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A section of one greatly
enlarged particle of a solid.
1
The monolayer
of adsorbed
molecules; typically
15 - 20% saturation.
2
The multilayer capillary condensation
stage approximately 70% saturation.
3
Total pore volume
filling; approximately
100% saturation.
4
Dimensions:
Weight:
Electrical:
Environmental:
Physical Specifications
Height: 29.0 inches (73.6 cm)
Height Open: 44.0 inches (111.6 cm)
Width: 25.25 inches (63.7 cm)
Depth: 21.0 inches (53.3 cm)
Depth Open: 26.2 inches (66.5 cm)
60 kg (132 lbs.)
100 - 240 VAC, 50/60 Hz
10 - 38°C operating range at
90% maximum relative humidity
Accessories
Regulator Assembly
Proper Quadrasorb functioning is assured
when high-quality gas regulators are used.
Quantachrome supplies complete assemblies
which include two-stage regulators with dual
gauges, cylinder connector, isolation valve
and 1/8" gas line connector. The regulators
feature stainless steel, non-venting diaphragms
and the appropriate CGA fitting for specific
gases. Different assemblies are available for
nitrogen (and other inerts including helium),
hydrogen, carbon monoxide, oxidizing gases
etc.
Vacuum Pump (standard applications).
All vacuum-volumetric gas sorption
analyzers
require a good vacuum pump and the Quadrasorb-
SI is no exception. The pump shall have the
capability to pull an ultimate vacuum of 10 millitorr
or below. Quantachrome can supply the correctly
sized pump complete with oil, hoses and fittings.
You are not required to purchase the necessary
vacuum pump from Quantachrome, but if you do
the entire system will have been qualified in our
factory as a set, ensuring consistent performance.
Vacuum Pumps (krypton/micropore model).
Low pressure applications require a deeper
vacuum that can only be achieved by a turbomolecular
pump. The turbo pump is included,
and the correct backing pump can be supplied by
Quantachrome either as a rotary oil pump or as
an oil-free, dry diaphragm (membrane) pump.
Storage Dewars
For the convenience of having larger quantities
of liquefied gases on hand, Quantachrome offers
storage dewars in sizes ranging from 5 liters to
30 liters, plus a transfer device and trolley for the
largest size.
The Gas Sorption Process
Before performing gas sorption experiments,
solid surfaces must be freed from contaminants
such as water and oils. Surface cleaning
(degassing) is most often carried out by
placing a sample of the solid in a glass cell
and heating it under vacuum or flowing gas.
Figure 1 illustrates how a solid particle
containing cracks, orifices and pores of
different sizes and shapes might look after
pretreatment.
Once clean, the sample is brought to a
constant temperature by means of an
external bath. Then, small amounts of a gas
(the adsorbate) are admitted in steps into
the evacuated sample chamber. Gas
molecules that stick to the surface of the
solid (adsorbent) are said to be adsorbed
and tend to form a thin layer that covers
the entire adsorbent surface. Based on the
well-known Brunauer, Emmett and Teller
(B.E.T.) theory, one can estimate the number
of molecules required to cover the adsorbent
surface with a monolayer of adsorbed
molecules, Nm (Figure 2). Multiplying Nm
by the cross-sectional area of an adsorbate
molecule yields the sample’s surface
area.
Continued addition of gas molecules beyond
monolayer formation leads to the gradual
stacking of multiple layers (multilayers).
The formation occurs in parallel to capillary
condensation (Figure 3). The latter process is
approximately described by the Kelvin equation,
which relates equilibrium gas pressure
to the size of capillaries capable of condensing
gas within them.
As the equilibrium gas pressure
approaches saturation, the pores largely
completely fill with adsorbate (Figure 4).
Knowing the density of the adsorbate, one
can calculate the volume it occupies and,
consequently, the total pore volume of the
sample. If at this stage one reverses the
adsorption process by withdrawing known
amounts of gas from the system in steps,
desorption isotherms are generated. The
resulting hysteresis
leads to isotherm shapes
that can be mechanistically related
to those
expected from particular poreshapes.
Older calculation methods such as the one
by Barrett, Joyner and Halenda (B.J.H.) allow
the computation of pore sizes from equilibrium
gas pressures. One can therefore take
experimental curves (isotherms) of adsorbed
gas volumes versus relative pressures
and
convert them to cumulative or differential
pore size distributions.
Modern pore size models are based on
Non-local Density Functional Theory
(DFT)- a statistical mechanics approach
that allows one to describe the sorption
of gas molecules in nanoporous materials
at a molecular level. Hence, the application
of such microscopic methods produces
the most accurate surface area and pore
size results.
QUADRASORB™ SI
