Adsorption and orientation at interfaces

The strong adsorption of such materials at surfaces or interfaces in the form of an orientated monomolecular layer (or monolayer) is termed surface activity. Surface-active materials (or surfactants) consist of molecules containing both polar and non-polar parts (amphiphilic). Surface activity is a dynamic phenomenon, since the final state of a surface or interface represents a balance between this tendency towards adsorption and the tendency towards complete mixing due to the thermal motion of the molecules. 

The tendency for surface-active molecules to pack into an interface favours an expansion of the interface this must, therefore, be balanced against the tendency for the interface to contract under normal surface tension forces. If it is the expanding pressure (or surface pressure) of an adsorbed layer of surfacant, then the surface (or interfacial) tension will be lowered to a value 

If the interfacial tension between two liquids is reduced to a sufficiently low value on addition of a surfactant, emulsification will readily take place, because only a relatively small increase in the surface free energy of the system is involved. If tt y0, a microemulsion may form (see page 269). 

In certain cases - solutions of electrolytes, sugars, etc. - small increases in surface tension due to negative adsorption are noted. Here, because the solute-solvent attractive forces are greater than the solvent-solvent attractive forces, the solute molecules tend to migrate away from the surface into the bulk of the liquid. 

The hydrophilic part of the most effective soluble surfactants (e.g. soaps, synthetic detergents and dyestuffs) is often an ionic group. Ions have a strong affinity for water owing to their electrostatic attraction to the water dipoles and are capable of pulling fairly long hydrocarbon chains into solution with them for example, palmitic acid, which is virtually un-ionised, is insoluble in water, whereas sodium palmitate, which is almost completely ionised, is soluble (especially above its Krafft temperature - see page 93). 

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