Structural and Transport Properties of Bola C-16 Micelles in Water and in Aqueous Electrolyte Solutions

Aq. solns. contg. a bolaform surfactant [α,ω-(4,7,10,13-pentaoxa-16-azacyclooctadecane)hexadecane], with and without electrolytes were investigated as a function of surfactant concn. and ionic strength. Small angle neutron scattering (SANS), NMR self-diffusion, and other phys.-chem. methods were used. From the anal. of SANS data it was inferred that in water the surfactant forms slightly charged ellipsoidal micelles, because of the partial hydrolysis of amino groups. The aggregates grow with the increase of concn., becoming more elongated. Due to the selective complexing ability of aza crown ether units, significant differences were obsd. upon addn. of LiCl, NaCl, and HCl. LiCl gives rise only to a screening of electrostatic repulsion between micelles. NaCl, in addn. to the screening effect, induces a redn. of micelle aggregation no. as a consequence of the increased repulsion between charged headgroups. The micelle size redn. is much stronger in the presence of HCl, which screens the micelle interactions once the surfactant is completely in ionic form. The aforementioned effects increase, as expected, on increasing the electrolyte concn. They are in line with information on the complexation of sodium and proton by the aza crown units, inferred by ionic cond. and potentiometric findings.