The solution behavior of a DTPA (diethylenetriamine pentaacetic acid)-based chelating surfactant, 4-C12-DTPA, has been studied by tensiometry and NMR diffusometry. In the absence of metal ions, the eight donor atoms in the headgroup are titrating, and the charge of the headgroup can thus be tuned by altering the pH. 4-C12-DTPA changes from cationic at very low pH, over a number of zwitterionic species as the pH is increased, and eventually becomes anionic at high pH. Around the isoelectric point, the chelating surfactant precipitated. The solution properties, studied above the solubility gap, were found strongly pH dependent. When increasing the amount of negative charges in the headgroup, by increasing the pH, the adsorption efficiency was reduced and the cmc was increased. An optimum in surface tension reduction was found at pH 5, due to a proper balance between protonated and dissociated groups. Anomalies between surface tension measurements and NMR diffusometry in determination of cmc revealed a more complex relation between surface tension, surface coverage, and cmc than usually considered, which is not in line with the common interpretation of the Gibbs adsorption equation. At some of the investigated pH levels, measurements of bulk pH could confirm the location of cmc, due to the increased protonation of micelles compared to monomers in solution. The adsorption of monomers to the air-water interface showed unusually slow time dependence, evident from decreasing surface tension for several hours. This is explained by rearrangements of the large head groups to reduce the headgroup area and increase the packing parameter.

Coordination complexes of some divalent metal ions with the DTPA (diethylenetriaminepentaacetic acid)-based chelating surfactant 2-dodecyldiethylenetriaminepentaacetic acid (4-C12-DTPA) have been examined in terms of chelation and solution behavior. The headgroup of 4-C12-DTPA contains eight donor atoms that can participate in the coordination of a metal ion. Conditional stability constants for five transition metal complexes with 4-C12-DTPA were determined by competition measurements between 4-C12-DTPA and DTPA, using electrospray ionization mass spectrometry (ESI-MS). Small differences in the relative strength between the coordination complexes of DTPA and 4-C12-DTPA indicated that the hydrocarbon tail only affected the chelating ability of the headgroup to a limited extent. The coordination of Cu(2+) ions was investigated in particular, using UV-visible spectroscopy. By constructing Job's plots, it was found that 4-C12-DTPA could coordinate up to two Cu(2+) ions. Surface tension measurements and NMR diffusometry showed that the coordination of metal ions affected the solution behavior of 4-C12-DTPA, but there were no specific trends between the studied divalent metal complexes. Generally, the effects of the metal ion coordination could be linked to the neutralization of the headgroup charge of 4-C12-DTPA, and the resulting reduced electrostatic repulsions between adjacent surfactants in micelles and monolayers. The pH vs concentration plots, on the other hand, showed a distinct difference between 4-C12-DTPA complexes of the alkaline earth metals and the transition metals. This was explained by the difference in coordination between the two groups of metal ions, as predicted by the hard and soft acid and base (HSAB) theory.

Mixtures of ionic surfactants and the chelating surfactant 2-dodecyldiethylenetriaminepentaacetic acid (4-C12-DTPA) have been examined in terms of interactions in mixed micellar systems. The amphoteric 4-C12-DTPA is zwitterionic with a negative net charge at the studied pH levels. The investigated ionic surfactants were the cationic dodecyltrimethylammonium chloride (DoTAC), the anionic sodium dodecyl sulfate (SDS), and the zwitterionic dimethyldodecylamine-N-oxide (DDAO). The surfactants all have the same hydrophobic chain lengths, and the results are evaluated in terms of headgroup interactions. 4-C12-DTPA interacts with different ionic surfactants by accepting or donating protons to the aqueous solution to increase the attractive interactions between the two surfactants; i.e., the protonation equilibrium of 4-C12-DTPA is shifted in different directions depending on whether there are predominant repulsions between positively or negatively charged groups in the mixed micelles. This was monitored by measuring pH vs concentration in the mixed systems. By measuring the pH, it was also possible to study the shift in the protonation equilibrium at increasing concentration, as the composition in the micelles approaches the composition in the total solution. Following the approach of Rubingh's regular solution theory, the interaction parameter β for mixed micelle formation was calculated from the cmc values determined by NMR diffusometry. Synergism in mixed micelle formation and negative β parameters were found in all of the investigated systems. As expected, the most negative β parameter was found in the mixture with DoTAC, followed by DDAO and SDS. The self-diffusion in the 4-C12-DTPA/DoTAC system was also discussed. The self-diffusion coefficient vs concentration plots show two distinctly different curves, depending on the surfactant that is present in excess.

Ion flotation was studied for the removal of cadmium, zinc, and strontium ions from aqueous solutions at pH 5–9 in a customized flotation cell, using an aminopolycarboxylic chelating surfactant, 2-dodecyldiethylenetriamine pentaacetic acid (C12-DTPA) in combination with two foaming agents: dodecyltrimethylammonium chloride (DoTAC) and dimethyldodecylamine-N-oxide (DDAO). The results from experiments showed that both Zn2+ and Cd2+ could be removed via ion flotation to 100% at pH 5, and Sr2+ could be removed via ion flotation to 60%–70% at pH 7–9. The removal of metal ions from the flotation cell was seen to vary with pH, but this was not exclusively related to the magnitudes of the formed metal ion-chelating surfactant conditional stability constants. The removal was also dependent on the foam properties of the samples that were found to vary over the investigated pH interval. The outcome of the investigation points to the chelating surfactant C12-DTPA having excellent chelating properties for all of the studied ions above pH 7. In combination with correctly chosen foaming agents, the optimized surfactant system could be expected to provide very efficient remediation of waters polluted with metal ions via ion flotation.

The correlation between interaction parameters and ion flotation efficiency in mixtures of chelating surfactant metal complexes and different foaming agents was investigated. We have recently shown that chelating surfactant 2-dodecyldiethylenetriaminepentaacetic acid (4-C12-DTPA) forms strong coordination complexes with divalent metal ions, and this can be utilized in ion flotation. Interaction parameters for mixed micelles and mixed monolayer formation for Mg(2+) and Ni(2+) complexes with the chelating surfactant 4-C12-DTPA and different foaming agents were calculated by Rubingh's regular solution theory. Parameters for the calculations were extracted from surface tension measurements and NMR diffusometry. The effects of metal ion coordination on the interactions between 4-C12-DTPA and the foaming agents could be linked to a previously established difference in coordination chemistry between the examined metal ions. As can be expected from mixtures of amphoteric surfactants, the interactions were strongly pH-dependent. Strong correlation was found between interaction parameter β(σ) for mixed monolayer formation and the phase-transfer efficiency of Ni(2+) complexes with 4-C12-DTPA during flotation in a customized flotation cell. In a mixture of Cu(2+) and Zn(2+), the significant difference in conditional stability constants (log K) between the metal complexes was utilized to selectively recover the metal complex with the highest log K (Cu(2+)) by ion flotation. Flotation experiments in an excess concentration of metal ions confirmed the coordination of more than one metal ion to the headgroup of 4-C12-DTPA.