Water and nitrogen in confinement.
The nature of a fluid in confinement at the nanometer-scale is still quite uncertain, and much less certain than the corresponding bulk fluid far away from any interfaces. Problems arise because the detailed atomic structure of the substrate is often unknown and difficult to characterise precisely. Gas adsorption measurements have been used for many decades to characterise the size and shape of a confining volume, but even here assumptions have to be made, for example, about the extent to which the fluid fills the pore, and the degree of microporosity present. Using a combination of neutron total scattering and computer simulation we have started to probe the structure of confining substrates, with particular reference here to the porous silica material, MCM41, and the way fluids such as nitrogen and water absorb in them. The results do not always give clear, unambiguous answers, but they do highlight some of the more pertinent questions that remain unanswered. Much work remains to be done on these complex environments. This puts into question many of the results from these systems that have been reported in recent years.