The kinetic pathways for coacervation and micelle formation are still not fully understood. Driven by electrostatic interactions and entropically driven counterion release, complexation of oppositely charged macromolecules leads to the formation of micellar nanostructures. Here we study the coacervation process, from initial formation and growth of stable micelles, on a nanometric length scale using time-resolved small-angle X-ray scattering (TR-SAXS). The micellar coacervates are formed through the complexation of anionic polyelectrolyte poly­(sodium 4-styrene­sulfonate) (PSSS) and cationic block–copolymer poly­(ethylene oxide)-block-poly­((vinylbenzyl)­trimethyl­ammonium chloride) (PEO-b-PVBTA). Mixing the polyelectrolytes in a stoichiometric 1:1 charge ratio resulted in the formation of stable spherical core–shell micellar-like coacervates consisting of a central core of complexed PSSS and PVBTA with a PEO corona. By use of synchrotron SAXS coupled to a stopped-flow mixing apparatus, the whole formation kinetics of coacervates could be followed in situ from a few milliseconds. The results of a detailed data modeling reveal that the formation of these polyelectrolyte coacervates follows a two-step process: (i) first, metastable large-scale aggregates are formed upon a barrier-free complexation immediately after mixing; (ii) subsequently, the clusters undergo charge equilibration upon chain rearrangement and exchange processes yielding micellar-like aggregates with net neutral charge that are pinched off to yield the final stable micelle-like coacervates. While the initial cluster formation is very fast and completed within the dead time of mixing, the subsequent rearrangement becomes significantly slower with increasing molecular weight of the PVBTA block. Interestingly, the overall kinetic process was essentially concentration independent, indicating that the rearrangement process is mainly accomplished via noncooperative chain rearrangement and chain exchange processes.

Using a double-disc chopper with a variable slit opening in concert with a velocity selector and the time-of-flight data acquisition mode, controlled variation of the wavelength spread Δλ/λ between 2 and 20% has become routinely possible at the KWS-2 SANS diffractometer of the Jülich Centre for Neutron Science at the Heinz Maier-Leibnitz Center.

The KWS-2 SANS diffractometer is dedicated to the investigation of soft matter and biophysical systems covering a wide length scale, from nm to µm. The instrument is optimized for the exploration of the wide momentum transfer Q range between 1x10 -4 and 0.5 Å -1 by combining classical pinhole, focusing (with lenses), and time-of-flight (with chopper) methods, while simultaneously providing high-neutron intensities with an adjustable resolution. Because of its ability to adjust the intensity and the resolution within wide limits during the experiment, combined with the possibility to equip specific sample environments and ancillary devices, the KWS-2 shows a high versatility in addressing the broad range of structural and morphological studies in the field. Equilibrium structures can be studied in static measurements, while dynamic and kinetic processes can be investigated over time scales between minutes to tens of milliseconds with time-resolved approaches. Typical systems that are investigated with the KWS-2 cover the range from complex, hierarchical systems that exhibit multiple structural levels (e.g., gels, networks, or macro-aggregates) to small and poorly-scattering systems (e.g., single polymers or proteins in solution). The recent upgrade of the detection system, which enables the detection of count rates in the MHz range, opens new opportunities to study even very small biological morphologies in buffer solution with weak scattering signals close to the buffer scattering level at high Q.In this paper, we provide a protocol to investigate samples with characteristic size levels spanning a wide length scale and exhibiting ordering in the mesoscale structure using KWS-2. We present in detail how to use the multiple working modes that are offered by the instrument and the level of performance that is achieved.

We present a small-angle neutron scattering (SANS) structural characterization of n-alkyl-PEO polymer micelles in aqueous solution with special focus on the dependence of the micellar aggregation number on increasing concentration. The single micellar properties in the dilute region up to the overlap concentration ϕ* are determined by exploiting the well characterized unimer exchange kinetics of the model system in a freezing and diluting experiment. The micellar solutions are brought to thermodynamic equilibrium at high temperatures, where unimer exchange is fast, and are then cooled to low temperatures and diluted to concentrations in the limit of infinite dilution. At low temperatures the kinetics, and therefore the key mechanism for micellar rearrangement, is frozen on the experimental time scale, thus preserving the micellar structure in the dilution process. Information about the single micellar structure in the semidilute and concentrated region are extracted from structure factor analysis at high concentrations where the micelles order into fcc and bcc close packed lattices and the aggregation number can be calculated by geometrical arguments. This approach enables us to investigate the aggregation behavior in a wide concentration regime from dilute to 6·ϕ*, showing a constant aggregation number with concentration over a large concentration regime up to a critical concentration about three times ϕ*. When exceeding this critical concentration, the aggregation number was found to increase with increasing concentration. This behavior is compared to scaling theories for star-like polymer micelles.