Confinement, nanoscopic pores

It is therefore reasonable to postulate that the confinement of carbene 46, perhaps within the nanoscopic pores of CyDs and FAUs, would inhibit the fragmentation reaction and foster the 1,3-CH insertion. This reasoning is twofold (1) there may not be enough space within the hosts cavities for the unraveling process, 46—>47, and (2) distortion of the carbene s topology might concomitantly disfavor the coarctate TS and allow the 1,3-CH insertion, 46->48, to finally occur. 

As mentioned before, the large interior volumes of mesoporous materials and their pore sizes are ideal for promoting molecular interactions, and confinement and selectivity can be achieved upon introduction of gates. This feature has led to the quest for smart designs of nanoscopic gates and their immobilization within the entrances to the mesopores of the materials. As will be seen, many research groups have responded successfully to that challenge. 

Salamacha and coworkers304 306 have carried out a series of studies on Lennard-Jones fluids confined to nanoscopic slit pores made from parallel planes of face centred cubic crystals. Grand canonical and canonical ensemble MC simulations have been used to determine the structure and phase behaviour as the width of the pore and the strength of the fluid-wall interactions were varied. The pore widths were small accommodating 2 to 5 layers of fluid molecules.304,305 The strength of the fluid-wall interaction is linked to the degree of corrugation of the surface, and it is found that the structure of the... 

The next slightly more complicated situation concerns a fluid confined to a nanoscopic slit-pore by structured rather than unstructured solid surfaces. For the time being, we shall restrict the discussion to cases in which the symmetry of the external field (represented by the substrates) i)reserves translational invarianee of fluid properties in one spatial dimension. An example of such a situation is depicted in Fig. 5.7 (see Section 5.4.1) showing substrates endowed with a chemical structm e that is periodic in one direction (x) but quasi-infinite (i.e., macroscopically large) in the other one (y). 

The simplest case that we shall be discussing hen in some detail is that of a fluid confined to a nanoscopic. slit-pore with homogeneous (infinitesimally) smooth substrate surfaces. For this prototypical model, it was shown in Section 1.6.1 that a mechanical expression for the grand potential exists. However, in what follows, it is more convenient to focus on the grand-potential density rather than on il itself. The former is defined through the relation... 

If we now confine the binary mixture to a slit-pore of nanoscopic dimension, we may, in fact, change the topolgy of the phase diagram. For example, by varying the degree of confinement (i.e., z in our current notation), it turns out to be possible to switch between various types of phase diagrams with profound consequences for liquid liquid and gas liquid phase equilibria. This phenomenon may have practical implications for the decomposition of mixtures of immiscible liquids in nanoporous matrices. 

The change of p between a pair of branches is discontinuous at characteristic pore widths where first-order transitions occur between these phases. The confinement-induced change in topology of tlie phase diagram may have important repercussions for the decompasition of binary mixtures in sorption experiments where one may envision pore condensation in nanoscopic solid matrices leading either to a mixed or deniixed liquid such that the physical nature of the confined phase depends solely on the pore width. 

A porous medium affects a liquid mixture not only by mere confinement to volumes of nanoscopic dimensions [91] but also by the energetic preference of the solid substrate for molecules of one of the components of the mixture [92, 93]. This selectivity causes an enrichment of the component in the proximity of the pore walls. For sufficiently wide pores, the decay length of the resulting concentration profile corresponds to the correlation length of concentration fluctuations [94]. In narrow pores, on the other hand, when the mean pore width D is less than concentration profiles near the pore walls overlap, thereby causing enhanced adsorption. 

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