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Abstract:
Capillary hysteresis in
cylindrical nanopores
has
been
studied using
MCM-41
as the
prime
example
of
a
mesoporous
material.
These
materials, due
to
their
regular
pore
structure,
can
be
considered
to
be
candidates
for
reference
adsorbents
for
standardizing adsorption measurements and
methods
for
characterization
of
porous
solids. They
provide
a
unique opportunity
for
verification
of
theoretical
models
employed
for
predicting phase equilibrium in
confined
geometry.
Three samples with
monodisperse
pore
channels have
been
synthesized and
examined
using X-ray
diffraction
(XRD).
Nitrogen
adsorption
isotherms
were
modeled
using
nonlocal
density functional theory
(NLDFT)
in
a
wide
range
of
pore
sizes
(18-80
A).
Theoretical isotherms
for pore
channels with
sizes corresponding
to
those identified
by
XRD
were
compared
with experimental isotherms
at
different
temperatures
between
70
and
82
K.
The
latter
have
been
measured
independently
on
two
different adsorption
setups.
The
theoretical
thermal
dependence
ofthe
thermodynamic
adsorption-desorption hysteresis
predicted
by
NLDFT
is
confirmed
by
the
experimental measurements.
It
is
shown
that
at
77.4
K
NLDFT
quantitatively predicts equilibrium phase transitions in
cylindrical
channels
of
MCM-41.
Theoretical and experimental results
prove
that
the
nitrogen hysteresis
observed
at
temperatures
below
77.4
K
is associated
with metastability
of
the
adsorption branch
of
the
isotherm.
The absence
of
experimental hysteresis
on
samples with
pore
size
of
about
40
A
at
temperatures
above
77.4
K
cannot
be
explained
by
the
capillary critical temperature
for
a
given pore
size being
achieved
as
was
assumed
previously
