Aggregation properties of cationic gemini surfactants with dihydroxyethylamino headgroups in aqueous solution

A series of cationic gemini surfactants, alkanediyl-α,ω-bis[di(2-hydroxylethyl) dodecylammonium] dibromide (abbreviated as 12-s-12 (OH), with s = 4, 6, 8 and 10 methylenes) have been synthesized, and their aggregation properties in aqueous solution have been studied by surface tension, electrical conductivity, isothermal titration calorimetry, 1H NMR, dynamic light scattering, and transmission electron microscopy. Two critical aggregation concentrations are observed for these surfactants. These surfactants start to form dimers at concentrations well below their critical micelle concentrations (CMC). Above the CMC, these surfactants can form both micelles and vesicles spontaneously with a micelle-to-vesicle transition. The hydrogen bond among the OH groups of headgroups and water molecules as well as the strong hydrophobic interaction among the hydrocarbon side chains should be the main origins for these unique aggregation behaviors of these gemini surfactants.