Chapter 17
Flexibility of Cetyltrimethylammonium 3,5-Dichlorobenzoate Micelles 1
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P. D. Butler , L. J . Magid , and J . B. Hayter 1
Downloaded by FUDAN UNIV on February 4, 2017 | http://pubs.acs.org Publication Date: December 9, 1994 | doi: 10.1021/bk-1994-0578.ch017
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Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600 Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
While aqueous cetyltrimethylammonium 2,6-dichlorobenzoate (abbreviated CTA2,6ClBz) forms Newtonian fluids of spherical micelles, the CTA3,5ClBz analog exhibits viscoelastic behavior even at very low concentrations (three to four millimolar). In this brief report we present light scattering data for the CTA3,5ClBz that indicate a persistence length on the order of 500Å. We also discuss a novel use of the bending rod plot (I(Q)*Q vs. Q) as an important qualitative tool for examining the data prior to applying quantitative techniques.
CTAX (cetyltrimethylammonium) surfactants having certain aromatic counterions (X) produce viscoelastic solutions at quite low concentrations (on the order of 1 or 2 wt %) in water (1) . These systems are interesting technologically as well as from the perspective of basic science. The CTA salicylate (Q-hydroxybenzoate) is the most extensively studied of these surfactants (2-4). When the counterion is a mono- or dichlorobenzoate, para and/or meta substitution produces viscoelastic solutions, while CTA2ClBz and CTA2,6ClBz produce Newtonian aqueous solutions containing globular micelles. We present data here on the CTA 3,5-dichlorobenzoate system, where disubstitution has been used to enhance the effect (onset of viscoelasticity is well below 0.5%, with a one of O.lmM). Small angle neutron scattering (SANS)(J,6) and cryo-TEM (7) have been used to look at this system. Cryo-TEM pictures show that the solutions do indeed contain very long threadlike micelles that have considerable flexibility. The micelles are too long to make it possible to determine micellar lengths from micrographs and flexibility is hard to quantify. The radius of the micelles obtained by cryo-TEM is 25À ± 5Â which is quite consistent with the SANS estimation of 23À ± 3À. Some estimates of length and persistence length from SANS have been attempted, but their validity is dubious at best. We have used light scattering (LS), 0097-6156/94/0578-0250$08.00/0 © 1994 American Chemical Society Herb and Prud'homme; Structure and Flow in Surfactant Solutions ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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Cetyltrimethylammonium 3,5-Dichlorobenzoate Micelles 251
BUTLER ET AL.
which has a lower Q range, to determine micellar lengths and persistence lengths. In this brief report we focus on micellar flexibility.
Downloaded by FUDAN UNIV on February 4, 2017 | http://pubs.acs.org Publication Date: December 9, 1994 | doi: 10.1021/bk-1994-0578.ch017
Experimental Materials. Preparation of the surfactant has been described elsewhere (5). A l l water used was from a Millipore Milli-Q water filtration system using house distilled water as the input. Samples were centrifiiged for 1 hour at approximately 1500g, but were not filtered because of sample viscosity. Samples were then thermostatted for at least 24 hours at 30°C in a constant temperature bath before being transferred to the cell holder where they were again thermostatted for 20 minutes. All measurements were made at 30°C due to the proximity of the Krafft temperature; all solutions contained 0.1M NaCl in order to screen electrostatic interactions. Spectrometer. The light scattering spectrometer is a standard setup described elsewhere (8) using a 2 Watt Coherent Innova 70 Argon Ion Laser operating at 514.5nm and a Brookhaven Instruments BI-2030AT autocorrelator for data acquisition. The sample cells are 10mm Wilmad NMR tubes polished to optical quality and immersed in a vat of toluene which serves as the index matched thermostating fluid. A sealed tube of benzene was used as the primary standard for putting all static data on an absolute scale. CS was used as a secondary standard, referenced back to the benzene, for more intensely scattering samples. 2
Method. The theory and technique of light scattering are thoroughly described in the literature (9). One special technique used should however be noted here. The method involves measuring a large number of very short runs (usually no longer than 10 s). The integrated (total) intensities are compared and any runs with an intensity deemed abnormal (in this study, usually 20% above the lowest intensity, or more than two sigma from the mean) are discarded. The total intensity of the remaining runs are averaged and an error bar is computed. This is in fact a dust discrimination technique devised due to difficulties caused by the lack of filtration. A similar technique has also been reported by Brown (10) and Russo (11). The Brookhaven software was modified to automate this special technique. Runs were performed, using benzene as the absolute intensity calibration standard, at angles ranging from 25° to 150°. The overlap concentration c* was determined by both static and dynamic light scattering by plotting apparent R (Rga ), apparent M ^ ( M ) , and D vs Q. The three methods, which are in good agreement, lead to a value of 2.4mM ± 0.4mM. g
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Results and discussion Bending Rod Plots. Figure la through le present static LS results for several concentrations in a bending rod (BR) plot representation. The utility of such a representation for wormlike micelles has been made clear by Burchard and coworkers(J2-i
