Radiotracer Studies of the Adsorption of an Ampholytic Surfactant, N-dodecyl-β-alanine, at the Aqueous Surface

A tritiated ampholytic surfactant, N-dodecyl-β-alanine, C10H21–(CHT)2–NH2+–(CH2)2–COO−, was synthesized, and the amount of adsorption on an aqueous surface was measured directly by the radiotracer method. The adsorption isotherm near the neutral pH region showed a saturation value of 3.73×10−10 mol/cm2 above the concentration of about 1/5 of the critical micelle concentration in the absence of added salt, and a value of 4.74×10−10 mol/cm2 above the critical micelle concentration in the presence of 1 mol/l NaCl. In the presence of 1 mol/l NaCl, no effect of pH on the adsorption was found. The Gibbs adsorption isotherm applicable to the aqueous ampholytic surfactant was derived. The observed amounts of adsorption in the presence and absence of NaCl were in good agreement with the values calculated from the Gibbs adsorption isotherm, using the surface tension data measured by the Wilhelmy plate method.