# Do Pores Exist?--Foundational Issues in Pore Structural Characterisation
## 2 Are Pores Real?
### Structural Reality of Pores
The advanced condensation effect means that pore bodies will fill at the same pressure as co-axial, immediate-neighbour pore necks, provided the pressure required to fill the pore body via a hemispherical meniscus is below that needed to fill the pore neck via a cylindrical sleeve meniscus, according to, say, the Cohan equations [1]. Hence, if both the pore-blocking and advanced condensation effects occur then the configuration of the adsorbed ganglia is the same for both the adsorption and desorption branches of the hysteresis loop, thereby meaning the measured NMR relaxation times would also be the same [24]. It is noted that the theory change to include the advanced condensation effect means that the suggestion of an ink-bottle pore geometry (of the mathematical model of representation) for the disordered mesoporous silica is still preserved. The presence of the pore-blocking and advanced condensation effects have been detected using scattering and magnetic resonance imaging techniques, respectively, which allow for the spatial resolution of the vapour sorption processes [24, 25, 26]. The theoretical pore structures obtained using indirect characterisation methods, such as gas sorption, for particular samples may differ in whether they require the existence of specific pores, and, thus, making a choice between them amounts to the assertion of the existence of particular pores or not. This will be discussed as follows. Separate measurements of the pore size distribution for the same disordered material sample are often obtained via two different indirect methods, such as gas sorption and mercury porosimetry, and subsequently compared. However, due to the plethora of adjustable parameters available in the typical analysis of these data, any apparent discrepancies can be easily dismissed on an ad hoc basis. For example, the Washburn equation, commonly used to interpret mercury porosimetry, requires an appropriate value of the mercury contact angle for a given sample, but this is often only vaguely known in advance. Hence, any discrepancy with a gas adsorption pore size distribution can be easily dismissed due to an incorrect contact angle. However, the availability of model, controlled-pore-size materials, such as controlled-pore glasses (CPGs), has permitted the proper calibration of mercury contact angles, even inside nanoporosity and for both intrusion and extrusion, using independent electron microscopy data [27,28]. This has meant that direct confrontation between gas adsorption and mercury porosimetry data is less easily avoided. The development of the fully integrated, serial gas sorption and mercury porosimetry method has enabled the comparison of sizes from these different techniques for the same pores, even when these are situated amidst a disordered, inter-connected network [29]. The particular sorption data for a small sub-set of (test) pores where mercury becomes entrapped can be extracted by subtracting the sorption isotherms obtained after mercury entrapment from those obtained beforehand [29]. Further, the development of mercury thermoporometry on the entrapped mercury has enabled the full triangulation of pore sizes obtained from three different structural characterisation methods [30]. A comparison of the modal pore sizes obtained from calibrated mercury intrusion and thermoporometry, with that obtained from conventional analyses (Barrett-Joyner-Halenda (BJH) and NLDFT) of the nitrogen adsorption isotherm, for disordered mesoporous silica showed that the latter tended to over-estimate the pore size [29]. The aforementioned discrepancy in pore sizes between mercury porosimetry and thermoporometry, on the one hand, and gas sorption, on the other, suggested that cap illary condensation arose at an unexpectedly (from the underlying pore geometry and characteristic size) high relative pressure. Two alternative theories have been proposed for this discrepancy.