A new relaxation effect with polymer depletion layers

Dynamic light scattering measurements of the apparent radius of small lipid liposomes in the presence of depletion layer forming dextran with a molecular weight of 110 and 500 kD demonstrated a relative decrease of the depletion effect with decreasing particle radius and increasing molecular weight. To explain this unexpected behavior the static view of the depletion effect near a resting wall has been modified by introducing the concept of a depletion layer formation time. If the latter is comparable to the time the particle requires to diffuse over a distance equal to the depletion layer thickness, the depletion effect should be reduced. This new effect was termed depletion layer relaxation. An estimate of the depletion layer formation time was derived by means of Brownian dynamics simulations of free dextran without and with hydrodynamic interaction. The power law of the autocorrelation time as a function of the polymer radius was calculated. At high polymer coverage, where ensemble averaging is meaningful, the depletion layer formation time is given by the autocorrelation time of the polymer radius. At low coverage the depletion layer formation time is larger by 1 order of magnitude. An analytical formula is provided in a linear approximation.