Cavities size distribution

Computer modelling of physisorption hysteresis is simplified if it is assumed that pore filling occurs reversibly (i.e. in accordance with the Kelvin equation) along the adsorption branch of the loop. Percolation theory has been applied by Mason (1988), Seaton (1991), Liu et al., (1993, 1994), Lopez-Ramon et al., (1997) and others (Zhdanov et al.,1987 Neimark 1991). One approach is to picture the pore space as a three-dimensional network (or lattice) of cavities and necks. If the total neck volume is relatively small, the location of the adsorption branch should be mainly determined by the cavity size distribution. On the other hand, if the evaporation process is controlled by percolation, the location of the desorption branch is determined by the network coordination number and neck size distribution. 

FIGURE 15.12 Comparison of active cavity size distribution estimated from cavity geometry measurements with that measured in boiling for contacting of 90°, 35°, and 18° (from Wang and Dhir [24], with permission of ASME). 

In particular, it has been suggested that the highly ordered structure of these salts may contain voids, and that these voids can accommodate small solute molecules. Furthermore, since the chains present on the cations are flexible they can move more rapidly than the whole cation, permitting a rapid diffusion of solutes from one void to another [18]. The formation of cavities (voids) in ionic liquids has been recently studied via Monte Carlo simulations [19]. Analysis of cavity size distribution functions shows that ionic liquids exhibit a large tendency to form cavities, a property which seems to be correlated to the attractive interactions between ions and, particularly, to the tendency of the ions to associate into ion aggregates. 

FIGURE 17.18 (a) Cavity-size distribution as a function of creep strain in alumina without a glassy phase (Lucalox). (b) Cavity-size distribution as a function of creep strain in alumina with a glassy phase (AD99). 

Figure 13.14 Cavity size distribution evolution as a function on strain for NT 154 sample tested at 1400°Cand 125 MPa [23],...

In our study [4] of cavity size distribution a sphere of diameter D is prescribed on every atomic center, and the cavity is evaluated as the pocket of contiguous space unoccupied by any of these spheres. The exclusion diameter D has to be equated to the van der Waals diameter Dvdw of a CH2 unit for the evaluation of the total cavity volume. When diffusion of small gas molecules through polymers is of interest, the unoccupied space available for inclusion of the small molecule can be evaluated by setting D equal to Dp + Dydw> where Dp is the diameter of the small probe molecule. The results of the cavity volume analysis were therefore presented [4] as a function of the exclusion ameter D varying over a range of values. 

Wang, X. Y, et al. (2004). Cavity Size Distributions inHighFree Volume Glassy Polymers by Molecular Simulation. Polymer, 45(11), 3907-3912. 

Wang, X. Y et al., A molecular simulation study of cavity size distributions and diffusion in para and meta isomers Pol m 2005, 46(21), 9155-9161. 

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