Functional Materials Group
for measuring pore size distributions.
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Dr. J.B.W. Webber, Prof. J.H. Strange.
We have developed a novel method of determining median pore size
and pore size distributions of porous materials, both on bulk
as a function of spatial position inside a sample with structured
NMR cryoporometry is based on the Gibbs equations; the technique involves
freezing a liquid in the pores and measuring the melting
temperature by Nuclear Magnetic Resonance. Since the melting
point is depressed for crystals of small size, the melting point
depression gives a measurement of pore size.
The method is non-destructive, and
is suitable for pore diameters in the scale range of a few
nanometers to just over 1 µm.
Studying Structured Matter : Nanoparticles, Porous Media
At Lab-Tools, and in the Functional Materials Group at the University of Kent, we
specialise in studying structured and divided matter, primarily by a
variety of Nuclear Magnetic Resonance, X-ray and
Neutron Scattering techniques. These
give complementary information. One may generate model porous structures and
study them by analytic and numerical techniques, and then compare the predicted
properties with the measured ones.
A crucial deciding factor on which technique to use is the length
scale of the structure that one wishes to study.
NMR Cryoporometry is well suited to studying structure on a scale of
about 2nm to 2 µm.
is a novel pore size distribution measurement technique
that we have developed at the University of Kent and at Lab-Tools Ltd.
It makes use of the fact that small crystals of a
liquid in the pores melt at a lower temperature than the bulk liquid.
This is the Gibbs-Thomson effect : The melting point depression is
inversely proportional to the pore size.
A liquid is imbibed into the porous sample, the sample cooled until all
the liquid is frozen, and then warmed slowly while measuring the quantity
of the liquid that has melted.
Nuclear Magnetic Resonance (NMR) may be used as a convenient method of
measuring the quantity of liquid that has melted, deep inside the porous mass,
as a function of temperature.
Figure 1 shows such a melting curve for water in a mesoporous
templated SBA-15 silica; the melting point for the water in the pores is
depressed by about 13C with respect to the melting point of the water
around the grains. SBA-15 silica has pores that are known to be cylinders on
a hexagonal lattice - the abrupt step in the melting curve indicates that
all the pore diameters are very similar.
NMR Cryoporometry is compatible with samples that can not be dried,
and can give the true pore volume for liquids,
with a pore size calibration that is in
good co-linear agreement with gas adsorption measurements.
Figure 1: Melting curve for water in an SBA-15 silica, showing that the water in the pores melts at about -12C.
Metrology of Nano-particles and Nano-pores.
Experimentally NMR Cryoporometry and Gas Adsorption pore-size
calibrations show good co-linear agreement.
Measurements for the melting point depression of water in a range of sol-gel
silicas, vs the pore diameter as measured by Gas Adsorption are in good agreement with the
Gibbs-Thomson equations that predict that the melting point depression is inversely proportional
to pore diameter; early results are shown in figure 2.
Cryoporometry and gas adsorption are both governed by the same set of
Gibbs equations; cryoporometry is the constant pressure case, and
gas adsorption the constant temperature case; cryoporometry uses the change
between the solid and liquid phases of the imbibed liquid, and
gas adsorption the change between the liquid and vapour phases.
The value of the calculated result returned is dependent on
four terms :
By measuring the melted liquid volume as a function of temperature,
the Gibbs-Thomson equation shows that we may then differentiate
and re-map this to a measurement of poresize distribution,
such that we obtain
pore volume as a function of pore size (figure 3).
Thermodynamic constants that are a property of the liquid -
this term is determined using a gas adsorption calibration.
There is some evidence that these thermodynamic terms may change their value
as atomic dimensions are approached.
a geometric term that is only dependent on the geometry of the
of the interface between the liquid and its solid, and hence is modified by
the structural geometry of the material being studied.
a surface interaction energy term that can be well approximated by the
contact angle that the interface between the liquid and its solid makes
with the substrate.
a dimensional term x, that gives a measure of the nano- through meso-
to micro-stuctural sizes in the sample.
NMR Cryoporometry offers the following advantages :
Although sol-gel and SBA-15 silicas may often give
a distribution that is close to Gaussian (figures 3, 5), other materials may
give very different shapes, with natural materials often showing a fractal
distribution (figure 6).
- The pore size calibration is in good co-linear agreement
with gas adsorption measurements (figure 2).
- NMR cryoporometry can measure over a wider pore size range
than gas adsorption or thermoporosimetry.
- NMR cryoporometry measures the true volume of pores for
- NMR cryoporometry measures a
static signal; thus the measurement can be made arbitrarily slowly to
improve signal-to-noise and resolution, whereas thermoporosimetry senses a
transient, dynamic signal, limiting the resolution.
- NMR cryoporometry has no 'preferred' pore distribution
shape, such as the Gaussian distributions often assumed by inversion
- NMR cryoporometry may be combined with standard NMR
imaging techniques to perform non-destructive 1, 2 or 3 dimensional
macroscopic spatial imaging of pore size distributions (figure 4).
Other methods (particularly Small Angle Neutron Scattering - SANS )
may however provide complementary information that is invaluable
in establishing a calibrated metrology.
This is particularly true when using NMR cryoporomery to study the size and packing
of particles, by measuring the size and distribution of the voids between the particles.
Commercially important porous materials that we have recently
studied include porous sol-gel
glasses, alumina and alumino silicates such as clays and
zeolites, activated and other porous carbons, cement,
and water and oil bearing shales, sandstones and limestones.
Experimentally NMR Cryoporometry and Gas Adsorption pore-size
calibrations show good co-linear agreement for sol-gel silicas.
Figure 3: Poresize distribution for the above SBA-15 silica, as measured by NMR Cryoporometry, using a calibration for cylindrical pores.
The intrinsic resolution of the technique is better than that of the
measured curve, which is fully resolved.
Figure 4: 2D resolved porosity -
we developed the first protocol for
macroscopically resolving volumetric mesostructure,
and performed measurements resolving 1D, 2D and 3D structure.
Figure 5: Normalised pore size distribution for example silicas, by NMR Cryoporometry. The blue curves are sol-gel silicas, the red curve is for an SBA-15, and the green curves are CPG silicas.
Figure 6: Pore size distributions for example rocks and marine sediments, by NMR Cryoporometry. Many natural materials show such a fractal pore distribution, with a sharp cut-off at a lower-bound pore size.
NMR Cryoporometry method
NMR Cryoporometry is, like gas adsorption, a thermodynamic method of measuring structure size. NMR Cryoporometry is closely related to thermoporosimetry, but has significant advantages.
These techniques are all governed by the same set of Gibbs equations:
Cryoporometry is the constant pressure case, and gas adsorption the constant temperature case;
Cryoporometry uses the change between the solid and liquid phases of the imbibed liquid, and gas adsorption the change between the liquid and vapour phases.
A liquid is imbibed into the porous sample, the sample cooled until all the liquid is frozen, and then warmed slowly while measuring the quantity of the liquid that has melted.
Nuclear Magnetic Resonance (NMR) is used as a convenient method of measuring the quantity of liquid that has melted, deep inside the porous mass, as a function of temperature.
equations that describe NMR Cryoporometry were established between
1850 and 1890, by Josiah Willard Gibbs and three Thomsons.
We have jointly, with Cambridge University, written a Review of NMR Cryoporometry,
now published in Physics Reports :
offers NMR Cryoporometry as a commercial characterisation service,
to determine poresize distributions, or to provide information on particle
size distributions and packing, for densely packed particles.
Lab-Tools is also very pleased to participate in academic funded projects, as part of its nano-science and nano-metrology
the School of Physical Sciences at the University of Kent can in addition
offer a range
of characterisation services, using both Chemical and Physical techniques.
One of our main strengths is the range of characterisation techniques we
can offer using Nuclear Magnetic Resonance, to study liquids, solids and
Users of these services
We have made physical analysis measurements for a number of international
Shell, Unilever, Coates Lorilleux, Lafarge Braas and Schlumberger.
Universities that we have worked with include Kent, Heriot-Watt, Leeds, Leicester and Imperial College.
Some of our
publications in the field of nano-science :
nano-scale to micro-scale structured matter, porous-media
and liquids in confined geometry.
For more information :
E-mail me : J.B.W.Webber@kent.ac.uk,
go to my research home page.
Also, see my Thesis.
Content, design & creation Dr. Beau Webber 2005.
2005-03-19 ... 2010-10-30