Effect of Counterions on the Shape, Hydration, and Degree of Order at the Interface of Cationic Micelles: The Triflate Case

• Published in: Langmuir Vol. 29; no. 13; pp. 4193 - 4203
• Main Authors: Lima, Filipe S, Cuccovia, Iolanda M, Horinek, Dominik, Amaral, Lia Q, Riske, Karin A, Schreier, Shirley, Salinas, Roberto K, Bastos, Erick L, Pires, Paulo A. R, Bozelli, José Carlos, Favaro, Denize C, Rodrigues, Ana Clara B, Dias, Luís Gustavo, El Seoud, Omar A, Chaimovich, Hernan
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 02.04.2013
• Subjects: Cations - chemistry; Chemistry; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Mesylates - chemistry; Micelles; Micelles. Thin films; Models, Molecular; Molecular Dynamics Simulation; Quaternary Ammonium Compounds - chemistry; Surface physical chemistry; Surface-Active Agents - chemistry; Micelle; Counter ion; Shape; Hydration; Interface
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la304658e

Specific ion effects in surfactant solutions affect the properties of micelles. Dodecyltrimethylammonium chloride (DTAC), bromide (DTAB), and methanesulfonate (DTAMs) micelles are typically spherical, but some organic anions can induce shape or phase transitions in DTA+ micelles. Above a defined concentration, sodium triflate (NaTf) induces a phase separation in dodecyltrimethylammonium triflate (DTATf) micelles, a phenomenon rarely observed in cationic micelles. This unexpected behavior of the DTATf/NaTf system suggests that DTATf aggregates have unusual properties. The structural properties of DTATf micelles were analyzed by time-resolved fluorescence quenching, small-angle X-ray scattering, nuclear magnetic resonance, and electron paramagnetic resonance and compared with those of DTAC, DTAB, and DTAMs micelles. Compared to the other micelle types, the DTATf micelles had a higher average number of monomers per aggregate, an uncommon disk-like shape, smaller interfacial hydration, and restricted monomer chain mobility. Molecular dynamic simulations supported these observations. Even small water-soluble salts can profoundly affect micellar properties; our data demonstrate that the −CF3 group in Tf– was directly responsible for the observed shape changes by decreasing interfacial hydration and increasing the degree of order of the surfactant chains in the DTATf micelles.