Structural characteristics of the aqueous mixed nonionic–cationic surfactants: Effect of chain length, head group and temperature

The micellar structural transition in cationic amphiphiles, viz. alkyltrimethylammonium bromides (C12-, C14-, C16- and C18TABr), hexadecylpyridinium bromide (C16PyBr) and hexadecyltriphenylphosphonium bromide (C16TPPBr) in the presence of polyoxyethylene dodecyl ethers (C12En, n = 3–8) was studied using viscosity and scattering (dynamic light scattering (DLS) and small angle neutron scattering measurements (SANS)) techniques. A progressive growth in micellar size was observed upon the addition of C12En particularly when n ≤ 5 and C12E3 was found to be the most effective in promoting the micellar growth attributed to increasing hydrophobicity of surfactants. These results are well supported by SANS measurements which reveal increase in major axis of the ellipsoidal micelle with increase in C12E3 concentration up to 20 mM, leading to large elongated micelles, above which the size remained nearly constant. Increased effectiveness of lipophilic nonionic surfactant is discussed in light of electrostatic and hydrophobic forces operating in solution, which are responsible for micellar growth process.