Chemisty of Stain Removal

I am just returning home from the 95th Canadian Society for Chemistry conference, which was hosted in Calgary, Alberta this year. One of the great things about going to Calgary for me (aside from visiting the zoo) is that my sister lives there. Instead of staying in one of the over-priced hotels near the conference centre, I was able to stay in one of the over-priced downtown apartments near the conference centre. This was great! And sure, the accommodation may not have had the most comfortable bed, it did come with free internet, home cooked meals, and a couple of cats to snuggle with. Oh, and family that I love. In this time though, my sister found that having a free-loading chemist as a house guest isn't all that bad. Not only will she leave her fabulous shoes for you to wear, she can apply her knowledge to help you remove a baked-in oil stain from one of your favourite sweaters.

The stain: in an expensive and great lululemon zip-up hoodie, an oil stain that was very visible was set in the pockets. The sweater had been machine washed and dried. Its distraught owner was quite sure nothing further could be done for the sweater. It was to fall victim to this villainous stain. 

Our heros: a chemist, canola oil, and dishsoap. The canola oil was rubbed into the stained areas. Then dishsoap was poured on top and also rubbed in. The sweater was then thrown back into the washing machine.

The outcome: one stain-free sweater, good as new.

That's right, we used oil to remove oil. Magic right? Not so. Here's the chemistry: because the oil had been washed and dried, it was now trapped in the fibers of the fabric and therefore  not accessible to soap when further washing would take place. This is because water and oil, as the saying goes, don't mix. This creates a barrier between the oil in the fibers and the soap. In order to get the oil out then, we need to find a medium that it does mix with. This is where the chemistry phrase "like dissolves like" comes in. Oils are soluble in other oils. Any oil will do: canola, olive, WD40, take your pick. By rubbing the oil onto the stain, it is able to dissolve the oil that is set into the fabric fibers. Once it has, you pour on the dishsoap, and the soap does what soap is designed to do, and form what are called "micelles" around the oil particles that can now be washed away in water. The result is a stain-free fabric. 

I talk a little bit more about some of the factors involved in my blog about water. The terms hydrophobic, hydrophilic all play a role in this chemistry. Oil is hydrophobic: water-hating. Soap is what is called "amphiphilic", meaning that one part of it likes water, while the other part doesn't. When placed in water it will arrange itself in the small spheres (micelles) with the hydrophilic (water-loving) part facing out into the water, the hydrophobic part facing in. Because oil is also hydrophobic, it ends up on the inside of these micelles, away from the water.    

So there you have, practical chemistry to save your clothing from destruction!

 

Water Water Everywhere

If there was one molecule that I could spend weeks writing about, it would be water. It isn't a complicated structure, like strychnine. It isn't a huge money making pharmaceutical like Lipitor. This is the molecule that is required for life, and yet there isn't a single atom of carbon-the element that forms the backbone of life-in it. The presence of water is the single most important indicator for the possibility of life on other planets. You can live weeks without food, you can only live days without water. What is so important about this simple molecule that some of us are able to take for granted? Let's talk about its chemistry.

Water, aqua, eau, dihydrogen monoxide, whatever your word for it is, is made up of two hydrogen atoms bonded covalently to a single oxygen atom. A covalent bond is one where the two atoms share electrons between them. This is different from an ionic bond, where electrons are transferred from one atom to another to create ions, one positive ion and one negative ion, and these ions are then attracted to each other via that whole "opposites attract" thing, known in the science world as electrostatic forces. Table salt, or sodium chloride, is an example of a compound held together by an ionic bond rather than a covalent bond. That was a lot of jargon, so to simplify things, you can think of an ionic bond like two atoms dating, or living together. Each atom is currently content with the arrangement, but if things should go awry they can easily separate themselves and go on their merry way. Conversely, covalent bonds are atom marriages. Much bigger commitment, everything is shared between the two, and breaking up is much more difficult. So back to water. Water has a polygamous marriage happening, with the two hydrogen atoms. Also, not all atom marriages involve a 50-50 sharing of assets (electrons). Some atoms tend to be a little needier (or greedier) and will hoard more of the assets (electrons). Oxygen is one such atom. Oxygen is what we call an electronegative atom. To be specific, it is the second most electronegative atom on the periodic table (fluorine is the most electronegative). This makes oxygen a big electron hog. This results in the two electrons that make up the oxygen-hydrogen bond spending most of their time on the oxygen end of the bond. The result is that the hydrogen end of the molecule has a partial positive charge (a full positive charge would mean that we now have an ionic species-we don't.) and the oxygen end has a partial negative charge. This polarity of the bonds is crucial! When you have many water molecules together, they each have this polar bond (partial positive on one end and partial negative on the other). The molecules will then order themselves such that the positive end of one molecule lines up with the negative end of another molecule, in a fashion similar to ionic bonds. Now these bonds are much weaker than covalent or ionic bonds, but are still extremely important in dictating the properties of water. These bonds are called hydrogen bonds. 

Hydrogen bonds are what makes water highly cohesive and gives it a high surface tension. The surface tension is what allows insects like water striders to walk on its surface, and also what it hurts so much to do a bellyflop into a pool. This is also why water has such a high boiling point (100 C) where as similar molecules, like H2S, are gases at 25 C. Hydrogen bonds are also what makes ice float. In liquid water, there remains a lot of disorder, with these hydrogen bonds continually breaking and reforming. As the water changes state from liquid to gas, the amount of order increases. The molecules are frozen in such a way to maximise the bonds. This makes the solid state much less dense than the liquid state, and thus the ice floats on top of the river, pond, sea. Imagine trying to go ice-fishing if this wasn't the case.

The polarity of the water molecule is also key to life. Humans are over 60% water by mass. All living cells have a significant water content. Cells are made up of, and defined by, a phospholipid bylayer called the cell membrane. Phospholipids are made up of a head group that likes water (hydrophilic) and a tail group that doesn't (hydrophobic). When these phospholipids are mixed with water they arrange themselves in a two-layered sheet with the the tails on the inside of the sheet away from the water and the heads remain on the outside edge mixing with the water, forming a spherical species called a vesicle, that has water on the inside and the outside, but not within the wall.  As these vesicles evolved into more complex structures we got life that eventually evolved into the multicellular beings that we are. Crazy to think that it was the properties of water that dictated that evolution, eh?

References:

Pratt, C. W.; Cornely, K. Essential Biochemistry 2004, John Wiley & Sons, Inc. Hoboken, NJ.