Adsorption of hexadecyltrimethylammonium bromide to mica: Nanometer-scale study of binding-site competition effects

We have changed the structure of an adsorbed surfactant layer by modifying the nature of the interface in situ. Muscovite mica contains surface anions that can bind to a variety of cations in aqueous solution. Using an atomic force microscope (AFM), we have investigated the influence of the adsorption of the salts HBr, KBr, and N(CH2CH3)(4)Br on the adsorption of hexadecyltrimethylammonium bromide (CTAB) to mica. In the absence of salt, at twice the bulk critical micelle concentration, CTAB initially forms cylindrical surface micelles on mica. The cylinders transform to a flat bilayer structure within 24 h. The introduction of 10 mM K+ produces cylindrical aggregates that are stable, and a further increase in the concentration of K+ produces defects in the cylinders. These defects consist of aggregate termini and changes in the direction of the long axis of single aggregate. More defects are introduced by H+ than by K+ (at the same concentration). This is consistent with the known higher binding constant of H+ to mica. Using the introduction of defects as an indicator of the adsorption of cations in the presence of CTA(4), we find that CTAB greatly slows adsorption of H+ but that the speed of K+ adsorption is not noticeably affected. The adsorption of K+ produces structures that are sensitive to the force that is applied by the AFM tip. At a critical repulsive force, the image changes discontinuously from a defective cylinder structure to a spherical or flattened disklike structure.