Chemistry Reference
In-Depth Information
4.4
SURFACE AND INTERFACE CHARACTERIZATION
Since the fi rst description by Luzzati in the early 1960s (Luzzati and Husson,
1962) studies of lyotropic liquid crystalline systems have predominantly
focused on the investigation of their interior nanostructure in solution, as
described in the earlier sections. However, in both biology and technological
applications, it is interfacial interactions that defi ne their properties (e.g., mem-
brane interactions and solute transport). As such, it is perhaps surprising that
relatively little attention has been given to interfacial characterization of LLC
systems. The application of advanced surface characterization techniques
would therefore seem to be a major opportunity in understanding and manipu-
lating LLC behavior. Herein, we will briefl y discuss two such techniques that
have recently been applied in this fi eld, atomic force microscopy and neutron
refl ectivity (Rittman et al., 2010; Vandoolaeghe et al., 2008, 2009a,b).
4.4.1
Atomic Force Microscopy
Characterization of cubic phases uses scattering techniques (SAXS and SANS)
allows the discrimination of different cubic phase symmetries and provides
information regarding dispersion particle size. However, SAXS is analogous
to powder diffraction in that it averages scattering over randomly oriented
micron-sized domains (i.e., dispersion particles or particle subdomains. The
data therefore provide little information regarding the interface of the cubic
phase with water or of the boundary between adjacent domains. Similarly,
many groups have imaged discrete single domains of cubic phase (Q
II
) as
cubosomes, using cryo-TEM (Gustafsson et al., 1996). However, it is not clear
to what extent they are representative of domains within a bulk polydomain
sample since they are stabilized by a co-surfactant that forms an additional
layer at the cubosome surface. The properties of domain boundaries and
defects are likely to strongly affect technological applications, viscoelastic
properties, and phase transition kinetics, yet understanding their nature has
been an enduring challenge both experimentally and theoretically (Andersson
et al., 1995; Boyd et al., 2007).
Atomic force microscopy (AFM) has the potential to address these chal-
lenges. It gives direct visualization and has suffi cient spatial resolution to
image individual water channels within a single domain, thus providing infor-
mation on domain orientation and epitaxy and domain boundaries. It can be
carried out under ambient conditions, in a temperature range consistent with
equilibrium lyotropic phase formation, under water or in air, and thus has
potential for structural analysis of LLC phases and dispersed particles, both
at equilibrium and during phase transitions.
AFM is a member of the family of scanned probe microscopies and was
fi rst developed by Binning, Quate, and Gerber in the 1980s (Strausser and
Heaton, 1994). The technique is based upon raster scanning a microscopically
sharp probe (a typical tip radius of curvature is
∼
10 - 50 nm) across a sample
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