Development and validation of convective‐diffusion models to analyse the transient behaviour of a packed bed: Factors influencing aqueous phase adsorption of a bisbiguanide

Near Equilibration Times in the Diffusion-Controlled Adsorption

“…An adsorption is the process of accumulation of molecules at the interface. − If the boundary between two phases is created suddenly, the adsorption will result in the diminished concentration of these molecules in the vicinity of the interface. This is the reason for the diffusion-controlled adsorption. , If the rates of adsorption and desorption are much higher than the rate of the interface creation, the former processes appear equilibrated at any measurement time. , This is local equilibrium, and both the amount of adsorbate and the concentration of dissolved surfactant at the interface increase under the influence of flux of adsorptive from the bulk of solution. , When the concentrations of later at the interface and in the bulk become equal, the flux vanishes and the general equilibrium is established. − It is expressed by the adsorption isotherm, that is, the equilibrium relation between the quantity of the adsorbed material and the concentration in the bulk solution at constant temperature. − Usually, the rate of equilibration is enhanced by convection so that the diffusion is restricted to the thin layer at the interface. − The term kinetics of adsorption frequently includes this transport component. − The rotating disk is a good solid–liquid interface because of precisely defined convection and diffusion fluxes. , The adsorption at this type of interface can depend on interactions between adsorbed molecules, which is described by Fowler–Guggenheim or Frumkin isotherm. − These isotherms are developed for the monolayer adsorption using the kinetic principle that the rates of adsorption and desorption are equal in the equilibrium. The first rate is proportional to the product of bulk concentration of dissolved surfactant and the free fraction of the surface of the adsorbent, while the second rate is proportional to the occupied fraction of the surface. If there is lateral interaction of the adsorbed molecules, the isotherm includes the term that corresponds to the energy of interaction. , The parameters of the isotherm are thermodynamic quantities and must be measured under bulk equilibrium conditions.…”