May, 1958
RADIOACTIVE TRACER STUDIES OF MONOLAYERS
(2) The difference in the ratio of observed to calculated exchange decreases in the order: aluminum, copper, silver. This indicates either that more active centers are produced on copper than on silver but less than on aluminum, or that the centers have more energy. Verification of these ideas will require additional work, but it is not illogical that the order should be as named, considering the activities of the metals. (3) In the adsorption measurements made at elevated temperatures, adsorption plateaus were noted a t both 48 and 60”. These plateaus occurred at less than monolayer surface coverage. To explain these plateaus as well as similar plateaus noted when most metals are machined under the most dilute acid solutions, it is necessary to postulate that a portion of the metal surfaces is inactive so far as adsorption is concerned. These “dead areas” are the result of the decaying of the Kramer effect. Now, a t elevated temperatures Kramer activity is initially very high but rapidly decays as the sites emit all their available e1e~trons.l~This rapid decay occurs before sufficient fatty acid can diffuse to the surface to form the saturated adsorbed layer, and thus adsorption plateaus are noted at less than a complete monomolecular layer. The situation is probably aggravated by a high desorption rate of the incomplete surface film. As time passes, both the desorption rate and the energy of the active centers decline. These rates are different. At forty minutes acid exposure time the Kramer effect has decayed to such an extent that it is no longer able to keep pace with the desorption, which requires a smaller amount of energy. As soon as this happens, the sharp departures from the adsorption plateaus indicated in Fig. 9 occur. (4) The low adsorption noted on aluminum surfaces now can be ascribed definitely to a very fast desorption rate of the soap from the metal surfaces. This may also be true for indium. This desorption rate probably is due partly to the solubilities of these materials in cyclohexane, and partly to the strained condition of the adsorbed soap molecules (14) I
