SELF-ASSEMBLY IN DOUBLE-TAILED SURFACTANTS IN DILUTE AQUEOUS-SOLUTIONS

The phase behavior and aggregation of dialkyldimethylammonium bromides (alkyl chain lengths 8, 10, 12 and 16) in dilute aqueous solutions (concentrations below 1 wt.%) were investigated by static light scattering, densitometry and freeze-fracture electron microscopy. Surface and interfacial tension measurements were performed by use of the drop weight and spinning drop methods. Phase diagrams of dialkylammonium bromide/water systems in the temperature range 20-60 degrees C were obtained, and it was found that four areas are present on the phase diagrams: (1) molecular solution; (2) vesicular solution with small unilamellar vesicles (D = 30-70 nm: depending on the alkyl chain length); (3) a two-phase region, where two populations of aggregates coexist: small vesicles and large liposomes; (4) a random lamellar L(3) phase. It is important to note that vesicle and liposome formation occurred spontaneously on consecutive dilutions of the L(3) phase. Surface and interfacial tension measurements have shown that aggregation in the bulk solutions were accompanied by unusually sharp decreases of the surface and interfacial tensions; the tension at the L(3) phase/octane interface dropped to an ultralow value (0.01 mN m(-1)). Langmuir balance studies of diC(12) derivative and diC(16) derivative monolayers on the aqueous subphase were performed, and some hysteresis in the compression-decompression cycles was observed, probably due to partial dissolution of amphiphiles. The minimum area per surfactant molecule in the condensed monolayers was found to be independent of the alkyl chain length, being equal to 68 Angstrom(2). The structural transformation vesicles/large liposomes/lamellar phase was illustrated by electron microscopy.