Vegetable Oil : The other household cleaner

A few weeks ago, my dog Lucy found a tube of printing ink and chewed it up. This resulted in some inky black paws! It was an oil-based ink, and soap and water wasn’t getting it out of her fur. So I mixed equal parts vegetable oil and soap in the palm of my hand and washed her feet with that. The ink came out!
Why does vegetable oil help remove ink?
To answer this question, we need to understand the wonderful concept of “hydrophobicity”. Hydrophobicity is a measure of how much a material “hates” water. Oil, grease, wax and oil-based ink are all hydrophobic. They do not mix with water at all. The opposite of hydrophobic is hydrophilic — meaning water loving — and that refers to any material that “loves” water — such as sugar, salt, alcohol, vinegar and antifreeze to name a few examples.
So what is the molecular basis for hydrophobicity? It all comes down to the quirky personality of water. You know the age old addage, “opposites attract”. Well, it is true on a molecular level, too! For example, sodium ion has a positive charge, and chloride ion has a negative charge — mix them together and you get salt. All of the substances I listed as hydrophilic (water loving) are either charged overall (such as the ions in salt) or have regions of charge (as in alcohol or antifreeze).
Hydrophobic substances do not have any charges. This means they are usually made of chemicals that have lots of carbon atoms but no oxygen atoms. You may have heard terms such as “unsaturated fats” or “saturated fats” — these words describe the arrangement of the strings of carbon atoms that are in fat.
What makes water quirky is that even though it has no charge, it loves other charged molecules. This is because water can quickly convert from H2O (with no charge) to just HO (with a negative charge) or to H3O (with a positive charge). In addition, although H2O is not charged, it is a little bit “polar” — this means that on one side it has a slight positive charge and on the other it has a slight negative charge. The flexible distribution of charges in water means that it can surround other charged molecules with ease (which is what happens when something hydrophilic dissolves in water).
What happens if you take a long molecule and put a charge at one end, but make the rest of it hydrophobic? You’ve created soap! The soap molecule can wrap up an oily molecule. When it does that, the charged part of the soap is the only thing left on the outside — so now water can surround it and make it dissolve.
When Lucy’s paw was covered in black ink, there was too much oily ink for the soap to penetrate. Plus, ink is a very thick sludgy mixture of oils. Vegetable oil, on the other hand is runnier and can be cleaned with soap more easily. By mixing vegetable oil in, the ink molecules mixed with the oil (because they are both hydrophobic) and loosened it away from Lucy’s fur. Then the soap and water was able to lift them both away.
Try mixing vegetable oil with dish soap next time you have grease or pine sap stuck in your hair or on your hands.