ESTIMATION OF FILM THICKNESS FROM COHESIVE PRESSURE AT AIR WATER INTERFACE

The pressure-area curves for neutral monolayers of butyric, valeric, caproic and caprylic acids at the airwater interface were obtained from the surface tension concentration data. Similar curves for the charged monolayers of sodium octanoate and sodium lauryl sulphate in the presence and absence of excess neutral salts were constructed on the basis of the results on surface tension. From the comparison of the pressure area curves of each of these monolayers at air-water and oil-water interfaces under identical conditions, the cohesive pressures of the monolayer at the air-water interface were calculated for different areas per adsorbed molecule using the equation of Davies. The cohesive pressure was observed to vary linearly with the square of the reciprocal area per adsorbed molecule or ion. From the slope of such linear plot, magnitude of the two-dimensional van der Waals constant in each case was estimated. The thickness of the interfacial film was estimated from the comparison of the values of two- and three-dimensional van der Waals constants. This is, however, found to be considerably greater than the length of the stretched molecules adsorbed at the interface. When correction for entropy, as suggested by Gershfeld is introduced, the film thickness becomes identical with the length of the molecule. © 1969 Dr. Dietrich Steinkopff Verlag.