Kinetic Studies of Adsorbed Films of Tritium-labelled Sodium Stearate at the Air/Water Interface

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Abstract
The adsorption of tritium-labelled sodium stearate at the air/water interface was directly measured radiometrically using an ultra-thin-windowed Geiger-Müller tube for varying soap concentrations, pH values, and temperatures. The changes in the amount of adsorption and in the surface tension were simultaneously traced with the lapse of time. The pressure-area curve was then obtained by the successive plotting of the surface pressure against the area at the same aging; the curve at equilibrium was also obtained. Thus, the pressure-area relationship of the adsorbed monolayer was automatically measured during the course of adsorption approaching equilibrium. A close resemblance was observed between the monolayer of stearic acid on-neutral water and the adsorbed films on the stearate solution of pH values from 8.85 to 9.30 at 30°C. The amount of adsorption measured for an alkaline solution often exceeds the value to be expected from the saturation value for a completely-ionized stearate monolayer. The pressure-area characteristics suggested a transition from the condensed state of the adsorbed film at a higher surface concentration to an expanded state at a lower concentration. It was also confirmed, by measuring the amount of adsorption in the atmospheres of both nitrogen and air containing carbon dioxide, that the atmospheric carbon dioxide acts to increase the adsorption of stearic acid on the aqueous surface. The diffusion coefficient of the adsorbing solute, calculated from the initial rate of adsorption, showed a smaller value than that obtained by conductometric measurements.