Chapter 7
Structure of Sheared Cetyltrimethylammonium Tosylate—Sodium Dodecylbenzenesulfonate Rodlike Micellar Solutions
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Richard D. Koehler and Eric W. Kaler Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, DE 19716
The viscoelastic behavior observed in aqueous solutions of the cationic/anionic surfactant mixture cetyl trimethylammonium tosylate(CTAT) and sodium dodecyl benzyl sulfonate(SDBS) provides evidence for the presence of rodlike micellar structures. In particular, highly viscous and viscoelastic gels form at low total surfactant concentrations. Small angle neutron scattering measurements reveal a strong interaction peak that becomes anisotropic with increasing shear rate. This anisotropy indicates an increase in the order of the micelles in a preferential direction. Model fits show that the rotational diffusion coefficients decrease with increasing total surfactant concentration for samples in the middle of the rodlike micellar region, but they increase on approach to the phase boundary. Thus micelles in the center of the micellar phase grow with increasing total surfactant concentration, while those near the boundary remain shorter. The shorter micelles are precursors to the vesicle and lamellar structures of nearby phases. Rodlike micelles form in ionic surfactant solutions at high salt concentrations(7,2). They also form in solutions containing strongly binding counterions such as salicylate(5-5) or tosylate(6). These ions contain a hydrophobic part that prefers to remain in the nonpolar core of the rodlike micelle. If the counterion is replaced with an oppositely charged surfactant, there is very strong "counterion" binding, and rodlike micelles, spontaneous vesicles, and lamellar structures can form(6). In the CTAT/SDBS mixture (Figure 1) decreasing the CTAT/SDBS ratio at a constant total surfactant concentration shows a progression from a rodlike micellar phase on the C T A T rich side, to a micelle/vesicle two phase coexistence region, and through a CTAT-rich vesicle lobe to the equimolar line. The phase behavior is similar on the other side of the equimolar line. The current emphasis is on the CTAT-rich micelle phase. Electron microscope images(6) and the viscoelastic response(7-5) of moderately concentrated rodlike micellar solutions suggests that they contain long and entangled threadlike micelles. Cates(7, 0.03A, so a fit to higher q data perpendicular and parallel to the flow direction provides information on the effective rotational diffusion coefficients of the rodlike micelles without the need for a detailed knowledge of the pair distribution function. Experimental Section Cetyl trimethyl ammonium tosylate (CTAT) was obtained from Sigma Chemical and recrystallized three times. Sodium dodecyl benzene sulfonate(SDBS) soft type was used as supplied from TCI. D2O was obtained from Cambridge Isotope Laboratories and also used as supplied. The phase diagram is reported elsewhere(o). SANS samples were prepared by completely mixing stock solutions of C T A T and SDBS in D2O to the desired weight ratio. Two samples were in the center of the one phase rodlike micellar region: sample 1 containing 1.5wt% 97/3 CTAT/SDBS and sample 2 containing 1.0wt% 97/3 CTAT/SDBS. Two other samples were prepared closer to the rodlike micelle phase boundary, sample 3 containing 1.5wt% 95/5 CTAT/SDBS and sample 4 containing 1.0wt% 94/6 CTAT/SDBS (Figure 1). Small angle neutron scattering measurements were performed on the 30-meter SANS spectrometer at the National Institute of Standards and Technology at Gaithersburg, M D . The sample to detector distance was 5.25m and a wavelength of 6Â with Δλ/λ=0.15 yielded a q range between 0.005Â and 0.08Â" . The samples were placed in a concentric cylinder shear cell with a 0.843mm gap width(24). Measurements were made at 25 C at steady shear rates. The samples were presheared for 300 seconds before the measurements began. The raw 2-D data was corrected by subtracting the appropriate experimental background. Circular averages were made on shells 10° wide around a direction perpendicular to the flow direction and parallel to the flow direction, and the data was put on absolute scale with the use of a polystyrene standard. For fitting, the experimental data was divided by the prefactor in equation 5. The persistence length or effective rod length was estimated using Bragg's law, L = ^ / q x , where q is the q value of the peak in the scattering curve(70). The radius of the rod was found by fitting the form factor, equation 4, to the high q data at zero shear where the decay of the intensity curve only depends on the particle radius. Effective diffusion coefficients at each shear rate were obtained by fitting equation 4 to data measured for q > 0.03Â both parallel and perpendicular to the flow direction. -1
1
e
m a
m a x
Results and Discussion Figure 4 shows representative scattering patterns at various shear rates from 1.0wt% 97/3 CTAT/SDBS in D2O (sample 2). At rest, the scattering from these micellar
Herb and Prud'homme; Structure and Flow in Surfactant Solutions ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
Sheared CTAT-SDBS
Rodlike Micellar Solutions
Downloaded by CORNELL UNIV on September 23, 2016 | http://pubs.acs.org Publication Date: December 9, 1994 | doi: 10.1021/bk-1994-0578.ch007
7. KOEHLER & KALER
Figure 4: Scattered neutron intensity, for 1.0wt% 97/3 CTAT/SDBS in D2O, showing an increase in anisotropy with shear rate, (a) shear rate 0 s , (b) shear rate 10 s , (c) shear rate 20 s", (d) shear rate 40 sr . _1
-1
1
1
Herb and Prud'homme; Structure and Flow in Surfactant Solutions ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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solutions is isotropic and displays a single strong correlation peak (Figure 4a). This indicates that the micelles are charged and highly interacting. The scattering patterns become increasingly anisotropic with increasing shear rate, and show increasing intensity in the direction perpendicular to the flow direction and decreasing intensity in the direction parallel to the flow direction. This change provides clear evidence that the rodlike micelles are aligning. The increase in anisotropy implies the rods align like the rungs on a ladder, thereby reinforcing the correlation peak in the direction perpendicular to the flow and weakening it in the direction parallel to the flow. Figure 5 shows scattering from sample 2 along lines perpendicular and parallel to the flow direction. The solid and dashed lines are the model fits of the form factor where the effective rotational diffusion coefficient is the only adjustable parameter; the radius and length are determined from the zero shear data. The good agreement between the shape of the experimental and fitted curves at high q suggests that the length estimated from the q value of the intensity peak is a good estimate of the persistence length of the micelles. Table I shows the results of this fit for the four samples studied. Table I. Rotational Diffusion Coefficients as a Function of Shear Rate Shear Rate (s- ) 1
5 10 20 50 100
-1
Rotational Diffusion Coefficient D (s ) wt% surfactant, ratio of CTAT/SDBS 1.5wt%,97/3 1.0 wt%, 97/3 1.5wt%,95/5 1.0wt%,94/6 0.7 1.7 1.0 1.7 1.3 1.0 1.3 1.7 0.4 1.7 2.7 2.0 6.7 1.2 0.6 2.0 10.0 8.3 1.0 2.0 r
The effective rotational diffusion coefficient decreases with increasing total surfactant concentration along the 97/3 CTAT/SDBS line, which is in the middle of the rodlike micelle region on the phase diagram. At higher SDBS ratios the effective diffusion coefficients increase with shear rate. The rotational diffusion coefficient, D , of a rod in dilute solution varies as L" while in a semi-dilute solution Dr ~ D (cL )- . The rotational diffusion coefficient of the rodlike micelles depends strongly on the length of the rod. The decreasing diffusion coefficient with increasing total surfactant concentration implies an increasing micelle length or an increase in entanglement. Near the phase boundary (samples 3 and 4), the observation of an increasing effective rotational diffusion coefficient with increasing shear rate suggests that the micelles are short or mobile enough to remain unaligned even at very high shear rates. The higher diffusion coefficient may be due to less entanglement or may be directly related to a decrease in rod length. The proximity of the phase boundary is consistent with the formation of shorter micelles and indicates that more endcaps can form near the phase boundary. ro
3
3
2
ro
Conclusions Rodlike micelles that can be aligned in a shear flow are present on the CTAT rich side of the CTAT/SDBS/D2O phase diagram. Modeling the neutron scattering data provides information about the effective rotational diffusion coefficient of these
Herb and Prud'homme; Structure and Flow in Surfactant Solutions ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
Sheared CTAT—SDBS Rodlike Micellar Solutions
Downloaded by CORNELL UNIV on September 23, 2016 | http://pubs.acs.org Publication Date: December 9, 1994 | doi: 10.1021/bk-1994-0578.ch007
7. KOEHLER & KALER
Herb and Prud'homme; Structure and Flow in Surfactant Solutions ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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rodlike micelles. The results indicate that longer, more entangled micelles are formed as the total surfactant concentration is increased at a constant ratio of CTAT/SDBS. Increasing the amount of SDBS at a constant total surfactant concentration leads to rodlike micelles which are much shorter and more mobile. Shorter micelles signal a change in the structures present as the phase transition is approached.
Downloaded by CORNELL UNIV on September 23, 2016 | http://pubs.acs.org Publication Date: December 9, 1994 | doi: 10.1021/bk-1994-0578.ch007
Acknowledgments This work was supported by E.I. DuPont de Nemours & Co. The authors thank Dr. R.G.Larson for supplying the FORTRAN code used in the calculation of the orientational distribution function of rods in a shear flow. The authors also acknowledge G. Straty and Dr. J. Barker for their help with the shear cell and SANS measurements. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the facilities used in this experiment. This material is based upon activities supported by the National Science Foundation under Agreement No. DMR-9122444. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 19. 20. 21. 22. 23. 24.
Kern, R.; Lemarechal, P.; Candau, S.J.; Cates, M.E. Langmuir, 1992, 8, 437. Candeau, S.J.; Hirsch, E.; Zana, R.; Adam, M. J. Coll. Interface Sci., 1988, 122, 430. Hoffmann, H.; Rehage, H.; Rauscher, A. In Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution; Kluwer Academic Publishers, Amsterdam, 1992, pp 493-510. Kern F.; Zana, R.; Candau, S.J. Langmuir, 1991, 7, 1344. Rehage, H.; Hoffmann, H. J. Phys. Chem., 1988, 92, 4712. Kaler, E.W.; Herrington, K.L.; Murthy, A.K.; Zadadzinski, J.A.N. J. Phys. Chem., 1992, 96, 6698. Turner, M.S.; Cates, M.E. J. Phys. II France, 1992, 2, 503. Turner, M.S.; Cates, M.E. Langmuir, 1991, 7, 1590. Herbst, L.; Hoffmann, H.; Kalus, J.; Thurn, H.; Ibel, K.; May, R.P. Chem. Phys., 1986, 103, 437. Kalus, J.; Hoffmann, H. J. Chem. Phys., 1987, 87, 714. Kaler, E.W. J. Appl. Cryst., 1988, 21, 729. Guinier, Α.; Fournet, G. Small-Angle Scattering ofX-Rays ; John Wiley & Sons, Inc., New York, 1955, p19. Kalus, J. In Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution; Kluwer Academic Publishers, Amsterdam, 1992, pp 463-492. Hayter, J.B.; Penfold, J. J. Phys. Chem., 1984, 88, 4589. Cummins, P.G.;Staples, E.; Hayter, J.B.; Penfold, J. J. Chem. Soc., Faraday Trans.1,1987,83,2773. Cummins, P.G.; Hayter, J.B.; Penfold, J.; Staples, E. Chem. Lett., 1987, 138, 436. Doi, M.; Edwards, S.F. The Theory of Polymer Dynamics; Oxford University Press, New York, 1986; pp 295,330. Perera, Α.; Kusalik, P.G.; Patey, G.N. J. Chem. Phys., 1987, 87, 1295. Canessa, E.; D'Aguanno, B.; Weyerich, B.; Klein, R. Molecular Physics, 1991, 73, 175. Doi, M.; Edwards S.F. J. Chem. Soc., Faraday Trans. 2, 1978, 74, 918. Larson, R.G. Recent Developments in Structured Continua; Longman: London, 1990; Vol 2. Larson, R.G.; Öttinger, R.G. Macromolecules, 1991, 24, 6270. Straty, G.C. J. Res. Natl. Inst. Stand. Technol., 1989, 94, 259.
RECEIVED July 8, 1994
Herb and Prud'homme; Structure and Flow in Surfactant Solutions ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
