Silica aerogel; synthesis, properties and characterization

Polysaccharide-based aerogels—Promising biodegradable carriers for drug delivery systems

Aerogel technology provides high added-value lightweight materials with outstanding textural properties (i.e., high surface area and open porosity). Aerogels are obtained from wet gels by using a suitable drying technology, usually supercritical drying process, able to avoid the pore collapse phenomenon in order to keep intact the porous texture of the wet material. In this sense, the study of the kinetic profile of the gel supercritical drying is regarded as a key aspect to be considered in the specific case of the design of aerogel-based systems. In this work, the drying profile with supercritical carbon dioxide (scCO 2 ) of alcogels (in ethanol) was determined using a customized supercritical fluid extraction equipment. The drying of alcogels from different precursors (inorganic-silica-, organic-starch-), densities (silica aerogel of densities 0.08 and 0.15 g/cm 3 ) and morphologies (cylindrical monoliths, microspheres) was studied. Depending on the nature of the gel precursor, the extent of drying (i.e., drying time duration) gave significant differences in the end textural properties of the dried gel.

Supercritical drying of aerogels using CO2: Effect of extraction time on the end material textural properties

Synthesis and physical properties of TEOS-based silica aerogels prepared by two step (acid–base) sol–gel process

Supercritical and compressed fluid technology provides a powerful tool for particle design and engineering. On the other hand, aerogels are nanoporous materials holding many world-class properties. However, the morphology of the end product can be the limiting factor for aerogels in some applications. The integration of different technologies for the production of particles can lead to a breakthrough in aerogel production. In this work, the combination of the emulsion–gelation process with supercritical fluid extraction was tested as a versatile technology for the production of microspherical aerogel particles. Novel natural product-based nanoporous materials (aerogels) based on starch were obtained using this method and were specifically targeted to chemical carriers for life science applications (e.g. pharmaceutics, tissue engineering, cosmetics, food, biotechnology, and agriculture). Aerogels in the form of cylinders and microspheres (400–800 μm) with the intrinsic outstanding textural aerogel properties (high surface area – Sa = 100–240 m2/g, low density – ρ = 0.10–0.25 g/cm3, large porosity – ɛ = 85–90%) were produced. The effect of micronization on the kinetics of the supercritical drying of the wet gel (in ethanol) was also studied. Thus the obtained aerogels were then tested for their use as active agent carriers regarding loading capacity using ketoprofen as model compound as well as the release behavior of the loaded active compound in aqueous media simulating physiological pHs.

Use of supercritical fluid technology for the production of tailor-made aerogel particles for delivery systems

Extraction of a primary solvent with liquid or supercritical carbon dioxide is the most difficult to control stage in low-temperature silica aerogel production. Diffusion of primary ethanol through alcogel structure to surrounding CO 2 was investigated in carefully controlled experiments with cylindrical alcogel samples. Changes of the alcohol concentration in carbon dioxide leaving the autoclave were followed with on-line chromatograph analysis and experiments were repeated for temperatures around the critical point of carbon dioxide. On the basis of the concentration histories during the drying, the temperature dependence of diffusion coefficient in the ethanol–CO 2 mixture inside the silica gel at constant pressure was identified.

Diffusion of ethanol–carbon dioxide in silica gel

In low-temperature aerogel production, replacement of an original solvent with liquid or supercritical carbon dioxide is the crucial stage, and it is one fully controlled by a diffusional mechanism. The diffusion of ethanol as the primary pore fluid through an alcogel structure to the surrounding near-critical CO 2 was investigated in carefully controlled experiments with cylindrical alcogel samples. Changes of the alcohol concentration in carbon dioxide leaving the autoclave were followed with an online gas chromatographic analysis. The experiments covered the range of temperatures from 292 to 315 K and pressure from 6.9 to 9.1 MPa. On the basis of the concentration histories during the drying, the diffusion coefficient in the ethanol–CO 2 mixture inside the silica gel was identified for each experiment. A comparison of measured concentrations with values derived from a mass transfer model is presented and discussed. The method for identifying an effective diffusion coefficient is described and the values of ethanol–CO 2 effective diffusivity in a silica network are presented. The correlation of effective diffusion coefficient in the porous structure of silica gel with the Schmidt number ratio covering the entire investigated range of temperature and pressure is proposed.

Effective diffusion coefficient in the low temperature process of silica aerogel production

A crucial stage in silica aerogel production in which extraction with liquid (or supercritical) carbon dioxide is used is alcohol diffusion through alcogel to CO 2 . The quality of samples produced ranged from perfect monoliths (long diffusion times) to completely damaged specimens (the shortest diffusion times). In intermediate conditions, a non-transparent damaged zone was observed inside samples. On the basis of the dependence of damage size on diffusion time, the values of the diffusion coefficient of ethanol in the liquid CO 2 inside the silica gel were identified for two temperatures.

Diffusion of ethanol-liquid CO2 in silica aerogel

The properties of diamond-like carbon layers deposited onto SiO2 aerogel

Gels dried in supercritical conditions give a new class of materials called aerogels. The process of silica aerogel production in which extraction with liquid (or supercritical) carbon dioxide is used, consists of four main stages: elution, diffusion, heating and evacuation. Aerogel production from silica gels prepared from tetraethoxysilane is described. The autoclave process which started with alcohol-liquid carbon dioxide exchange was followed by supercritical evacuation. Some important technical aspects of the process are considered. The effect of elution rate, temperature and duration of diffusion is investigated.

G. Rogacki

Papers

Diffusion of ethanol–carbon dioxide in silica gel

Effective diffusion coefficient in the low temperature process of silica aerogel production

Diffusion of ethanol-liquid CO2 in silica aerogel

The properties of diamond-like carbon layers deposited onto SiO2 aerogel

Drying of Silica Alcogels by Liquid and Supercritical C02