Figure: Simulation snapshots of the investigated ionic surfactants model, at low, intermediate and high hydration levels, from left to right. Lamellar, cylindrical and micellar self-organized phases are evident. The hydrophobic tail of surfactants is represented in gray, the charged hydrophilic head in green. Water molecules are in blue, the hydronium cations in red.
Confinement at the nano-scale and interaction with interfaces profoundly alter structure and dynamics of water. Grasping the true nature of these modifications at the molecular level is crucial, in contexts as diverse as biology or materials science. In particular, these phenomena are everywhere in polymer electrolyte membranes used in fuel cells, and their control is the key for optimizing performances. Transport properties of protons, in fact, are expected to correlate to the peculiar state of the adsorbed water molecules, which shows significant deviations from that typical of the bulk phase. This behaviour, in turn, strongly impacts morphology and functional response of the confining medium. Insight on this complex interplay can be obtained by computer simulation.
We have recently performed Molecular Dynamics simulation work on a self-assembling numerical model for ionic surfactants. These materials are by now recognized as simplified templates for the nano-scale organization of the more complex ionomers, including Nafion. By following the evolution of water confined in environments with tunable topology (see Figure), we have analysed in depth the main dynamical features, also in terms of quantities measured in scattering experiments with Neutrons. Interestingly, we have found clear evidences of anomalous sub-diffusion, a phenomenon substantially overlooked in previous studies. We have identified water molecules lying at the charged interfaces with the hydrophobic confining matrix as the main responsible for this unusual feature, thus demonstrating and fully rationalizing the existence of spatially heterogeneous dynamics in the ionic domains.
We believe that our findings contribute to a deeper understanding of transport mechanisms active in these soft matter systems, and provide, in perspective, sensible guide-lines for the rational design of new materials for energy technologies.
This work is a collaboration between the INAC/SYMMES/PCI (SL and SM) and the Theory Group of the Laue Langevin Institute (SH). We acknowledge funding for a Ph.D. studentship (SH) from the ILL.
Samuel Hanot, Sandrine Lyonnard, and Stefano Mossa
Nanoscale 8, 3314 (2016)
Soft Matter 11, 2469 (2015)
Stefano Mossa (email@example.com) and Sandrine Lyonnard (firstname.lastname@example.org)
Maj : 18/04/2016 (1168)