given by

Ec/kBT ) Σ/πDd2

(5)

Since Σ is reported to be ∼0.7 nm2,31 we obtain (Ec/kBT) Z 0.09. The free energy change due to the dissolution of two molecules of C12H24 from hydrocarbon solvents into water is 110 kJ/mol,49 which is equal to 44 kBT at 298.15 K. Since this value is in the same order of magnitude as the energy of the hydrophobic interaction of DDAB molecules constituting the aggregates, the Ec/kBT is probably much smaller than the energy for the hydrophobic interaction. Thus, the hydrophobic interaction between DDAB molecules is so strong that the DDAB aggregates are not destroyed by the polymer confinement. Consequently, the view illustrated in Figure 8 is energetically reasonable. In eq 4, the electrostatic interaction energy between PAA- and a charged wall of DDAB aggregates is not taken into account. Even in the absence of the polymers, most of the DDA+ constituting the aggregates probably adsorbs bromine anion. Thus, the replacement of Br- with PAAdoes not induce a drastic change in the electrostatic interaction energy between the surfactant molecules, and eq 5 may be used to estimate the free energy for the confinement of the polymers. Figure 8 is a schematic view of the structure of the present DDAB/polymer composite. The diameter Dd and the average length L of DDAB aggregates are 1.6 and 10 nm, respectively. PAA-Na and PAA-H are dissolved in the water pool of DDAB aggregates without changing the Dd. PAA-Na is elongated in the rodlike aggregates. The elongated length is ∼10 nm, which is similar to the average length of the rodlike structure of DDAB aggregates. PAA(49) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; John Wiley & Sons: New York, 1973.

Polyelectrolyte and Ionic Surfactant Fluids

Na is a typical polyelectrolyte and takes a rather extended conformation in the aqueous phase. This results in the large value of ξPAA-Na,aq in the aqueous phase. However, this large ξPAA-Na,aq value vanishes in the microemulsion due to the strong electrostatic interaction between DDA+ and PAA-. PAA-Na adsorbs DDA+ in the microemulsions, and it acts like an uncharged polymer. In the PAA-H system, Dd is not influenced by the presence of PAA-H. The length of the PAA-H in the cylindrical water pool with Dd ) 1.6 nm is much longer than L of the DDAB aggregates. Thus, the PAA-H chain connects the rodlike aggregates of DDAB. This is responsible for the large value of ξPAA-H,s and for the increase in ηsp,m with increasing C m. Conclusion The structure and properties of the systems composed of the cationic surfactant DDAB and anionic polymers PAA-H and PAA-Na were investigated in the organic phase. Despite the strong electrostatic interaction, both polymers were dissolved in the water pool of DDAB aggregates without changing the original rodlike structure of DDAB microemulsions. PAA-Na is a strong electrolyte,

Langmuir, Vol. 14, No. 17, 1998 4743

but it behaved as an uncharged polymer in DDAB microemulsions by adsorption of the DDAB cation. PAANa was confined within the water pool of DDAB aggregates because the length of the PAA-Na in the cylindrical water pool was similar to the average length of the DDAB aggregates. Thus, the characteristic sizes obtained from the SAXS and the DLS experiments and the viscosity were not affected by the presence of PAA-Na. The solubilization of PAA-H in the DDAB microemulsion did not cause a considerable structural change for the rodlike structure. However, the PAA-H-chain connects the rodlike aggregates composed of DDAB because the length of the PAA-H confined within the cylindrical water pool is much longer than the average length of the rodlike aggregates. This results in the presence of the slow-decay mode in the photon correlation function and an increase in the viscosity with adding the PAA-H. Acknowledgment. The authors gratefully acknowledge financial support from a Grant in Aid for Scientific Research, Ministry of Education, Science, Sports and Culture, Japan. LA971283K