Structural and trasport properties of Bola C-16 micelles in water and in aqueous electrolyte solutions

Abstract

Aqueous solutions containing a bolaform surfactant [R,ω-(4,7,10,13-pentaoxa-16-azacyclooctadecane)- hexadecane], with and without electrolytes have been investigated as a function of surfactant concentration and ionic strength. Small angle neutron scattering (SANS), NMR self-diffusion, and other physical-chemical methods were used. From the analysis 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 concentration, becoming more elongated. Due to the selective complexing ability of aza crown ether units, significant differences were observed upon addition of LiCl, NaCl, and HCl. LiCl gives rise only to a screening of electrostatic repulsion between micelles. NaCl, in addition to the screening effect, induces a reduction of micelle aggregation number as a consequence of the increased repulsion between charged headgroups. The micelle size reduction 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 concentration. They are in line with information on the complexation of sodium and proton by the aza crown units, inferred by ionic conductivity and potentiometric findings.