Thermoporometry and cryoporometry

Thus, if a liquid is imbibed into a porous material, and then frozen, the melting temperature will provide information on the pore-size distribution.

The detection of the melting can be done by sensing the transient heat flows during phase transitions using differential scanning calorimetry – DSC thermoporometry,[1] measuring the quantity of mobile liquid using nuclear magnetic resonance – NMR cryoporometry (NMRC)[2][3] or measuring the amplitude of neutron scattering from the imbibed crystalline or liquid phases – ND cryoporometry (NDC).

[1] NMRC is a recent technique (originated in 1993) for measuring total porosity and pore size distributions.

[2][3] Nuclear magnetic resonance (NMR) may be used as a convenient method of measuring the quantity of liquid that has melted, as a function of temperature, making use of the fact that the

Thus NMRC cryoporometry is similar to DSC thermoporosimetry, but has higher resolution, as the signal detection does not rely on transient heat flows, and the measurement can be made arbitrarily slowly.

[6] Note: the Gibbs-Thomson equation contains a geometric term relating to the curvature of the ice-liquid interface.

[7] It is also possible to adapt the basic NMRC experiment to provide structural resolution in spatially dependent pore size distributions, by combining NMRC with standard Magnetic resonance imaging protocols,[8] or to provide behavioural information about the confined liquid.

[9] Modern neutron diffractometers have the capability to measure complete scattering spectra in a couple of minutes, as the temperature is ramped, enabling cryoporometry experiments to be performed.

[4] ND cryoporometry has the unique distinction of being able to monitor as a function of temperature the quantity of different crystalline phases (such as hexagonal ice and cubic ice) as well as the liquid phase, and thus can give pore-phase structural information as a function of temperature.

[12] NMR cryoporometry (external cryoporometry website) is a very useful nano- through meso- to micro-metrology technique (nanometrology, nano-science.co.uk/nano-metrology) that has been used to study many materials, and has particularly been used to study porous rocks (i.e. sandstone, shale and chalk/carbonate rocks), with a view to improving oil extraction, shale gas extraction and water abstraction.

A currently exciting application for NMR Cryoporometry is the measurement of porosity and pore-size distributions, in the study of carbon, charcoal and biochar.

Biochar is regarded as an important soil enhancer (used since pre-history), and offers great possibilities for carbon dioxide removal from the biosphere.

Figure 1: Cryoporometry melting mechanisms for a through ink bottle type pore where the imbibed material wets the surface of the porous material. The light blue shows the frozen phase and the dark blue shows the molten phase. Position A shows melting via sleeve shaped menisci for the necks and body. Position B shows the necks in the molten phase and advancing hemispherical menisci melting the large pore body; this is the advanced melting mechanism [ 12 ]