Kinetics of Adsorption of Cationic Surfactants at Silica-Water Interface

Abstract

The kinetics of adsorption of cationic surfactants (CTAB, MTAB and DTAB) at silica surface has been studied at various values of bulk surfactant concentration (Ct2), pH, ionic strength, and temperature and in presence of different electrolytes and urea. The adsorption process has been found to follow a two-step first-order kinetic rate equation with two different rate constantsk1andk2. From the variation ofk1andk2with temperature, values of energies of activationEa1andEa2for both the kinetic steps have been evaluated. The corresponding values of enthalpies of activation (ΔH1#and ΔH2#), entropies of activation (ΔS1#and ΔS2#) have been evaluated using Eyring's equation for absolute reaction rate. It has been found that for both the kinetic steps, ΔH1#<TavΔS1#and ΔH2#<TavΔS2#, which means that activation reaction is largely entropy controlled. Again, for both kinetic steps, ΔH#varies linearly withTavΔS#, and ΔG1#and ΔG2#vary between 70 and 88 kJ/mole of surfactant, respectively. Thus there is a entropy–enthalpy compensation effect in the adsorption process.