Kinetics of adsorption of cationic surfactants at silica-water interface

The kinetics of adsorption of cationic surfactants (CTAB, MTAB and DTAB) at silica surface has been studied at various values of bulk surfactant concentration (C-2(t)), 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 constants k(1) and k(2). From the variation of k(1) and k(2) with temperature, values of energies of activation E-a1 and E-a2 for both the kinetic steps have been evaluated. The corresponding values of enthalpies of activation (Delta H-1# and Delta H-2#), entropies of activation (Delta S-1# and Delta S-2#) have been evaluated using Eyring's equation for absolute reaction rate. It has been found that for both the kinetic steps, Delta H-1# < T(av)Delta S-1# and Delta H-2# < T(av)Delta S-2# which means that activation reaction is largely entropy controlled. Again, for both kinetic steps, Delta H# varies linearly with T(av)Delta S#, and Delta G(1)# and Delta G(2)# vary between 70 and 88 kJ/mole of surfactant, respectively. Thus there is a entropy-enthalpy compensation effect in the adsorption process. (C) 1998 Academic Press.