THE PHYSICS AND CHEMISTRY OF SURFACES* ERIC
K. RIDEAL.THE UNIVERSITY, CAMBRIDGE, ENGLAND
When an insoluble substance, whose molecules possess a greater attraction for water than for other molecules of the substance itself, is placed on a clean water surface, it spreads out into a surface film. The film so formed will be one molecule thick, for when every molecule reaches contact with the water the affinitiesof the molecules will be better satisfied than in any other way. Substances which form monomolecular surface films with ease are those which have water-soluble groups such as COOH, NH2, etc. The polar head of a long-chain paraffin molecule having such a group a t one end tends to bury itself deeply in the water while the rest of the molecule stands steeply oriented to the surface, since long paraffin chains resist immersion. The formation of a stable, monomolecular surface film may be regarded as the solution of the polar end-group of a molecule in the water, the rest refusing to be dragged in. Langmuir obtained force-area curves in which the two-dimensional pressure of a surface film against a floating barrier is plotted against the area per molecule in square Angstrom units. Such curves were found to be exactly anaiogous to the pressure-volume curves of three-dimensional systems. With the aid oi the Langmuir apparatus it has been possible to demonstrate the existence of two-dimensional films in the solid, smectic, liquid, and vaporous states, to observe the phenomenon of two-dimensional dimorphism, to study the conditions of phase equilibria and to measure the entropy changes associated with the process of two-dimensional fusion, vaporization, and sublimation. There is thus a complete thermodynamics of two-dimensional films entirely analogous to the thermodynamics of three-dimensional systems. The rigidity of solid films may he demonstrated by scattering talc upon the film and then blowing upon the film. There is a discrepancy between the measurements of the cross-sectional areas of the hydrocarbon chain as measured by the film method and by means of X-rays which suggests that the long chains of fatty acids adhere to one an other in solid surface films in such a way that the zig-zags of tht chains interlock, for then the mutual potential energy is a t a minimu$ When the area per molecule obtained from a force-area curve is correate~ for the tilt resulting from such interlocking, it agrees exactly with the dross sectional area per molecule derived from the X-ray analysis of crystals o fatty acids. In the case of the solid film of palmitic acid, the area per molecule extrapolated to zero compression is 20.6 square k g s t r o m units. Evidence has been obtained to show that the molecules in gaseous films lie flat on the surface. Kinetic theory shows that the equation of state of an ideal gaseous film is FA = RT where F is the two-dimensional pressure and
* Dohrne lecture delivered at The Johns Hopkins Univmity, October 20, 1930, as reported from notes taken by Willard E. Bleick. 2156
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A the area of the film, R and T having their usual significance. This is analogous to the equation of state P V = RT of an ideal three-dimensional gas. When FA is plotted against F for an actual gaseous film, curves of deviation from the perfect gaseous state are obtained which are of the same form as those obtained by Amagat who plotted P V against P for actual three-dimensional gases. Dr. Rideal next described a method of investigating the architecture of surface films which has been developed into a method of precision by himself and his collaborator J. H. Schulman. It has long been known that interphasic potentials which are not of inconsiderable magnitude exist between air and liquid. The present method consists in measuring the potential differencebetween a platinum wire situated above the surface of a glass Langmuir trough and a calomel electrode establishing contact with the liquid in the trough. The trough is placed on insulated stands in a wellearthed Faraday cage. Above the trough is suspended from a sulfurinsulated drawbridge a platinum wire upon which a small quantity of polonium is electrodeposited. Connection between the platinum wire and the needle of a Lindemann electrometer placed on an insulated bracket inside the Faraday cage is made by means of a magnetically operated switch. With a plate voltage of 70 volts the potential difference between the air electrode and the calomel can be measured to *2 millivolts, using the Lindemann electrometer as a null instrument, the movement of the fiber being observed through a periscope placed above the Faraday cage. The whole apparatus was mounted on brick pillars in a small cellar maintained at constant temperature. With this apparatus i t is found that reproducible air-liquid potential differencescould be obtained, independent within relatively wide limits of variation of the height of the polonium-coated wire above the surface of the liquid, usually N/100 HCI in the trough. The calomel electrode, to eliminate liquid junction potentials, was filled with the same electrolyte as was present in the trough. Solutions of palmitic and myristic acids in freshly distilled petrol ether served as stock solutions of film-forming :aterials. In carrying out an experiment the surface of the electrolyte in he trough was scraped with a waxed glass slide in the usual manner in conducting experiments with a Langmuir trough, the drawbridge holding the "air electrode" was lowered into position by means of a silk string and the interphase potential difference was measured with the aid of a Wheatstone bridge and the Lindemann electrometer. If the surface of the liquid be clean, repetition of the operations of scraping and measurement yield the same value for the potential difference. A film is now spread on the surface of the liquid and after evaporation of the petrol ether theinterphase potential is again determined. The film can be expanded or compressed by means of a waxed bamer in a manner
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identical with that employed by Langmuir and the change in the interphase potential simultaneously determined. When the change in interphase potential, AV, caused by the presence of the films of palmitic and myristic acid, is plotted as a function of the molecular area, a curve is obtained having pronounced breaks which correspond exactly to the breaks in the force-area curves for films of these acids. Inspection of the arrest points of the curves and a comparison with those obtained in a Langmuir trough reveal the fact that over periods of phase transitions, expanded to liquid condensed, and expanded to the vapor state, the interphase potential remains constant. Since in the first case the number of molecules per sq. cm. is increasing and in the second decreasing as the phase change progresses, i t necessarily follows that the molecular contribution of the fatty acid film to the interphase potential must change with the state of the film. Although the distributions of the electric field in a terminal carboxyl group immersed in the water are a t present unknown, we may regard their composite effects, as contributing a component of electric moment, p, vertical to the water surface. If the number of molecules, each contributing a component of electric moment, p, ben per sq. cm., we obtain from the Helmholtz equation, AV = 4mp, themean electric moment, p, from the observed values of AT' and n. When these values of electric moment of myristic acid are compared with the state of the film as derived from the force-area curve we note that the electric moment in the expanded phase is constant and equal E. S. U. In the liquid condensedphase a t zero compression to 2.36 X E. S. U., while a t the moment before the molecule it is equal to 1.46 X assumes the horizontal position of the vaporous state, where the moment becomes very small, i t possesses the large value of 3.7 X lO-I9 E.S. U. It is also noted that part of the transition from vapor to liquid or ex panded and the complete transition of the liquid or expanded to the liquic condensed form takes place with aregular change in the mean moment, i. e. the molecules of each phase can exert their own characteristic moleculal moment when two phases are present. The variation in the electri~ moment with compression over the region vapor, liquid expanded, an( h liquid condensed forms is compatible with the view that the change in t moment is due to a progressive tilt in the molecule. Only on compresslox of the solid condensed phase does there appear to be any considerabl' mutual interaction of the head groups. Dr. Rideal concluded by describing recent attempts to investigate by thi method the nature of films of iodine on a mercury surface and of films of a1 coho1 on solid metallic surfaces. It has been shown that when a crystal c iodine is placed on a mercury surface, spreading occurs to form a monomolecular film until a definite equilibrium spreading pressure is reached. This pressure is a direct measure of the attraction of the iodine molecule for the mercury surface.
