What Does Micelle Mean?
A micelle is a roughly spherically shaped grouping of amphiphilic molecules contained in a liquid. These supermolecular assemblies form due to the molecules having a polar end that likes water and a non-polar end that likes oil. The surface of the micelle is comprised of the end of the molecule that interacts favorably with the solvent and the other end packs together in the micelle's core. The formation of micelles within a colloidal mixture depends on the concentration of the amphiphilic surfactant, temperature, pH and ionic strength of the mixture.
Chemical corrosion inhibitors that are injected inside pipelines often form micelles, but they function best before too many micelles form. Understanding the properties of micelles is important in the prevention of internal corrosion.
Corrosionpedia Explains Micelle
Surfactant molecules have a polar side that likes water (hydrophilic) and a non-polar side that dislikes water (hydrophobic). Mixing surfactants with water drives the minimization of unfavorable interactions between water and the non-polar end. At low concentrations, the surfactant molecules primarily lay at the interface of the water. The polar end interacts with the water, while the non-polar end is not exposed to the water's surface. As more surfactant is added, less of the surface of water is open to interact with the surfactant. Thus, more molecules mix with the water, creating non-favorable interactions between the water and the non-polar end. To counteract the unfavorable interactions, the surfactants group together by packing their non-polar ends together, forming a shell of polar ends, and composing a micelle.
For each surfactant, a concentration exists called the critical micelle concentration (CMC). At this concentration, the amount of surfactant on the surface of the liquid is at a maximum and the addition of more surfactant only creates micelles. Chemical corrosion inhibitors must work at the surface of the pipes where internal corrosion occurs. When inhibitors form micelles, they are removed from the surface, lowering their ability to block corrosion. As such, inhibitors work best at concentrations slightly lower than the CMC of the inhibitor.
Determining micelle concentration and the CMC is difficult due to the dispersion of micelle sizes ranging from 10 to 300 nanometers. Diagnostic field testing of a pipeline may utilize fluorescence response. Here, a small amount of fluorescent compound is added to a sample. Typically, the chosen fluorescent molecule's fluorescent activity is low within solution, but highly active when trapped inside the micelle. Thus, an increased fluorescence response in the sample correlates to the micelle concentration. Using such techniques allows facilities to monitor and test the usage of inhibitors within their pipelines.