Distinctive effect of maleic acid and fumaric acid on structural transitions in cationic micellar solution

Isomeric additives, maleic and fumaric acids (MA and FA) showed a distinctive influence on the structural transition induced by n-octanol in aqueous cetyltrimethylammonium bromide (CTAB) solution. With MA, significant increase in viscosity was observed on successive addition of n-octanol. Samples with high n-octanol concentration showed viscoelasticity. However, no such dramatic change was observed with FA. Rheological studies revealed the existence of entangled threadlike micelles in CTAB/MA/n-octanol system. Newtonian flow behaviour exhibited by the FA-containing samples indicated the presence of spherical micelles. The existence of threadlike micelles in MA containing samples was further confirmed by cryogenic transmission electron microscopy and small-angle neutron scattering studies. The favourable geometry and better acidic strength of MA are regarded as the factors synergistically facilitating unidimensional micellar growth in the presence of n-octanol. In contrast, the stronger hydrophobicity of FA resulting from trans geometry and feeble ionization, modifies the solvent properties and retards micellar growth.