Open access publications in the SLU publication database

Abstract

Self-assembly of alkyl-N-iminodiacetic acid (R-IDA) system, R = -CH₃, -C₆H₁₃, -C₁₂H₂₅ and -C₁₈H₃₇, has been characterized by means of X-ray crystallography. The solid state presents highly ordered tilted interdigitated bilayer structures with saturated hydrocarbon chains parallel aligned from opposite directions. The polar head groups are intermolecularly hydrogen bound mediated by imino nitrogen and carboxylate groups. The zwitterionic units are cross-linked by short strong hydrogen bonds, d(O(-H)...O) ≈ 2.46 Å, forming a polymeric structure, and two-dimensional sheets by weaker bonds, d(N(-H)...O)≈ 2.8 Å. The alkyl-N-iminodiacetic acid systems are present as monomers in water, CH₃-IDA and C₆H₁₃-IDA, have acidic constants in the expected range, pKa₁ ≈ 1.6, pKa₂ ≈ 2.3 and pKa₃ = 9.6-10.4. The long tailed n-octadecyl-N-iminodiacetic acid system has a very acidic first proton, a substantially weaker second acidic constant, pKa₂ = 5.6-7.6 and pKa₃ = 9.4-10.6, strongly dependent on ionic strength. These results show that the short strong hydrogen bonds are maintained in aqueous systems. The complex formation, acid-base properties and the structures of the copper(II)- and zinc(II)-alkyl-N-iminodiacetic acid systems have been studied as well. EXAFS refinement presents a uniform short range order around copper(II), d(Cu-O/N) = 1.94-1.98 Å, nearly independent of composition and hydrocarbon chain length. On the other hand, the zinc(II) system has a wider range of coordination numbers, 4-6, and Zn-O/N bond distances, 1.99-2.08 Å. The acidic constant of the second copper(II)-alkyl-N-iminodiacetate complex has been determined to pKa = 5.3-6.3. For the zinc(II)-n-hexyl-N-iminodiacetate acid system (1:2) a buffer region of two protons, pKa~4.0 and 7.0 is observed. The stability of the n-octadecyl-N-iminodiacetate acid system was investigated by potentiometric methods. The C₁₈H₃₇-IDA aggregates in aqueous suspension are totally unaffected by the presence of copper at pH values below 4, weakly affected in the region 4-7, while complex formation takes readily place above pH ≈ 7, but kinetically controlled. An analytical method to determine the concentration of compounds forming strong complexes eith copper in the μM range has been developed.