Surfactant adsorption kinetics by total internal reflection Raman spectroscopy. 2. CTAB and Triton X-100 mixtures on silica.

Total internal reflection (TIR) Raman spectroscopy has been used to study the kinetics of adsorption, desorption, and displacement of mixed surfactant systems at the silica-water interface. The limited penetration depth of the evanescent wave provides surface sensitivity while the chemical sensitivity of Raman scattering permits the determination of the time-dependent composition of the adsorbed film. Principal component analysis is used to deconvolute the Raman spectra with a time resolution of 2 s and a precision of 5% of a monolayer. Both equilibrium and kinetic measurements are presented for the cetyltrimethylammonium bromide (CTAB)/Triton X-100 system over a range of concentrations and compositions. For a total concentration of 2 mM, the adsorption isotherm shows strong synergistic behavior with the addition of small amounts of CTAB (∼2% of the total surfactant) doubling the adsorbed amount of Triton X-100. This synergism has a marked influence on the kinetics: for example, when Triton X-100 replaces CTAB, the Triton X-100 surface excess overshoots its equilibrium value and returns only very slowly to equilibrium. For systems above the cmc, the repartitioning of surfactant between micelles and monomers results in unexpected behavior during exchange or rinsing of mixed surfactant solutions. For example, during rinsing, the more rapid diffusion of CTAB away from the surface leads to a local increase in the monomer concentration of Triton X-100, resulting in a temporary spike in the Triton X-100 surface excess. Displacement kinetics of CTAB by TX-100 and vice versa are generally slower than the adsorption or desorption of the pure surfactants but cover a wide range of kinetic time scales, depending on the details of the compositions and concentrations of the initial and final solutions.