STUDY OF THE ADSORPTION OF CATIONIC SURFACTANTS ON AQUEOUS LAPONITE CLAY SUSPENSIONS AND LAPONITE CLAY MODIFIED ELECTRODES

The adsorption of the cetyltrimethylammonium (CTA+) cationic surfactant by deionized or 0.05 M Na2SO4 aqueous laponite clay suspensions is compared with the adsorption of CTA+ in a laponite clay modified electrode (LCME). Incorporation of CTA+ in a LCME is achieved by soaking the modified electrode in an electrolytic aqueous solution of CTA+. The cyclic voltammetry parameters of the ferrocene (Fc), methylviologen (MV2+), and ferricyanide (Fe(CN)6(3-)) electroactive probes at the resulting CTA+-LCME readily correlate with the adsorption isotherms of CTA+ in laponite suspensions and with the steady-state pyrene probe fluorescence experiments. It is concluded that in the presence of 0.05 M Na2SO4, similar mechanisms govern the adsorption of CTA+ in a laponite clay film or a laponite clay suspension. Adsorption of CTA+ onto a laponite clay suspension leads first to flocculation because of face to face aggregation of the elementary laponite platelets and then to redispersion when adsorbed CTA+ exceeds the cation exchange capacity (cec) of laponite. Elementary platelets are not recovered and bigger colloidal laponite particles are obtained even at maximum CTA+ adsorption density. As a consequence, adsorbed CTA+ are located in two different environments: a fraction is intercalated in the interlamellar spaces as one-, two-, or three-layer complexes with a parallel orientation relative to the laponite basal plane surface. On the external surface of the laponite particles, an aggregative adsorption phenomenon of CTA+ occurs in a process similar to the hemimicellization of surfactants onto nonswelling solid surfaces. Steady-state pyrene fluorescence quenching and time-resolved pyrene fluorescence decay experiments are used to discriminate these two different organizations of adsorbed surfactants. Electroactivity of CTA+-LCME is proposed to arise from species located at the external surfaces of the laponite particles which compose the clay film, that is from the stacking defect zones.