DOCs are the waste molecules skimmers are designed to remove; these are produced as byproducts from the breakdown of biological materials. This pollution arises from not only the deliberate input of foods in our tank but also from decaying organic matter (bacteria, algaes, etc). DOCs are bipolar molecules; these surfactants are attracted to air/water interfaces, i.e., bubbles. A bipolar molecule contains one or more atoms attracted to air, and one or more atoms attracted to water. A skimmer exploits this difference in the following manner:
"As an air bubble moves through the column of organic-laden water, the electrically charged protein molecules (which contain electrically polar and electrically nonpolar regions) are attracted to the air/water interface of the bubble. The polar regions of the molecule (made up of nitrogens, oxygens, etc.) are attracted to the air/water interface and these polar "tails" stick out away from the air bubble into the water column. The nonpolar regions stick out into the air bubble because it does not "like" to be in contact with the polar solvent (i.e. water). If you could look at this bubble under high enough magnification down to the molecular level, the entire air bubble would look like a fuzzy ball with protein tails and other electrically charged tails sticking out from the surface of the air bubble. The polar regions outside of the air bubble stabilize the air bubble very much like a soap bubble in your kitchen sink or your washing machine. This is the reason why foam begins to build up at the surface of the skimmer. As the protein-laden bubble reaches the top of the protein skimmer, the proteins begin to accumulate which creates a stable foam bubble. These stable foam bubbles take a long time to pop. Thus, the proteins slowly are concentrated at the top of the skimmer where they are slowly pushed through the "throat" of the protein skimmer and into the collection cup."
