Discussion on Surface Phenomena: Films

The concept of the orientated monolayer as constituting the state of matter, in what was termed by the late Sir William Hardy the boundary state, is such an important one in the interpretation of the phenomena involving both equilibria and kinetics at interfaces that it is necessary to examine with some care the premises on which it rests. From the evidence accumulated with the aid of the Langmuir trough we can infer that a film of a long chain fatty acid floating on a water surface consists of a monomolecular array or monolayer of molecules with their hydrophilic groups immersed in the water and the hydrophilic chain extended above the surface. The method, however, provides no information on film uniformity, and it seemed desirable to check this assumption by direct methods. Two direct methods are available: an optical and an electrical. By determining both the relative phase retardation and the ratio of the relexion coefficients, which a beam of polarized light suffers on reflexion from a clean water surface and from one covered with a monolayer, and with the aid of Strachan's development of the Drude equations it is possible to determine the scattering indices of the film material in the planes both parallel and perpendicular to the surface. The following figures give the values obtained for a myristic acid film:— Acid in bulk .... 3·5 10-22 cc-1 In plane of film ... 2·8 10-22 cc-1 Normal to film .... 4·1 10-24 cc-1 It is important to note that in the plane of the film the material possesses optical properties similar to those obtained in the bulk phase, but reveals a much smaller scattering component in the direction perpendicular to the surface. These measurements provide an optical confirmation of the Hardy Langmuir principle of molecular orientation and, further, the method permits us to examine the film for uniformity.