The structures of aggregates formed in aqueous solutions of an anionic surfactant, sodium dodecyl sulfate (SDS), with the addition of a cationic hydrotropic salt, p-toluidine hydrochloride (PTHC), have been investigated by small angle neutron scattering (SANS). The SANS spectra exhibit a pronounced peak at low salt concentration, indicating the presence of repulsive intermicellar interactions. Model-independent real space information about the structure is obtained from a generalized indirect Fourier transformation (GIFT) technique in combination with a suitable model for the interparticle structure factor. The interparticle interaction is captured using the rescaled mean spherical approximation (RMSA) closure relation and a Yukawa form of the interaction potential. Further quantification of the geometrical parameters of the micelles was achieved by a complete fit of the SANS data using a prolate ellipsoidal form factor and the RMSA structure factor. The present study shows that PTHC induces a decrease in the fractional charge of the micelles due to adsorption at the micellar surface and consequent growth of the SDS micelles from nearly globular to rodlike as the concentration of PTHC increases.
Bibliographical noteFunding Information:
The authors acknowledge the support of the National Institute of Standards and Technology, US Department of Commerce, for the use of neutron research facilities. We are also thankful to J.A. Silas, B. Baser, and M. Labourt-Ibarre for assistance. We thank Dr. Steve Kline (NIST) for many fruitful discussions. This work was supported by the National Science Foundation (CTS-9814399).
- Quasielastic light scattering
- Small angle neutron scattering
- Sphere-to-rod transition