Immiscible Anionic Gemini Surfactant-Perfluorinated Fatty Acid Langmuir Monolayer Films

Abstract

A new member of the N,N,N′,N′-dialkyl-N,N′diacetate ethylenediamine family of anionic gemini surfactants has been synthesized, and its miscibility with the model perfluorocarbon, perfluorotetradecanoic acid (PF), has been investigated in monolayer films at the air−water interface. Thermodynamics of mixing and the accompanying changes in the mixed film structure have been probed using a combination of compression isotherm measurements supported by Brewster angle microscope imaging and X-ray scattering measurements, and results have been compared with those collected for a previously studied, shorter tail chain variant of the surfactant. Thermodynamic measurements showed that the gemini surfactant and perfluorotetradecanoic acid were immiscible, with weak repulsive interactions, manifesting as small positive deviations from ideal mixing, observed between the two film components. Films were highly textured, with micrometer-scale, phase-separated domains readily detectable. Grazing incidence X-ray diffraction measurements showed that the gemini surfactant was disordered in the monolayers, whereas the perfluorocarbon formed discrete crystallites in the disordered matrix. Despite the small deviations from ideal mixing detected in the thermodynamic measurements, the X-ray measurements indicated that the presence of the gemini perturbs the PF crystal lattice from that of pure PF. Finally, X-ray reflectivity measurements showed that the addition of equimolar PF to the gemini monolayer induces a significant increase in the nominal head group thickness of the film, suggesting that interactions between the two surfactants can lead to structural rearrangements of gemini's head group near to the water surface.

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

“…The larger limiting area of Ace(18)‐2‐Ace(18) in comparison with stearic acid can be attributed to the larger head group cross section of the molecule. The larger value in comparison with Ace(12)‐2‐Ace(12) is consistent with previous X‐ray reflectivity measurements, which suggest that the Ace(18)‐2‐Ace(18) tends to “flatten” on the subphase surface (Rehman et al, 2019).…”

“…Resulting electron density profiles were fit using a standard two‐slab model in which one slab of electron density corresponds to the monolayer head group and the second slab to the tail group; fitting data are summarized in Table 3. The overall film thickness and slab‐fitting results for the monolayer on pure water were comparable with the previously measured values for Ace(18)‐2‐Ace(18), and the tail layer length was greater than that of the shorter Ace(12)‐2‐Ace(12) surfactant, as expected (Rehman et al, 2017b, 2019). The key observation from film thickness measurements was that a small (~ 4 Å) but significant decrease in tail layer length was observed when switching to an Fe 3+ ‐enriched subphase.…”

“…As noted in the isotherm data, binding of the metal ion might result in a significant conformational change in the structure of the molecule, which would also perturb the measured SP. We note that the XR measurements reported previously (Rehman et al, 2019) suggest that Ace(18)‐2‐Ace(18) on pure water has a disordered tail group region, and the head group adopts a “flattened” conformation, similar to what is reported for cationic alkyl ammonium gemini surfactants with an appropriately sized spacer group (Alami et al, 1993). Thus, we have carried out additional structural characterization measurements on the films in the presence of Fe 3+ to supplement the SP measurements.…”

“…Previously reported grazing incidence X‐Ray diffraction measurements of Ace(18)‐2‐Ace(18) monolayers indicates that these surfactants form liquid (nondiffracting films) at the air–water interface at all surface pressures, likely because the bulky head groups pack poorly (Rehman et al, 2019) and the lack of discontinuities in the isotherms is consistent with the absence of liquid‐to‐solid or similar phase transitions. The maximal values for C s −1 extracted from the isotherms are consistent with liquid‐state films under all measurement conditions.…”

“…FeCl 3 (reagent grade, 97%) and chloroform (spectrophotometric grade) were purchased from Sigma Aldrich (Oakville, Ontario, Canada), and HCl (ACS grade) was purchased from EMD Merck. We have reported the synthesis, purification, and characterization of Ace(18)‐2‐Ace(18) previously (Rehman et al, 2019). Stock solutions (1 mM) of Ace(18)‐2‐Ace(18) were prepared by dissolving the solid surfactant in chloroform.…”