Diverse Applications of Surface and Colloid Chemistry in Science and Industry - Surface and Colloid Chemistry: Principles and Applications - page 217

Afmsilica

Silica

FIGure 10.3 AFM sensor with SiO 2 sphere (schematic).

These data showed that the isolectric point (IEP) of SiO 2 was around pH 2, as
expected. The binding of phosphate ions to the chrome surface were also estimated
as a function of pH and ionic strength (Choi et al., 2004). Further, both STM and
AFM have been used to investigate the corrosion mechanisms of metals exposed to
the aqueous phase. Since both STM and AFM can operate under water, this provides
a variety of possibilities.

10.2.1 d o m a I n p a T T e r n S I n m of in of m of l e c u l a r f I l m a S S e m b l I e S

Self-assembly monomolecular films (SAMs) at the air-water interface are struc-
tures that are very sensitive to external forces. For example, if these monomolecular
assemblies are compressed to certain values, some drastic rearrangements will have
to take place. Monolayers of lipids exhibit changes in structures near or after the
collapse state, which has been designated as domains . The spontaneous formation
of domain assemblies in monomolecular films of amphiphiles at the air-water (oil-
water interface needs) interface has evoked great interest. During the past decade,
an extensive number of investigations have been carried out by using different
microscopes with varying sensitivities. The latter leads to different kinds of domain
dimensional sizes.
There are two kinds of domains:

Macro domains
Molecular domains

as determined by the detection method.
Macro domains of micrometer size have been reported when using an ordinary
microscope. The typical procedure is to use 2% of a fluorescent lipid analog, which
renders visible a domain pattern. This, of course, assumes that the fluorescence moi-
ety has no effect on the assembly structure. There is (generally) no information about
the thickness of the domains. The shapes of domains are varied, and very complex
(circular or near-circular domains, parallel stripes, or more irregular, wormlike, or
similar kinds of structures).
Molecular domains have been observed using the AFM analyses of LB films of a
collapsed state (Birdi, 1997, 2003). For example, cholesterol films showed half-but-
terly -shaped domains (each domain consisting of 10 7 molecules; Figure 10.4). This
quantity was estimated from the following data: the height of the domains was 90
Å, which corresponds with six layers of the cholesterol molecule (length of the mol-
ecule is found to be 15 Å from molecular models). AFM image analysis is capable