The pore size you can evaluate by performing SEM and analyzing the images. For porosity in general, I'd do X-Ray Reflectivity measurements to evaluate the density of the film.

You could compare your known area of this film (a.b mm - taking into account 2 sides and the edges if it's on a glass slide substrate, for example) with a section measured by BET surface area - the N2 explores all the pores of the material and will give a larger result/estimate of the surface area then the simple geometric size).

It will depend on the thickness and porosity of your film.

I have measured the porosity of paper by using a total of 25 paper-disks of 1" diameter.

To avoid measuring the porosity between the sheets (disks), I place another disk of a coarse wire mesh between them. Please realize that you will probably need about 0.1 ml in total pore volume to achieve a reasonable resolution/quality of yoru data with Hg-porosimetry.

Once you prepare your sample in that way, you just use the standard techniques, like nitrogen adsorption or mercury porosimetry.

Alternatively you can use the previously mentioned x-ray technique or the is also a thermal transpiration technique, which measure the temperature gradient and then deducts the porosity from the temperature data. However the later techniques are not as widely established and you will have a harder time to "compare" your results.

SEM is always an alternative but it is vert time consuming.

How did you deposite the film? If you use screen-printing method, you can print a thick film with the same process and then measure the porosity of the thick film by the normal method.

Atomic Force Microscopy (AFM)  and  BJH(ads)  measurements might used to determine  pore size

Hi, to identify the type of silica you should go back to the BET analysis(Brunauer-Emmett-Teller) ..on which it determines the surface area, pore diameter, pore size distribution, and volume. All these parameters are closely linked to silica gel performance in separating compounds.

The efficiency of silica gel depends on surface condition. a large surface area results in greater contact or interactions with the analyte, and increasing separation of different products.   Pore diameter, distribution, and volume permit semi size exclusion chromatography, where smaller molecules fit into pores more easily than larger ones (e.g. in peptide separations). This justifies the use of several types of silica gels to achieve better discrimination in chromatographic processes.

with respect to its purification, you should make sure that the silica used doesn't have a lot of metal ions like copper, cadmium, zinc, and aluminum.. may also present as lewis acid which will lead to destroying the product. This silica gel is analyzed by ICP-MS(Inductivity Coupled Plasma-Mass Spectroscopy) for over 40 metals, to ensure that every batch has a very low metal content. 

Thank you 

Dear all, thank you so much for your useful explanations.

First of all, sorry for my late answer, two years after the question !

If you think about film thickness less than 1 µm, thus deposited on a thick support, and about nanoporosity, i.e. microporosity (pore size < 2 nm) and mesoporosity (2 nm < pore size < 50 nm), BET analysis with commercial apparatus based on volumetric measurements is not sensitive enough or you need to analyse a very large amount of surface to analyse enough porous material. A small number of techniques exists to overcome this issue. They are generally based on techniques that couple adsorption-desorption (like BET measurements) with a technique dedicated to thin film analysis. It can be quartz crystal microbalance (QCM), spectroscopic ellipsometry, X-ray reflectometry (XRR), small angle X-ray diffraction, neutron diffusion, ...

For instance, I can advise to consult the article

Three characterization techniques coupled with adsorption for studying the nanoporosity of supported films and membranes

V. Rouessac, A. van der Lee, F. Bosc, J. Durand, A. Ayral, Microporous Mesoporous Mat. 111 (2008) 417-428.

but also more recent articles.

Regards,

Vincent