Surface free energy of a fluid

A fluid has a surface energy only when it exists in a sufficiently condensed state. Because of the uniform energy distribution in a gas, no difference exists between an internal molectUe in the center of a gas volume and a molecule located near a wall therefore a gas has no surface energy. 

The theory for the forces acting between molecules was put forward by Letmard-Jones and Devonshire [98]. The liquid molecule is assumed to be located in a cage formed by the neighboring molecules, and it is constantly under the influence of their fields yet being sufficiently free to execute translatory and rotary movements. 

Each molecule in a liquid volume is surrounded by molecules on all sides, and hence is subjected to attractive forces acting in all directions. Generally speaking, a uniform attraction in all directions is exerted by every molecule for a period of time which is relatively long compared with periods of vibration. 

Very different conditions obtain at the surface. The molecules ate attracted back towards the liquid and also from all sides by their neighbors, yet no attraction acts outwards to compensate for the attraction towards the center. Each surface molecule is subjected to a powerful attraction towards the center acting, for reasons of symmetry, in a direction normal to the surface. 

The work required to increase the area of the surface by an infinitesimal amount dA. at constant temperature, pressure and composition is done against a tension y, generally known as the surface tension which can be defined from the poim of view of energy involved [99,100]. The free energy change dF is equal to the reversible work done  

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