Effects of amphiphiles on surface and interfacial tension

The molecules at the surface of a liquid are not completely surrounded by other molecules as they are in the bulk of the liquid. As a result there is a net inward force of attraction exerted on a molecule at the surface from the molecules in the bulk solution, which results in a tendency for the surface to contract. 

The contraction of the surface is spontaneous that is, it is accompanied by a decrease in free energy. The contracted surface thus represents a minimum free energy state and any attempt to expand the surface must involve an increase in the free energy. The surface free energy of a liquid is defined as the work, tv, required to increase the surface area A by 1 m  

A similar imbalance of attractive forces exists at the interface between two immiscible liquids. Table 6.1 lists surface tensions of various liquids and also interfacial tensions at the liquid/water interface. The value of the interfacial tension is generally between those of the surface tensions of the two liquids involved, except where there is interaction between them. Table 6.1 includes several such examples. The interfacial tension at the octanol/water interface is considerably lower than the surface tension of octanol owing to hydrogen bonding between these two liquids. 

Amphiphilic molecules in aqueous solution have a tendency to seek out the surface and to orientate themselves in such a way as to remove the hydrophobic group from the aqueous environment and hence achieve a minimum free energy state (see Fig. 6.1). A consequence of the intrusion of surfactant molecules into the surface or interfacial layer is that some of the water molecules are effectively replaced by hydrocarbon or other nonpolar groups. Since the forces of inter-molecular attraction between water molecules and nonpolar groups are less than those 

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