Wormlike micellar and vesicular phases in aqueous solutions of single-tailed surfactants with aromatic counterions

The aggregation behavior of a series of alkyltrimethylammonium 5-ethylsalicylate surfactants (C(n)TA5ES, n = 12, 14, 16) in aqueous solution has been investigated employing turbidity measurements, fluorescence anisotropy experiments (using 3-hydroxynaphthalene-2-carboxylate, HNC-), and cryo transmission electron microscopy (cryo TEM). Critical aggregation concentrations icac) were determined by surface tension experiments; they are smaller than those of their salicylate analogues. C(n)TA5ES surfactants form vesicles immediately above the cac rather than wormlike micelles such as their salicylate counterparts. C(12)TA5ES was found to form large unilamellar vesicles (LUVs which transformed into surfactant layers upon increasing surfactant concentration. The transition was characterized by an increase in turbidity. C(14)TA5ES and C(16)TA5ES were found to form LUVs and long wormlike vesicles which are assumed to be responsible for the viscoelasticity of the solutions. Upon increase of the surfactant concentration, a transition to large multilamellar vesicles (LMVs) was observed. In addition, plate structures and elongated Vesicles were present. The transition from unilamellar to multilamellar vesicles is accompanied by an increase in turbidity. HNC- fluorescence anisotropy values (r) were zero below the cac of the surfactants and showed an increase as soon as surfactant aggregates were formed. C(n)TA5ES surfactants exhibit temperature-dependent aggregation behavior. Aqueous solutions of C(16)TA5ES are turbid at low temperatures and undergo a transition to a clear and viscoelastic phase upon increasing the temperature. The turbidity changes are attributed to a vesicle-to-micelle transition.