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

Abstract

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 head group 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 counter-ions 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 head group 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 head group 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 head group.

Impact of the Amphoteric Nature of a Chelating Surfactant on its Interaction with an Anionic Surfactant: A Surface Tension and Neutron Reflectivity Study of Binary Mixed Solutions

“…our previous work. 2,3 In accordance with the hypothesis put forward for this study, the amphoteric nature of the chelating surfactant greatly influences its surface activity, the interactions with another surfactant, and consequently also the composition of mixed surface layers. The composition of the surface layer of a mixture containing an amphoteric surfactant may change dramatically with concentration since this involves a change in the ionic strength and subsequently a change in the balance between competing entropic influences in the system.…”

Synthesis, physicochemical characterization and aquatic toxicity studies of anionic surfactants derived from amino and α-hydroxy acids

“…It is important to note that alkali metals do not form coordination complexes with carboxylic groups and that the interactions are purely electrostatic. 91…”

“…It is important to note that alkali metals do not form coordination complexes with carboxylic groups and that the interactions are purely electrostatic. 91 Glutamic acid derivatives showed diverse foaming properties. C 10 Glu formed stable foam in the pH = 2-4 range, similar to C 10 Asp (Fig.…”

Removal of Cd2+, Zn2+, and Sr2+ by Ion Flotation, Using a Surface-Active Derivative of DTPA (C12-DTPA)

“…18 On its own, C 12 -DTPA does not foam very well, probably because of its comparatively low CPP value, and it generally does not form the good planar lamellar structure that is required for the formation of liquid films in the foam. 37 In the foamability and foam stability experiments performed, the cationic surfactant DoTAC and the zwitterionic surfactant DDAO were mixed together with the chelating surfactant C 12 -DTPA and different metal ions at various pH values. Compared to single surfactant systems, the properties of surfactant mixtures in solution can be influenced by the synergistic effect; for instance, mixtures can show lower surface tensions than single compounds.…”