Surface Activity of Complexes Formed with N-Dodecyl-β-alanine and Copper(II) Ion in Aqueous Solution

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Abstract
The surface activity of the equimolar mixed solutions of N-dodecyl-β-alanine (NDA) and inorganic salts was studied at 30 °C. The surface tension of the NDA–CuCl2 solution decreased more than that of the single NDA solution, whereas the surface tensions of the NDA–NaCl, NDA–CaCl2, and NDA–MgCl2 solutions were almost the same as that of the single NDA solution in the concentration region studied. The experiments with the bulk solution have shown that the 1 : 1-complex ion formed with NDA and the copper(II) ion is soluble and more surface-active than NDA alone, but the 1 : 2-complex is not effective in the surface activity because of its extremely low solubility. The formation constant of the 1 : 1-complex ion was determined, by potentiometry with both copper(II) and hydrogen ion-selective electrodes, to be 5.5×10−5, while the standard free energy of the complex formation was found to be 24.7 kJ mol−1. The composition and structure of the 1 : 2-complex were studied by means of X-ray diffractometry, IR spectroscopy, and elemental analyses for the precipitate. The total amounts of NDA adsorbed at the air/aqueous solution interface were determined by radiometry using tritiumlabeled NDA, and the amount of the 1 : 1-complex adsorbed was calculated by the Gibbs adsorption isotherm. Thus, the compositions were determined in both the adsorbed and bulk phases. It was seen that the relative adsorbability of the 1 : 1-complex was larger than that of NDA by a factor of about 1.5×102 and, further, was larger than that of a 1 : 1-complex formed with NDA and sodium dodecylsulfate by a factor of about 3.7.

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