Abstract
In previous communications the results of examination of the surface potentials of monolayers under different states of compression and on different substrates were presented. It was shown,
inter alia
, that not only could information as to the orientation of the surface phase be obtained even in such complex cases as monolayers of proteins, but the effects of alteration of inclination as well as of adhesion of the polar groups to the substrate were capable of quantitative measurement. Marked changes in the surface potentials are obtained when the polar group undergoes chemical reaction such as in the formation of oxonium compounds by a ketonic oxygen or on ionization of a carboxyl group, whilst changes of minor character are associated with a variation in the electrokinetic potentials of the film forming material. It is evident that it should be possible to follow the alteration of surface potential with time in cases where a chemical reaction involving a change in the summation of the vertical components of the groups forming the effective double layer is proceeding in a monolayer. Reactions in monolayers at air-liquid and liquid-liquid interfaces present, apart from their importance in biological processes, certain unusual aspects of interest in the sphere of reaction kinetics. In order to visualize these aspects in more detail we will consider a simple case, a monolayer of oleic acid distended at an air-liquid interface undergoing oxidation by means of a dilute solution of an oxidizing agent such as acid permanganate present in the substrate. The oxidation proceeds in bulk phase quite smoothly, the double bond undergoing progressive oxidation to a dihydroxy and ketonic acid respectively before rupture of the chain ensues. In a highly expanded film of oleic acid the double bond is extended on the substrate and the molecular orientation of the reactant is such that the double bond in every molecule is equally accessible to the oxidizing agent, the MnO¯
4
ion. On compression of the film the surface density of molecules increases and eventually when the molecular area attains some 30 A.
2
the limit of film stability is reached and the film collapses. Corresponding to the increase in the number of molecules of oleic acid per square centimetre one should anticipate an increase in the velocity of oxidation of the film, provided that we ensure the constancy of the active mass of the other reactant, the MnO¯
4
ion. The velocity constant should also vary exponentially with the temperature providing a measure of the apparent energy of activation in the usual manner. A closer consideration of the system, however, reveals the fact that at some point during the process of compression the reactive group, in this case the double bond, will be removed from the water surface by the compression of the molecules constituting the monolayer. If it were not for the fact that the molecules were in thermal agitation, once this critical point had been reached all the double bonds in the surface would be rendered inaccessible to the oxidant, and reaction would cease. On further compression the hydrocarbon barrier now erected between the layer of double bonds and the oxidant in the substrate increases in thickness until either the molecules in the film become vertically orientated or, as generally happens, the film collapses. Owing to thermal agitation, however, the surface molecules are not at rest, and double bonds can penetrate the hydrocarbon barrier and attain access to the oxidant. The ease of penetration will depend both on the thickness of the barrier and on the mobility of the molecules, which will be influenced both by the temperature and their cohesion. We have under these conditions a reaction, the velocity of which is determined not only by the usual factors such as the collision frequency, and the probability, and magnitude of the energy transfer involved in activation, but in addition by what may be defined as an accessibility factor, the accessibility coefficient being unity in highly distended films but zero in highly compressed rigid films. It may not be without biological significance that in the systems examined in this communication a very large change occurs in the accessibility coefficient, and thus in the magnitude of the reaction velocity coefficient as a result of a relatively small change in the compression or surface tension of the interface. We may note that this molecular orientation as an important factor in reaction kinetics must often be present in heterogenous catalysis at solid surfaces,
e.g
., in the dependence on the pressure of the rates of hydrogenation of unsaturated hydrocarbons examined especially by Schuster,* but as yet we have no exact knowledge of the actual configuration and change in orientation or alteration in the surface density of the adsorbate in such systems. Similar considerations are clearly applicable to reactions in bulk phases where only specific portions of large and complex molecules are involved, as examples of which may be cited the polymerization of unsaturated hydrocarbons or in the combination between the dextro and lævo forms of complex organic reactants discussed in detail by Mills.