A Surface Active Agent can be described as a substance that can modify the surface properties of liquids or solids. In cleaning applications, these agents work at the boundary layer between soil and solvent. For aqueous based products, surfactants aid water overcoming its difficulty in dissolving oils and greases. By design, surfactant molecules have two chemical groups. On one end, there is an hydrophobic component that is attracted to the oil/grease. The other group is hydrophilic and compatible with water. Due to the strong interactions between the water molecules arising from dispersion forces and hydrogen bonding acting cooperatively, the hydrocarbon tail is squeezed out of the water. Hence the tail is usually termed hydrophobic.
In a cleaning solution, the hydrophobic end of the surfactant molecule orients toward the soil. Many surfactant molecules will attack the soil, breaking it up into small pieces and completely surrounding it. The hydrophilic ends of the surfactant molecules project into the solvent (i.e. water), causing the soil to be broken up, removed from the surfaces, lifted, and suspended into the cleaning solution. The outside ends on the detergent molecule chains are attracted to water, and the inside ends prefer oil, forming a cluster of surfactant molecules around the oil. In addition to the assembly at the interfaces, surfactants can undergo a self-assembly process known as micellization resulting in a sequestering of the hydrophobic end.
Surfactant technology represents a vibrant and challenging area of physicochemical science where the exact mechanisms of well-established processes are often not fully understood or are at least the subject of continued debate. This is partly due to the complex nature of even the simplest commercially available surfactants—there is still a degree of empiricism when selecting surfactants for certain applications.