Written by Yuhang Jin, AP225 2011 Fall.
"Amphiphilic" is used to describe the properties of a certain group of chemicals. A chemical substance being amphiphilic possesses both hydrophilic and lipophilic functional groups. As a result, amphiphilic compounds show good affinity for both water and oil.
Structure and examples of amphiphilic molecules
In amphiphilic molecules, the hydrophilic group is usually charged and highly polar. Examples include carboxylates -CO2- and sulfonates -SO3- with negative charges and amines -NH3+ with positive charges. Sometimes a structure containing a large number of polar, non-charged hydroxyl groups etc. can also serve as the hydrophilic part. The lipophilic part is usually a long chain of nonpolar hydrocarbon structures . The structure of a typical amphiphilic molecule SDS is shown in Figure 1.
The most common examples of amphiphilic chemicals are soaps, detergants and surfactants [1,2]. Their special structures lead to relatively high solubility in both polar solvents like water and a large variety of nonpolar solvents. Amphiphilic molecules may form aggregates or micelles in water. An example of a spherical micelle is presented in Figure 2. More importantly, due to their unique properties, amphiphilic molecules as surfactants can be absorbed to water-oil interfaces, reducing the interfacial energy and facilitating the formation of emulsions such as droplets. This role of amphiphilic molecules has been vastly utilized in droplet microfluidics.
 Witten, T. A. & Pincus, P., "Structured Fluids", Oxford University Press, 2004