Wa. Ducker et Ej. Wanless, Adsorption of hexadecyltrimethylammonium bromide to mica: Nanometer-scale study of binding-site competition effects, LANGMUIR, 15(1), 1999, pp. 160-168
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 adsorpti
on of the salts HBr, KBr, and N(CH2CH3)(4)Br on the adsorption of hexadecyl
trimethylammonium bromide (CTAB) to mica. In the absence of salt, at twice
the bulk critical micelle concentration, CTAB initially forms cylindrical s
urface micelles on mica. The cylinders transform to a flat bilayer structur
e within 24 h. The introduction of 10 mM K+ produces cylindrical aggregates
that are stable, and a further increase in the concentration of K+ produce
s 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 consi
stent with the known higher binding constant of H+ to mica. Using the intro
duction of defects as an indicator of the adsorption of cations in the pres
ence of CTA(4), we find that CTAB greatly slows adsorption of H+ but that t
he speed of K+ adsorption is not noticeably affected. The adsorption of Kproduces structures that are sensitive to the force that is applied by the
AFM tip. At a critical repulsive force, the image changes discontinuously f
rom a defective cylinder structure to a spherical or flattened disklike str
ucture.