Porous glasses pore size distribution

The mercury porosimetry intrusion plot of a mesoporous glass membrane is given in Fig. 2. That curve and the isotherm shapes in Fig. 1 are indicators for a relatively narrow pore size distribution of the ultrathin porous glass membranes. 

Ultrathin porous glass membranes with variable texture properties were prepared from a Si02-rich sodium borosilicate initial glass by careful fine timing of the conditions of heat treatment for phase-separation. Pore sizes between < 1 and 120 nm can be realized. The membranes are characterized by a narrow pore size distribution. The transport, optical and mechanical properties vary with the pore size. The tailorable texture and transport characteris-... 

Chord length distribution and pore size distribution of porous VYCOR glass... 

Analysis of behavior in single pores is certainly an excellent place to start an understanding of adsorption hysteresis. On the other hand, real porous materials eu e in most cases not simply described in terms of single pore behavior. At the very least a distribution of pores of different sizes should be contemplated. The first analysis of hysteresis loops using a theory of adsorption in single pores together with a pore size distribution was the independent domain theory of Everett and coworkers (Everett, 1967). The most sophisticated application of this kind of approach was made by Ball and Evans (1989) who used density functional theory for adsorption in a distribution of cylindrical pores and compared the hysteresis loops obtained with those for xenon adsorbed in Vycor glass. 

Such more realistic models of porous materials can also be used to rigorously test existing characterization methods. The model material is precisely characterized (we know the location of every atom in the material, hence the pore sizes, surface area and so on). By simulating adsorption of simple molecules in the model material and then inverting the isotherm, we can obtain a pore size distribution for any particular theory or method. Such a test for porous glasses is shown in Figure 8, where the exactly known (geometric) PSD is compared to that predicted by the Barrett-Joyner-Halenda (BJH) method, which is based on the modified Kelvin equation. 

The amorphous (nonordered) mesoporous materials such as ordinary SiC>2 aerogel and porous glass possess mesopores, but the channels or pores are irregular and the pore sizes distribute over a wide range. Most macroporous materials such as ceramics and cement have the same characteristics irregular pores and wide pore-size distribution. 

The modelling of gas permeation has been applied by several authors in the qualitative characterisation of porous structures of ceramic membranes [132-138]. Concerning the difficult case of gas transport analysis in microporous membranes, we have to notice the extensive works of A.B. Shelekhin et al. on glass membranes [139,14] as well as those more recent of R.S.A. de Lange et al. on sol-gel derived molecular sieve membranes [137,138]. The influence of errors in measured variables on the reliability of membrane structural parameters have been discussed in [136]. The accuracy of experimental data and the mutual relation between the resistance to gas flow of the separation layer and of the support are the limitations for the application of the permeation method. The interpretation of flux data must be further considered in heterogeneous media due to the effects of pore size distribution and pore connectivity. This can be conveniently done in terms of structure factors [5]. Furthermore the adsorption of gas is often considered as negligible in simple kinetic theories. Application of flow methods should always be critically examined with this in mind. 

E. Grosgogeat, J.R. Fried, R.G. Jenkins and S.T. Hwang, A method for the determination of the pore size distribution of molecular sieve materials and its application to the characterization of partially pyrolyzed polysilastyrene/porous glass composite membranes. /. Membr. Sci., 57 (1991) 237. 

E. Vernon Ballou and T. Wydeven, Solute rejection by porous glass membranes. II. Pore size distributions and membrane permeabilities. /. Colloid Interface Sci., 41 (1972) 198. 

The characteristic feature of controlled porosity glasses is a very narrow pore size distribution [2,3,5,16], well seen when compared with silica gel or other porous materials produced by polymerization or by crosslinking of monomers-cf. Fig. 5. 

The so called porous silica is made by the acid etching of annealed sodium borosilicate glass. The bulk structure is glass mainly of silica and has a uniform pore size distribution in the mesopore region. These are used as a source of silica glass. 

Once the model porous glass structures have been assembled using quench MD simulation, their geometric pore size distributions can be determined by sampling the pore volume accessible to probe molecules... 

Mason, G. (1988). Determination of the pore-size distributions and pore space interconnectivity of Vycor porous glass from adsorption—desorption hysteresis capillary condensation isotherms. Proc. R. Soc. Lond. A, 415, 453-86. 

Figure 14. Pore-size distribution curves of silica gel and gel-derived porous silica glasses. The gel was obtained from a solution with mole ratios TMOS H2O HCl CH3OH of 1 1.53 0.40 2. The gel was dried at 40 °C for 7 days and heated at a rate of 0.5 °C/min to various temperatures.

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