Formation of Stable Bilayer Assemblies in Water from Single-Chain Amphiphiles. Relationship between the Amphiphile Structure and the Aggregate Morphology

The aggregate morphology of 62 single-chain amphiphiles prepared in these laboratories was examined by electron microscopy, and the relationship between the structural elements of the amphiphile and the aggregate morphology was discussed. The aggregate morphology includes globules, multi-, and single-walled vesicles, rods, tubes, and disks. Some of the morphologies are designed as pre-lamella, pre-rod, pre-disk, etc., because of their incomplete assemblage. The basic building block of these morphologies is the bilayer assembly. The structural elements of the amphiphile are conveniently divided into (1) flexible tail, (2) rigid segment, (3) hydrophilic head group, (4) spacer group, and (5) additional interacting group. The first three elements are essential for stable self-assembly, but all five elements affect the aggregate morphology. The flexible tail is linear methylene chains (or their related structures) of C7 or longer. The rigid segment usually consists of two benzene rings (biphenyl, diphenylazomethine, azobenzene, etc.) and affects the aggregate morphology by its dipolar property and geometry (conformation). The bilayer formation is promoted by dipolar rigid segments and the high-curvature aggregate (rods and tubes) results from bent rigid segments. The disk-shaped aggregate can be obtained from a combination of the extended and bent conformations of a rigid segment. This is supported separately by the morphology change of a two-component system which contains the extended and bent rigid segments. Among the hydrophilic head groups used (trimethylammonium, modified ammonium, phosphate, sulfonate, phosphocholine, and poly(oxyethylene)), the trimethylammonium group appears to be least effective for the molecular alignment. The spacer group is the methylene chain inserted between the rigid segment and the head group. Longer spacers (usually C10) yield well-defined aggregates more effectively than the shorter ones. A most notable example of the additional interacting group is given by the ester group at the tail end, which transforms globular aggregates into vesicles. The aggregation behavior other than the morphology was studied for typical ammonium amphiphiles with different rigid segments. Finally, the light-induced dynamic control of the aggregate morphology was discussed in relation to the static control based on the amphiphile structure. The temperature dependence of the fluorescence intensity of the biphenyl rigid segment was shown to depend on the aggregate morphology.