Pulsed field gradient (PFG) NMR at high field was utilized to directly observe a transition between two different diffusion regimes in a Nafion 117 membrane loaded with water and acetone. Although water selfdiffusivity at small water loadings was observed to be diffusion timeindependent in the limit of small and large diffusion times, it showed a significant decrease with increasing diffusion time at intermediate times corresponding to root mean square displacements on the order of several microns. Under our experimental conditions, no self-diffusivity dependence on diffusion time was found for water at large water loadings and for acetone at all studied acetone loadings. The diffusion time-dependent self-diffusivity at small water concentration is explained by the existence of finite domains of interconnected water channels with sizes in the range of several microns that form in Nafion in the presence of acetone. The domain sizes and permeance of transport barriers separating adjacent domains are estimated based on the measured PFG NMR data. At large water concentrations, the water channels form a fully interconnected network, resulting in time-independent self-diffusivity. The absence of such a percolation-like transition with increasing molecular concentration for acetone is attributed to a difference in the regions available for water and acetone diffusion in Nafion. The diffusion data are correlated with and supported by structural data obtained using small-angle X-ray and neutron scattering techniques. These techniques reveal distinct water channels with radial dimensions in the nanometer range increasing upon water addition, while acetone appears to be in an interfacial perfluoroether region, reducing the size of the radial channel dimension.
Lateral organization
of lipids in the cell membrane appears to
be an ancient feature of the cell, given the existence of lipid rafts
in both eukaryotic and prokaryotic cells. Currently seen as platforms
for protein partitioning, we posit that lipid rafts are capable of
playing another role: stabilizing membrane physical properties over
varying temperatures and other environmental conditions. Membrane
composition defines the mechanical and viscous properties of the bilayer.
The composition also varies strongly with temperature, with systematic
changes in the partitioning of high and low melting temperature membrane
components. In this way, rafts function as buffers of membrane physical
properties, progressively counteracting environmental changes via
compositional changes; i.e., more high melting lipids partition to
the fluid phase with increasing temperature, increasing the bending
modulus and viscosity, as thermal effects decrease these same properties.
To provide an example of this phenomenon, we have performed neutron
scattering experiments and atomistic molecular dynamics simulations
on a phase separated model membrane. The results demonstrate a buffering
effect in both the lateral diffusion coefficient and the bending modulus
of the fluid phase upon changing temperature. This demonstration highlights
the potentially advantageous stabilizing effect of complex lipid compositions
in response to temperature and potentially other membrane destabilizing
environmental conditions.
Pulsed field gradient (PFG) NMR in combination with quasielastic neutron scattering (QENS) was used to investigate self-diffusion of water and acetone in Nafion membranes with and without immobilized vanillic acid...
The shear viscous response of water is closely associated with changes in network connectivity on the sub ps timescale. The bulk viscous response is shown here to be associated with local density fluctuations and rotational motion around 1–3 ps.
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