Verification of Interfacial Monolayer Conformation of Weakly Hydrophilic Diamide Derivatives Possessing a Fluorocarbon-Sandwiched Hydrocarbon

Abstract Gemini-type diamides with two hydrocarbons have been studied as candidates of thixotropic additives. Monolayer behavior of Gemini-type diamides containing fluorocarbons is interesting for understanding the formation of higher-order structure related to these thixotropic properties. We investigated the interfacial monolayer conformation of diamide derivatives possessing a hydrocarbon, which was sandwiched in fluorocarbons of different chain lengths on a water surface. The fluorocarbon chain-containing diamide derivatives, employed in this study, were recently synthesized as compounds with a hexane or heptane linker length. Two fluorocarbon chain lengths of each compound were obtained by controlling their interfaces, from CF3– to CF3(CF2)8–. The surface pressure-area (π-A) isotherm of these monolayers was confirmed to exhibit remarkable dependence on the fluorocarbon chain length; the two-dimensional surface morphology changed from domain- to network-like, because of the fluorocarbon chain length. From the results obtained from X-ray diffraction (XRD), it was expected that the molecular chains in the condensed phase functioned as an extended chain, which appeared to have formed an unstable conformation. Here, the diamide site did not function as a hydrophilic group. In the standing extended-chain conformation, it was expected that the conformation had been achieved by the hydrogen bond network of the diamide group coupled with the relatively weak van der Waals interaction between the fluorocarbon chains. Further, in the derivative with a small chain length, it was considered that two-dimensional micelles were formed in the low-surface-pressure phase in accordance with the stratified dipole-array theory.