Spontaneous localization of two kinds of hard spheres in giant vesicles

Abstract

A phase separation in a dispersion of two kinds of particles has recently been reported to result from entropic interactions between particles and between particles and surfaces in closed spaces. In many cases, the phase separation took place localization of large particles in the vicinity of a compartment. However, phase separation in closed systems has been limited to systems in which the number of small particles was much larger than the number of large particles. In contrast, we prepared giant vesicles (GVs) in which the volume fraction of large particles was higher than that of the small particles. As a result, we observed a new phenomenon in which small particles localized spontaneously and stably in the vicinity of the vesicular membrane. To explain this phenomenon theoretically, we assumed that an equilibrium osmotic pressure was realized between an outer phase containing a relatively large number of small particles and another inner phase. The osmotic pressure was estimated from the free energy change due to the excluded-volume effect. There was good agreement between the distribution ratio of the number of large and small particles in the phases calculated from fluorescent microscopy images and the prediction of the osmotic equilibrium model.