We all learn pretty early on that oil and water don’t mix. Add a glug of cooking oil to a glass of water and you’ll end up with the oil sitting on top of the water1. Stirring or shaking this two-layered mixture will form droplets of oil in the water and you could argue that creation of these small droplets constitutes ‘mixing. But if left to their own devices the droplets will eventually merge and rise to the top to reform the layer of oil. Usually, we mean ‘mixing’ to be something else, a more permanent state of affairs. No amount of stirring or shaking will cause the cooking oil to ‘mix’ with the water in such a way that it doesn’t re-separate.
Now, it’s likely that we’ve not told you anything new so far: you probably already knew that you can’t mix oil and water. But, here’s the interesting thing: if you think about it, you spend a significant amount of time in your daily life trying to do just that, and using various substances to help. For example:
- You clean the oily residues from plates and dishes after eating, using washing-up liquid (dish soap for our American readers…)
- You (hopefully) clean oily ‘dirt’ from your skin when you shower using soap or shower gel
- You clean oily stains from your clothes using detergent
All of these activities involve ‘mixing’ oil with water in order to remove the oil and clean something. But if oil and water don’t mix, how do the substances (soap, detergents, etc.) mentioned above manage to clean things? Let’s find out…
“Like dissolves like”
Why don’t oil and water mix? It’s a simple enough question, but like many simple questions in science, the answer is quite complicated. To give a proper answer, we’d need to dive into the world of thermodynamics and look at the enthalpy and entropy changes associated with various processes. We’ll spare you from this discussion for now and save it for another blog post. Instead, let’s take a more empirical approach: “like dissolves like” is a common expression used by chemists when describing why things mix or dissolve. Put simply, solutes (things that dissolve) tend to have similar properties to their solvents (things that dissolve them). This line of reasoning isn’t very scientific (and shouldn’t be treated as such!) but it does work as a simple explanation for why things dissolve in, or mix with, other things. For example, we can reason that sugar dissolves in water because, like water, it is polar2. On the other hand, oils, fats and other non-polar materials do not tend to dissolve in water. This is why chemists often call non-polar materials hydrophobic (‘water-fearing’) and call materials things that dissolve in water hydrophilic (‘water-loving’).
Based on the discussion above, we can see that one way to make oil mix with water would be to somehow make the oil polar. However, in order to do so we’d need to change the chemical structure of the oil, in a way that is not easily done; this is not how the products that we mentioned above work. Instead, those products contain materials called surface active agents, or ‘surfactants’ for short.
Introducing surfactants
Surfactants are molecules that contain two parts: one part that is hydrophilic and another part that is hydrophobic, as shown below.
To understand how surfactants can help us to mix oil and water, let’s take the oil out of the equation for a second – if we take a beaker of water and add a small amount of surfactant, the majority of the surfactant molecules will sit at the air-water interface, with the hydrophilic head groups in the water, and their hydrophobic tails pointing into the air, as shown below3.
This kind of arrangement is energetically favourable because:
- It allows the hydrophilic parts of the surfactant molecules to interact with the water (and each other)
- It keeps the hydrophibic parts away from the water whilst allowing them to interact with one another
Although it’s not relevant to this post, you might be interested to know that this type of structure is called a ‘Langmuir monolayer’ (see this link for more information). In fact, it was Irving Langmuir’s work on these monolayers that won him the 1932 Nobel Prize in Chemistry! This arrangement is also used to create ‘Langmuir-Blodgett’ films, which are monolayers on solid substrates that have all sorts of interesting applications – maybe we’ll write another post about those later. As another side note, the Langmuir Monolayer arrangement shown in the image above has the interesting effect of reducing the surface energy (by reducing the surface tension) of the water, which is why we call the molecules that form these structures ‘Surface Active Agents’ or surfactants for short.
That’s all very interesting, but what does it have to do with mixing oil with water? Well, if we keep adding surfactant there comes a point at which something different happens – the surfactants begin to form structures in the water.
There are many different types of structure that the molecules of the surfactant can form, but we’ll focus on perhaps the most well-known here, which is a spherical structure with the hydrophilic heads of the surfactants on the outside of the sphere and the hydrophobic tails on the inside of the sphere. A cross section of such a spherical structure is shown below.
This arrangement has similar advantages to the monolayer arrangement that we looked at previously in that it allows the ‘like’ bits (the water and the hydrophilic part of the surfactant) to interact and also shields the ‘unlike’ bits from each other. We call this structure a ‘micelle’ (or a ‘spherical micelle’, to be exact) and refer to the tipping point at which micelles form (at a given temperature) as the ‘critical micelle concentration’.
Now lets bring our oil back and imagine that we’d formed our micelles in a mixture of oil and water – what would happen then? Well, here’s the crucial point: because the inside the micelle is hydrophobic, they are capable of encapsulating small droplets of oil (remember that “like dissolves like”). But remember that the outside of the micelle is hydrophillic, meaning that it is ‘like’ water. As a result, our oil-filled micelles sit quite happily in the water layer and, to the naked eye, this layer still looks like looks like a homogeneous solution. However, if we could zoom in to the microscopic level we’d see that it’s actually a dispersion of oil-filled micelles in water.
In other words, we have ‘mixed’ oil and water by creating a colloid – an emulsion of oil in water stabilised by micelles! (If you’re not sure what a colloid is, see last week’s post). Here, our surfactants here are behaving as an ’emulsifying agent’, much like egg yolk does in mayonnaise or mustard does in a vinaigrette (both egg yolk and mustard contain chemicals that act as surfactants).
Wash up
So, whenever you successfully mix oil and water, you’re probably creating an colloid (an emulsion) stabilised by micelles. The formation of micelles is the process behind many of your daily activities: washing up, washing the car, showering, washing your clothes – basically all washing, as well as a lot of other stuff4. You might have noticed that micelles are actually in fashion at the moment, with several brands proudly displaying ‘contains micelles’ or ‘micellar’ on the front of their products.
The reality, however, is that micelle formation is the science behind the way that ALL soaps and detergents work – not just the ones that claim to form micelles!
1 We should point out that the non-aqueous layer doesn’t always sit on the top; in biphasic mixtures involving liquids that have a greater density than water (e.g. dicholormethane) the aqueous layer sits on top of the non-aqueous layer.
2 See this link for more information about polarity
3 In reality, not all of the surfactant molecules will sit at the surface, a small proportion will dissolve in the water.
4 The ability of micelles to promote the mixing of oil and water mean that they have applications well beyond just washing! Micelles have an imporatant role to play in all sorts of areas of science. We might write about these applications in a later post.