Interfacial Layer and Surface Tension

In Chapter 4, we considered the thermodynamic properties of homogeneous phases with consistent properties through the entire phase. It should be obvious that this is not the case at the interface between two different phases of a system. The area of contact between two phases where the molecules of both phases are interacting is called the interfacial layer, which is usually considered to be a few molecules thick in a neutral species. If we are considering the bulk properties of the phase, the effect of this region on the properties of the phase can be considered to be vanishingly small. 

The molecules at the surface of a liquid experience a different net attractive force than those of the bulk solution. The surface molecules tend to be attracted into the bulk liquid since the liquid s attractive force is stronger than the vapor layer above it. This phenomenon gives rise to a surface contraction (a curving of the surface) and a corresponding force at the surface of the liquid. This force is termed the surface tension of the liquid. Surface tension is responsible for the formation of liquid droplets, capillary rise, and many other physical phenomena. 

To increase the area of the surface, work must be done on it. Recall the definition of work from basic physics is 

Diagram of an apparatus used to determine the surface tension of a liquid. 

Measurement of surface tension can be carried out in a variety of ways. Two of the most common, simple, and relatively accurate methods of calculating surface tension are measuring the rise of liquid in a capillary (capillary rise method) and the pull on a thin vertical plate partially immersed in the liquid (Wilhelmy plate method). 

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