Aggregation Behavior of Monomeric Surfactants and a Gemini Cationic Surfactant by NMR and Computer Simulation Data

Abstract

Aggregation of decyltrimethylammonium bromide and cetyltrimethylammonium bromide (CTAB) in D₂O has been studied. Spin–lattice relaxation time and self-diffusion coefficient of surfactant molecules were measured at concentrations below and above surfactant critical micelle concentration. The aggregation properties of conventional surfactant, CTAB, examined by nuclear magnetic resonance (NMR) and molecular dynamic (MD) simulation, were compared with the properties of double-tail analog, N,N,N′,N′-tetramethyl-N,N′dihexadecyl-1,4-butan di-ammonium di-bromide (BCTA). Both NMR and computer simulation methods suggest that micellization is a stepwise process and the pre-micellar aggregates take place in a solution at concentration below critical micelle concentration. According to MD simulation Gemini surfactant, BCTA, forms worm-like micelles, whereas CTAB, which may be considered as its “monomer”, forms only elongated micelles.