This NBC Learn video explains how ammonia works with water to dissolve fatty acids, like stearic acid, in greasy dirt; includes animation illustrating deprotonation, and explaining the Brønsted-Lowry definition of acids and bases.
The Dirt on Ammonia as a Cleaning Agent
BETH NISSEN, reporting:
Spring cleaning. The annual household ritual of banishing dirt, scrubbing off grime, cutting through grease and washing windows…all tasks made much easier by chemicals in household cleaning products, one chemical in particular, in many commercial, home-made and industrial-strength cleaners: ammonia
This is how ammonia is most commonly seen: as a liquid, heavily diluted with water, so you may not know that ammonia is a gas made up of one atom of nitrogen, the N in its chemical formula, and three atoms of hydrogen, the H3.
Like nitrogen and hydrogen, ammonia is colorless, but not odorless. Ammonia has a strong wet diaper smell, sharp and stinging enough to wake you up from a dead faint. In fact, the key ingredient in smelling salts is a solid form of ammonia.
What makes ammonia such a strong cleaner? Let’s start by getting the dirt on dirt. Greasy dirt, like what collects on kitchen surfaces when you fry a hamburger. If you could see that dirty, greasy surface on a molecular level, you’d see a dense mat of dirt particles, bacteria, and grease molecules, things like fatty acids, for example, stearic acid, found in animal fat like that in ground beef. In high heat, this normally solid fatty acid melts, spatters; later, it congeals.
You can try to use water alone to clean up this mess but it won’t work well. And here’s why. This is the stearic acid molecule. Except for this little part here, the molecule, this whole long chain of hydrocarbons, is hydrophobic: it repels water. Like most fatty acids, stearic acid doesn’t want to dissolve in water. And H2O doesn’t want to, actually can’t dissolve it. H2O is a polar molecule, with a slightly negative and a slightly positive side. It wants to dissolve other polar molecules and ions, which to remind you, it does like this.
All the negative sides of the molecules in a substance being dissolved – for example, table salt – are surrounded and isolated by a mass of H2O molecules with their positive sides showing. The negative sides of H2O molecules swarm the positive sides of the solute molecules. All that swarming and surrounding breaks the solute into tiny isolated components which can then be carried away by the cohesive H2O mob.
But stearic acid, like most fatty acids, is neutral: none of its sides or ends has a positive or negative charge to speak of. H2O molecules don’t have any strongly electrically-charged handles for their positive or negative sides to swarm, surround and break apart. If only there was some way to get the stearic acid molecules to give up their neutrality, become electrically charged. There is. And the words give up are key.
Fatty acids, like all molecules, are made of atoms, you know, protons and neutrons in the center; electrons orbiting around. Turns out, if the stearic acid molecule gives up one proton, the nucleus in the very center of this hydrogen atom, here at this end, it will become negatively charged, which will make it soluble.
H2O wants the proton – but is too weak to pull it off. If you were wondering when we were going to get back to ammonia, it’s right now: NH3 is going to do what H2O can’t. Ammonia is a stronger proton grabber – it’s attracted to protons. It pulls at the proton. The stearic acid molecule can’t hold onto it, lets it go – or as chemists say, donates the proton. What’s happened here is an example of classic acid-base chemistry – using what’s called the Brønsted-Lowry definition of acids and bases.
You may have learned that substances with a pH below 7 were acids; those with a pH above 7 were bases. The Brønsted-Lowry definition of acids and bases can be boiled down to this: Substances that donate protons are acids; substances that accept protons are bases.
So by definition, stearic acid and other fatty acids are acids, as their names might also suggest, molecules capable of donating a proton. And ammonia able to, and here’s a word not often heard in conversation, deprotonate the stearic acid molecule – is a base. A Brønsted-Lowry base.
By the way, here’s what happens after the deprotonation. The stearic acid molecules become stearates, stearic acids in salt form, that have a negative charge. As they’re mobbed by the H2O molecules, the stearates start to dissolve into the water in the form of micelles, which look a little like dandelion puffballs. The parts of the original molecule that hated water? They still do, they’re on the inside of the micelles. The negatively-charged bits – newly-made from that one part of the molecule not hydrophobic they’re on the outside. The mass of greasy dirt is soon broken into bits, dispersed in the ammonia water, ready to be poured out, rinsed away, wiped up leaving the kitchen surface damp but clean.
Remember: ammonia is a gas. It evaporates with the water, and evaporates quickly leaving no residue to attract dirt and dust, or dry into streaks on glass, which is what makes ammonia cleaner so good for doing windows. So, that’s the basic story of this common household cleaner and why ammonia shines.
