Soap in action

Your hands are greasy and you are trying to wash away the grease. You try to wash it with water, but you see droplets of water on your hand and it still feels oily. You use soap and problem solved. Why does soap work?

Before we think about how soap works, let's think about what happens when there is no soap. Think about mixing oil and water. They don't mix. Oil and water behave very differently compared to say alcohol and water. Most of you probably have never seen near pure alcohol, but when you mix alcohol and water... you don't see different parts to it. These parts are typically called phases. In the oil and water system, there are two phases - the oil phase and the aqueous (water) phase. Suppose you added salt and mixed it. You should still observe two phases. But taste the oil before and after. The salt preferentially goes into the aqueous phase and "avoids" the oil phase. Intuitively, this is because the two phases have  different capacity to interact with the salt molecule. Since water can interact more favorably, water dissolves the salt better. This is called partitioning. In a sense, this is why water and oil form two distinct phases (and are not miscible). Water doesn't dissolve completely in oil and oil doesn't dissolve completely in water. But, it should be noted that to some degree water does dissolve in the oil. When the two phases form, water is saturated* with oil and the oil is saturated with water. It might be a very very low concentration but there is some water in the oil phase and some oil in the water phase. *Saturation in chemistry means it has reached the thermodynamically maximum concentration. In more simple terms, more of it cannot dissolve. Think about adding a lot of sugar into water and leaving it for a really long time. The sugar crystals remain and there is no net dissolution of sugar crystals.

Okay, I think I was a little long winded, but the main point was that some things dissolve in certain phases - oil or water. Typically not both. But what if you had a thing that dissolves in water and connected it to something that dissolves in oil? You get an amphiphilic thing. It's like having a cat-dog. Half of it loves water and the other half loves oil. It's like a glue that brings the two things together. This amphiphilic thing called surfactant (ex. SDS) acts like a linker between water and oil and now the two different phases are no longer visible. When you wash your hands, the oil is washed away with the water because surfactants are helping the oil "attach" to the water.

Next time you see soap in water you know how things work in the molecular level. Surfactants are very interesting species (btw in chemistry, things like chemical entities are called species... I found this really weird and difficult at first but it's a very generic term referring to almost anything. The word "thing" doesn't sound scientific enough I guess). They can come together in very interesting ways. Think about adding special surfactant in a particular liquid and these surfactants build some very ordered and defined structure as shown in Figure 1. This process is called self-assembly.

Figure 1. adapted from http://www.rsc.org/chemistryworld/Issues/2003/July/amphiphiles.asp

Possibly, self-assembly is a interesting topic for tomorrow. DNA is also capable of undergoing self-assembly and there are some really cool research out there that try to build nano-machines using DNA as building blocks.