Condensation in pores has been studied fairly extensively, but when the temperature is lowered below freezing point strange things start to happen. Studies on the NIMROD instrument at the ISIS neutron and muon source have shown that in the case of CO2 in a mesoporous material as the temperature cools, instead of freezing or remaining liquid, it escapes from the pores altogether. The process is also reversible. The work has been published in Physical Review Letters.
Understanding the behaviour of fluids in confined environments is crucial in several processes, including gas storage and separations, catalysis, oil recovery and geological water management. Water confined within mesoporous materials has been extensively studied. Current understanding is that at low temperatures the water crystallises in the pore centre while surface water remains liquid well below the bulk freezing point. This research set out to understand how CO2 responds to decreasing temperatures when confined in mesoporous silica. Using neutron diffraction the results seemed to show that the CO2 neither froze nor remained liquid, but actually escaped from the pores.
Dr Theodore Steriotis from NCSR “Demokritos”, Greece says, “We know fairly well what happens for CO2 sorbed in mesoporous matrices above the triple point. But this is the first time anyone has attempted to monitor the phase behaviour of CO2 in an ordered mesoporous material below 216K.”
The experiment was carried out on the NIMROD instrument at ISIS. NIMROD is unique because it can perform small and wide angle neutron scattering simultaneously. This is ideal for studying both the spatial distribution of the adsorbed phase and the molecular structure of the confined phase.
The mesoporous silica sample, SBA-15, was studied dry, partially filled and fully filled at a range of temperatures and pressures using neutron diffraction. It was also studied gravimetrically. Similar studies of water confined in SBA-15 showed that the water freezes within the pores at temperatures below the bulk freezing point. However, when CO2 was used instead the results strongly implied that as the temperature falls some CO2 remains as a film on the pore walls, however in the centre of the pores the CO2 is neither liquid or frozen, but has escaped the pores altogether.
Raising the temperature shows that the effect is reversible, albeit with some temperature hysteresis. Gravimetric studies appear to confirm this result. Dr Steriotis says, “In many cases things work quite differently at the nanoscale and thus thermodynamic s of confined nanophases is a field of intense research. The phenomenon observed may be relevant to a wide range of applications. We are now starting to study materials with different pore sizes and geometries to understand the underlying mechanisms at play.”
Research date: January 2016
Anomalous Depletion of Pore-Confined Carbon Dioxide upon Cooling below the Bulk Triple Point: An In Situ Neutron Diffraction Study, PRL 116, 025502