Definition
The spreading coefficient (S) indicates the ability of one liquid to spread over another. It is defined as:
where:
γS is the surface tension of the sublayer liquid.
γL is the surface tension of the spreading liquid.
γSL is the interfacial tension between the two liquids.
Interpretation
Positive SSS: Spreading occurs spontaneously.
Negative SSS: No spontaneous spreading.
Applications of Spreading Coefficient
Topical Formulations: Influences the spreadability of creams and ointments.
Coating Processes: Affects film formation on solid surfaces.
Emulsion Stability: Determines the ease of emulsification.