BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 100

secondary micropores. Since the recent recipes for the preparation and manu-
facturing of new carbonaceous adsorbents are still developed, the studies of the
methods for porosity characterization should be improved and this improvement
should enhance the method sensitivity. It is not surprising that the above topics
attract a lot of attention. Moreover, a systematic numerical investigation of the
effect of bimodality of PSD and its sensitivity on the reconstruction has not been
published yet. Some researchers observed that the micropore size distributions
determined from experimental isotherms (GCMC and DFT) usually show minima
near two and three molecular diameters of the effective pore width (1 nm), re-
gardless of the simulation method used. The proposed explanation was that this
is a model induced artifact arising from the strong packing effects exhibited by a
parallel wall model. Moreover, the inclusion of surface heterogeneity in the DFT
model used to generate the local adsorption isotherms (while more realistic) did
not change this observation significantly. Scientists argued that these artificial
minima are primarily due to the homogeneous nature of the model (a sharp mono-
layer formation occurring at approximately the same relative pressure in most of
the pores, and the second layer existence in pores between 1.0 and 2.0 nm) that
can accommodate four or more layers forming at the relative pressure of ca. 0.1.In
the other words, since all theoretical isotherms in wide pores exhibit a monolayer
step at about the same pressure, the contribution from pores that fill at this pres-
sure would have to be reduced. This compensation effect is responsible for the
observed minima on the pore size distribution curves, and also for the deviations
between best fit and experimental isotherms. Similarly, less pronounced minima
occur for pores, which fill at the relative pressures corresponding to the formation
of the second, third, and higher layers. In addition, scientists investigated the ef-
fect of heterogeneity of the pore walls (differences in the pore wall thicknesses)
on the PSD shape. They pointed out that the influence of the pore wall thickness is
significant, especially for lower values of relative pressures since the intermolec-
ular potential is dominated by interactions with this wall. Consequently, the pore
size distributions are shifted to smaller pores as the pore wall thickness decreases.
However, the minima at 1and 2 nm were observed for all systems studied. The
impact of the boundary values of nitrogen relative pressure (p/ps) was analyzed
on the basis of the observed alterations in PSDs by many scientits.
They observed that the changes in microporosity and mesoporosity of acti-
vated carbons can be described adequately only when the range of p/p s is as wide
as possible. The PSD curves can be broadened with shifted maxima especially for
micropores and narrow mesopores when adsorption data start at relatively high p/
ps value. However, the differences between the respective pore size distribution
functions are insignificant. The influence of a random noise on the stability of
the solution of the integral equation (calculation of PSD) was also studied on the
basis of theoretically generated isotherms. For low and medium noise levels, the
reproducibility of some perturbed isotherms was good. Summing up, the “true”