The interpahase is the contact region between two different phases of a system, such as solid–liquid, liquid–gas, or solid–gas. It is not simply a two-dimensional geometric boundary, but rather a thin, three-dimensional zone where physical and chemical properties gradually change from one phase to the other.

The interpahase is crucial because it is the site where many chemical and physical processes occur: heterogeneous reactions, mass and heat transfer, adsorption phenomena, emulsion formation, crystal nucleation, and growth. For example, in heterogeneous catalysis, the interface between a solid catalyst and a gaseous or liquid reactant is where the chemical reaction takes place.

The properties of the interface—such as surface energy, interfacial tension, and molecular structure—depend on the nature of the interacting phases and environmental conditions (e.g., temperature, pressure, concentration).

A special case involves colloidal interfaces, where a finely dispersed phase interacts with a continuous medium. In such systems, the interface plays a key role in determining overall stability.

In industrial applications, controlling interfacial characteristics is essential in fields like materials science, pharmacology, paint and coating production, and nanotechnology.

The interpahase is an active and complex region, often invisible, but essential for understanding and controlling chemical phenomena in multiphase systems.