AFM study of adsorption of cationic surfactants and cationic polyelectrolytes at the silica-water interface

The adsorption of cationic polyelectrolytes and cationic surfactants from a queous solution onto silica substrates was examined using atomic force micr oscopy (AFM) in surface force mode and in surface imaging mode. The polymer s were poly(diallyldimethylammonium chloride) (PDADMAC), poly-L-lysine hydr obromide, and polyvinylbenzyltrimethylammonium chloride, and the surfactant s were hexadecyltrimethylammonium chloride (CTACl) and hexadecyltrimethylam monium bromide. CTACl forms micelles at the interface between micellar CTAC l solutions and silica. These micelles desorbed when the CTACl solution was replaced with water. Am adsorbed layer of CTACl hindered adsorption of PDA DMAC. This is because CTACl generates a surface change that has the same si gn as the polymer. When PDADMAC adsorbed in the absence of CTACl, it formed a featureless, neutral layer. The PDADMAC did not desorb, even after exten ded rinsing with water. When an adsorbed layer of PDADMAC was exposed to CT ACl solution above the critical micelle concentration, the AFM image and su rface force are very similar to those observed when CTACl adsorbs to silica . This adsorbed layer is either spheres or hemispheres.