Probing the Three-Dimensional

Introduction
Atomic force microscopy (AFM)
provides a unique direct-visualiza-
tion tool to study the three-dimen-
sional structure of adsorbed sur-
factants on solid surfaces. In the
limited body of reported topogra-
phy data, however, the heights of
observed surfactant aggregates
were significantly lower than
expected values.
The reduced corrugation on an
AFM topography image is usually
due to the fact that the surface
adsorbates are soft compared to
the spring constant of the can-
tilever and the applied force is too
high. The technique commonly
used to image weakly adsorbed
surfactant aggregates in contact
mode AFM is to maintain a force
within the pre-contact double-
layer electrostatic interaction
range (1, 2). Generally, the force is
on the order of 101
to 102
pN. The
difficulty encountered in obtaining
uncompressed topography images
on adsorbed surfactant aggregates
seemed to suggest that such small
forces were still too high for the
soft surfactants.