Investigation of the alkyl chain lengths of surfactants on their adsorption by montmorillonite (Mt) from aqueous solutions

In this study, the behaviors of adsorption of six different quaternary ammonium salts onto montmorillonite (Mt) have been investigated. One of six cationic surfactants is branched (tetrabutylammonium bromide (C4)4ABr) and the others are linear (with five different hydrocarbon chain lengths, i.e. C16TABr, C14TABr, C12TABr, C10TABr, and C8TABr).The organoclays were characterized by spectroscopic techniques such as XRD and FT-IR. Their surface morphologies were also determined by using the SEM images. To reveal the adsorption mechanism, the measurements of contact angle and zeta potential of the samples were carried out. When the alkyl chain length increased, the adsorption ability of the surfactant adsorbed onto Mt surface increased. The effects of temperature, surfactant initial concentration and contact time onto the adsorption were investigated. The adsorption isotherms obtained show that the experimental isotherms exhibit a good fit with the Langmuir, Freundlich and Temkin adsorption models at all the temperatures, but the fit with models decreases with the decrease of chain length. The isosteric enthalpy and entropy changes were evaluated. The adsorption of surfactants onto Mt was found to be exothermic and spontaneous at all temperatures except for C10TABr adsorption. The interlayer distance and the packing density of surfactant ions in the interlayer region increased with increased chain length, as confirmed by the XRD patterns. FTIR spectrum and contact angle measurements of raw montmorillonite and organoclays indicate the incorporation of surfactants and the change in hydrophilicity of the organoclays. The values of zeta potential imply the presence of hydrophobic interactions between their hydrocarbon tails for C16TABr and partly for C14TABr adsorption. The negative values of zeta potential of other surfactants (C14TABr, C12TABr, C10TABr, C8TABr and (C4)4TABr) except for C16TABr compared to the raw clay indicate that the ion exchange is more effective according to electrostatic interactions in terms of the adsorption process.