Light absorption in solutions of cetyltrimethylammonium and cetylpyridinium bromides

Abstract

Absorption of monochromatic light in solutions of cetyltrimethylammonium (CTAB) and cetylpyridinium (CPB) bromides has been studied in the wavelength λrange of 190–1000 nm. The investigation has been performed at 30°C and surfactant concentrations ranging from 5 × 10⁻⁵ to 3.5 × 10⁻³ M. Spectra of solutions of LiBr, KBr, and CTAB in the premicellar concentration region have been shown to coincide with each other. They are unimodal, and the heights of their maxima at λ_max ≈ 193 nm depend on Br⁻ concentration alone. In contrast to CTAB solutions, the UV spectra of CPB solutions are characterized by the presence of two absorption maxima near λ_max1 = 191 nm and λ_max2 = 259 nm, as well as a shoulder at 210–218 nm. Their existence is caused by the presence of both Br⁻ anions and CP⁺ cations in the solutions. The dependences of the integral absorption in the examined wavelength range on CPB concentration exhibit inflections both in the premicellar region and upon the transition to micellar solutions, with these inflections characterizing the critical dimerization and micellization concentrations, respectively. For CTAB, this regularity has only been observed at the critical micellization concentration. The data on the concentration dependence of the light absorption have resulted in the proposal of a method for determining the degree β of counterion binding by micelles of ionic surfactants. The β values calculated for CTAB and CPB are equal to 0.89 ±0 0.1.