Direct Observation of the Formation of Surfactant Micelles under Nonisothermal Conditions by Synchrotron SAXS

Abstract

Self-assembly of amphiphilic molecules into micelles occurs on very short times scales of typically some milliseconds, and the structural evolution is therefore very challenging to observe experimentally. While rate constants of surfactant micelle kinetics have been accessed by spectroscopic techniques for decades, so far no experiments providing detailed information on the structural evolution of surfactant micelles during their formation process have been reported. In this work we show that by applying synchrotron small-angle X-ray scattering (SAXS) in combination with the stopped-flow mixing technique, the entire micelle formation process from single surfactants to equilibrium micelles can be followed in situ. Using a sugar-based surfactant system of dodecyl maltoside (DDM) in dimethylformamide (DMF), micelle formation can be induced simply by adding water, and this can be followed in situ by SAXS. Mixing of water and DMF is an exothermic process where the micelle formation process occurs under nonisothermal conditions with a temperature gradient relaxing from about 40 to 20 °C. A kinetic nucleation and growth mechanism model describing micelle formation by insertion/expulsion of single molecules under nonisothermal conditions was developed and shown to describe the data very well.

Probing the Self-Assembly of Unilamellar Vesicles Using Time-Resolved SAXS

“…This allowed probing the millisecond range structural dynamics involved in micellization of copolymers in selective solvents [18], self-assembly of surfactant micelles [19], morphological transformation of block-copolymer micelles [20], self-organization of DNA and lipid membranes [21], etc. Nevertheless, the potential of this approach in the elucidation of self-assembly processes is only beginning to be explored and this chapter is intended to provide a brief overview of experimental requirements.…”

Monitoring the Transition from Spherical to Polymer‐like Surfactant Micelles Using Small‐Angle X‐Ray Scattering

“…Micelle kinetics have been measured under conditions further away from equilibrium using modern time‐resolved small‐angle neutron scattering (TR‐SANS)19–22 and small‐angle X‐ray scattering (TR‐SAXS) 23–26. In contrast to other techniques, TR‐SAXS/TR‐SANS is able to capture the structural evolution on the order of milliseconds.…”

Concentrating membrane proteins using ultrafiltration without concentrating detergents

“…This significantly lowers passage of micellar detergent across the membrane so that a greater fraction of the detergent transmitted is in the unimeric rather than micellar form compared to stirred cell UF. The unimeric detergent concentration is independent of bulk detergent concentration beyond the cmc and approaches the value of the cmc (Jensen et al, 2013) and approaches the cmc. Thus, a higher fraction of detergent is in the unimeric form in 0.4 versus 1 % DM.…”