Adsorption of anionic surfactants onto sepiolite

Anionic surfactants constitute the main ingredients of detergents and a number of surfactant formulations used in a spectrum of diverse industries. The aim of this study is to examine the amenability of natural sepiolite to the adsorption of anionic surfactants, sodium dodecylsulfate (SDS) and sodium dodecylbenzenesulfonate (SDBS). Adsorption isotherms exhibit three regions with distinctly different slopes. The first region is characterized by the complexation of anionic surfactants with Mg²⁺ ions at the octahedral sheet or hydrogen bonding between the oxygen groups of anionic head groups of surfactant and H⁺ of the bound or zeolitic water. The Mg²⁺ ions released from sepiolite leads to the precipitation of magnesium salt of surfactant in the second region. The third region marks both the beginning of plateau region and micellar dissolution of the precipitate. The effect of temperature on surfactant adsorption for SDS/sepiolite system was utilized to calculate such thermodynamic parameters as the free energy of adsorption (Δ G_ads^°) and the heat of adsorption (Δ H_ads^°). The low value of Δ H_ads^° (1.87 kJ/mol) is an evidence for the physical adsorption of anionic surfactants onto sepiolite. The relatively large value of entropic contribution (- T Δ S_ads^°) indicates that the adsorption of anionic surfactants onto sepiolite is entropically governed.