We often hear the word “hydrophobic” bandied around. Well surprise, surprise, it means just what you think it means – water-hating. Or to be more precise; “tending to repel or fail to mix with water”.
The opposite is a word used less often: hydrophylic. Something is hydrophilic if it mixes easily with water.
This has to do with how well the mineral “wets” in water. White mineral fillers are generally not soluble, so they do not dissolve. Rather, the particles disperse in the water. The solid particles separate from each other and are suspended in the water, sometimes with the aid of a dispersant.
A hydrophilic mineral wets well and disperses easily and quickly. A hydrophobic mineral like talc tends to sit on the surface of the water and does not want to mix. It can usually be persuaded to go into the water with stirring, however.
As you’d expect from minerals that come out of the ground, most of them are hydrophilic with the odd exception:
| Titanium dioxide | Hydrophilic |
| Calcium carbonate | Hydrophilic |
| Talc | Hydrophobic |
| Kaolin | Hydrophilic |
| Dolomite | Hydrophilic |
So if you are making a waterproofing compound that must repel water, you may choose talc over kaolin, for example.
The main reason a mineral surface likes water is because it is polar. This means it has positive and negative charges which attract the OH- and H+ molecules in water.
A hydrophobic mineral is generally nonpolar, having little or no charge. So it repels water. Organic compounds like fats and oils and petroleum products are also nonpolar hence hydrophobic.
The interesting thing is we can coat the surface of a hydrophilic mineral to make it hydrophobic. For calcium carbonate, this is usually done with stearic acid.
Stearic acid is a clever molecule which is polar one end and nonpolar the other end. This allows it to stick to the hydrophilic surface of the calcium carbonate. The nonpolar ends of the stearic acid then face outwards, turning the mineral from water-loving to water-hating. Amazing, hey?
So instead of liking water, the coated CaCO3 starts preferring organic compounds, such as plastic polymers. This makes it a common filler in plastics.
Paint chemists are also concerned that their coatings should be hydrophobic. They need to reject water to protect the substrate underneath. However they would also like the coating to reject organic compounds, like oil or fingerprints. In fact the surface should be omniphobic – it must just hate everything!
These guys went one step further and invented the word SUPEROMNIPHOBIC – it rejects any and all substances or dirt. That’s what makes it self-cleaning. Wouldn’t it be great when they invent this property for little boy’s clothes?
How to make a superomniphobic surface involves manipulating the contact angle of the water or oil droplet when on the surface. But that’s a subject for another day…