# Do Pores Exist?--Foundational Issues in Pore Structural Characterisation
## 2 Are Pores Real?
### Structural Reality of Pores
First, it may be that, while the single-pore capillary condensation theory (e.g., Cohan equations, BdB, or NLDFT) is still correct, the multi-layer build-up has been suppressed in some way in the particular pores isolated by the serial experiment, relative to the rest of the void space and/or relative to that usually envisaged for such materials as incorporated into the so-called 'universal' \(t\)-layer curves [29]. This theory sites the discrepancy in the (poor) wetting properties of the silica test pore walls. Alternatively, it may be that, since the void space is an inter-connected network, the pore-to-pore co-operative adsorption effect known as (network) delayed condensation may be operating [9]. The latter implies that capillary condensation in the test pores is delayed (to higher relative pressure than expected) due to the lower pore potential along the axis of these pores due to the existence of (the gaps at) junctions with side-arm pores along its length, compared with a pore of the same size and underlying (cylindrical, say) geometry but with completely solid walls. The second theory posits the existence of additional pores compared with the first [9]. The existence of this set of additional pores was preferred after comparing the findings from serial adsorption and mercury porosimetry experiments conducted, in turn, with each of two different adsorbates, namely nitrogen and argon, on the same sample [9]. This is because argon fails to wet the new pore wall created when the side-arm pores are filled with entrapped mercury following porosimetry, while nitrogen does, and, thereby, then perceives a solid wall rather than something with wetting properties more like the original gap. While both theories retain the presence of the test pore, and thus have some underlying structural similarity, the difference in the behavior of the argon and nitrogen after mercury is entrapped is best explained by also invoking the presence of the additional pores/connections. The limitations on the resolution and contrast currently possible with tomography methods makes the study of sorption behavior of individual mesopores within the extensive, complex network of macroscopic samples impossible at present. Theories for pore structure characterisation are often extrapolated to domains in which they do not apply, or are insufficient. Nanocasting and templating techniques have provided model materials with supposedly known geometry with which to test theories of gas adsorption-based characterisation methods [17; 18]. However, it has also been found that these supposedly ordered model materials are often more complex in structure than first envisaged, such as possessing surface corrugations in supposed straight, isolated cylindrical pores, or microporous walls [19]. Further, conclusions drawn based upon simplified geometries and topologies for the model templated materials do not necessarily carry over to much more complex disordered, inter-connected networks, and new effects entirely may arise for the latter [1; 9]. It is thus necessary to independently, explicitly demonstrate the presence, and measure the extent, of network-specific effects, such as pore-to-pore cooperative effects like advanced condensation or delayed condensation, within disordered materials [1; 9]. The single pore hysteresis effects emphasised by NLDFT-based methods of PSD determination [17; 18] tend to be non-existent (as pores are too long) in, or only a minor contribution, if any, to, hysteresis within disordered porous materials [1; 9], and thus are insufficient for full characterisation. Further, network modelling alone, without the prior active, independent detection of co-operative adsorption effects, has too much of a plethora of potential, empirically adjustable parameters with which to match experimental (boundary and scanning) isotherms to be considered a rigorous demonstration of a comprehensive understanding of gas sorption in such materials.