We are all taught at school that there are three states of matter, although those of you who read our recent post on the topic will know that there are more. Even so, we tend to label the materials that we encounter in our everyday lives as solids, liquids and gases and don’t often need to think too hard about it. But there are cases in which things aren’t so straightforward…
Take gels, for instance. The term ‘gel’ is used to describe pretty much any material that is jelly-like (or jello-like for our American audience!). Gels are pretty common materials – many of us style our hair with hair gel, wash ourselves with shower gel, eat jelly babies and brush our teeth with gelatinous toothpaste afterwards. But here’s an interesting question: how do gels fit into our solid, liquid, gas view of the world? Are gels liquids, solids or something else?
I’d wager that, if asked to classify a gel as a liquid or a solid, many people would opt for liquid. After all, many things that we think of as gels do exhibit some liquid-like properties. Most shower gels, for example, will eventually conform to the shape of the container that they’re put in – a characteristic property of a liquid. The problem is that these relatively liquid-like gels clearly have some solid-like properties in that they are self-supporting to some extent. Thicker gels like set jelly/jello, for example, don’t really behave like liquids, and jelly babies certainly don’t!
You can probably see where this is going – gels have some properties that are liquid-like and some properties that are solid-like. They don’t really fit in to our solid/liquid/gas labeling system, which is why they have a special name. But if gels aren’t really solids or liquids, what are they? To begin to answer this question, we’ll need to learn a little bit about how gels are made.
How are gels made?
To make a gel, we almost always need to start with a liquid. In principle we can use any liquid, but in practice water or common organic solvents are usually good choices. The aim is to make our chosen liquid less liquid-like and give it more solid-like properties. There are two main methods we can use to achieve this, and both involve the formation of a 3D structure.
In the first method, we start by dissolving a material called a ‘gelator’ (the stress is on the second syllable; it does not rhyme with He-Man’s nemesis) in a hot solvent and cool the resulting solution. This method will be familiar to you if you’ve ever made jelly/jello, where gelatine is the gelator and water is the solvent. How does this work? Put simply, gelators are materials whose constituent molecules have a tendency to ‘self-assemble’ into some sort of structure that is much larger than an individual molecule. The driving force for this self-assembly process is often the formation of hydrogen bonds1 between molecules. When the gelator is dissolved in the hot solvent any hydrogen bonds that form are quickly broken due to the thermal energy of the solution. However, as the solution cools, hydrogen bonding reestablishes itself and eventually a 3D network is formed in the liquid. This 3D network spans the whole sample and if we were able to zoom in, we’d typically see that it’s composed of very thin fibres that are entangled and interconnected. The solvent in which this network has been formed sits between these fibres (see schematic below). Crucially, due to strong surface interactions between the fibres and solvent molecules, the liquid phase isn’t able to flow freely as it normally would. Instead, it’s somewhat ‘trapped’ by the surrounding network. This is what leads to the formation of our gel.
The second method of gel formation is similar to the first, but instead of the 3D network being formed by self-assembly of gelator molecules, it’s formed by polymerisation. This involves taking a solution of monomers and polymerising them with a cross-linking agent to form a cross-linked polymer2.The monomers react with each other to form long polymer chains, and the cross-linking agent joins these chains together. This results in the formation of a covalently bonded (rather than hydrogen bonded) 3D polymeric network which immobilises the liquid phase. We call gels made using this method ‘chemical gels’ because the 3D network is held together by chemical bonds. Gels made using the first method we looked at are called ‘physical gels’.
Introducing colloids
Ok, so we’ve seen how gels are made, but how does this help us with our classification of gels as materials? Well, in both physical and chemical gels we have two components:
- A 3D network that is a solid
- Water, or an organic solvent, which is a liquid
This leads us to the answer to our question about what a gel actually is: a gel is a liquid dispersed in a solid.
Now, you might be wondering if we can make dispersions of other states of matter. As it turns out, we can! If we disperse a gas in a liquid, for instance, we make a foam. The foam on the top of a pint of beer is just gas from the beer dispersed within the beer itself. We can even disperse the same state of matter in itself. An ‘emulsion’ is a liquid dispersed in a liquid and a thing called a ‘solid sol’ is a solid dispersed in a solid (note that we can’t disperse a gas in a gas since gases are all miscible)3. The general name for materials in which one state of matter is dispersed in another (or in itself) is a ‘colloid’. The table below shows the colloids formed by dispersing solids, liquids and gases. You’ve probably heard of most of these colloids, but who knew ‘solid foam’ was a thing? Apparently Samuel Stephens Kistler did, because he won a bet with Charles Learned when he created a solid foam called aerogel. This involved stripping the liquid out of a gel to leave air dispersed in the 3D solid network. Aerogel is a really interesting material – you can read more about it here.
[table id=2 /]
So, that’s colloids – one state of matter dispersed in another. They’re fascinating materials with many of interesting applications. We’ll explore some of the colloids listed in the table above and their applications in future posts, but in the meantime keep an eye out for materials that don’t fit into the traditional solid/liquid/gas labelling system – you’ll notice them all over the place and chances are they’re colloids!
1 A hydrogen bond is an interaction that occurs between a hydrogen atom on one molecule and a different atom (usually oxygen or nitrogen) on a neighbouring molecule.
2 Monomers are small molecules that react to form long chains that we call polymers. Cross-linking agents are small molecules that are able to join these long chains together to create large interconnected molecules.
3 It’s worth pointing out that you can’t form colloids with things that are miscible with one another, or things that dissolve in each other. For example, table salt (NaCl) dissolves in water to form a solution – it doesn’t disperse in water to form a sol.