Surface radioactivity measurements of monomolecular films

Abstract

In the last several years there has been a resurgence of interest in surface films at the air/water interface. The use of radioactive tracers in interfacial studies is attractive due to the sensitivity to detect small quantities of matter. The radiotracer method, therefore, has been utilized in various surface studies at the air/water interface including adsorption from solution [ 1,2], surface chemical reactions [ 31, monolayer desorption [ 4,5], surface diffusion [ 4-81, and surface concentration of insoluble monolayers [ 91. In this communication, we report simultaneous measurements of film loss from a sparingly soluble monolayer by radiotracer and Wilhelmy plate methods. Our findings indicate that greater caution should be exercised in the interpretation of surface radiotracer measurements of monomolecular films because of a previously unrecognized mass transport process, when a Geiger-Miiller (G-M) tube is employed, due to a possible electric field effect. Myristic [ l-‘*Cl acid (Rosechem Products, 59 mCi mmol-l) was deposited from a purified n-hexane spreading solution onto a 0.01 N HCl solution at 20-21 ‘C. The experimental apparatus is described in detail elsewhere [ 81. Compression barriers were not used in order to eliminate any possible film leakage past barriers. The monolayer was spread to an initial surface pressure of 5.5 mN m-‘, and then the surface pressure was measured as a function of time at constant area by the Wilhelmy plate method. Experimentally measured surface pressure-specific surface area (l7-A ) isotherms were then used to obtain the surface concentration dependence with time. The results were evaluated by plotting the ratio r/r, versus &, where r is the surface concentration of the myristic acid molecules, r,, is the initial surface concentration, and t is time. Simultaneously, the beta radiation emanating from the radioactive monolayer was measured by an end-window G-M tube ( Amperex 18526) and decade scaler, and the ratio of the surface concentrations, r/r,, is equal to the ratio of the monolayer activities after subtraction of the background count. Figure 1 shows the film loss measurements using the Wilhelmy plate and