Langmuir 1994,10, 100-108
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Polymer-like Lecithin Reverse Micelles. 1. A Light Scattering Study Peter Schurtenberger' and Carolina Cavaco Institut fur Polymere, ETH Zentrum, CH-8092 Zurich, Switzerland Received August 6, 1993. In Final Form: November 3, 1993" We report the results of a systematic static and dynamic light scattering study of polymer-like lecithin/ cyclohexane reverse micellar solutions. The composition and concentration dependence of static and dynamic quantities such as the osmotic compressibility and the static and hydrodynamic correlation lengths are analyzed using theories for dilute and semidilute polymers. At low surfactant concentrations, we present a detailed analysis of the water-induced micellar growth found in these solutions, and we show how the effect of micellar growth and intermicellar interactions on the light scattering data can be taken into account in a self-cossistent way. At high surfactant concentrations, we demonstrate the existence of universal scaling functions for these quantities and discuss the close analogy to semidilute polymer solutions.
Introduction In a number of aqueous surfactant systems it has been shown that giant wormlike micelles exist under certain circumstances, and it has been suggested that these micelles can entangle and form a transient network above a crossover concentration @* quite analogous to the behavior of flexible pblymers in solution. Several reports have indeed demonstrated that the results from static and dynamic light scattering experiments performed on viscoelastic surfactant solutions can be successfully interpreted in terms of theories originally used to describe the behavior of semidilute polymer solutions.14 However, while static properties of micellar solutions such the osmoticcompressibility, (dII/d@)-1,or the static correlation length, &, directly obey the same simple scaling laws as do classical polymers, the dynamic behavior of micellar solutions can be very different. This is due to two fundamental differences between polymers and micelles: (i) in micellar systems, the average aggregation number, h',is not constant but depends on surfactant concentration and temperature, and (ii) micelles are transient structures which can break and recombine and thus offer additional stress relaxation mechanisms. A theoretical model which describes the dynamic behavior of "equilibrium polymers", Le., flexible linear macromolecules that can break and recombine, has recently been developed by C a t e ~ . ~It a has then been suggested that wormlike micelles could serve as good experimental model systems for equilibrium polymers,and several attempts were made to compare experimental results from ionic or nonionic aqueous micellar solutions with the theoretical predictions for Cates' model.lv9 While numerous aqueous surfactant systems are known to exhibit e Abstract published in Aduance A C S Abstracts, December 15, 1993. (1) Cabs, M. E.; Candau, S. J. J. Phys.: Condens. Matter 1990, 2, 6869-6892, and references therein. (2) Candau, S. J.; Hirsch, E.; Zana, R. J . Colloid Interface Sci. 1986, 105, 521-528. (3) Candau, S. J.; Hirsch, E.; Zana, R.; Adam, M. J. Colloid Interface Sci. 1988, 122, 430-440. (4) Marignan, J.; Appell, J.; Baesereau, P.; Porte, G.; May, R. P. J. Phys. (Paris) 1989,50,3553-3566. (5) Appell, J.; Porte, G. Europhys. Lett. 1990, 12, 185. (6) Imae, T. Colloid Polym. Sci. 1989, 267, 707-713. (7) Catas, M. E. Macromolecules 1987,20, 2289-2296. (8) Catas, M. E. J. Phys. (Paris) 1988,49,1593-1600. (9) Kern, F.; Lemarchal, P.; Candau, S. J.; Cates, M. E. Langmuir 1992,437-440.
polymer-like properties, there are only few reports on equilibrium polymers in organic solvents.lOJ1 In contrast to aqueous micellar systems, reverse micelles or waterin-oil microemulsions at moderately high values of surfactant concentration and low values of the molar ratio of water to surfactant, WO, are generally believed to have a droplet-like structure. A noteable exception can be found in the system lecithin/ organ'ic solvent/water, where a formation of gel-like, viscoelastic reverse micellar solutions can be observed.12 We were able to explain their unusual polymer-like properties with a water-induced one-dimensional micellar growth into very long and flexible cylindrical reverse micelles, Le., a characteristic sphere-to-flexible cylinder transition normally observedin aqueous solutions ~ n l y . ~ ~ ~ ~ At high enough lecithin volume fractions, a, these giant polymer-likereverse micelles entangle and form a transient network similar to semidilute polymer solutions, which explains at least qualitatively the tremendous increase in zero shear viscosity observed at higher values of wo and @. A schematic representation of the proposed simple model for the structural properties of lecithin reverse micelles is shown in Figure 1. We were able to test this structural model successfully for lecithin/isooctane solutions using a variety of different techniques.lg16 We then recently extended our measurements to lecithin/cyclohexane solution^,^^-^ for which much higher values of wo can be achieved, and where the maximum of the viscosity is shifted to approximately wo2 12 (isooctane, w ~= 3.0) , (For ~ a ~detailed description of the extension of the isotropic reverse micellar phase (10) Terech, P.: Schaffhauser, V.: Maldivi.. P.:. Guenet. J. M. Langmuir 1992.8, 2104-2106. (11) Zhou,Z.;Georgalis,Y.;Liang,W.;Li,J.;Xu,R.;Chu,B. J.Colloid Interface Sci. 1987.116.473-484. (12) Scartazzini,.R.; Luisi, P. L. J. Phys. Chem. 1988,92, 829-833. (13) Schurtenberger, P.; Scartazzini, R.; Luisi, P. L. Rheol. Acta 1989, 28,372-381. (14) Schurtenberger, P.; Scartazzini, R.; Magid, L. J.; Leser, M. E.; Luisi, P. L. J . Phys. Chem. 1990,94,3695-3701. (15) Schurtenberger, P.; Magid,L. J.; Penfold,J.;Heenan, R. Langmuir 1990,6,1800-1803. (16) Ott,A.; Urbach, W.;Langevin,D.; Schurtenberger,P.;Scartazzini, R.; Luisi, P. L. J . Phys.: Condens. Matter. 1990,2, 5907-5912. (17) Schurtenberger, P.; Magid, L. J.; King, S.; Lindner, P. J . Phys. Chem. 1991,95,4173-4176. (18) Schurtenberger, P.; Magid, L. J.; Lindner, P.; Luisi, P. L. Prog. Colloid Polym. Sci. 1992,89, 274-277. (19) Schurtenberger, P.; Peng, Q.; Leser, M.; Luisi, P. L. J . Colloid Interface Sci. 1993, 156,43-51. (20) Schurtenberger, P.; Cavaco, C. J. Phys. 11 1993, 3, 1279-1288.
0743-7463/94/2410-0100$04.50/00 1994 American Chemical Society
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Polymer-like Lecithin Reverse Micelles organic solvent
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the SLS and DLS results is possible and that we can obtain information on the micellar size, structure, and intermicellar interactions over a wide range of concentrations. At low values of 9,we shall present a detailed analysis of the water-induced micellar growth and present an estimate of the micellar flexibility and polydispersity. At high values of a, we shall verify the universal properties and scaling behavior of (dII/dW1, &,and [h, and demonstrate the close analogy to semidilute polymer solutions.
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Figure 1. Location of the viscoelasticmicellar phase (Lz)in the ternary phase diagram for soybean lecithin/cyclohexane/water, where concentrations are given in weight percent (see ref 19 for details). Also shown is a schematic description of the transition from small and almost spherical to giant and flexible cylindrical reverse micelles upon addition of water at constant surfactant volume fraction, and the formation of a transient network upon an increaseof the volumefraction of the dispersed phase (lecithin + water) which is analogousto the dilute-semidilute transition in classical polymers. (Lz-phase) in these systems see refs 14 and 19). Using cyclohexane as the solvent, it is now possible to vary the macroscopic and microscopic properties of the micelles enormously with the choice of wg. On the basis of dynamic and static light scattering experiments at low values of @ we were able to verify the water-induced growth in lecithin/ cyclohexane reverse micellar solutions. At low values of wg,the micelles were found to be quite small and the solutions exhibit static and dynamic properties which are typical for classical micellar or colloidal solutions. At high values of wg,the micellar size can be extremely large, and the micelles have polymer-likestructural properties which are well described by the wormlike chain model. Using a combination of light scattering and SANS, we were able to directly confirmthe existing structuralmodel and obtain an estimate of the flexibility (or persistence length, lp).17J8 Since the system is oil-continuous and no complicating effects arise due to additional contributions from electrostatic interactions or salt effects, we postulated that reverse micellar solutions of lecithin in organic solvents such as isooctane or cyclohexane serve as good model systems for structural and dynamic studies of equilibrium polymer solutions. Here we now present the results of a very detailed and systematic study of the W Q and 9 dependence of static and dynamic quantities such as (an/ d@)-l, t8,or the hydrodynamic correlation length, [h, in lecithin/cyclohexane solutions using static and dynamic light scattering. The results are analyzed using theories for dilute and semidilute polymers. We shall in particular make use of the fact that we have recently shown that one can directly apply the results from conformation space renormalization group theory for semidilute polymer solutions to equilibrium polymers such as wormlike micelles or microemulsions if one takes into account the fact that their size distribution is concentration dependent.20 We were able to obtain detailed and quantitative information from static light scattering experiments on both the micellar size distribution and the intermicellar interaction effects, Le., on the weight average molecular weight, M,, and on the static structure factor, S(0). Here we now use our new data as a further quantitative test of our previous findings. We shall show that within this theoretical framework a self-consistent interpretation of
Materials and Methods Soybean lecithin was obtained from Lucas Meyer (Epikuron 200) and used without further purification. Cyclohexane (spectroscopic grade) was purchased from Fluka. Samples were prepared as follows: The lecithin was dissolved in the organic solvent overnight using a magnetic stirrer. The appropriate amount of water was then added with a microliter syringe (Hamilton)under gentle stirring. A dispersion of the water was then achieved by subsequent vigorous stirring of the solutions for a few minutes. During this process a steep increase of the viscosity can be observed. Completemixing was finally obtained by slow stirring for a few hours. Depending upon the values of WO, samples were finally equilibrated for a few days at a temperature of 25 "C. While samples with volume fractions of 5 'P 5 0.15 were prepared directly as described above, 2.2 X solutions with lower values of CP were prepared using a previously equilibrated stock solution with @ = 2.2 X lo-*and diluting it with organic solvent to the required final concentration. Staticlight scattering measurementswere madewith a Malvem 4700 PS/MW spectrometer, equipped with an argon ion laser (Coherent,Innova200-10,XO = 488nm) and a computercontrolled and stepping motor driven variable angle detection system. Measurements were usually performed at a temperature of 25.0 f 0.1 OC. Approximately 1mL of solution was transferred into the cylindrical scattering cell (10 mm inner diameter). The scattering cell was then sealed and centrifuged between 20 and 420 min at approximately 5000g and 25 OC in order to remove dust particles from the scattering volume. Static light scattering (SLS)experiments were generally performed at 13different angles (30' I 0 5 150°),and 30 to 100 individualmeasurementsweretaken and averagedfor eachangle. The data was then corrected for background (cell and solvent) scatteringand convertedinto absolutescatteringintensities AR(0) (Le. "excess Rayleigh ratios") using toluene as a reference standard. The excessRayleighratio of the samplewas calculated using
where A(I(0)) and (ZA8)) are the average excess scattering intensity of the solution and the average scattering intensity of the reference solvent toluene, = 39.6 X lo-' m-1 is the Rayleigh ratio of toluene, and n and w a r e the index of refraction of the solution and the reference solvent, respectively.z1 Plots of Kc/AR(@ versus Q'/3 were extrapolated to Q = 0 to give intercepts Kc/AR(O), where K = 4a2n'(dn/dc)a/(Nn.Xo'), dn/dc is the refractive index increment, lQl = ( 4 d h ) sin(O/t) is the scattering vector, and c is the concentration of surfactant plus water, respectively. Values of dn/dc for soybean lecithin in cyclohexane were measured as a function of wowith a modified Brice-Phoenix differential refractometer with a resolution better than An = 2 X W . The values used are summarized in Table 1. The static correlation length 4. was determined from the intercept and slope of the plot W A R ( @versus Q*/3 using a Lorentzian scattering law of the form
we)
For samples with high values of wo and low values of 'P, Le., for (21)Schurtenberger,P.;Augueteyn,R. C.Biopolymers 1991,31,12291240.
Schurtenberger and Cauaco
102 Langmuir, Vol. 10, No. 1, 1994 Table 1. wo-Dependenca of the Index of Refraction Increment dn/dc at T = 28.0 OC wo dnldc (makg-1) Wo dn/dc (makg') 2.0 5.41 X lod 10.0 3.72 X 1O-a 4.0 6.0 8.0
5.06 X 1od 4.64 X 106 4.37 x 1O-a
12.0 14.0
c
3.38 X 10-5 3.00 x 1O-a
lOOO/
very large micelles in dilute solutions where f a is large, the data points at high values of Q were not included in the fit. Dynamic light scattering (DLS) experimenta were generally performed at 4 to 7 scattering angles and a temperature of T = 26.0 f 0.1 "C, and 3-10 intensity autocorrelation functions were individually analyzed using a second-order cumulant analysis.Z2 The resulta were then averaged for each value of Q, and a cooperative diffusion coefficient Dc = l i m w ( r)/Q2 was determined. From D, a hydrodynamic correlation length &, was calculated using
(3) where TO is the viscosity of the solvent.14
Results PhenomenologicalObservations. Here we first give a short summary of some general phenomenologicalresults. Soybean lecithin can form clear and optically isotropic reverse micellar solutions in a variety of different organic solvents. These solutions can be transformed into transparent, highly viscous and thermodynamically stable gellike systems by adding very small quantities of water. In general, the zero shear viscosity qs increases dramatically with increasing w o and reaches a distinct maximum at a well-defined value wommar.The value of qsat wowax depends strongly upon lecithin concentration, temperature, and nature of the organic solvent used. The ratio of q$qo can . further addition of water, be as high as lo6at W O , ~ Upon 7,generally decreasesand phase separation can be observed. (Itis interestingto note that a t higher values of wo,a liquidliquid type phase separation into two macroscopically separated and optically clear phases can be observed for isooctane, whereasphase separation into a reverse micellar and a nearly pure water phase occurs for cyclohexane as ~seems , to~ be quite ~ ~ the solvent.) The value of w independent of lecithin concentration, but it does depend upon the nature of the organic solvent used. A detailed report on the rheological behavior of lecithin reverse micellar solutions will be presented elsewhereSz3 Dynamic Light Scattering. The results from systematic DLS measurements of lecithin/cyclohesane reverse micellar solutions are summarized in Figure 2, where t h is plotted as a function of the volume fraction of the dispersed phase for different values of WO. At low volume fractions @ 5 6 X we observe a very pronounced increase of f h with increasing wowhich primarily reflects the strong water-induced micellar growth from relatively small reverse micelles with f h = 30 A at wo = 2.0 to giant wormlike particles with f h = 400-700 %I at wo 1 6.0. At these low values of @, we also see that f h increases with increasing concentration, which indicates a concentration dependence of the micellar size distribution. At volume fractions 9 L 6 X 103, f h first reaches a maximum and becomes more and more independent of wo. At even higher values of @, f h then decreases with increasing @, and the @-dependencecan now be described by a power law of the form f h -a", where x = 0.65 f 0.05 for all values of wo. The crossover to the power law dependence provides us (22) Koppel, D. E. J. Chem. Phys. 1972,57, 4814-4820. (23) Cavaco, C.; Schurtenberger, P. Manuscript in preparation.
4
.0001
,001 .01 .1 volume fraction
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Figure 2. Hydrodynamiccorrelation length, h,as a function of volume fraction, @, and water to lecithin molar ratio, WO: (X) W O = 2.0; (e)Wo = 4.0; (*) Wo = 6.0; (0) Wo = 8.0;( 0 )Wo = 12.0; (A) wo = 14.0. Data seta for @ = constant and wo = constant are connected by dashed and solid lines, respectively. The surface marked as @*indicates the crossover from the dilute to the semidilute regime.
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14 1111
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.0001
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.......:
,001 -01 .1 volume fraction
wo '4 ,
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I , / /
1
Figure 3. Static correlation length, f., as a function of volume fraction, @, and water to lecithin molar ratio, WO: (e)wo = 4.0; (*) Wo = 6.0; (0) Wo = 8.0;(H) Wo = 10.0;(0) zuo 12.0; (A)Wo = 14.0. Data seta for @ = constant and W O= constant are connected by dashed and solid lines, respectively. The surface marked as @* indicatesthe crossover from the diluteto the semidiluteregime. with a first estimate of the entanglement threshold @* as a function of wo,and we see from Figure 2 that @* decreases with increasing WO. Static Light Scattering. We have previously presented the concentration dependence of the osmotic compressibility (all/a@)-lfor different values of woezoWe were able to show that (dll/d@)-' reflects the water-induced growth at low values of @ and becomes independent of w o at high values of @. For CP 1 @*,(dII/d@)-1 was found to decrease with increasing @ following a power law of the form (dII/a@)-l @-Y, where y = 1.30 f 0.05. A similar behavior can also be observed for the static correlation length f s determined from the Q-dependence of the scattering intensity (eq 2). The experimental data are summarized in Figure 3, where f a is plotted as a function of @ for different values of WO. f 8 shows qualitatively the same trends as does f h , with a strong wo-dependence at 9 < @*, where f 8 increases from f a = 75 A at w o = 4.0 to f a = 680 A at w o = 14.0. For @ 1 @*, f s becomes again independent of w o and decreases with a power law of the form f a where z = 0.70 f 0.1. However, our ability to measure f s accurately is restricted to a relatively small
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Langmuir, Vol. 10, No. 1, 1994 103
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increase with increasing surfactant concentration with a These models predict power law of the form N , an exponential size distribution of the form F ( N ) exp(-N/Nw),which results in Nw/Nn = 2, where Nn is the number-average aggregation number. Therefore, we have included polydispersity in our analysis using a SchulzFlory "most probable d i s t r i b u t i ~ n " ~ ~
F ( N ) = ~ ( ~ - ' )p () l where the "extent of reaction" p is given by 0.5 0
400
200
600
800
Rh
Figure 4. RdRh as a function of the hydrodynamicradius Rh for soybean lecithin in cyclohexane at T = 25.0 O C . The data were obtained with different values of w o and for @ 9*. Shown are (A) t h versus @, (B)& versus 9, and (C)A ( Z ( O ) ) / ( Z d O ) ) versus 9, respectively, for various values of WO: (0) wo = 6.0;( 0 )w0 = 8.0; (A) w o = 12.0; (m) wo = 14.0. Also shown are data for soybean lecithin in isooctane at T = 25.0 O C , wo = 1.5 (A), and fits of a power law to the experimental data as solid lines.
and classical polymer solutions. We obtain quantitative agreement between theoretical calculations on the basis of results from renormalization group theory for semidilute polymers and the experimental data for different static and dynamic quantities, which providesadditional support for the previously postulated20very strong concentrationand water-induced micellar growth. This demonstrates that the influence of micellar growth and intermicellar interactionscan successfullybe incorporated in an analysis of scattering experiments provided that the "equilibrium polymer" feature, i.e., the existence of a composition dependent micellar size distribution, is taken into account. The present analysis could thus be extended far beyond a simple confirmation of scaling laws for 3 > 3*generally done in light scattering studies of polymer-like micellar solutions. However, several aspects of the system still remain to be studied in more detail. First of all, the incorporation of intramicellar excluded volume effects has been justified in an indirect way only, and we are thus currently conducting an additional careful investigation of the Q-dependence of the static and dynamic structure factor with light and neutron scattering experiments. Furthermore, we have restricted the theoretical and
experimental characterization of our model system either to static or to dynamic properties on a very short characteristic time scale, where the finite lifetime of the aggregates can be neglected. Clearly this is not the case for dynamic properties such as the zero shear viscosity or the long-time self-diffusion coefficient. This can easily be seen from a comparison of the rheological behavior (see refs 19 and 23) and Figure 2, where the DLS results at high values of wo level off and thus reflect the influence of water on the micellar growth, whereas the zero shear viscosity decreases dramatically for wo > 12. An additional experimental and theoretical effort on the understanding of dynamic properties on long characteristic time scales is therefore required and will be reported in a forthcoming article.23
Acknowledgment. This work was supported in part by the Portuguese National Fund for Technological and Scientific Research through the Grant BD/1256/91-IC. We are most grateful to E. Blochliger for her help with the sample preparation and for her skillful assistance in making the figures. We are indebted to Professor H. C. Oettinger for illuminating discussions.
