Friday, April 13, 2012

Surfactants: How does simple soap work?

I am currently reviewing a book about intellectual property for a trade publication. It specifically focuses on chemistry patents which I know very little about outside of my master's level knowledge of basic chemistry. Interestingly, I have learned about scientific patents in general from my involvement in The Association for Women in Science (AWIS). Several PhD level female scientists have taken the patent agent exam and worked for law firms as patent agents. It seems to be a logical, lucrative and practical solution to the problem of an educated woman in the business world. For some reason the patent/law world is able to accommodate women much better than academia or even industry science is able to do.

Partly for this reason, I volunteered to read and review the book for Technical Communication. This brings me to the focus of this post- surfactants. Apparently surfactants are used to solve a nanotechnology/quantum problem. This is fascinating to me because the two areas of science seem so distant from each other, yet apparently in this case, someone wrote a patent using surfactants for their more electricity/nano oriented problem. (This also gives you an idea of where I get ideas for these posts. Usually I'm reading about something that sparks my interest into writing about a related topic.)

Surfactants help make something intermix with something else. The most obvious example of this is when you are washing your dishes. (Believe me, I'm the queen of DLD right now as a stay-at-home mom- this is coined as DLD or "dinner, laundry, dishes") You have a stack of plates covered in greasy, leftover food. Hopefully, you remove the excess food before you place them in hot, soapy water to wash. Why not just hot water? In most cases, some of the grease is still left if you just wash them in hot water. On a molecular level, here is why:

The grease on the plates is full of hydrogen/carbon chains that have the properties of something  "hydrophobic." Generally, the chemical properties of carbon/hydrogen don't mix with the properties of water (made of hydrogen and oxygen). This is related to the concept of polarity/electronegativity that is discussed here in my other blog.Generally, polar compounds mix evenly with other polar compounds and nonpolar compounds (pure covalent compounds) mix with other nonpolar compounds. Water is extremely polar while any kind of grease or fat is extremely nonpolar.

The detergent molecules (surfactants) create an interface between the polar and nonpolar so that the grease molecules can be removed from the place.  The detergent generally has a polar head that will "stick" to the polar water molecules while it has a nonpolar carbon/hydrogen tail which will "stick" to the grease molecules. By intermixing with both polar and nonpolar components of the grease/water mixture the soap is able to engulf the grease and send it down the sink.

 As you can see in the picture, the left side of the soap molecule is a carbon/hydrogen chain while the right side (the head) has oxygen-containing functional groups. It is these oxygen functional groups that will intermix with the water while the greasy tail (left side) will combine with the grease on your plates.

Here is a picture of nonpolar greasy molecules in the same container with polar molecules:
My book likens it to a mixture of magnetic and nonmagnetic particles. Can you think of anything else that behaves this way?

I have discussed polarity and dipole moments in other posts. Here are links:
Ionic and Covalent Bonding
VSEPR Shapes