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 (Ct<INF POS="STACK">2), 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 k1 and k2. From the variation of k1 and k2 with temperature, values of energies of activation Ea1 and Ea2 for both the kinetic steps have been evaluated. The corresponding values of enthalpies of activation (DeltaH<INF POS="STACK">1# and DeltaH<INF POS="STACK">2#), entropies of activation (DeltaS<INF POS="STACK">1# and DeltaS<INF POS="STACK">2#) have been evaluated using Eyring's equation for absolute reaction rate. It has been found that for both the kinetic steps, DeltaH<INF POS="STACK">1# < TavDeltaS<INF POS="STACK">1# and DeltaH<INF POS="STACK">2# < TavDeltaS<INF POS="STACK">2#, which means that activation reaction is largely entropy controlled. Again, for both kinetic steps, DeltaH# varies linearly with TavDeltaS#, and DeltaG<INF POS="STACK">1# and DeltaG<INF POS="STACK">2# vary between 70 and 88 kJ/mole of surfactant, respectively. Thus there is a entropy-enthalpy compensation effect in the adsorption process. Copyright 1998 Academic Press.