Fundamental physical chemical properties of monolayers formed from a new anionic gemini surfactant with a minimal-length (single-bond) spacer unit have been investigated at the air−water interface and compared with those of monolayers formed from affiliated comparator surfactants. The minimal spacer surfactant, dubbed C 18 -0-C 18 , exhibited strikingly different packing characteristics from an anionic gemini surfactant with a comparatively bulkier headgroup, including the formation of closepacked, crystalline films, and shared similar characteristics to simple fatty acid-based monolayers. Monolayers of C 18 -0-C 18 also exhibited good stability at the air−water interface and transferred with reasonable efficiency to solid substrates, although the film integrity was compromised during the transfer. Results from this work suggest that the single-bond spacer approach might be more broadly useful for designing gemini surfactants that pack efficiently into ordered monolayers.

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 Fex(OH)y(3x−y)+ complexes with the monolayer surface as has been reported with other charged and uncharged monolayers.

Phase-separated monolayers of 10,12-pentacosadiynoic acid and perfluorotetradecanoic acid can be photopolymerized to produce micrometer-sized, fluorescent polydiacetylene fibers at the air−solid interface. The photopolymer fibers were not uniformly fluorescent but rather showed a series of fluorescent spots along their lengths. The spots exhibited the classic properties of single-molecule fluorescence emission, including diffractionlimited size and fluorescence intermittency ("on−off blinking"). We have analyzed the fluorescence blinking dynamics of these spots using a variety of single-molecule analysis approaches, including fluorescence intensity histograms, autocorrelation analysis, as well as cross-correlation analysis as a function of distance between individual transition dipole moments, and propose a simple physical model for the fiber structure based on the observed blinking dynamics, in which the polymer fibers contain numerous structural defects. The model was supported by grazing incidence X-ray diffraction measurements of the mixed monolayer films at the air−water interface, in which it was observed that the presence of perfluorocarbon in the mixed monolayers significantly inhibited the ability of the 10,12-pentacosadiynoic acid to polymerize.

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