In Situ Characterizations of Porous Media for Applications in Biofuel Cells: Issues and Challenges - Porous Media: Applications in Biological Systems and Biotechnology - page 505

φ ) can thus be obtained or estimated. One
may often find that the derived surface mass density can be a useful and
easy-to-comprehend parameter to represent the surface property of the film
or porous medium as Γ = d

The porosity (1 − θ )or(1 −

ρ i is the intrinsic density of the
material constitutes the mass of the film or the porous medium. Depending
on the purpose of study and experimental conditions, there are variations
in the data reduction to reach reliable interpretation (de Feijter et al. 1978;
Cuypers et al. 1978; Stenberg et al. 1980; Cuypers et al. 1983; Stenberg and
Nygren 1983). More detailed description of these variations can be found in
the discussion by Arwin (2005).
A very useful application in the biofuel cell development is to understand
the behavior of adsorption of bio- or macromolecules into a porous medium.
Due to the shear size of the biomacromolecules, their interaction with a porous
medium or thin film creates a complicate boundary that makes the ellipsomet-
ric optical model dicult to apply. Microscopically, this interaction between
macromolecules and the pore surface makes no difference to the adsorption
of such molecules on any model surface. Macroscopically, in the ellipsometric
perspective, this interaction can be treated as adsorption into porous medium
with volume filling in an effective-medium theory as proposed by Bruggeman
(von Bruggeman 1935). Before adsorption, the porous medium needs to be
characterized with spectroscopic techniques to obtain the porosity profile and
relevant pore size information. There are three constituents involved in the sys-
tem: the porous medium bulk, the ambient medium, and the macromolecules,
of which their optical properties need to be determined. If the porous struc-
ture does not possess an iso-symmetric geometry, anisotropic effects due to
form birefringence needs to be considered. There might be other anisotropic
effects including those arising from ordering of the alignment of the molecules
in the adsorption layer (den Engelsen 1976; Sano 1988; Tronin and Konstanti-
nova 1989; Schubert 1996). Another important issue is the surface roughness,
which often makes the optical model interpretation dicult due to light scat-
tering that undermines the quantitative sensitivity. As a matter of fact, the
presence of macromolecules itself creates an inherent roughness toward light
scattering, and such a technical surface roughness is of the order of or larger
than the macromolecule. In the case that the lateral roughness is larger than
the wavelength of the incident light, the loss of the intensity due to scattering
can be determined by the porous structure characterization as reflected in
the optical property measurements of the porous medium. Depending on the
severity and lateral size of the roughness, this rough boundary can be modeled
as an equivalent thin effective-medium layer. Nevertheless, roughness remains
a tough issue for quantitative analysis of the adsorption in porous medium
with rough surface.
Recently, Murray et al. (2002) made a comparison of complementary tech-
niques among laser-generated surface acoustic waves (LSAW), ellipsomet-
ric porosimetry (EP), Rutherford backscattering (RBS), and nanoindenta-
tion to study a range of mesoporous xerogel low- k dielectric films for their

ρ i θ (or φ ), where