pseudoternary

Pseudoternary, or pseudo-ternary, diagrams are a type of phase diagram used to represent the composition and phase behavior of multicomponent mixtures in a three-component space. They are commonly employed in colloid and interface science for oil–water–surfactant systems, and in pharmaceutical and cosmetic formulation.

In a true ternary system, the three components vary at fixed temperature and pressure. However, many practical systems contain four components: oil, water, surfactant, and cosurfactant. To create a ternary plot, the surfactant and cosurfactant are treated as a single effective component, forming a pseudo-component. The three vertices then represent oil, water, and the combined surfactant–cosurfactant mixture. The interior points correspond to all possible combinations that keep total mass or volume constant.

Use and interpretation: The diagrams delineate regions where stable oil-in-water emulsions, water-in-oil emulsions, bicontinuous microemulsions, or other phases occur. They aid formulation by guiding the selection of component ratios to achieve desired phase behavior. They are constructed by systematic mixing and observing phase behavior at a given temperature, salinity, and pH; often researchers use them in phase-behaviour studies and formulation optimization.

Limitations: The pseudo-component hides the detailed composition of the surfactant and cosurfactant and assumes approximate ideal mixing; results depend on how the pseudo-component is defined and on experimental conditions. The diagram may not capture all interactions or be valid when significant chemical reactions occur.