What are aerogels

Institute of Thermal Separation Processes

Mohammad Alnaief

Aerogels

Airgel, frozen smoke, is a truly extraordinary material. It is the lightest solid known to date and holds 14 entries in the Guinness Book of World Records, including the best insulator and the lowest density solid. Aerogels consist of 99.8% air, but aerogels cannot be regarded as conventional foam, but as a porous material with pore sizes in the micrometer range. Aerogels are composed of individual particles in the nanometer range that are connected in a three-dimensional network.

Aerogels can be made from silicon dioxide (silica aerogels) as well as from various organic and inorganic substances such as titanium oxide, aluminum oxide, carbon, starch, alginate, chitosan, agar, pectin and cellulose.

These new materials have many outstanding properties, such as low thermal resistance, refractive index and speed of sound, in addition to a large surface area and thermal stability.

Aerogels can be made with a density three times the density of air. Furthermore, they can be used in many areas of science and industry. One of the most impressive applications of the aerogels is carried out by NASA, here they act as insulation material for space shuttles. Many scientific working groups are currently working with aerogels, which then present their results at the International Conference on Aerogels, ISA, which takes place every three years. The synthesis and innovative application of aerogels are currently of great interest. Due to their environmentally friendly and non-toxic properties, aerogels can also be used in the pharmaceutical industry. The large surface and the open pore structure of the aerogels make them an ideal carrier material.

Silica airgel properties

  • Density: 0.003-0.35 g / cm³
  • Inner surface: 600 - 1000 m² / g
  • Open pore network
  • Primary particle diameter: 2 - 5 nm
  • Average particle diameter: ~ 20 nm
  • Nontoxic
  • Almost transparent; diffuses blue light
  • Thermal tolerance: shrinkage starts at 500 ° C; the melting point is> 1000 ° C
  • Dielectric constant: 1.1
  • Refractive index: 1.0 - .05
  • Brittle - quickly disintegrates into dust
  • Will be destroyed in contact with water
  • May disintegrate with rapid pressure fluctuations
  • Non-flammable
  • Can be made in almost any shape
  • No risk of injury - airgel particles are smooth and round

Manufacturing methods

The easiest way to describe the airgel synthesis is with the words of the man who first succeeded in the synthesis, Steven S. Kistler: “Apparently, in order to produce airgel, the liquid has to be replaced by air in such a way that the surface of the liquid is kept away from it to sink back into the gel. If a liquid is pressurized so that it is always higher than the vapor pressure of the liquid and if the temperature is constantly increased, the liquid will turn into a gas when the critical temperature is reached, without two phases being created during the process. (SS Kistler, J. Phys. Chem. 34, 52, 1932).
The standard process for airgel production is the so-called sol-gel process. Here, all the necessary components are mixed with a solvent, which then enter into a chemical reaction and form highly cross-linked particles. At the beginning of the reaction the mixture is liquid, but then becomes more and more viscous as the reaction continues. After the reaction has ended, the mixture loses its fluidity and turns into a gel. This gel consists of a three-dimensional network in which the solvent is embedded. During supercritical drying, the solvent is extracted from the gel body, leaving behind an air-filled, solid network that maintains its initial shape and size.

Airgel production can be summarized in four simple steps (see following drawing):

introduction

Due to their excellent properties, aerogels are very versatile. Each individual property represents an innovative application and opens up new possibilities for researching new areas of application. This thesis deals with the pharmaceutical application of aerogels. Since aerogels have a large surface area (up to 1200 m² / g), have an open pore structure and are non-toxic, they are ideally suited as drug carriers.

Objective of the project

The aim is to develop tailor-made drug carriers for specific applications. This includes the loading and release of the drug in the desired organ of the human body. Aerogels from various organic and inorganic raw materials, such as silica, starch, alginate, chitosan, agar, pectin or cellulose, are used for this. In the next step, biodegradable polymers are to serve as starting materials for the aerogels.

methodology

Pharmaceutical substances have different chemical structures, which leads to different affinities with regard to the carrier materials, in this case aerogels. Accordingly, various carrier materials have to be produced in order to load them with the various pharmaceutical substances in the desired amount. Another possibility is to change the properties of existing carrier materials.

The surface properties of the airgel can be adjusted by means of surface functionalization; various functional groups can be used to improve the surface properties so that the aerogels meet the needs of any pharmaceutical substance. One of the goals of this study is to control both surface coverage and the type of functional group.

Functionalization can be divided into two classes:

  1. Pretreatment: The desired functional group already takes part in the sol-gel reaction, so that a functionalized gel is created.
  2. Post-treatment: The surface properties of the airgel that has already been produced are changed by functionalization reactions

Each functionalization class provides its own methods and applications, which offers a wide range of possibilities for surface modification of aerogels.