FORMATION OF CATIONIC ANIONIC MIXED SURFACTANT BILAYERS ON LAPONITE CLAY SUSPENSIONS .2. FLUORESCENCE PROBE STUDIES

The fluorescent pyrene probe is used to gain information about the mixed adsorption of anionic dodecylsulfate DS- and cationic cetyltrimethylammonium CTA+ or dioctadecyldimethylammonium DODMA+ onto a laponite clay colloid. The experimental results from pyrene probe fluorescence, zeta potential measurements and quenching experiments correlate with the adsorption isotherms and correspond with the formation of a mixed cationic/anionic surfactant bilayer: the first layer of cationic surfactant is cation-exchanged on the anionic clay sites. The second layer of anionic DS- is adsorbed tails to tails through van der Waals interactions with the hydrophobic first layer and projects its sulfate headgroups into the water phase. At the onset of DS micellization, the CTA+-DS- bilayer desorbs and leads to the formation of mixed CTA+-DS- micelles in the bulk. In contrast, with DODMA+ which is unable to form mixed micelles with DS-, the DODMA+-DS bilayer does not desorb from the clay surface in the presence of DS- micelles. The formation of a mixed bilayer leads to the redispersion of the cationic surfactant exchanged laponite flocs and allows the formation of a very stable colloid in which the hydrophobic nature of the clay is dramatically improved. A fluorescence emission study demonstrates that the mixed bilayers provide a micelle-like environment to solubilized pyrene. Excimeric emission and steady-state quenching experiments give data on the micelle-like properties of the bilayer.