This is my latest drawing, inspired by more diagrams and hours spent peering through a microscope than I'd care to count.
One of the areas of science that has always fascinated me is complex fluids. These are liquids which have structures inside them, but the structures and order are so soft and fragile that the tiniest bit of force (actually stress) is enough to break them apart. Complex fluids will behave like liquids when they're flowing, but if they rest for a while, the fragile structures reform.
My drawing is not an accurate technical diagram - not by any stretch of the imagination. But if the little lines were molecules with, for example an acid on one end and a carbon and hydrogen chain on the other they would be a type of molecule called an amphiphile.
Amphiphiles are very good at sorting themselves into fragile structures within fluids. Have you heard the phrase "oil and water don't mix"? Well, amphiphiles have an end that likes water and another end that likes oil. The two different ends are chemically stuck together, and can't escape one another. So what do they do?
The answer: They form spherical blobs, tubes, layers and other more complex patterns within a liquid. The "goal" of the amphiphile is to cluster together a bunch of water-liking ends from a lot of different amphiphiles to create a little water liking area where their ends can be happy. At the same time the oil-liking ends also want to be together. The different patterns that form are the result of the amphiphiles balancing out the goal of making little areas for their mismatched ends with surface area and volume constraints.
Have you ever used an amphiphile? Soap is an example of an amphiphile. If you have a mixture of oil-liking and water-liking molecules making up the dirt on something dirty pure water won't rinse the dirt off. The oily parts will simply repel the water and stay put. How do you get oily dirt to rinse off in water? Oil and water don't mix!
An amphiphile, like soap, can form structures around the parts of the dirt with the oil-liking ends surrounding the oily dirt and mixing with it. While the oily end of the amphiphile encapsulates the dirt inside a fragile little sphere, the water-liking ends poke out - they're all on the outside of the dirt capsule. When water "sees" this dirt capsule, what does it interact with? The outside, which is covered with amphiphile ends that like water. The water carries away the capsule and the oily dirt hidden on the inside, held by the oily ends of the amphiphiles.
So what does this have to do with paint?
Both water miscible oil paints and acrylic paints use surfactants to make them mix well with water. Surfactants are, in general, molecules that reduce the surface energy between two liquids that don't want to mix. They help the two liquids "like each other" well enough to mix. For example if you add an amphiphilic soap to a mixture of oil and water, the amphiphiles will do their little encapsulation trick and help keep the oil from separating from the water. If you could see the actual molecules in the mixture of oil, water, and amphiphiles, you'd see spherical amphiphile containers holding the oil and surrounded by the water. As the proportions of the mixture change the shapes of the containers will change and eventually invert, but that's a detail tangential to paint.
Acrylic paints are based on soft little beads of acrylic polymer. If you recall from an earlier post, Oil paint as a polymer, a polymer is a long rope-like molecule created when a bunch of smaller molecules link together. The acrylic polymer actually doesn't like water very much. It doesn't dissolve in water, which is a good thing or your paintings would also dissolve in water! Surfactants and other chemical additives are used to keep the acrylic polymer beads from separating out of the water and forming little clumps.
Have you ever wondered why some acrylic paints change their color and darken as they dry? Or why there are acrylic media that are white and milky when you pour them, but they dry transparent? The particles in acrylic are large enough to scatter light. The mechanism for the white color is very similar to the source of "whiteness" in milk or to the white color you get when you scratch up clear glass. Think about glass as an example. You haven't added any pigment to the glass by scratching it, but the scratches make it harder for light to find it's way right through the glass. Instead of passing right through a transparent piece of glass, light is scattered by the scratches.
When acrylic paint is wet, the surfaces of all of the little beads act like the scratches in glass and scatter light. When it dries, it's been formulated so that the acrylic polymer beads merge together. No more edges = no more scattering, and the paint dries clear.
As a fluid and as a polymer-based technology, acrylic paint and its development are really fascinating. Look for a few more tidbits on acrylic chemistry