Molecular Organization of an Adsorbed Layer: A Zwitterionic, pH-Sensitive Surfactant at the Air/Water Interface

Neutron and X-ray reflection measurements have been used to study the structure of the adsorbed layer of a chelating surfactant at the air/liquid interface. The chelating surfactant 2-dodecyldiethylenetriaminepentaacetic acid (C₁₂-DTPA) has a large headgroup containing eight donor atoms that can participate in the coordination of metal ions. The donor atoms are also titrating, resulting in an amphoteric surfactant that can adopt a number of differently charged species depending on the pH. Very strong coordination complexes are formed with metal ions, where the metal ion can be considered as part of the surfactant structure, in contrast to monovalent cations that act as regular counterions to the negative net charge. Adsorption was investigated over a large concentration interval, from well below the critical micelle concentration (cmc) to five times the cmc. The most striking result is the maximum in the surface excess found around the cmc, which is consistent with previous indications from surface tension measurements. Adding divalent metal ions has a limited effect on the adsorption at the air/liquid interface. The reason is the coordination of the metal ion, resulting in compensating deprotonation of the complex. Small variations in the headgroup area of different metal complexes are found, correlating to the conditional stability constants. Adding sodium chloride has a significant effect on the adsorption behavior, and the results indicate that the protonation equilibrium is more important than the ionic strength effects. From combined fits of the neutron and X-ray data, a model that consists of a thick headgroup region and a relatively thin dehydrated tail region is found, and it indicates that the tails are not fully extended and that the limiting area per molecule is determined by the bulky headgroup.