ABSTRACT
This chapter describes the importance of interfacial tension in pharmaceutical formulation. A boundary between two phases is termed an interface. Liquid–liquid interfaces result from the contact of mutually immiscible liquids. Liquid–solid interfaces result from the contact of an insoluble solid with the liquid. The phenomena resulting from and at the boundary of the two phases are termed interfacial phenomena. Interfacial tension is the force per unit length that must be applied in parallel to the interface to expand the interface, counterbalancing the net inward pull of the two phases. While the term surface tension is reserved for liquid–gas and solid–gas interfaces, the term interfacial tension is commonly used for liquid–liquid interfaces. At the liquid–gas interface, cohesive forces are generally greater than adhesive forces, leading to an inward pull on the molecules toward the bulk. Usually, the interfacial tension (liquid–liquid) of a hydrophilic liquid is less than its surface tension (liquid–vapor).
