Introduction and overview of film formation

The general scheme to obtain microporous membranes (pore diameter 2 nm) is identical to that for mesoporous ones. Starting with a specific precursor, a wet film is formed by either film casting or slip casting, which is followed by 

The drying rate for these very thin microporous layers is large and this causes probably rather dense, low porosity membranes (see below). The drying rate is, however, also controlled by the liquid content of the support system which acts as a reservoir similar to that discussed for mesoporous membranes. 

The porosity can be controlled in two ways. The first method is based on the scaling of mass Mf and size tf of the mass fractal particles. Since density equals mass/volume, the density pf of a mass fractal object varies in three-dimensional space as  

Thus the porosity of a mass fractal decreases with its size and when complete interpenetration is avoided (which requires Df 1.5) the porosity can be controlled by the size of the branched specimen during drying. Examples of this procedure are given by Brinker et al. [42]. This discussion reveals a dilemma to obtain the smallest pores, interpenetration should be large and is obtained by D 1.5 and low condensation rates. This leads however to low porosities. 

High quality microporous membranes are almost exclusively reported for silica or for binary silica-titania or silica-zirconia systems [42,46]. This is due to the very fast hydrolysis and condensation rates of the metal organic precursor of the metals relevant for membrane synthesis (Ti, Zr, Sn, Al). This usually results in too large particles in the precursor solution. Though many authors claim to have produced microporous materials by sol-gel methods (see e.g. Section 8.2.3), only a few have shown the synthesis of membranes of these materials and a still smaller number has characterised them with appropriate separation properties to be reasonably defect free. Therefore in the remainder of Section 8.2.1 a focus will be given to silica-based membranes. 

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