A Nonionic Microemulsion with Adsorbing Polyelectrolyte - Langmuir

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A Nonionic Microemulsion with Adsorbing Polyelectrolyte H. Bagger-Jo¨rgensen*,† and U. Olsson Physical Chemistry 1, Center for Chemistry and Chemical Engineering, University of Lund, P. O. Box 124, S-221 00 Lund, Sweden

I. Iliopoulos Laboratoire de Physico-Chimie Macromole´ culaire, Universite´ P. et M. Curie, CNRS URA-278, ESPCI, 10 rue Vauquelin, F-75231 Paris cedex 05, France

K. Mortensen Physics Department, Risø National Laboratory, DK-4000 Roskilde, Denmark Received May 20, 1997. In Final Form: August 1, 1997X Structure and interactions were investigated when small amounts of a hydrophobically modified poly(sodium acrylate) (HMPA) were added to a droplet microemulsion-lamellar system comprising nonionic surfactant. As demonstrated by small angle neutron scattering and NMR self-diffusion the aggregate structure was unaffected by HMPA, accounting for a temperature shift in the phase boundaries. Viscoelastic gels are formed in the droplet microemulsion-HMPA mixtures above a certain polyelectrolyte concentration. HMPA is soluble in the lamellar phase at high dilution. Upon increasing the bilayer concentration a phase separation is induced where excess bilayers form a separate phase. The lamellar phase with HMPA collapses when adding salt, probably as a result of bridging. A viscous microemulsion solution can be transformed to a viscoelastic gel upon increasing the temperature. A dramatic change in surfactant aggregate structure is responsible for this spectacular effect.

1. Introduction Polymer-surfactant interaction has received considerable interest during recent years.1-16 Generally it appears that nonionic surfactants present a very low affinity toward polyelectrolytes,17 and association occurs mainly in cases where hydrophobic interactions6 between polymer and surfactant are operating. When the surfactant concentration is increased, the phase behavior also becomes dependent on the geometry of the surfactant film. For example, at low polymer concentrations both polyacrylate and poly(ethylene oxide) are soluble in a micellar † Present address: Astra Draco AB, P.O. Box 34, S-221 00 Lund, Sweden. X Abstract published in Advance ACS Abstracts, September 15, 1997.

(1) Nystro¨m, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994. (2) Zana, R.; Binana-Limbele´, W.; Kamenka, N.; Lindman, B. J. Phys. Chem. 1992, 96, 5461. (3) Goddard, E. D. Colloids Surf. 1986, 301. (4) Carlsson, A.; Karlstro¨m, G.; Lindman, B. Colloids Surf. 1990, 47, 147. (5) Brooks, J. T.; Cates, M. E. J. Chem. Phys. 1993, 99, 5467. (6) Iliopoulos, I.; Olsson, U. J. Phys. Chem. 1994, 98, 1500. (7) Singh, M.; Ober, R.; Kleman, M. J. Phys. Chem. 1993, 97, 11108. (8) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149. (9) Kabalnov, A.; Olsson, U.; Wennerstro¨m, H. Langmuir 1994, 10, 2159. (10) Kabalnov, A.; Olsson, U.; Thuresson, K.; Wennerstro¨m, H. Langmuir 1994, 10, 4509. (11) Bagger-Jo¨rgensen, H.; Olsson, U.; Iliopoulos, I. Langmuir 1995, 11, 1934. (12) Clegg, S. M.; Williams, P. A.; Warren, P.; Robb, I. D. Langmuir 1994, 10, 3390. (13) Eicke, H.-F.; Quellet, C.; Xu, G. Surf. Sci. Technol. 1988, 4, 111. (14) Holmberg, A.; Piculell, L.; Wessle´n, B. J. Phys. Chem. 1996, 100, 462. (15) Thalberg, K.; Lindman, B. In Surfactants in Solution; Mittal, K. L., Shah, D. O., Eds.; Plenum Press: New York, 1991; Vol. 11. (16) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B. Prog. Colloid Polym. Sci. 1992, 89, 118. (17) Lindman, B.; Thalberg, K. In Polymer-Surfactant Interactions; Goddard, E. D., Ananthapadmanabham, K. P., Eds.; CRC Press: Boca Raton, FL, 1992; p 203.

S0743-7463(97)00506-4 CCC: $14.00

phase, but none of them is soluble in a lamellar phase.11,18 An important observation is that the solubility in this phase may be significantly increased if the polymer can adsorb onto the surfactant film, e.g., for hydrophobically modified water soluble polymers,6 resulting in solubility of previous insoluble polymers in lamellar phases and in bicontinuous microemulsions.10,11,18,19 Besides an increased solubility, the hydrophobically modified polymers are also very competent as viscosity enhancers already at low polymer and surfactant concentrations. This has been observed in a number of systems, both in water16,20,21 and in oil13,14,22 continuous phases. The phenomenon has been interpreted as an interconnection of the surfactant micelles by the polymer, resulting in micellar gels. Also the physical cross-linking of vesicles, resulting in stiff gels, has been reported.23 The present study deals with effects on structure and interactions when adding a hydrophobically modified polyelectrolyte to an aqueous solution comprising the nonionic surfactant pentaethylene glycol dodecyl ether (C12E5) and oil (decane). The polyelectrolytes, which we denote HMPA, are hydrophobic modifications of poly(sodium acrylate), denoted PA, having a small fraction of hydrophobic side chains randomly grafted to the PA backbone. The C12E5/decane ratio is constant and equal to φs/φo ) 0.815, where φs and φo are the surfactant and oil volume fractions, respectively. The partial phase diagram of the C12E5/decane/water system, redrawn from (18) Bagger-Jo¨rgensen, H.; Coppola, L.; Thuresson, K.; Olsson, U.; Mortensen, K. Langmuir 1997, 13, 4204. (19) Rajagopalan, V.; Olsson, U.; Iliopoulos, I. Langmuir 1996, 12, 4378. (20) Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991,7, 617. (21) Gradzielski, M.; Rauscher, A.; Hoffmann, H. J. Phys. IV 1993, 3, 65. (22) Quellet, C.; Eicke, H.-F.; Xu, G.; Hauger, Y. Macromolecules 1990, 23, 3347. (23) Sarrazin-Cartalas, A.; Iliopoulos, I.; Audebert, R.; Olsson, U. Langmuir 1994, 10, 1421.

© 1997 American Chemical Society

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Figure 1. (a) Partial phase diagram of the system C12E5-water-decane (redrawn from ref 24), which we here refer to as the reference system. The C12E5/decane ratio φs/φo ) 0.815, where φs and φo denote the surfactant and oil volume fractions, respectively, is kept constant. The diagram is drawn as temperature versus total volume fraction surfactant plus oil, φ ) φs + φo. For the various phases the following notations are used: L1 is a liquid microemulsion phase similar to the normal micellar phase found in binary surfactant-water systems. LR is a lamellar liquid crystalline phase. L3 is a liquid phase having a multiply connected bilayer structure. (b) Addition of 0.2 wt % 3-C18 (wp(3-C18) ) 0.002) to the reference system, redrawn from ref 11. Note that the monophasic lamellar (LR) phase has a limited extension. At low φ a region with very viscous, strongly flow birefringent, and slightly bluish samples is detected, while at higher concentrations an equilibrium between two lamellar phases, LR′ + LR′′, is found. The previous horizontal phase boundaries are here strongly concentration dependent, especially at low φ. (c) Addition of wp(1-C18) ) 0.006 to the reference system, redrawn from ref 11. The phase behavior is similar to (b).

ref 24, is shown in Figure 1a for φ ) φs + φo < 0.35. Here we are mainly concerned with the microemulsion phase, denoted L1, and the lamellar liquid crystalline phase, denoted LR. The L1 phase is stable between 23.5 and 30 °C. Below 23.5 °C it phase separates with excess (almost) pure oil. Along the lower phase boundary the surfactant and oil form spherical droplets of low polydispersity. With increasing temperature there is a minor growth of the droplets and for φ > 0.15 there is also, within the L1 phase, a transition to a bicontinuous structure above approximately 27 °C.26,27 As a result of a decrease of the preferred curvature of the C12E5 monolayer film with increasing temperature, there is a transition to a lamellar phase, where planar bilayers are stacked with one-dimensional periodic order, around 30 °C. Small additions (less than 1%) of HMPA to the surfactant system have significant effects on the phase diagram.11 As an illustration we show in parts b and c of Figure 1 the phase diagrams when 0.2 wt % 3-C18 and 0.6 wt % 1-C18, respectively, have been added to the surfactant system, where the notations 3-C18 and 1-C18 correspond to PA where octadecyl (C18) chains have been grafted to 3/100 and 1/100 of the monomer units, respectively. Several interesting features can be observed from the phase diagrams. (i) The HMPA is soluble in all the phases, in contrast to the unmodified PA, which, for example, is insoluble in the lamellar phase. (ii) Phase boundaries are strongly shifted to higher temperatures (24) Olsson, U.; Schurtenberger, P. Langmuir 1993, 9, 3389. (25) Bagger-Jo¨rgensen, H.; Olsson, U.; Mortensen, K. Langmuir 1997, 13, 1413. (26) Leaver, M. S.; Olsson, U.; Wennerstro¨m, H.; Strey, R. J. Phys. II 1994, 515. (27) Leaver, M.; Furo´, I.; Olsson, U. Langmuir 1995, 11, 1524.

at lower φ in the presence of HMPA. (iii) Adding HMPA to the lamellar phase at higher φ induces a phase separation with two lamellar phases in equilibrium. In the present paper we have investigated some physical properties of the mixed system. Using small angle neutron scattering (SANS) and NMR self-diffusion measurements, we first address the question if the addition of polyelectrolyte induces a structural change in the microemulsion. These techniques also probe the interactions between HMPA and microemulsion droplets. Using small angle X-ray scattering (SAXS) we have also investigated the equilibrium between the two lamellar phases at high bilayer concentration and the contraction of a HMPAcontaining lamellar phase upon the addition of salt. Finally, we also present some preliminary results on the rheology of the microemulsion phase and a gel forming at lower φ and higher temperature in the region denoted LR+ in the phase diagrams. 2. Materials and Methods Materials. C12E5 was obtained from Nikko Chemicals Co., Ltd., Tokyo, decane (>99%) was obtained from Sigma, and D2O (99.8% isotopic purity) was obtained from Norsk Hydro. All chemicals were used without further purification. Poly(acrylic acid) (PAA) was purchased from Polysciences. The average molecular weight is, according to the supplier, 1.5 × 105. The poly(acrylic acid) was hydrophobically modified by adding octadecylamine in an aprotic solvent, 1-methyl-2-pyrrolidone (MPD), in the presence of a coupling agent, N,N′ -dicyclohexylcarbodiimide (CDI). The experimental details are described elsewhere.28 The modified polyelectrolytes, containing randomly distributed octadecyl (C18) side chains, were obtained in their (28) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988, 20, 577.

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sodium salt form and have the same polymerization degree as the precursor polyelectrolyte. From measuring the pH-value of modified and unmodified PA in water solution (pH ≈ 8-9), we conclude that protonation is negligible; i.e., the polyelectrolyte is fully charged. Sample Preparation. Samples were prepared by weighing the desired amounts into test tubes, which were immediately sealed. Surfactant and oil concentrations are denoted by their respective volume fractions, φs and φo, respectively, and the total surfactant plus oil volume fraction is denoted φ()φs + φo). For the conversion from weight to volume fractions the following densities (g/cm3) have been used: 0.967 (C12E5); 0.73 (decane); 1.105 (D2O); 0.998 (H2O). The effect of the added polyelectrolyte on the solvent density has been neglected. The polyelectrolyte concentration is expressed as weight fraction, wp, since its density is not accurately known. Rheology. All samples were prepared at least 2 days before use and were kept at temperatures between 25 and 40 °C in order to equilibrate in the homogeneous L1 phase. A controlledstress rheometer, Carri-Med CSL100, equipped with a cone and plate geometry, was used for the rheological measurements. To minimize solvent evaporation, a solvent trap filled with water was used. Temperature was controlled within (0.1 °C. The shear storage, G′, and shear loss, G′′, moduli were, unless otherwise stated, recorded in the linear viscoelasticity range, where G′ and G′′ are independent of the strain amplitude. In contrast to the phase diagram determinations and NMR, SANS, and SAXS measurements, which were performed on samples with D2O as solvent, the samples used in the rheological measurements were prepared with protonated water of Millipore quality. Replacing D2O with H2O shifts the phase boundaries approximately 2 °C upward.29 NMR Self-Diffusion. The proton self-diffusion studies were performed at 100 MHz on a modified Jeol FX-100 FT NMR spectrometer equipped with a home-built pulsed field gradient. The temperature was controlled by a thermostated air flow with a stability better than (0.5 °C and measured by a calibrated copper-constantan thermocouple. The pulsed gradient spinecho (PGSE) sequence for measuring self-diffusion coefficients is based on the basic spin-echo sequence where a 90° radiofrequency (rf) pulse at time t ) 0, followed by a 180° rf-pulse at t ) τ gives rise to a refocusing of the transverse magnetization at t ) 2τ of magnitude I(2τ) ) I(0) exp{-2τ/T2}. Here I(0) is the initial magnetization and T2 is the transverse relaxation time (assuming exponential relaxation). In the PGSE sequence two field-gradient pulses are placed on either side of the 180° rf pulse with duration δ and separation ∆. Translational diffusion in between and during the field-gradient pulses results in an incomplete refocusing and the echo intensity is given by30

I ) I0 exp{-2τ/T2 + kD}

(1)

where k ) γ2G2δ2(∆ - δ/3). Here γ is the magnetogyric ratio, G is the magnitude of the magnetic field gradient, and D is the self-diffusion coefficient. The echo intensity is measured in the Fourier transformed spectrum of the second half of the echo, where individual resonances (corresponding to C12E5, decane, and water) can be resolved. Self-diffusion experiments were carried out by following the echo decay as a function of δ, keeping ∆ ) τ ≈ 0.1 s fixed. The receiver coil has a slightly different characteristic in the presence of the field gradients, and hence the initial magnetization, I(0), was treated as a fitting parameter. Small Angle X-ray Scattering (SAXS). SAXS experiments were performed on a Kratky compact small angle system equipped with a position sensitive detector with 1024 channels. The sample to detector distance is 277 mm and the wavelength 1.54 Å, allowing measurement down to a scattering vector q ≈ 0.01 Å-1. The slit-smeared spectra were desmeared by using the direct method of beam height correction.31 Small Angle Neutron Scattering (SANS). SANS experiments were performed on the small angle instrument at Risø National Laboratory. The raw data were treated according to standard procedures (masking and radial averaging) and were (29) Leaver, M. S.; Olsson, U.; Wennerstro¨m, H.; Strey, R.; Wurz, U. J. Chem. Soc., Faraday Trans. 1994, 91, 4269. (30) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1. (31) Singh, M. A.; Ghosh, S. S., Jr. J. Appl. Crystallogr. 1993, 26, 787.

finally put on an absolute scale by dividing the spectra with the scattering of H2O, taking the relevant transmission factors into account. At high q-values the scattered intensity approached a constant value, which was mostly due to incoherent scattering from hydrogen. The coherent part was obtained by subtracting the incoherent background term from the total intensity.

3. Length Scales Before the experimental results are presented and discussed, it is helpful to consider the various length scales involved in these complex mixtures of polyelectrolyte and surfactant aggregates. Surfactant Aggregates. A useful property of the C12E5 surfactant is that it has an essentially invariant and wellknown area, as, per molecule at the polar-apolar interface. This quantity is often expressed in terms of the surfactant length ls ≡ vs/as ) 15 Å,25,32,33 where vs ) 702 Å is the surfactant volume.34 The polar-apolar interface where the area is found to be curvature independent corresponds to the interface between the alkyl and pentaethylene oxide blocks of the surfactant. The polar and apolar blocks of the surfactant have essentially equal volumes. For an oil-swollen micelle, the radius of the hydrocarbon core, containing oil and the surfactant alkyl chains, is thus given by

Rhc ) 3φhcls/φs

(2)

where φhc ) φo+ 0.5φs is the hydrocarbon or apolar volume fraction. φo and φs are the oil and surfactant volume fractions, respectively. In the present system, φs/φo ) 0.815 (φhc ) 0.775φ, where φ ) φo + φs) and Rhc ) 75 Å. The droplet hydrocarbon core is covered by “end-grafted” pentaethylene oxide chains, and the droplets are further characterized by a hydrodynamic radius RH ) 94 Å and a hard sphere radius RHS ) 86 Å.24 The structural length scale of a homogeneous droplet dispersion can be estimated from assuming a body-centered cubic (bcc) lattice. The nearest neighbor center of mass separation is then given by

ξd )

31/2 2 2 N/V

1/3

( )

(3)

where N/V is the number density. With the present droplet size we obtain approximately

ξd ≈

140 Å φ1/3

(4)

The surfactant and oil may also form bilayers in the lamellar and L3 phases. The bilayer hydrocarbon thickness is given by

dhc ) 2φhcls/φs

(5)

which in the present system corresponds to dhc ) 52 Å. The bilayer repeat distance in the lamellar bilayer stack can for example be obtained from

d)

dhc 2ls 67 ) ≈ Å φhc φs φ

(6)

Polyelectrolyte. The typical structure of the modified sodium polyacrylate is illustrated in Figure 2. We denote the pure sodium polyacrylate PA and the modified ones (32) Rajagopalan, V.; Bagger-Jo¨rgensen, H.; Fukuda, K.; Olsson, U.; Jo¨nsson, B. Langmuir 1996, 12, 2939. (33) Strey, R.; Schoma¨cker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86, 2253. (34) Olsson, U.; Wu¨rz, U.; Strey, R. J. Phys. Chem. 1993, 97, 4535.

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Figure 2. (a) Typical structure of hydrophobically modified poly(sodium acrylate), denoted HMPA. x is the degree of modification, in monomer percent, by C18-tails. (b) Schematic illustration of the HMPA. The negatively charged, hydrophilic backbone has hydrophobic C18-chains randomly attached. 〈l〉 is the average curve linear repeat distance between two adjacent hydrocarbon tails.

1-C18 and 3-C18, respectively. The numbers indicate the monomer percentage of hydrophobic side chains and in our case are 1 and 3%, respectively. The distribution of the alkyl groups along the PA chain is random.35 The degree of polymerization is approximately 2000, giving a contour length, L ≈ 5000 Å, assuming a monomer length of 2.5 Å. The 1-C18 and 3-C18 polyelectrolytes contain approximately 20 and 60 alkyl chains, respectively. The average curve linear repeat distance, 〈l〉 ≈ 250 Å for 1-C18 and 80 Å for 3-C18. The effective size, e.g., radius of gyration, Rg, of polyelectrolytes decreases with increasing concentration as the electrostatics get screened and is therefore not easy to quantify. Additionally, the size of the HMPA might be smaller than that of the unmodified PA, as indicated by measurements of the osmotic pressure.36 In dilute solutions a relevant length scale is the average center of mass separation, ξp. Assuming again a bcc packing we obtain

ξp ≈

66 Å φp1/3

(7)

where we have used a density of 1.45 g/mL.37 The overlap concentration φ* then corresponds to ξp ≈ 2Rg. A common polyelectrolyte concentration in this study was 0.2 wt % corresponding to φp ) 1.4 × 10-3, for which we obtain ξp ≈ 600 Å. Hence, if Rg is smaller than ≈300 Å, the polymer solution is dilute, whereas if it is larger, the solution is semidilute. 4. The Microemulsion (L1) Phase Invariant Droplet Size and Shape. An important question in the present system is whether the addition of HMPA affects the microemulsion structure. The lower phase boundary of the microemulsion phase (L1/(L1 + O)) has a particular significance since it corresponds to the so-called emulsification failure boundary (EFB) discussed by Safran et al.38 Here the microstructure consists of oilswollen spherical micelles of low polydispersity as has also been confirmed in the present microemulsion up to high volume fractions.24-26 However, the EFB temperature, which is essentially independent of φ in the absence of polyelectrolyte, shifts strongly to higher temperatures (35) Magny, B.; Lafuma, F.; Iliopoulos, I. Polymer 1992, 33, 3151. (36) Bagger-Jo¨rgensen, H. Thesis, University of Lund, 1997. (37) L’Alloret, F. thesis l’Universite´ de Paris 6, 1996. (38) Turkevich, L. A.; Safran, S. A.; Pincus, P. A. In Surfactants in Solution; Mittal, K. L., Bothorel, P., Eds.; Plenum Press: New York, 1986; Vol. 6; p 1177.

Figure 3. Neutron scattering spectrum from a sample containing φ ) 0.078 and wp(1-C18) ) 0.01 (O) at 26 °C (close to EFB), drawn as the absolute intensity divided by the volume fraction, I/φ, versus the scattering vector, q. For comparison, the experimental scattering for a sample without HMPA, with φ ) 0.065, at 24 °C is also shown in the figure (0). The solid line is a fit of a spherical form factor to the experimental intensity, yielding a mean micellar hydrocarbon radius of 75 Å, a shell thickness of 15 Å, and a relative polydispersity of 16%. The mismatch at low q-values is due to intermicellar interaction (i.e., the structure factor S(q) * 1).

at lower φ upon addition of HMPA (Figure 1). This effect is in close analogy with addition of ionic surfactant and is an electrostatic effect caused by the adsorption of HMPA to the surfactant film.32 The effect of HMPA on the microemulsion droplets along the EFB was investigated by small angle neutron scattering (SANS), a powerful technique for measuring the size and shape of colloidal particles. Due to its relatively low concentration ( 0.03 Å-1) demonstrating that the microemulsion droplets remains spherical with the same size (Rhc ) 75 Å) and polydispersity (relative standard deviation 16%) as in the absence of HMPA. The situation is illustrated in Figure 3 where we present the scattering from a sample with φ ) 0.078 and wp(1-C18) ) 0.01, where wp(1-C18) denotes the weight fraction of 1-C18. The scattering, normalized with the droplet volume fraction, is compared with the scattering from a sample without HMPA and φ ) 0.065. The samples were prepared with heavy water (D2O) and a mixture of perdeuterated and normal decane, yielding the same scattering length density as D2O. The only coherent scattering is therefore that from the surfactant film. The two curves coincide at higher q. At lower q, on the other hand, the scattering from the sample with HMPA is significantly lower than that from the sample without HMPA. The decrease in scattered intensity when adding HMPA shows that the polyelectrolyte induces an effective long range repulsion between the droplets, resulting in a slightly more ordered arrangement. The invariance of the droplet size and shape is further illustrated in Figure 4 where the normalized high q scattering from two samples, φ ) 0.026 and φ ) 0.065, respectively, each containing wp(3-C18) ) 0.002, are compared with that from a sample with φ ) 0.026 and no HMPA. Here the samples were made with D2O but with normal protonated decane only, and the particle form factor is therefore different compared to the case of Figure

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Figure 4. Intensity normalized by volume fraction surfactant and oil, I/φ, versus q for three different samples along EFB: φ ) 0.026 and wp ) 0 at 24 °C (O), φ ) 0.026 and wp(3-C18) ) 0.002 at 35 °C (0) and φ ) 0.065 and wp(3-C18) ) 0.002 at 25 °C (4). All points fall on each other, showing that the micellar microstructure is essentially the same in all samples. Due to the limited q-range, the different intermicellar interaction upon HMPA addition is not detected in this experiment.

Figure 5. Self-diffusion coefficients of surfactant, Ds (b), and oil, Do (0), along EFB with wp(3-C18) ) 0.002, drawn versus φ. The slightly different values of Ds and Do at lower concentrations are within experimental uncertainty. For comparison the diffusion coefficients in the pure microemulsion are shown (2); here Ds and Do are almost identical and are not shown separately. The micellar diffusion coefficient for one sample with φ ) 0.065 and wp(PA) ) 0.002 is also shown in the figure ([).

3. The normalized scattering at high q is also here identical, confirming the invariant droplet size and shape. Trapping of the Microemulsion Droplets. Another useful tool for investigating microemulsion structure is NMR self-diffusion.39,40 Besides being essentially the only experimental method which can prove bicontinuity and accurately probe the droplet-to-bicontinuous structural transition, it can also be used to investigate droplet size and interactions via their diffusion coefficient. In the present study NMR self-diffusion measurements were applied to investigate the interaction between the microemulsion droplets and the HMPA. For this purpose the self-diffusion coefficients of the surfactant, Ds, and oil, Do, were measured as a function of φ in a system with wp(3-C18) ) 0.002. The results are presented in Figure 5, where we compare with the corresponding micellar selfdiffusion coefficients in the absence of polymeric additive24 (Ds ) Do and the value corresponds to the diffusion coefficient of the micelles, Dmic). In the absence of HMPA the micellar diffusion coefficient decreases monotonically with increasing φ from the value (39) Lindman, B.; Olsson, U. Ber. Bunsenges. Phys. Chem. 1996, 100, 344. (40) So¨derman, O.; Stilbs, P. Prog. Nucl. Magn. Reson. 1994, 26, 445.

Bagger-Jo¨ rgensen et al.

Figure 6. Same data as in Figure 5 but now drawn as the micellar diffusion (taken as the mean value of Ds and Do) in the 3-C18 system, divided by the micellar diffusion in the pure microemulsion, yielding the relative micellar mobility F ) DmicHMPA/Dmic (O). The amount of free micelles, Pb, calculated according to eq 11 is shown as (9). The lines are just guides for the eye.

Dmic0 ) 2 × 10-11 m2 s-1 at high dilution. The concentration dependence arises from hard sphere excluded volume interactions. In the system with HMPA we first note that Ds ) Do, which confirms that the surfactant aggregates consist of discrete oil-swollen micelles. Ds ) Do therefore corresponds to a micellar diffusion coefficient, Dmic, also in this case. The micellar diffusion coefficient decreases upon addition of HMPA. At the lowest concentration, φ ) 0.026, Dmic is an order of magnitude lower in the presence of HMPA compared to without. The difference, however, decreases with increasing φ as illustrated further in Figure 6 where we have plotted the ratio, F, between the micellar diffusion coefficient with and without HMPA as a function of φ. The ratio levels off at a value of ≈0.5 at higher φ. Since the SANS data showed that the micellar size remains constant upon addition of the HMPA, the ratio, F, therefore has to be a measure of the interaction between HMPA and the micelles. In fact, the low value ( Ds as the microstructure gradually transforms into a bicontinuous network. In the system with HMPA the minimum in diffusion coefficients is deeper, due to the trapping of the droplets. However Ds and Do become the same in the two systems for φ > 0.3, as expected when the structure is bicontinuous. This observation is im(42) Olsson, U.; Schurtenberger, P. Prog. Colloid Polym. Sci. 1997, 104, 157.

Figure 8. Plot of the storage modulus, G′, (filled symbols), and the loss modulus, G′′ (open symbols), versus frequency in for two samples at the lower phase boundary. In (a) φ ) 0.06 was constant with wp(3-C18) ) 0.002 (circles) and wp(3-C18) ) 0.01 (squares). In (b) φ ) 0.23 was constant and had the same HMPA concentrations as in (a).

portant since it shows that HMPA is soluble in a bicontinuous structure at relatively high surfactant concentrations, which is not the case for the lamellar structure (see section 5 below). Rheology. An important application of the general material hydrophobically (hydrophilically) modified water soluble (oil soluble) polymers is for rheology control. If the grafted hydrophobic (hydrophilic) groups are long enough, the “stickers” of the modified polymers may associate with or adsorb onto particles like micelles. If “stickers” from more than one polymer molecule are attached to the same micelle, the micelle acts as a physical cross-link. Above a certain polymer concentration and within a certain concentration range of micelles, an infinite network can be obtained where the system behaves as a viscoelastic physical gel.6,16,43 As a preliminary rheological study we have here investigated two concentrations of 3-C18 at some different droplet concentrations along the lower boundary of the L1 phase, where the droplets are spherical. At wp(3-C18) ) 0.002 the samples behaves essentially as viscous solutions within the whole range of droplet concentrations studied. In parts a and b of Figure 8 are shown the frequency dependence of the storage (G′) and loss modulus (G′′), obtained from oscillation measurements, for two samples with wp(3-C18) ) 0.002 and φ ) 0.06 and 0.23, respectively. As is seen, G′′ . G′ in almost the whole frequency range. From the frequency dependence of G′′ at lower frequencies we can obtain a value for the low shear (Newtonian) viscosity, η0. The values are η0 ) 70 and 35 mPas for φ ) 0.06 and 0.23, respectively. At φ ) 0.02 and the same (43) Biggs, S.; Selb, J.; Candau, F. Langmuir 1992, 8, 838.

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Table 1. Calculated Number of HMPA Molecules per Micelle, NHMPA/Nmic, and Number of Octadecyl Chains per Micelle, NC18/Nmic, as function of O and wp(3-C18) φ

wp(3-C18)

NHMPA/Nmic

NC18/Nmic

0.024 0.06 0.12 0.23 0.024 0.06 0.12 0.23

0.002 0.002 0.002 0.002 0.01 0.01 0.01 0.01

0.55 0.22 0.11 0.06 2.7 1.1 0.55 0.27

35 14 7 3.5 173 69 35 17

HMPA concentration, η0 ≈ 20 mPa s. For comparison a solution of 3-C18 at this concentration has a viscosity of 2 mPa s,44 and solutions of the microemulsion droplets alone have a viscosity of 1.1 mPa s at φ ) 0.06 and 2.5 mPa s at φ ) 0.23.45 The same droplet concentrations were also investigated at the HMPA concentration wp(3-C18) ) 0.01. With this concentration of 3-C18 the effect of different surfactants has been studied previously.6,16,20,23,46 The results from oscillation measurements are shown together with those from the lower HMPA concentration in Figure 8. At this HMPA concentration, the sample at φ ) 0.06 acts as a physical gel with G′ > G′′ in the whole frequency range. At φ ) 0.23, on the other hand, we find G′′ > G′ in the whole frequency range, although they approach each other at higher frequencies. In mixtures of micelles and HMPA, the rheology depends on the relative number of micelles to polyelectrolyte chains.6,16,43 These numbers are presented in Table 1. There is an optimum ratio of micelles to polyelectrolyte chains for forming the network. At low micellar concentration there is an insufficient cross-linking density, while at high micellar concentration (excess of micelles) the hydrophobic polyelectrolyte side chains are saturated with micelles and there is no cross-link formation. A network formation of course also requires that the polyelectrolyte concentration is sufficiently high so that there is overlap between different polyelectrolytes. This requirement is probably not met in the wp(3-C18) ) 0.002 case. From eq 7 we obtained ξp ≈ 600 Å, indicating that the effective polyelectrolyte size cannot be much larger than this number. At wp(3-C18) ) 0.01 on the other hand we obtain a physical gel with φ ) 0.06, corresponding to ≈1 micelle per HMPA, indicating that a sufficient polyelectrolyte overlap has been obtained. When the droplet concentration isincreased to φ ) 0.23, corresponding to ≈4 micelles per HMPA, the hydrophobic side chains have been saturated with micelles and there is no longer an infinite network. 5. The Lr Phase HMPA can be solubilized in the lamellar phase, in contrast to the unmodified PA.11 Solubilizing HMPA in a dilute lamellar phase results in a dramatic reduction of the turbidity. This is presumably due to the introduction of an electrostatic double layer interaction resulting from HMPA adsorbing onto the bilayer surface. Two Lamellar Phases in Equilibrium. It has been observed that incorporating a non-adsorbing polymer into a lamellar phase in some cases leads to a coexistence between two lamellar phases with different periodicity.47,48 (44) Wang, T. K.; Iliopoulos, I.; Audebert, R. In Water-Soluble Polymers; Shalaby, S. W., McCormick, C. L., Butler, G. B., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991; p 218. (45) Leaver, M. S.; Olsson, U. Langmuir 1994, 10, 3449. (46) Loyen, K.; Iliopoulos, I.; Olsson, U.; Audebert, R. Prog. Colloid Polym. Sci. 1995, 98, 42. (47) Ficheux, M.-F.; Bellocq, A.-M.; Nallet, F. J. Phys. II 1995, 5, 823.

Figure 9. Desmeared SAXS scattering pattern from a twophase sample in the LR′ + LR′′ region, containing φ ) 0.445 and wp(3-C18) ) 0.002. Around q ) 0.024 Å-1 the first-order diffraction peak from the dilute, HMPA containing a LR′ phase is detected, and at exactly twice the value q ) 0.048 Å-1 a distinct second-order peak is seen. At q ) 0.054 Å-1 the firstorder peak from the more concentrated, HMPA free LR′′ phase is seen, but no second-order peak.

Also in the presence of HMPA an equilibrium between two lamellar phases is obtained at higher φ (Figure 1b,c). In a previous study it was shown that the HMPA was predominantly dissolved in one of the phases, which also had the highest water content.11 Moreover, the surfactantto-oil ratio was found to be similar in the coexisting phases and thus, the surfactant and oil can be approximated as a pseudocomponent. In this study we have investigated this equilibrium further by performing SAXS measurements as a function of φ in the system with wp(3-C18) ) 0.002. Macroscopic phase separation of the two LR phases is a very lengthy process (of the order of weeks in a table centrifuge) and was therefore not performed in the present study. The slow separation kinetics however makes it possible to perform X-ray experiments without macroscopic phase separation. When a sample is brought into the two-phase region (LR′ +LR′′), a microscopic phase separation almost immediately takes place, resulting in a homogeneously turbid solution. This emulsion consists of microdomains of each phase. Since the microdomains are much larger than macromolecular dimensions, the X-ray scattering pattern is a superposition of the scattering curves from each of the two phases. In our experiments the samples were loaded into quartz capillaries in the homogeneous L1 phase. The capillaries where then placed in the instrument and heated up to the desired temperature, where measurements were performed. Complete microscopic phase separation took place in a few minutes, and after this time the scattering pattern and intensity were independent of time. In Figure 9 we present the desmeared SAXS pattern obtained at φ ) 0.445 in the two-phase region. From the dilute lamellar phase (containing HMPA) we observe a first- and a second-order correlation peak at scattering vectors q ) 0.024 and 0.048 Å-1, respectively. The third peak in Figure 9, at q ) 0.054 Å-1, corresponds to the first-order reflection from the concentrated (HMPA free) lamellar phase. No higher order reflection is observed from this phase even though the bilayer concentration is approximately three times higher. The (one-dimensional) structure factor, S(q), of lamellar phases depends on inter bilayer interactions.49,50 With a (48) Ligoure, C.; Bouglet, G.; Porte, G. Phys. Rev. Lett. 1993, 71, 3600. (49) Roux, D.; Safinya, C. R. J. Phys. (Paris) 1988, 49, 307. (50) Nallet, F.; Roux, D.; Milner, S. T. J. Phys. (Paris) 1990, 51, 2333.

Microemulsion with Adsorbing Polyelectrolyte

Figure 10. Bilayer repeat distances in the LR phases, drawn versus total φ. (b) refers to the HMPA containing LR′ phase, and (0) refers to the HMPA-free LR′′ phase. The repeat distance for the homogeneous LR phase at φ ) 0.23 is indicated as ([). The dashed line shows the expected repeat distances in the HMPA-free system, obeying one-dimensional swelling.29

strong, for example electrostatic, repulsive interaction S(0) is low, the correlation peaks are sharp, and several orders can be observed. In the case of weak repulsive interactions such as the undulation force, S(0) is relatively high and usually only a broad first-order peak is observed which becomes broader the lower the bilayer concentration. The fact that we observe two sharp correlation peaks from the dilute phase containing HMPA indicate strong interactions which we attribute to the confinement of the polyelectrolyte layer. From the more concentrated lamellar phase only a single correlation peak is observed, which is consistent with only a low (or zero) HMPA concentration in this phase. Here the repulsive force is expected to be essentially due to undulations. In Figure 10 we present how the average repeat distance, d, in the two coexisting lamellar phases varies with φ. At φ ) 0.23 the system is a single phase and only one repeat distance, d ) 320 Å, is observed. At φ ) 0.28 we observe two repeat distances, d′ ) 310 Å and d′′ ) 130 Å. Both d′ and d′′ stay almost constant (only a weak decrease is measured) as φ is increased further, indicating that the tie lines in this area are roughly parallel to the dilution line. Collapse upon Addition of Salt. The electrostatic interaction can be screened by the addition of salt. In the previous study11 it was observed that for a NaCl concentration of 100 mM, the lamellar phase containing HMPA was nonswelling. Instead, a concentrated lamellar phase containing the HMPA was found to coexist with excess brine. Here we have investigated this collapse in more detail, by following the equilibrium swelling of the HMPA containing lamellar phase with SAXS as a function of the concentration of NaCl. The HMPA is 3-C18 and wp(3C18) ) 0.002. The results are shown in Figure 11, where we have plotted the water layer thickness, dw ) d - db, where db ) 67 Å is the bilayer thickness, as a function of the aqueous NaCl concentration. The experiments were performed with a constant total bilayer concentration of φ ) 0.12. For NaCl concentrations 20 mM), there is only a weak decrease in dw as the NaCl concentration is increased. The swelling limit of the lamellar phase corresponds to a balance of repulsive and attractive interactions. Model-

Langmuir, Vol. 13, No. 22, 1997 5827

Figure 11. Water layer thickness, dw, in the LR phases containing φ ) 0.117 and wp(3-C18) ) 0.002, as a function of electrolyte (NaCl) concentration. The water layer thickness decreases dramatically as the salt concentration is increased. The solid line is the fitted maximum water layer thickness, dwmax, calculated according to eq 12 and 14, yielding Heff/ae2 ) 2.4 × 1012 J m-4. The dashed line illustrates the maximum swelling dictated by the bilayer concentration.

ing the adsorbed HMPA as an effective (weak) charge density of the bilayers, e/ae, where e is the elementary charge and ae is the effective area per charge, the repulsive electrostatic pressure can be approximated by51

Πel ≈

2e2 exp{dw/λD} 0ae2

(12)

Here,  is the dielectric constant, 0 the permittivity of vacuum, and λD is the Debye length. The van der Waals (vdW) interaction in multilayer systems has been analyzed by Ninham and Parsegian.52 In the limit of low φ (thin bilayers) and no retardation effects, the vdW free energy density can be approximated with

gvdW ) -

5Heff 12πδ3

φ5

(13)

The corresponding pressure is obtained from Π ) φ(∂g/∂φ) - g, yielding

ΠvdW ) -

5Heff 3πδ3

φ5

(14)

The swelling limit, dwmax, obtained from the balance ∏el + ∏vdW ) 0, may now be compared with the experimental data using Heff/ae2 as an adjustable parameter. The solid line in Figure 11, which shows a good agreement with the data, corresponds to Heff/ae2 ) 2.4 × 1012 J m-4. The (nonretarded) Hamaker constant for oil across water is approximately 6 × 10-21 J. Taking this value for Heff, we obtain ae ) 5000 Å2. This charge density corresponds to one effective charge for every 16 monomer units, which is close to two effective charges per hydrocarbon side chain. At shorter separations we expect a strong steric repulsive interaction associated with the confined polyelectrolyte layer. This regime, which here corresponds to the collapsed state, starts at 20 mM NaCl (dw ) 180 Å). The relatively weak decrease of dw with salt concentration >20 mM may be understood as a salt dependence of the polyelectrolyte flexibility. (51) Israelachvili, J. Intermolecular & Surface Forces; Academic Press: London, 1991. (52) Ninham, B. W.; Parsegian, V. A. J. Chem. Phys. 1970, 53, 3398.

5828 Langmuir, Vol. 13, No. 22, 1997

Above we have considered a balance between electrostatic and van der Waals forces only. In a previous study, where the bilayers were made charged by adsorbing small amounts of ionic surfactant, the addition of salt did not lead to a collapse of the lamellar phase as observed here. Instead, when the electrostatic repulsion was screened with brine, the system remained swollen by going back to the undulation force as the dominating long range repulsion. There are two possible explanations why this does not occur in the present system. One is that the adsorbed polyelectrolyte layer gives a higher rigidity of the surfactant film, thereby reducing the undulation force which cannot overcome the attractive van der Waals interaction. This was implicitly assumed in the analysis above. The other possibility is that we are dealing with a “bridging-flocculation”.53 (A third possibility is of course a combination of the two.) An observation which speaks against a significant increase in the surfactant film rigidity is that the lamellar phase may still swell with oil. This indicates that it is actually polymer bridging which is responsible for the collapse of the lamellar phase with salt. With a bridging attraction included we still expect a similar strong salt dependence; however, the effective charge density obtained above should be seen as a lower limit. An additional observation indicating that bridge formation may occur in the lamellar phase is the formation of gels in the LR+ region of the phase diagrams. This will be discussed in the following section. 6. Thermal Gelation In nonionic surfactant systems which have a dilute lamellar phase, one often finds a region between the micellar and lamellar phases where dilute samples are macroscopically homogeneous and scatter light strongly.54,55 The properties of the samples, however, depend on how it has been prepared indicating nonequilibrium. This region in the phase diagram, which often is denoted LR+, is also found in the presence of HMPA and is indicated in the phase diagrams of parts b and c of Figure 1. Most likely, it is related to the so-called onion phases discussed by Simons and Cates,56 and self-diffusion54 and freeze fracture electron microscopy studies57-59 have indeed demonstrated the presence of vesicles.54,57-59 Vesicles and onions have also been reported in surfactant systems with weakly charged surfactant bilayers.60,61 Vesicles and onions may be favored when the spontaneous curvature of the surfactant film is weakly toward oil.56,62 The true equilibrium state of the LR+ region is however still unclear. When the surfactant film is weakly charged, as in the presence of small amounts of ionic surfactant32 or HMPA, the effective spontaneous curvature is strongly concentration dependent and it is also possible to go from LR to L1, passing the LR+ region, by dilution at constant temperature. In the “pure” nonionic systems on the other hand, the spontaneous curvature depends on temperature alone and the L1-LR+-LR sequence is only observed as a function of temperature.57 (53) Rossi, G.; Pincus, P. A. Macromolecules 1989, 276. (54) Olsson, U.; Nakamura, K.; Kuneida, H.; Strey, R. Langmuir 1996, 12, 3045. (55) Jonstro¨mer, M.; Strey, R. J. Phys. Chem. 1992, 96, 5993. (56) Simons, B. D.; Cates, M. E. J. Phys. II 1992, 2, 1439. (57) Strey, R. Ber. Bunsenges. Phys. Chem. 1996, 100, 182. (58) Schoma¨cker, R.; Strey, R. J. Phys. Chem. 1994, 98, 3908. (59) Loyen, K. Thesis, Pierre et Marie Curie University, Paris, 1997. (60) Hoffman, H.; Thunig, C.; Schmiedel, P.; Munkert, U. Langmuir 1994, 10, 3972. (61) Oberdisse, J.; Couve, C.; Appell, J.; Berret, J. F.; Ligoure, C.; Porte, G. Langmuir 1996, 12, 1212. (62) Wennerstro¨m, H.; Anderson, D. M. In Statistical Mechanics and Differential Geometry of Micro-Structured Materials; Friedman, A., Nitsche, J. C. C., Davis, H. T., Eds.; Springer Verlag: Berlin, 1991.

Bagger-Jo¨ rgensen et al.

Figure 12. Plot of the storage modulus, G′ (b), and the loss modulus, G′′, (O) versus frequency for a sample with φ ) 0.059 and wp(3-C18) ) 0.002 at 42 °C.

With HMPA present it has been observed that a sample composition in the LR+ region may behave as a viscoelastic gel while the same composition is a viscous solution, with a significantly smaller low shear viscosity, in the L1 phase at lower temperatures.23,46,63 Hence, the material shows the spectacular phenomenon of thermal gelation. Such observations were also made in the present systems. In Figure 12 we present G′ and G′′, obtained from oscillatory measurements, as a function of frequency for a sample containing φ ) 0.06 and wp(3-C18) ) 0.002 at 42 °C. We observe that G′ . G′′ in the whole frequency range, showing that the sample is a viscoelastic gel. The sample was prepared in the rheometer by heating from the L1 phase where it behaves as a viscous solution (Figure 8a). We note that the gel structure appears to be sensitive to shear. The experiments were performed at relatively low applied stress, σ. The following values were used: σ ) 0.5 Pa for 10-3 Hz < f < 10-1 Hz, σ ) 1.5 Pa for 5 × 10-2 Hz < f < 1 Hz, σ ) 3 Pa for 1 Hz < f < 10 Hz, σ ) 12 Pa for 5 Hz < f < 15 Hz, σ ) 50 Pa for 10 Hz < f < 40 Hz. In spite of the low stress we are not in the linear viscoelastic regime for the whole frequency range, as seen by the poor overlap in G′ around f ) 10-1 Hz. The self-diffusion coefficients of water, Dw, and oil, Do, were measured on a sample containing φ ) 0.10 and wp(3-C18) ) 0.002 in the LR+ region. The surfactant diffusion could not be measured here because of too rapid transverse relaxation, an indication of very large surfactant aggregates. The presence of large aggregates is also supported by Do, which in the LR+ region was found to be approximately an order of magnitude lower compared to the L1 phase at 25 °C. Also Dw was found to be lower in the LR+ compared to the L1 phase. While the relative diffusion coefficient, Dw/Dw0, where Dw0 is the diffusion coefficient in pure water, is close to unity in the L1 phase, a value of ≈0.6 ( 0.05 was obtained in the LR+ region (35-40 °C). In a vesicle solution there are two sites for the solvent molecules, corresponding to the inside and the outside. In the case of fast exchange on the experimental time scale (here ≈0.1 s), only an average diffusion coefficient is observed. If the vesicles are large (low diffusion coefficient), only the outside site will contribute significantly to Dw and Dw/Dw0 is approximately given by Dw/Dw0 ≈ 1 - φv, where φv is the vesicle volume fraction. Taking into account also the obstruction64 due to the (spherical) vesicles on the solvent diffusion outside we have54 (63) Loyen, K.; Iliopoulos, I.; Audebert, R.; Olsson, U. Langmuir 1995, 11, 1053. (64) Jo¨nsson, B.; Wennerstro¨m, H.; Nilsson, P.-G.; Linse, P. Colloid Polym. Sci. 1986, 264, 77.

Microemulsion with Adsorbing Polyelectrolyte

φv )

2(1 - Dw/Dw0) 2 + Dw/Dw0

Langmuir, Vol. 13, No. 22, 1997 5829

(15)

From φv we can estimate the average vesicle size from

(

1-

)

db Rout

3

)

φv - φ φv

(16)

where Rout is the outer radius of the vesicle. From Dw/Dw0 ) 0.6 we obtain with eq 15 φv ) 0.3. Inserting this value together with φ ) 0.1 and db ) 67 Å into eq 16 gives Rout ≈ 500 Å. A SANS experiment in the LR+ region was performed on a sample with φ ) 0.078 and wp(1-C18) ) 0.01. The experiment was, as in the droplet case, performed using D2O and a mixture of C10D22 and C10H22, and thus, the only coherent contribution to the scattered intensity is from the surfactant monolayer. The absolute scaled intensity is shown in Figure 13. The scattering curve decays approximately as q-2 with two broad humps, one at q ≈ 0.01 Å-1 and a second one at q ≈ 0.1 Å-1. The second hump corresponds to a distance 2π/0.1 ≈ 60 Å, which is the expected monolayer separation in a bilayer. This together with the overall q-2 decay is a strong indication of a bilayer structure. If interpreted as a form factor oscillation of a spherical vesicle, the position of the first hump is consistent with a vesicle radius of approximately 450 Å. For comparison we have, assuming the structure factor to be unity, calculated the scattering from a dispersion of spherical homogeneous double shells of width ) ls ) 15 Å, separation 52 Å, and outer radius 450 Å, including a 20% polydispersity in the radius. An illustration of the radial scattering length density profile with ∆F ) F(C12E5) - F(D2O) ) 6.28 × 1010 cm-2 is shown as an insert in Figure 13. The calculated scattering curve is drawn as a solid line in Figure 13. The calculated curve shows a reasonable agreement with the experimental curve. At low q, deviations are expected since the structure factor has been neglected in the calculation. At high q, the systematic deviation seen is due to the diffuse nature of the surfactant shell.25,65,66 Although the fit to this simple model is satisfying, the obtained polydispersity is rather high and the presence of vesicles with more than one shell (onions) cannot be omitted. The thermal gelation is associated with a structural transition of the surfactant aggregates. The spherical oil-in-water droplets, formed at lower temperatures, correspond to the smallest possible aggregates at a given φs/φo. As indicated by the self-diffusion coefficients and the SANS data, the aggregates formed in the LR+ are significantly larger, and hence the number of aggregates is reduced. For HMPA-micellar gels the micelle-topolyelectrolyte ratio is an important parameter. At wp(3-C18) ) 0.002, however, there is no gel formed at any micellar concentration. Hence, the gel formation has to be associated with the increase in aggregate size rather than a reduction in the number of aggregates. As pointed out previously,23 the gel in the LR+ region may probably be viewed as large surfactant aggregates interconnected by bridging HMPA molecules. (65) Gradzielski, M.; Langevin, D.; Magid, L.; Strey, R. J. Phys. Chem. 1995, 99, 13234. (66) Strey, R.; Winkler, J.; Magid, L. J. Phys. Chem. 1991, 95, 7502.

Figure 13. SANS spectrum from a sample located in the LR+region, containing φ ) 0.078 and wp(1-C18) ) 0.01 at 38 °C. Two characteristic distances are detected, one around q ≈ 0.01 Å-1 and a second around q ≈ 0.1 Å-1. The shallow peak around q ≈ 0.1 Å-1 is evidence for a bilayer structure. The solid line is the calculated form factor of a solvent-filled, unilamellar, spherical vesicle with an outer radius of 450 Å, bilayer thickness db ) 67 Å, and a polydispersity in radius and bilayer thickness of 20%. The insert shows the radial scattering length density profile used in the form factor calculation.

7. Concluding Remarks We have investigated the impact on structure and rheology on a surfactant/water/oil system on adding HMPA. The main observations and conclusions can be summarized as follows: (i) Addition of HMPA does not, if a shift in the phase boundaries are taken into account, change the structure of the surfactant aggregates in the L1 phase. The change in phase boundaries are in close analogy with the addition of ionic surfactant. (ii) Addition of HMPA slightly increases the effective droplet-droplet repulsion, as revealed by SANS, and may trap the microemulsion droplets, as found by NMR selfdiffusion measurements. (iii) Addition of HMPA raises the viscosity of the samples in the L1 phase substantially. At low HMPA concentration the solutions are viscous, whereas at higher HMPA concentration viscoelastic solutions are formed. (iv) The LR phase containing HMPA has a limited swelling with water, which is also in analogy with addition of ionic surfactant. At high bilayer concentration two LR phases are found in equilibrium, one enriched in bilayer and one enriched in HMPA (v) The LR phase containing HMPA deswells on addition of electrolyte. Presumably polymer bridging is operating, but a simple DLVO analysis shows that the collapse is possible without bridging, taking into account that HMPA adsorption makes the bilayers stiffer, thereby reducing the undulation force, combined with the screening of the long-range electrostatics on salt addition. (vi) When entering the LR+ region the sample undergoes thermal gelation. Water self-diffusion and neutron scattering experiment indicate vesicle formation in this area. Acknowledgment. Krister Thuresson is gratefully acknowledged for his kind assistance with the rheological measurements. We thank Bo Jo¨nsson and Håkan Wennerstro¨m for helpful discussions. This work was supported by the Swedish Natural Science Research Council (NFR) and the Human Capital and Mobility program, Contract CHRX-CT94-O655 (European Union). LA9705064