Nov., 1956
EFFECTS OF SPREADING SOLVENT ON MONOLAYERS
1533
EFFECTS O F THE SPREADING SOLVENT ON MONOLAYERS AS DETERMINED BY PRESSURE-AREA AND RADIOACTIVITY-AREA ISOTHERMS BYHURLEY D. COOKAND HERMAN E. RIES, JR. Research Department, Standard Oil Company (Indiana), Whiting, Indiana Received M a y 16, 1068
Effects of spreading solvents on pressure-area isotherms for monomolecular films are not completely understood. Experiments with stearic acid show that benzene, n-hexane and chloroform cause no significant differences in the intermediate and high-pressure portions of the isotherm. At low pressures, observed differences in isotherm shape could not be directly related to volatility, solubility or polarity of the solvent. At very low pressures, radioactivity-area isotherms for radiostearic acid spread from the three solvents show a non-homogeneous distribution of the monolayer. The uncompressed monolayers seem to consist of large clusters of extremely small islands. During compression, clusters may coalesce and islands may deform t o a continuous solid film of low compressibility. The structure, packing and deformation of islands under compression may give rise to the various shapes of the isotherms in the low-pressure region. The solvent could thus alter the shape of the isotherm by affecting the size, shape and distribution of island structures, rather than by being retained in the monolayer.
Introduction In film-balance studies of two-dimensional packing and phase transformation of monolayers, the position and shape of the pressure-area isotherms are fundamental.’-8 Position determines the apparent area per molecule in the plane of the substrate surface at different stages of compression. Shape reveals the packing characteristics of the film-forming molecules and thus reflects inter- and intramolecular structure. Both position and shape may be altered by retention of spreading solvent. An ideal spreading solvent would be totally insoluble in water, would be free of film-forming contaminants, would homogeneously disperse the material to be spread, would not be retained in the monolayer, and would not be so volatile as to cause significant errors in measuring the concentration of the solution or the quantity spread. Benzene is widely used because it dissolves many polar compounds only slightly soluble in n-hexane. Although chloroform dissolves most polar materials readily, its high volatility and solubility in water make it less suitable. Most investigators have assumed that the solvents commonly used to spread the film-forming material evaporate almost completely within a few minutes and exert a negligible effect on the isotherm a t all pressures. Jaffe4 placed a number of highly purified solvents on a water surface and measured the pressure of each as a function of time. Benzene exerted less than 0.001 dyne per cm. after four minutes. Archer and LaMer5 have suggested that solvent molecules may remain in the monolayer, even under pressure. They interpret variations in water-evaporation rate through a film with changing film pressure to be a solvent effect. Solvent molecules are thought to be occluded in the monolayer when the film is spread. They attribute the decrease in the rate of water evaporation with increasing pressure to the expulsion of these occlusions on compression of the film. ( 1 ) N . I
