Filming the self-assembly of macromolecules in slow motion!

In an article published in ACS Macro Letters, a team of physicochemists describes a novel approach which uses neutron and light scattering to monitor in situ the self-assembly of amphiphilic block copolymers in near-equilibrium conditions. These results offer a wealth of possibilities for observing and better understanding the organisation of complex biological and synthetic macromolecules in aqueous solutions.

Micelles, vesicles, polymersomes, lipid membranes... these are just some of the self-assembled structures formed by amphiphilic macromolecules. These molecules contain both a hydrophilic (“water-loving”) part and a hydrophobic (“water-fearing”) part and are widely used in everyday life products from paints and pharmaceutical products to food and cosmetics. They make it possible to disperse an oily phase into an aqueous phase and to stabilise nano-objects – and structures with all kinds of geometries – in different media. Their spontaneous self-assembly is driven by weak interactions between the hydrophobic parts, which cluster together to minimise their contact area with water, while the hydrophilic parts repel each other.

The self-assembly mechanisms of amphiphilic macromolecules have been widely studied and are, in theory, predictable, provided they are controlled by thermodynamics. In practice, however, the mobility and reorganisation dynamics of large molecules are very slow. Often, the structures obtained are not the most thermodynamically stable structures, but those which form the fastest.

These non-equilibrium structures depend very much on the experimental conditions in which they are formed, prompting the question:

"how can we slow down this process sufficiently to be able to observe which structures really are the most stable, and monitor the different stages of their formation?"

To find the answer to this question, physicochemists expert in soft matter from 4 CNRS laboratories* teamed up with scientists from the Institut Laue Langevin (ILL) to develop an innovative experimental protocol using a dialysis set-up developed at the ILL, which allows water to diffuse slowly in a mixture containing a miscible solvent.

The very gradual increase in the proportion of water slowly activates the attraction between the hydrophobic parts, which in turn triggers the self-assembly of macromolecules in solution. What makes this set-up so unique is that it allows the self-assembly mechanisms to be observed in situ using a combination of dynamic light scattering (DLS) and small-angle neutron scattering (SANS), both of which are suitable for the investigation of nanostructures ranging from 1 to 100 nm. Scientists were therefore able to monitor the self-assembly of chains of amphiphilic diblock copolymers made of poly(ethylene glycol) and poly(dimethylsiloxane) (PEG-PDMS) throughout the process, and establish their phase diagram under quasi-thermodynamic equilibrium. The study was rounded off by cryo-electron microscopy and surface tension analysis of the structures formed.

Presented in ACS Macro Letters, this method can be applied to all types of synthetic and biological macromolecules and colloids. Indeed, the dialysis technique implemented on the large dynamic range SANS diffractometer D22 at the Institut Laue-Langevin enables the slow diffusion not only of solvents but also of any other molecules capable of triggering self-assembly, such as acids or bases, ions, hydrogen bond donors/acceptors and biological ligands. For those in the physical chemistry community seeking to understand the self-assembly mechanisms of these systems, this new technique opens up exciting new prospects.

ILL Instrument: D22, the Large dynamic range small-angle diffractometer

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ILL Contact: Lionel Porcar

Contact:Christophe Schatz

*Laboratoire de chimie des polymères organiques (LCPO, CNRS/Bordeaux INP/University of Bordeaux), Centre de recherche Paul Pascal (CNRS/University of Bordeaux), Institut Charles Sadron (CNRS/INSA Strasbourg/University of Strasbourg), Laboratoire Léon Brillouin (LLB, CNRS/CEA)