Phase behavior of crowded like-charged mixed polyelectrolytes in a cell-sized sphere

We studied the phase behavior of a mixture of two semiflexible negatively charged polyelectrolytes, giant DNA and alginate, under crowded condition in a cell-sized sphere (5--40 $\ensuremath{\mu}$m in diameter), where the persistence length of giant DNA is 50 nm and that of alginate is 5 nm. Through microscopic observation, we found that the polymer mixture exhibits a unique phase behavior, which depends on the size of the sphere, whereas the mixture remains homogeneous and isotropic in bulk solution. When the sphere is small, DNA is completely depleted on the surface. When the sphere is medium sized, a portion of the DNA is depleted on the surface, and the remainder stays within the sphere. By introducing a curvature-dependent term for the interaction between DNA and the surface into the Flory-Huggins model, we interpret the observed characteristics of the phase behavior in terms of the relative importance of the surface-to-volume effect.