Homogeneous length scale of shear-induced multilamellar vesicles studied by diffusion NMR

A recently developed protocol for pulsed gradient spin echo (PGSE) NMR is applied for the size determination of multilamellar vesicles (MLVs). By monitoring the self-diffusion behavior of water, the technique yields an estimate of the homogeneous length scale λhom, i.e. the maximum length scale at which there is local structural heterogeneity in a globally homogeneous material. A cross-over between local non-Gaussian to global Gaussian diffusion is observed by varying the experimentally defined length- and time-scales. Occasional observation of a weak Bragg peak in the PGSE signal attenuation curves permits the direct estimation of the MLV radius in favorable cases, thus yielding the constant of proportionality between λhom and radius. The microstructural origin of the Bragg peak is verified through Brownian dynamics simulations and a theoretical analysis based on the center-of-mass diffusion propagator. λhom is decreasing with increasing shear rate in agreement with theoretical expectations and results from 2H NMR lineshape analysis.

NMR measurements of restricted diffusion are used to determine the characteristic size of structures in microheterogeneous materials such as multilamellar vesicles.119Research highlights► The homogeneous length scale of close-packed multilamellar vesicles is estimated by diffusion NMR. ► PGSE NMR diffusion-diffraction is observed for multilamellar vesicles. ► The homogeneous length scale is proportional to the radius of multilamellar vesicles.