Doctoral Dissertations

Date of Award

12-1991

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Linda J. Magid

Abstract

Both dynamic and static light scattering (LS) were used to characterize the micellar structure of three different cetyltrimethylammoniumchlorobenzoate surfactants (abbreviated CTAa,bCl where a and b represent the position of the chlorine on the aromatic ring) which are known to form viscoelastic solutions at very low concentrations: CTA4C1; CTA3,4C1; CTA3,5C1. The concentrations studied ranged from 0.3mM to 27mM. The CTA3,4 and CTA3,5C1 were found to grow much more rapidly than the CTA4C1. The micelles formed by the surfactants with the dichloro counterions were found to already be relatively large at the very lowest concentrations with radii of gyration (&Rbar;g) of several hundred Angstroms, corresponding to a cylindrical contour length (&Lbar;c) on the order of 1000Å assuming the radius of 23Å measured by small angle neutron scattering. The rods were found to grow rapidly with increasing concentration, and were shown to become too large to measure by static LS once the concentration exceeded 2mM. Overlap concentration was reached at around 3mM for the dichlorobenzoate micelles. Rough estimates of &Lbar;c at overlap indicate a size of 3,000 to 10,000 nm. Scaling laws were shown not to be applicable in the overlap region of these systems. The persistence length for the CTA3,5C1 system was found to be 500±100Å, with some indication that it is somewhat smaller at the lowest concentrations and may also increase again at the highest concentrations measured. Internal modes were shown to contribute to the dynamic scattering, but no attempt was made to extract them from the data. Despite the existence of internal modes preventing the use of the second cumulant to measure polydispersity, substantial polydispersity was shown to exist, perhaps for the first time, by a careful analysis of the static scattering. A salt study was performed on the CTA3,4C1. Four concentrations of NaCl were used ranging from 0.01M NaCl to 0.2M NaCl. The indications were that above a certain threshold concentration of added salt, the micellar size decreases on addition of salt.

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