The Sol-Gel Process
Almost all aerogels are derived from gels made through sol-gel chemistry. Here we’ll explain what the term “sol-gel” means and how the sol-gel process works.
What is Sol-Gel?
The term sol-gel (say “sahl-jell”) refers to a process in which solid nanoparticles dispersed in a liquid (a sol) agglomerate together to form a continuous three-dimensional network extending throughout the liquid (a gel). The term sol-gel is sometimes used as a noun to refer to gels made through the sol-gel process, but this is somewhat of an abuse of the term, since pretty much all gels are made through the sol-gel process.
Sols, Gels, and Aerogels are Colloids
A colloid is a mixture in which at least two different phases are intimately mixed at the nanolevel. The term “phase” generally refers to a solid, liquid, or gas form of some substance. A colloid typically has a continuous phase in which something else with a different phase is dispersed (the “dispersed phase”). Different phases can still be the same phase of matter, for example, two different phases could both be liquids, just not miscible liquids.
Colloids are different from homogeneous solutions, in which a substance is dissolved or mixed with another substance and does not separate out, in that the components of colloids are nanoparticles or macromolecules (giant molecules), typically with a length or diameter ranging from a few nm to several hundred nm in diameter.
A sol is a liquid. The continuous phase in a sol is a liquid and the dispersed phase is a solid. The difference between a sol and a non-colloidal liquid is that solid nanoparticles are dispersed throughout the liquid in a sol. If you put a sol in a centrifuge, you can force the nanoparticles dispersed in the liquid to precipitate out. This will not happen for a non-colloidal liquid solution, for example, salt dissolved in water.
An example of a sol is black inkjet ink (carbon black dispersed in water).
A gel is a wet solid-like material in which a solid network of interconnected nanostructures spans the volume of a liquid medium. The continuous phase is a solid network and the dispersed phase is a liquid. Gels tend to be mostly liquid in composition and typically exhibit the density of a liquid as result but have cohesiveness like a solid.
An example of a gel is Jell-OTM gelatin.
An aerogel is solid with air pockets dispersed throughout. Aerogels are essentially the solid framework of a gel isolated from the gel’s liquid medium. Some aerogels, such as carbon aerogels and iron aerogels, are derived from other types of aerogels, but the aerogels they’re derived from came from a gel directly.
Production of Sols
Sols of all sorts of compositions can be made several different ways. Nanoparticles of any solid dispersed in any liquid in such a way that the solid phase does not spontaneously precipitate or settle out is considered a sol.
Generally there are two ways sols are made:
- Nanoparticles are grown directly in a liquid. This happens when you make Jell-O, or a silica gel. Basically you mix ingredients that contain molecules that can interconnect together to form bigger molecules and eventually nanoparticles. These nanoparticles then hook up together to form a gel network. See the Silica Aerogel article under Flavors of Aerogel for a detailed example.
- Nanoparticles are synthesized and then dispersed in a liquid. This is how more advanced gels are made. Nanoparticles such as quantum dots or carbon nanotubes are made through some process and then dissolved in a liquid directly or dispersed using the help of a surfactant (a detergent, like shampoo or dish soap). This is how metal chalcogenide aerogels and carbon nanotube aerogels start out.In the case of quantum dots, these nanoparticles are usually synthesized in a liquid, centrifuged out, and then redissolved in the desired liquid medium for whatever you’re using them for. Carbon nanotubes, on the other hand, are grown at high temperatures outside of a liquid medium.
The Sol-Gel Transition
A sol can become a gel when the solid nanoparticles dispersed in it can join together to form a network of particles that spans the liquid. This requires that the solid nanoparticles in the liquid, which are constantly bouncing around in random directions because of temperature (that is, they are undergoing Brownian motion), bump into each other and stick together when they do. For some nanoparticles this is easy, almost automatic, since they contain reactive surface groups that condense together to form bonds. For other nanoparticles, however, this can be tricky and requires the addition of an additive to “glue” the particles together or removal something from the particle surfaces so that they stick together when collide, either by bonding together or by electrostatic forces (static electricity).
As a sol becomes a gel, its viscosity approaches infinity and finally becomes immobile (that it is, it stops being able to flow and fill its container, although it might still wobble back and forth). This transition from sol to gel is called gelation. The point in time when the particle network extends across the entire volume of the liquid causing it to immobilize is called the gel point. The time required for a gel to form after mixing stuff together to make the gel is called the gel time.
Factors that Affect Sol-Gel Chemistry
Sol-gel chemistry tends to be particularly sensitive to the following parameters:
- pH. Any colloidal chemistry that involves water is sensitive to pH. In the case of silica gel formation, this has to do with the hydrolysis step of the silica precursor that results in silanol groups, which are what connect together to produce silica nanoparticles and eventually the gel network. See the Silica Aerogel article under Flavors of Aerogel for more details.
- Solvent. As molecules assemble together (polymerize) into nanoparticles, the solvent needs to be able to keep the nanoparticles dissolved so that they don’t precipitate out of the liquid. Also, the solvent can play a role in helping nanoparticles connect together. As a result, the solvent makes a big difference in ensuring a gel network can form.
- Temperature. The chemical kinetics of the different reactions involved in the formation of nanoparticles and the assembly of nanoparticles into a gel network are accelerated by temperature, meaning the gel time is affected by temperature. If the temperature is too low, gelation may take weeks or months. If the temperature is too high, the reactions that join nanoparticles together into the gel network occurs so quickly that clumps form instead and solid precipitates out of the liquid.
- Reaction-generated heat released from chemical reactions involved in the formation of nanoparticles and gel networks can feed back into the solution and cause things to react faster, releasing even more heat, causing things to react even faster, etc.
- Time. Depending on the type of gel being made, different steps in the gel formation process work differently over different time scales. In general, slower is better for sol-gel. If a gel is allowed to form slowly, it usually has a much more uniform structure. This often means a stronger gel and, in the case of potentially transparent gels like silica, results in a clearer gel that Rayleigh scatters less (appears less blue). Speeding reactions up too much causes precipitates to form instead of gel network, and can make a gel cloudy and weak or simply not form.
- Catalysts. A catalyst is a chemical that accelerates a chemical reaction but does not get used up in doing so. In a lot of sol-gel chemistry, both acids (H+) and bases (OH–) are catalysts, but accelerate chemical reactions by different mechanisms. This is another reason why sol-gel chemistry is usually pH sensitive. In silica gel chemistry, fluoride ion (F–) can catalyze gel formation, as it exploits a special ability of silicon to temporarily form five bonds. Small amounts of catalysts (“catalytic amounts”), on the order of milligrams per tens of milliliters of solution can cause drastic changes in gel time–in many cases reducing gel time from hours, days, or weeks to minutes.
- Agitation. Mixing a sol as it gels is important to ensure that the chemical reactions in the solution occur uniformly and that all molecules receive an adequate supply of chemicals they need for the reactions to proceed properly. However, as a sol gels, there are microscopic and macroscopic domains of partially-formed gel network throughout the liquid, and agitation can sometimes disrupt the formation of these domains, meaning they get broken up, however usually the broken network fragments regrow and a solution-wide network results.
The Rest Depends on the Recipe!
There are lots of different types of gels and so the details of what chemicals get mixed to make a sol-gel reaction happen depends on the gel. Read through the Flavors of Aerogel section and the Aerogel Recipes section to get a sense for what gets mixed together to do all this.