Direct observation of the self-assembly of surfactant micelles

The applications of surfactants are closely linked to their ability to self-assemble into nanometre-sized micelles.  The hydrocarbon tails of the surfactants form the core of the micelles while the hydrophilic head groups form the shell exposed to water or some other polar solvent. As the self-assembly process occurs on very short time scales, the kinetics of the associated structural evolution is very challenging to observe experimentally.  The Aniansson-Wall mechanism proposed in the 70s, involving a stepwise insertion of single surfactant (unimer), is often believed to be the dominating mechanism in micelle formation [1]. However, as direct observation of the micelle formation is extremely challenging, neither the concept of the Aniansson-Wall mechanism nor competing pathways, such as fusion/fission [2], have been properly verified against experimental structural information.

Here we have performed a time-resolved study where the complete micelle formation kinetics could be observed in situ on the nanometre scale with millisecond time resolution. The experiments were carried out at beamline ID02 and took advantage of the high brilliance and low instrumental background of the SAXS instrument [3].  Unlike other methods such as fluorescence, NMR, or conductivity experiments, the structural evolution could be observed directly, facilitating a direct evaluation of kinetic pathways.

An experimental system was designed using a sugar based surfactant dodecyl maltoside (DDM) which was dissolved in dimethylformamide (DMF). Micelle formation was initiated by a rapid mixing with water using a stopped-flow device. The mixed solution was transferred to a capillary cell for in situ SAXS measurements during the micelle formation process as illustrated schematically in Figure 1. The time delay from mixing to the first measurement was about two milliseconds, which is rapid enough to capture the self-assembly process.

Figure 1. Schematic representation of the stopped-flow rapid mixing experiment and time evolution of the SAXS intensities.

The resulting time-resolved SAXS data contain information on the average number of surfactant molecules per micelle (or aggregation number, p_w) at any time, as well as the structure (shape, size, etc.) of the micelles. The data clearly show the micelle formation with an increasing scattered intensity characteristic of micelles. For the lowest values of the surfactant concentrations, no sign of micelles was observed for the initial data frames, meaning that the entire evolution from singly dissolved surfactants to final micelles was covered in the experiment.  Interestingly, the data could be described by a simple time-dependent superposition of singly dissolved surfactants and micelles without any sign of either small ‘pre-micelles’ or large micelles that would be expected for fusion/fission pathways.

The SAXS results were compared to a nucleation and growth model based on the step-wise insertion/expulsion of single surfactant molecules, i.e. the Aniansson-Wall mechanism. A complicating factor in the present study was that the mixing of the solvents (DMF and water) is exothermic, i.e. releases heat. Hence a non-isothermal nucleation and growth model had to be considered, which was implemented by expanding a previously reported micelle formation model [4] where the time-dependent temperature (determined by independent SAXS measurements) was expressed in the chemical potential controlling the nucleation. A set of parameters for the free energy of the surfactant molecules was optimised to give the best possible fit of the model to the average aggregation numbers obtained from the SAXS data. The fits are displayed in Figure 2a. The corresponding theoretical evolution of the full distribution of aggregation numbers was calculated and is shown in Figure 2b and c. Two main species are found in the solution at any time: singly dissolved surfactants and micelles close to equilibrium. Smaller ‘pre-micelles’ give no significant contribution to the distribution, which is consistent with the structural information content of the SAXS data.

Figure 2. a) Fit of the kinetic model to the average aggregation numbers obtained by SAXS. b) and c) Typical time evolution of the distribution of aggregation numbers of surfactant molecules derived from the kinetic modelling of the experimental data for a surfactant concentrations of 0.77 and 1.5 wt%, respectively.

In conclusion, the combination of stopped-flow mixing and synchrotron SAXS has resulted in unique structural details of the formation and evolution of surfactant micelles. The data confirm the classical Aniansson-Wall theory for the mechanism, which is based only on single insertion events rather than fusion of smaller ‘pre-micelles’. It was further shown that the rate for the insertion of the surfactant molecule into a micelle is not only limited by the diffusion, but an additional significant kinetic barrier exists. This barrier could be related to the prerequisite of a special orientation of the surfactant molecule, with the hydrophobic tail pointing towards the micelle, for a successful insertion event.

Principal publication and authors
Direct observation of the formation of surfactant micelles under nonisothermal conditions by synchrotron SAXS, G.V. Jensen (a), R. Lund (b,c), J. Gummel (d), M. Monkenbusch (e), T. Narayanan (d), and J.S. Pedersen (a), J. Am. Chem. Soc. 135, 7214–7222 (2013).
(a) Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Aarhus (Denmark)
(b) Donostia International Physics Center, Donostía-San Sebastián (Spain)
(c) Department of Chemistry, University of Oslo (Norway)
(d) ESRF
(e) Jülich Centre for Neutron Science, Forschungszentrum Jülich (Germany)

References
[1] E.A.G. Aniansson, and S.N. Wall, J. Phys. Chem., 78, 1024 (1974).
[2] E. Lessner, M. Teubner, and M. Kahlwelt, J. Phys. Chem., 85, 1529 (1981).
[3] P. Panine, S. Finet, T.M. Weiss, and T. Narayanan, Adv. Colloid  Interf. Sci., 127, 9 (2006).  
[4] R. Lund, L. Willner, M. Monkenbusch, P. Panine, T. Narayanan, J. Colmenero, and D. Richter, Phys. Rev. Lett.,  102, 188301 (2009).

Top image: Watching micelle formation with time-resolved SAXS.