Concept of surface tension - applications

Surface tension (or surface energy) is defined as the work, W, required in order to isothermally increase 

The non-balance of forces at the interface between two immiscible liquids is represented by the liquid-liquid interfacial tension (or interfacial energy). This interfacial tension usually (but not always) lies between the individual surface tensions of the two liquids. For fiiUy miscible liquids, like water and methanol or water and ethanol, the interfacial tension is zero. 

The above picture implies a static state of affairs. However, the actual state is that of great turbulence on the molecular scale as a result of two-way traffic between the bulk of the liquid and the surface, and between the surface and the vapour phase (for a gas-liquid surface). The average hfetrme of a molecule at the surface of a liquid is about 10 s. 

Introduction to Applied Colloid and Surface Chemistry, First Edition. Georgios M. Kontogeorgis and S0ren Kiil. 2016 John Wiley Sons, Ltd. Published 2016 by John Wiley Sons, Ltd. 

We sometimes use the term surface tension for hquids or solids in contact with air (or vacuum) and the term interfacial tension when we have two condensed phases in contact, e.g. hquid-hquid or hquid-solid. Nowadays, both terms are used, with the term interfacial tension or energy being the more rigorous one. 

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