SANS and TEM studies of aqueous micellar solutions and ternary microemulsions

Author

Jeffrey Calvin Gee

Date of Award

12-1991

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Linda J. Magid

Abstract

The techniques of small-angle neutron scattering (SANS) and transmission electron microscopy (TEM) were combined to investigate the microstructures of aqueous micellar systems and ternary microemulsions. The primary goals were to determine the sizes and shapes of micelles formed by cetyltrimethylammonium-3,5-dichlorobenzoate (CTA 3,5ClBz), CTA 3,4ClBz, and CTA 4QBz, and to determine the microstructure in different regions of the phase diagram of the microemulsions composed of hexadecyloctyldimethylammonium bromide (C₈₁₆DAB), hexane, and water. SANS curves for the aqueous micellar systems were fit to models for monodisperse rigid cylinders and for polydisperse rigid cylinders. The fits indicated that the micelles formed by each surfactant were cylindrical, and that the radius in each system was about 23-24 Å, independent of NaCl and surfactant concentrations. The micelle lengths were too large to be determined, but the data indicated that they were greater than 1000 Å for each system at all surfactant concentrations studied. Further analyses of the scattering curves suggested that the micelles were flexible rather than rigid, and that the persistence lengths in each system were about 200 Å. Cryo-TEM analyses of the micellar systems provided direct images of the micelles in vitreous ice. For all three surfactants, images of long, thread-like micelles were obtained at each concentration investigated. Radii measured from the micrographs were about 25 Å, in excellent agreement with the SANS analyses, and micelle lengths ranged from about 6000 Å to greater than 28000 Å. Images acquired for solutions of CTA 2,6ClBz showed that these micelles were spherical at 10 mM surfactant, but that the micelles became increasingly shear-sensitive as the surfactant concentration was increased. The micelles grew into long threads as the surfactant concentration was increased, and coexisting spheres and cylinders were observed at 40 mM surfactant, which was the first documented case of coexisting spheres and cylinders in an aqueous binary surfactant system. Images resembling fingerprints were obtained at 100 mM surfactant SANS measurements for the water, C₈C₁₆DAB, hexane system were obtained for samples across three water dilution lines of constant surfactant to oil ratios: s/o = 0.238, 0.615, and 1.00. Analyses of the surfactant film showed that the interface between water and surfactant was sharp, except at the lowest water concentrations. At low water contents, the monomers were staggered around the water regions, and the interface was considerably diffuse. The surfactant/oil interface was shown to be diffuse at all compositions. Hts to both the Teubner-Strey and Vrij scattering models indicated that water droplets predominated across most of the phase diagram. At high surfactant and low water concentrations, the scattering results were inconclusive. Freeze-fracture electron microscopy investigations of the water, C₈C₁₆DAB, hexane system revealed droplet structures across most of the phase diagram. Lamellar structures were seen in samples having high surfactant and low water concentrations, and droplets were visible in all other samples. No evidence for a bicontinuous network was observed.

Recommended Citation

Gee, Jeffrey Calvin, "SANS and TEM studies of aqueous micellar solutions and ternary microemulsions. " PhD diss., University of Tennessee, 1991.
https://trace.tennessee.edu/utk_graddiss/11112