A gel is a colloidal system consisting of a continuous solid phase that encloses or permeates a liquid phase (often water). It behaves like a solid and is elastic, even though it consists largely of liquid. Gels are formed by the cross-linking of polymers or colloidal particles, which create a three-dimensional network that encloses the liquid.
Gels are classified as non-Newtonian fluids, which means that their flow properties do not follow the laws of classical Newtonian viscosity. Unlike Newtonian fluids, whose viscosity remains constant, non-Newtonian fluids such as gels respond differently to shear forces. Their viscosity can change under mechanical stress, such as pressure or stretching. As a result, gels exhibit viscous properties under slow deformation but tend to behave in a more solid or elastic manner under rapid or intense stress.
The cross-linking in the liquid phase gives it its macroscopically “solid” character. Gels come in various types and forms and are frequently used in pharmaceuticals or as adhesives.
A special type of gel is the so-called xerogel, from which the swelling agents are typically removed. Examples of xerogels include dried silica gel and gelatin.
Gels can be easily prepared using differential scanning calorimetry (DSC), as is often done in quality control.
The curve shows a DSC scan of aluminum oxide nanoparticles in a gel matrix, heated at a linear heating rate of 10 K/min in a nitrogen atmosphere.
The signal shows two significant effects during the run that are worth examining more closely:
In the range up to 120 °C, water loss occurs, which is due to the change in mass of the sample Cp change caused by thechange in massof the sample.
As a result of this effect, a dry gel matrix containing nanoparticles remains—known as a xerogel. At approximately 200 °C, a phase transition of the nanoparticles from an ordered to an amorphous aluminum oxide structure, which can be identified as a small, sharp peak.
These two effects are reproducible and provide a good characterization of the nanoparticle gel.
