Biomedical Engineering Reference
In-Depth Information
3.3.1.4
Porosimetry by Gas Absorption
The gas absorption technique for pore measurement is well established and sound. It
requires dry specimens so is useful for dry biomass applications but has limited utility for
understanding wet processing such as enzymatic hydrolysis of lignocellulose in biorefin-
ing. The technique is based on the Brunauer-Emmett-Teller (BET) equation (41), where
the volume of gas absorbed in a monolayer,
v
m
, is related to the ratio of the equilibrium
(
P
) and saturation (
P
0
) pressures and the total gas volume absorbed,
υ
:
P
P
0
1
v
[
(P
0
/P )
−
c
−
1
v
m
c
1
v
m
c
1]
=
+
(3.5)
Therefore
v
m
and
c
(BET constant) can be obtained from fitting Equation (3.5) to the
experimental data of
P/P
0
and
v
. The specific surface is then evaluated using
(v
m
Ns)
V
S
BET,total
=
(3.6)
where
N
is Avogadro's constant,
s
is adsorption cross section (area per adsorbed
molecule), and
V
is molar volume of the adsorption gas. Water sorption on cellu-
lose powder was studied by comparing the adsorption of water and nitrogen with BET
theory (42).
3.3.1.5
Enzymatic Hydrolysis Rate
The rate of enzymatic degradation of cellulose is especially important for those attempt-
ing to use biomass-derived glucose for fermentation. This assay typically applies a
standard cellulase to biomass and measures the rate of cellulose depolymerization.
Reducing aldehydes produced by the depolymerization are commonly detected by color
reactions with reagents such as Cu(II) or ferrocyanide. For further discussion, we sug-
gest a recent review (43) because there are too many variations on this method to discuss
here. Access of the enzymes to the substrate is critical to hydrolysis, and this accessi-
bility can be elegantly assayed (43). Caution should be used when comparing cellulase
activities because the size of the substrate, in addition to porosity, can influence observed
cellulase activity when diffusion is limiting (27).
3.3.2
Cellulose Crystallinity
Cellulose crystallinity is often measured because cellulose crystals (crystallites, whiskers)
have a slow degradation rate, high strength, high aspect ratio, and ability to form chiral
nematic liquid crystals. Assessing the mass fraction and size of crystals is commonly
accomplished by NMR or X-ray diffraction, but Raman and infrared (IR) methods can
also be used. Crystal size can be increased by allowing structures to relax at high tem-
perature and humidity, such as during kiln drying of wood, and decreased by mechanical
damage, such as ball milling or swelling. Although we discuss cellulose crystals, a more
proper term might be 'ordered domains' because they do not technically meet all the
requirements of a perfect crystal. Not only is there a large number of defects in the
directions perpendicular to the cellulose chain, but there appears to be a slow twist to
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