Role played by the head size of some non‐ionic surfactants on their adsorption onto montmorillonite clay

Abstract

AbstractSorption behavior of polyoxethylene(n)monooleate series [Ol(EO)n] onto montmorillonite clay was studied at 25°C to investigate the influence of the surfactant's head size on the sorption process. All the tested surfactants exhibited L‐shaped isotherms that means a strong interaction between the adsorbent and the adsorbate. Also, all the obtained isotherms ended with a drastic increase in the isotherm slope at nearly constant equilibrium concentration. This abrupt increase reflected the fairly high affinity of the tested surfactants to the clay surface at high bulk concentration region. The maximum amount adsorbed at the plateau region, Γmax, was calculated according to the Langmuir adsorption theory and followed the order: Ol(EO)14 > Ol(EO)20 < Ol(EO)40 < Ol(EO)80. In case of short ethylene oxide (EO) chain, Γmax decreased with the increase in the chain length; but a reverse result was obtained in case of surfactants with longer EO chain length (20 to 80 units). The free energy of adsorption, ΔG°ad, had negative values indicating the spontaneous adsorption of surfactant molecules onto clay. The values of ΔG°ad increased with increasing EO units from 14 to 20 units and decreased with further elongation in the EO chain from 20 to 80 units. Copyright © 2004 John Wiley & Sons, Ltd.