How liposomes diffuse in concentrated liposome suspensions

Building upon the observation that liposomes of zwitterionic lipids can be stabilized against fusion by the adsorption of cationic nanoparticles (Yu, Y.; Anthony, S.; Zhang, L.; Bae, S. C.; Granick, S. J. Phys. Chem. C2007, 111, 8233), we study, using single-particle fluorescence tracking, mobility in this distinctively deformable colloid system, in the volume fraction range of ψ = 0.01 to 0.7. Liposome motion is diffusive and homogeneous at low volume fractions, but separable fast and slow populations emerge as the volume fraction increases beyond ψ ≈ 0.45, the same volume fraction at which hard colloids with sufficiently strong attraction are known to experience gelation. This is reflected not only in scaling of the mean square displacement, but also in the step size distribution (van Hove function) measured by fluorescence imaging. The fast liposomes are observed to follow Brownian motion, and the slow ones follow anomalous diffusion characterized by a 1/3 time scaling of their mean square displacement.