Self-Assembly of Bilayers from Double-Chain Fluorocarbon Amphiphiles in Aprotic Organic Solvents: Thermodynamic Origin and Generalization of the Bilayer Assembly

We designed new fluorocarbon amphiphiles, 3-7, that undergo spontaneous bilayer formation in organic media. They are comprized of double fluorocarbon chains and a single oleyl chain as solvophobic and solovophilic moieties, respectively. These two molecular modules were linked through the chiral L-glutamate residue. When dispersed in chlorocyclohexane or benzene, these amphiphiles formed stable bilayer assemblies that gave rise to morphologies such as tubes, tapes, rods, and particles as confirmed by electron microscopy. These dispersions showed bilayer/monomer phase transitions as inferred by differential scanning calorimetry. This phase transition was accompanied by changes of the helical pitch of the fluorocarbon conformation and of intermolecular hydrogen bonding among the glutamate moiety. From the surface tension analysis of adsorbed monolayers at the air/liquid interface, we concluded that the bilayer formation in bulk chlorocyclohexane was driven by the enthalpic change. Limited miscibilities between fluorocarbon chains and hydrocarbon solvents produced the main enthalpic driving force of the molecular association, and the hydrogen bonding and van der Waals force of the aromatic moieties served as secondary driving forces. The molar fraction at CMC (critical membrane or micelle concentration), which reflects instability of aggregates, was greater than those of aqueous bilayers (below 10^-7) but smaller than those of aqueous micelles (above 10^-4). The chromophoric bilayers 6 (Phe-Ole) and 7 (Azo-Ole) showed large molecular ellipticities of 10⁵-10⁶deg-cm²-dmol^-1in their CD spectra. Molecular ordering in the novel bilayers was not inferior to those of conventional aqueous bilayers, although their bilayer stability does not surpass that of aqueous bilayers.