Langmuir 1990, 6, 1800-1803
1800
Letters Shear Aligned Lecithin Reverse Micelles: A Small-Angle Neutron Scattering Study of the Anomalous Water-Induced Micellar Growth Peter Schurtenberger,*vt Linda J. Magid,* Jeffrey Penfold,$ and Richard Heenan5 Znstitut fur Polymere, ETH Zentrum, CH-8092 Zurich, Switzerland, Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996- 1600, and Neutron Science Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX,England Received August 7, 1990. Zn Final Form: September 26, 1990
We present results from a small-angleneutron scattering study of shear-alignedviscoelastic reverse micellar solutions of lecithin in isooctane. We obtain direct evidence for the presence of cylindrical reverse micelles and a water-induced anisotropic growth in these solutions. A substantial increase of the micellar contour length L with increasing water to lecithin molar ratio wo can be observed.
Introduction A number of surfactants are known to form reverse micelles in apolar solvents.' These micellar aggregates can solubilize considerable amounts of water and are generally believed to have a dropletlike structure up to moderately high values of the surfactant volume fraction, 9,and the molar ratio of water t o surfactant, WO. Under these conditions, the micellar radius depends primarily upon w o and is almost independent of surfactant concentration. The addition of a small amount of water to reverse micellar solutions usually induces a spherical growth of the micelles, with no significant changes of macroscopic quantities such as the viscosity. However, a completely different behavior can be found for lecithin reverse micelles in a number of different organic solvenh2 These solutions can be transformed into transparent, highly viscous, and thermodynamically stable viscoelastic systems by adding very small quantities of water. With isooctane for example, the viscosity 17 increases by as much as a factor of lo6 upon the addition of three molecules of water per lecithin molecule. In general, I] increases dramatically with increasing wo and reaches a distinct maximum at a welldefined w ~ , ~ ~ ~ . ~ , ~ We previously investigated the structure and phase behavior of lecithin/isooctane solutions by means of a combination of small-angle neutron scattering (SANS), polarizing microscopy, and 3IP and 2HNMR.4 On the basis of these investigations,we then postulated that the addition of water to lecithin/isooctane solutions induces onedimensional aggregation of the lecithin molecules into long cylindrical reverse micelles. Above a crossover lecithin volume fraction, 9*, these micelles subsequently entangle and form a transient network similar to semidilute polymer solutions5 or aqueous solutions of ionic micelles a t high ionic strength,6-10which would explain the viscoelastic E T H Zentrum. t University of Tennessee. 8 Rutherford Appleton Laboratory. (1)Luisi, P. L.; Magid, L. J. CRC Crit. Rev. Biochem. 1986,20,409. (2)Scartazzini, R.;Luisi, P. L. J. Phys. Chem. 1988,92,829. (3)Schurtenberger, P.; Scartazzini, R.; Luisi, P. L. Rheol. Acta 1989, 28. 372. (4)Schurtenberger, P.; Scartazzini, R.; Magid, L. J.; Leser, M. E.; Luisi, P. L. J. Phys. Chem. 1990,94,3695. t
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nature of these systems and the very high values of 7.The results from systematic SANS measurements a t different values of w o and 9 were in agreement with the presence of locally cylindrical micelles in these solutions. Furthermore, this structural model was strongly supported by QLS and SANS measurements of the concentration dependence of the hydrodynamic and static correlation lengths, &, and &,respectively, in the semidilute regime.4 However, the hypothesis of a water-induced cylindrical growth for the lecithin molecules could only be supported indirectly by making analogies to classical polymer theory. In particular the observed decrease of 9*with increasing w o was in agreement with our model, where we assumed that the micellar contour length L increases with increasing wo at constant lecithin volume fraction. In this article we now report direct evidence for a waterinduced anisotropic micellar growth in these solutions. Anisotropic micelles can be aligned in a shear gradient G.''-13 The presence of partially or fully aligned rodlike aggregates in solution creates anisotropic scattering patterns along the two orthogonal directions Q1 and 811,i.e. perpendicular and parallel t o the flow direction. Since the shear dependence of the micellar alignment and thus the scattering anisotropy is very sensitive to the rod length, we would expect to find a much stronger shear dependence and higher values of the ratio Z(Ql)/Z(QII) at w ovalues close to WO,max as compared to w o > 1that we can expect to find full alignment of the particles. For noninteracting rodlike micelles under shear, the relative time spent by the micelle in a given orientation (e,4) (in polar coordinates, with the origin at the center of the micelle, for details see ref 11)can then be described by a probability function p(e,d;r),which depends on r. The neutron beam illuminates two regions of the sample when traversing the Couette cell designed by Staples et al., with the direction of the shear gradient reversed in the second region. The resulting scattering intensity I(Q)on the detector will thus be the sum of two uncorrelated intensities, with"
p(Q,y-)]sin 0 dB (1)
where A is an experimental constant, F is the form factor for a micelle a t a given orientation with respect to the scattering vector 8, and y is the angle between 8 and the cylinder axis. In their theoretical model Hayter and Penfold developed an expression for p(O,$;r)that was based on work originally used for flow birefringence experiments.16 Using their expression, we can now calculate the shear rate dependence for different of the scattering intensity along Q1and 81, values of the micellar contour length L. We can then adjust L so as to obtain the best agreement between experimental and theoretical intensities I(Q1)and I(Q1,). Assuming a value for L, we first have to calculate the corresponding Dr. Theoretical expressions for Dr are available for different particle shapes such as cylinders, flexible chains, or ellipsoids. We have used an equation for cylinders with L > 2R
D, = 3k,T(s
-t)/(8~7,(L/2)~)
(2)
s>2
where s = log ( L / R ) ,t = 1.57 - 7(0.28 - l / ~ and ) ~ 70 , is the solvent v i s ~ o s i t y .A~ value ~ of R = 26 A was chosen for the radius of the cylinder independent of lecithin concentration and w0.4 This choice of R is based on a deduction of the cross-sectional radius of gyration, R G , ~ , from plots of In ( Q Z(Q)) vs Q2 at different values of CP and wo (data not shown) and assuming uniform cylinders with R G , =~ R/2't2. An additional difficulty in such an analysis is the contribution of intermicellar interactions to the rotational diffusion coefficient Dr of the micelles. The model of Hayter and Penfold is valid for noninteracting rigid rods only. In the case of overlapping rodlike micelles, the orientational probability of the individual micelles will no longer be exactly described by p(e,$;r). However, if we assume (16) Peterlin, A.; Stuart, H. In Hand- und Jahrbuch der Chemischen Physik; Akad. Verlagsgesellschaft Becker und Euler: Leipzig, 1943; V O ~8, . pp 44-51. (17) Broersma, S. J. Chem. Phys. 1960, 32, 1626.
1802 Langmuir, Vol. 6, No. 12, 1990 I
Letters
T
0
e g
20
20
0
n 0.08
0.04
Q
[A']
0.08
0.04
a
[A.']
Figure 2. Measured scattered intensities along the QL ( 0 )and 811(0) directions for soybean lecithin in isooctane, @ = 0.021: (A) w o = 2.0, G = 5000 s-l; (B)wo = 3.0, G = 2500 s-1. The theoretical curves are for cylinders with R = 26 8, and (A) L = 2000 8, or (B)L = 4500 A, respectively.
that the form of p(O,r$;I')for hindered rotation is similar as in the dilute ideal case, the effect of collision-hindered rotation can then be incorporated formally by using DAG) as a phenomenological, shear dependent parameter.12At low shear rates, collisions will be more important, and DAG) can be estimated, for example, using the expression of Doi and Edwards for the rotational diffusion coefficient of rigid rods in semidilute solutions
0 [A
'I
Q
[A"]
Q
L 5000
shear gradient G [ 5''
(la) Doi, M.;Edwards, S. F. The Theory of Polymer Dynamics; Oxford University Press: New York, 1986.
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Figure 3. Dependence of the scattering intensities Z(QL) ( 0 ) and Z(QII) (0) on the shear rate G for soybean lecithin in isooctane, @ = 0.021 and wo = 3.0 (A) G = 42 s-l; (B)G = 120 s-1; (C) 750 s-l. The theoretical curves are for cylinders with R = 26 8, and L = 4500 8,. A shear-dependent rotational diffusion coefficient D,(G) was used.
0.010
where D,,o is the rotational diffusion coefficient in dilute solutions, c is the number density of rods, and P is a numerical constant.18 However, a t high shear rates and thus almost complete alignment, the micelles are essentially noninteracting and D, should approach Dr,o.12We can therefore try to estimate L from the observed scattering anisotropy at high shear rates. For @ = 0.021, we obtain values of L = 2000 A for w o = 2.0 and L = 4500 A for w o = 3.0, respectively. Such an increase of L with increasing w o is in good agreement with previous indirect evidence from rheological measurements and from the dependence of the overlap threshold @* on w0.4Figure 2 shows the experimental data used for these calculations and the theoretical scattering intensities obtained with these parameters. A close agreement between experimental and calculated data could be achieved, although a small but systematic deviation a t the lowest Q values indicates possible contributions from polydispersity and/or entanglement effects. In our estimate of L we had to rely on the shear dependence of the scattering anisotropy at higher Q values and assume noninteracting rods at these high shear rates. It would thus be most desirable to extend our measurements to concentrations @
