The pore structure of silica aerogels is difficult to describe in words. Unfortunately, the available methods of characterizing porosity do only a slightly better job. The International Union of Pure and Applied Chemistry has recommended a classification for porous materials where pores of less than 2 nm in diameter are termed “micropores”, those with diameters between 2 and 50 nm are termed “mesopores”, and those greater than 50 nm in diameter are termed “macropores”. Silica aerogels possess pores of all three sizes. However, the majority of the pores fall in the mesopore regime, with relatively few micropores. The pore size distribution of a single-step silica aerogel is shown below:
Pore Size Distribution of Silica Aerogel
It is very important when interpreting porosity data to indicate the method used to determine the data. Various measurement techniques can give differing results for the same sample. The entire range of characterization methods has been applied to silica aerogels, including:
Gas/Vapor adsorption: This is the most widely available and utilized method for determining aerogel porosity. In this technique a gas, usually nitrogen, at its boiling point, is adsorbed on the solid sample. The amount of gas adsorbed depends of the size of the pores within the sample and on the partial pressure of the gas relative to its saturation pressure. By measuring the volume of gas adsorbed at a particular partial pressure, the Brunauer, Emmit and Teller (BET) equation gives the specific surface area of the material. At high partial pressures the hysteresis in the adsorption/desorption curves (called “isotherms”), the Kelvin equation gives the pore size distribution of the sample. The pore size distribution shown above was determined using a 40-point nitrogen adsorption/desorption analysis. Gas adsorption methods are generally applicable to pore in the mesopore range. However, microporosity information can be inferred through mathematical analyses such as t-plots or the Dubinin-Radushevich method. Gas adsorption can not effectively determine macropores. For a detailed description of this procedure, see the IUPAC guidlines for “Reporting Physisorption Data for Gas/Solid Systems” in Pure and Applied Chemistry, volume 57, page 603, (1985).
Mercury Porosimetry: This technique is generally not effective for aerogels. The high compressive forces encountered in forcing mercury into the pores of an aerogel cause its structure to collapse.
Scattering Methods (x-ray, neutron and visible light): Scattering methods involve the angle dependent deflection of radiation by features within the sample. These features can be solid particles or pores. Scattering efficiency is greatest when the wavelength of the radiation used is comparable to the features being studied. X-ray and neutron scattering are particularly well suited for determining the fractal geometry of the aerogel pore network. See the section on Optical Properties for a discussion of visible light scattering.
Other methods: Gas/solid NMR, electron microscopy of replicants, and atomic force microscopy have also been used to characterize the pore network of silica aerogels with limited success.
Because of the limitations of these methods, a major problem in aerogel science remains unresolved. If the mass, density, and total pore volume of an aerogel are measured, it is apparent that there is a substantial amount of porosity that is not accounted for. This obviously results from the drawbacks of using gas adsorption to determine the pore volume. It is assumed that the “missing porosity” lies in the micro- or macropore regimes, areas not measured effectively by this method.
One final important aspect of the aerogel pore network is its “open” nature and interconnectedness. Pores in various materials are either open or closed depending on whether the pore walls are solid or themselves porous (or at least “holey”). A macroscopic example of a open-pored material is a common sponge, while “bubble wrap” packaging is an example of a closed-pore material. In a closed-pore material, gases or liquids cannot enter the pore without breaking the pore walls. This is not the case with an open-pore structure. In this instance, gases or liquids can flow from pore to pore, with limited restriction, and eventually through the entire material. It is this property that makes silica aerogels effective materials for gas phase catalysts, microfiltration membranes, adsorbents, and substrates for chemical vapor infiltration.