Isoelectric point (pI)

The isoelectric point (pI) is a concept that is not only important in biochemistry, but also in surface science. When it comes to surfaces, the isoelectric point refers to the pH value at which a surface is electrically neutral. This means that the number of positive and negative charges on the surface is equal.

The charges on the surface are mainly due to functional groups or chemical species present on the surface. When the pH of the surrounding solution is below the isoelectric point, the acidic functional groups are protonated, resulting in a positive surface charge. Conversely, when the pH is above the isoelectric point, the basic groups deprotonate and the surface carries a negative charge.

Understanding the isoelectric point is important to control the charge states of surfaces and to understand how they interact with their environment. For example, the charge of a surface can influence how molecules or particles adsorb or repel on it. This is particularly relevant in areas such as colloid and interface science, biomaterials research and surface coating.

The isoelectric point of a surface can be determined by experimental techniques such as zeta potential measurements, surface charge density measurements or pH titration curves. Knowledge of the isoelectric point enables scientists and engineers to specifically modify surfaces and materials in order to control their interactions with other substances and thus achieve the desired properties for a variety of applications.