Micromeritics TRISTAR II PLUS Operator's Manual page 288

B DFT Models
P S
ORE IZE
It is well established that the pore space of a mesoporous solid fills with condensed adsorbate at
pressures somewhat below the prevailing saturated vapor pressure of the adsorptive. When
combined with a correlating function that relates pore size with a critical condensation pressure,
this knowledge can be used to characterize the mesopore size distribution of the adsorbent. The
correlating function most commonly used is the Kelvin equation. Refinements make allowance for
the reduction of the physical pore size by the thickness of the adsorbed film existing at the critical
condensation pressure. Still further refinements adjust the film thickness for the curvature of the
pore wall.
The commonly used practical methods of extracting mesopore distribution from isotherm data
using Kelvin-based theories, such as the BJH method, were for the most part developed decades
ago and were designed for hand computation using relatively few experimental points. In general,
these methods visualize the incremental decomposition of an experimental isotherm, starting at
the highest relative pressure or pore size. At each step, the quantity of adsorptive involved is
divided between pore emptying and film thinning processes and exactly is accounted for. This
computational algorithm frequently leads to inconsistencies when carried to small mesopore
sizes. If the thickness curve used is too steep, it finally will predict a larger increment of adsorptive
for a given pressure increment than is actually observed; since a negative pore volume is non-
physical, the algorithm must stop. Conversely, if the thickness curve used underestimates film
thinning, accumulated error results in the calculation of an overly large volume of (possibly
nonexistent) small pores.
The use of equation (1) represents an improvement over the traditional algorithm. Kernel
functions corresponding to various classical Kelvin-based methods have been calculated for
differing geometries and included in the list of models.
M I
ODELS NCLUDED
Kelvin Equation with Halsey Thickness Curve
N2 - Halsey Thickness Curve
Geometry:
Substrate:
Category:
Method:
The kernel function is calculated using the Halsey equation with standard parameters:
The nitrogen properties used in the Kelvin equation are:
B - 14
Slit
Average
Porosity
Nitrogen 77 K
TriStar II Plus Operator Manual
303-42800-01 (Rev M ) — Sep 2023