Iron Binding in an Ethylenediaminetetracetic Acid-Based Gemini Surfactant Monolayer Film

The ability of Fe3+ to bind to Langmuir monolayers of an N, N, N ', N '-dialkyl-N, N '-diacetate ethylenediamine gemini surfactant has been explored. Fe3+ in the subphase resulted in the formation of compacted, liquid-phase monolayers with a mesh-like morphology at the micron length scale in comparison with expanded, unstructured liquid-phase monolayers in the absence of iron. The response of the monolayer to subphase Fe3+ was different from that reported for Na+ and Ca2+ for an affiliated, shorter-tail gemini surfactant. Combined surface potential and X-ray reflectivity measurements indicated that Fe3+ induced minor conformational changes in the monolayer, suggesting ionic association with the head group. Direct evidence for the binding of iron was provided by total reflection X-ray fluorescence measurements, which revealed that multiple ions were associated with each head group as opposed to chelating with 1:1 stoichiometry as observed with bulk ethylenediaminetetracetic acid. Cumulative data suggest the adsorption of Fe-x(OH)(y)((3x-y)+) complexes with the monolayer surface as has been reported with other charged and uncharged monolayers.