Biomedical Engineering Reference
In-Depth Information
another for the accurate characterization of the sorption properties of
a material. In gravimetric and volumetric adsorption measurement,
the sample density or volume used defines the position of the
Gibbs
dividing surface
. This defines the surface that separates the adsorbent
from the gas phase, and is typically determined using helium. This
issue has attracted considerable attention in the literature due to
the errors that can be introduced into adsorption measurements,
in general, by inappropriate consideration of the potential effects
of helium adsorption. There is currently no standard method for
addressing the helium adsorption issue for microporous materials,
but we will briefly summarize some of the recent literature on the
topic below.
Firstly, however, we should note the practical difficulties in
determining and defining the density of real materials. Although
the theoretical density of a material is a relatively straightforward
property to calculate, in practice density determination can be
rather more complicated. We will consider four different density
definitions, following the discussion of Lowell
[6]. There are
two definitions that are of interest to us with regard to measurement
accuracy, and a further two that are of interest generally in the
application of materials for gas storage. The most important density
definition, in terms of measurement accuracy, is known commonly
as the
et al.
. This is the ratio of the mass to the volume
occupied by the sample, excluding the volume of any open pores. It is
the density that should be used in the buoyancy effect (Section 1.5.10)
and dead volume (Section 1.5.11) corrections, in the gravimetric and
volumetric techniques, respectively. This may be different to the
skeletal density
true
density
of a material, if we define the true density as the ratio of the
mass to the volume occupied by the sample, excluding all pores, both
open and closed.
4
The true and skeletal densities will be equal if the
material contains no blocked pore regions. The second definition,
which has come to prominence in recent research into hydrogen
storage using metal-organic frameworks, but is perhaps less relevant
to carbon nanomaterials, is the
. This
is the density calculated from the mass and the volume occupied
by the solid, including all internal pore space. Meanwhile, the two
definitions of practical interest for gas storage, which are therefore
relevant in the context of the comparison of the performance of
envelope
or
geometric density
4
Closed pores
, in this context, are internal volumes and voids that are inaccessible to
the adsorptive molecules.
