(506d) Adsorption Isotherms And Dynamics Of Adsorption For Aqueous Ctab And C12E6 Surfactants On Three Surfaces: Experimental Data And Theoretical Interpretation

Several prior studies were conducted to understand the driving forces for the adsorption of surfactants on different surfaces [Paria and Khilar, Advances Colloid Interface Science, 110 (2004) 75]. Such studies generally compare adsorption isotherms for one surfactant on different surfaces, or adsorption isotherms for various surfactants on one surface. We report here experimental studies on the adsorption from aqueous solution of two surfactants (one ionic and one nonionic) on three different surfaces. We employ the quartz crystal microbalance technique (QCM-D) to measure the equilibrium adsorption of CTAB and C12E6 on gold, alumina and silica surfaces. The QCM-D method allows us not only to assess the equilibrium adsorption isotherms, but also, because the sensing elements are flat and non-porous, to monitor in real time the dynamics of adsorption. The results obtained show that the adsorption of nonionic surfactant is slower than that of the cationic surfactant. The equilibrium adsorption isotherms strongly depend on the chemical composition of the solid surfaces. Silica adsorbs slower than the two other surfaces. Dry AFM, XPS and contact-angle experiments were performed on the adsorbing surfaces to determine the origin of the differences observed among the adsorption isotherms. AFM results show a somewhat important difference in the roughness between the three crystals, i.e., silica surfaces appear somewhat rougher at the molecular scale. We will discuss our results in perspective of molecular theories for both the equilibrium and kinetics of surfactant adsorption, with important applications in the optimization of industrial processes that involve surfactant adsorption.