Kinetics of Adsorption of Cationic Surfactants at Charcoal-Water Interface

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Authors

  • Department of Food Technology and Biochemical Engineering Jadavpur University, Calcutta-700 032 ,IN
  • Centre for Cellular and Molecular Biology, Uppal Road, Hyderabad - 500 007 ,IN

DOI:

https://doi.org/10.18311/jsst/1998/2101

Abstract

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 (Ct2), 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 Eal and Ea2 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 Tav 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 < TavΔS#1, so that in this concentration range the first step is controlled by entropy. But as Ct2 approaches the me of CTAB, the value of ΔH#1, becomes greater than TavΔS#1 and TavΔS#2 increases with increase of (Ct2), 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 Ct2, is altered from 1.5 to 6.0 mM, the values of TavΔS#1 and TavΔS#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 TavΔS#1 and TavΔS#2. An entropy-enthalpy compensation effect in the CTAB-Charcoal interaction has also been noted.