Molecular transport properties in two-dimensional membranes are characterized by the molecular size of the diffusing object, R, the molecular diffusion constant, D, and the membrane viscosity, ηm. Continuum hydrodynamic models, such as Saffman and Delbrück theory, are well accepted to model molecular diffusion in membranes when R is relatively large compared to the membrane constituent molecules. The model assumes continuum solvent and membrane fields, characterized by their viscosities η and ηm, and cylindrical molecules with the size R and the thickness corresponding to that of the membrane as a diffusing object. When R becomes small, such a continuum picture breaks down, and some theories predict existence of additional contributions to the drag due to the effects from the molecular order parameter or interactions. Experimental observations for such a contribution have never been achieved yet, as an experiment beyond hydrodynamics is challenging. Here, we employ incoherent and coherent quasi-elastic neutron scattering techniques to access such conditions. The neutron backscattering technique was used to estimate D from the self-motion of H atoms in membranes by measuring incoherent neutron scattering. On the other hand, ηm was determined from the collective membrane fluctuations by measuring coherent neutron spin echo signals. Interestingly, the temperature dependences of D and ηm show some differences, despite our expectation that the same frictional forces dictate these two parameters. Furthermore, a direct comparison between these two parameters indicates that additional drag on top of the hydrodynamic drag is necessary to explain the observed data.

Lipid molecular diffusion and membrane viscosity measured by quasi-elastic neutron scattering

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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