Simulation of adsorption at rotating disk

Abstract Diffusion-controlled adsorption at the rotating disk is calculated for Frumkin isotherm. The near equilibrium isotherms are simulated and their time components are analysed. The purpose of the work is to investigate the physical meanings of S-shaped isotherms that appear for strong attractions in the adsorbed layer. It is demonstrated that the surface coverage that is created gradually, by the diffusion process, consumes the concentration gradient of the dissolved surfactant in the diffusion layer. Once the equilibrium with the bulk of the solution is established, the driving force of diffusion-controlled adsorption vanishes and no additional adsorption is possible. The diffusion layer thickness on the rotating disk tends to the constant value and the flux depends only on the concentration of dissolved adsorbent at the disk surface. Hence, the equilibrium with the bulk of solution is established faster than at the stationary disk. This equilibrium takes an infinite amount of time to reach, but the system can approach close to the equilibrium within a definite period of time. The near equilibration time depends on the bulk concentration of surfactant. These relationships exhibit sharp maxima under the influence of attractions in the adsorbed layer.