Kinetics of Adsorption of Cationic Surfactants at Charcoal-Water Interface

Kinetics of adsorption of cationic sufactants, cetyltrimethyl ammonium bromide (CTAB), myristyltrimethyl ammonium bromide (MTAB) and dodecyltrimethyl ammonium bromide (DTAB) at the hydrophobic surface of charcoal have been studied at various values of bulk surfactant concentration (C^t₂), pH, ionic strength and temperature. The adsorption process has been found to follow 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 E_al and E_a2 for both the kinetic steps have been evaluated. The corresponding values of enthalpies of activation (ΔH^#₁, and ΔH^#₂) and entropies of activation (ΔS^#₁, and ΔS^#₂) have been evaluated usign Eyring's equation for absolute reaction rate. It has been found that the value of T_av ΔS^# decreases with increase in the value of concentration of CTAB below its cmc. At the concentration range 0.15 mM to 0.30 mM, ΔH^#₁ < T_avΔS^#₁, so that in this concentration range the first step is controlled by entropy. But as C^t₂ approaches the me of CTAB, the value of ΔH^#₁, becomes greater than T_avΔS^#₁ and T_avΔS^#₂ increases with increase of (C^t₂), when it approaches the cmc of CTAB and also below cmc, so that the second step in the activated state is mainly controlled by entropy. Above cmc of CTAB, when C^t₂, is altered from 1.5 to 6.0 mM, the values of T_avΔS^#₁ and T_avΔS^#₂ vary in an irregular manner. Both pH and ionic strength have significant effects on the values of ΔH^#₁ and ΔH^#₂ The free energies of activation ΔG^#₁ and ΔG^#₂ are found to vary between 62 to 85 kJ/mole and ΔH^#₁ and ΔH^#₂ vary linearly with T_avΔS^#₁ and T_avΔS^#₂. An entropy-enthalpy compensation effect in the CTAB-Charcoal interaction has also been noted.