Transient light scattering in aqueous surfactant systems

Abstract

A new phenomenon in aqueous surfactant systems is presented. Laser-induced timedependent changes in the H ν , V ν , and D ν light scattering at 90° have been measured in aqueous solutions of ionic surfactants both above and below the CMC. Parameters of the effect were determined in detail for cetyltrimethylammonium bromide, but the effect was seen in cetyltrimethylammonium chloride, sodium oleate, sodium dodecyl sulfate, and N,N,N-trimethylaminododecanimide as well. Other surfactant systems (oleyl- , tridecyl- , and nonylphenolethylene oxide adducts) did not exhibit the phenomena. The H ν and V ν intensities decayed in an exponential fashion from the time of first illumination with a time constant of the order of ∼2 sec for incident power densities of ≥50 W/cm 2 from an argon ion laser. Lesser incident powers produced longer decays. Evaluations of the dependence on temperature, concentration, and sampling were also made. The effect seems most consistent with a low concentration of a highly anisotropic structure in solution even below the CMC.