In Situ Characterizations of Porous Media for Applications in Biofuel Cells: Issues and Challenges - Porous Media: Applications in Biological Systems and Biotechnology - page 499

13.4 Needs for in situ Characterization: Issues and
Challenges
As many porous media in various forms and structures are being used as elec-
trode supports, immobilization matrices, or separators, and having unique
functionalities operable in aqueous solution at nearly neutral pH, their per-
formance and behavior should be better understood by in situ characteriza-
tion techniques in a noninvasive manner. In the quest of finding useful in
situ characterization techniques that can be applied to studying the behavior
of these porous media, one often found that many commonly known in situ
techniques might not be useful or feasible for such studies in aqueous solution.
For instance, although atomic force microscopy (AFM) can be a useful tool
for studying the surface morphology of a solid, with high spatial resolutions,
it would be awkward to study a porous medium, where open pores are part
of the morphological features. AFM is also dicult for studies in a dynamic
situation where its temporal resolution is poor for a large surface area. It is
therefore interesting to realize that there are only few in situ characterization
techniques available for studying solid-liquid interfaces in solution to extract
physical and chemical information with temporal and spatial resolutions ten-
tative for kinetic and dynamic studies.
There is further complication for in situ characterizations due to a sample's
small quantity or physical dimensions. In light of more recent interests in nano-
materials and nanotechnologies, such in situ characterizations may become
problematic for porous media in small quantities or thin film forms. Tradi-
tional porosimetry (gas adsorption Brunauer-Emmett-Teller (BET) or mer-
cury porosimetry) usually require a large amount of sample. These techniques
are also ex situ, thus the sample properties could be altered by the preparation
or characterization process. For in situ characterizations of porous films (in
solution), it is more desirable to determine pore structure in solution or oper-
ating environment, in which factors that can affect the bulk (such as swelling
due to hydration) and interfacial (such as wettability due to hydrophobic and
hydrophilic interaction) properties can be investigated. One should anticipate
that the properties determined in the solution and operating environment
could be substantially different from those under dry and/or rigid conditions,
especially the properties associated with a solid/gas or solid/liquid interface.
Such a detailed solid/liquid interface characterization is a critical point of
discussions in this chapter.

13.5 Techniques

13.5.1 Spectroscopic Imaging Ellipsometry
Ellipsometry is an optical technique that is known for a century, versatile and
powerful for the investigation of a material on its dielectric properties (complex

Applicable in situ