Semi-flexible chain condensation by neutral depleting agents: role of correlations between depletants

We revisit the problem of semi-flexible chain condensation by neutral depleting agents (e.g. colloidal spheres or flexible polymers) by using a simple formalism that allows us to address its main features without specifying the kind of depleting agents. Correlations between depleting agents are shown to produce a reswelling of the chain at high enough volume fraction, consistent with an earlier analysis by Grosberg et al. (Biopolymers, 21:2413, 1982) in the context of DNA condensation induced by neutral polymers. It is also shown that the conditions under which spherical colloids can condense a semi-flexible chain are strongly restrictive, unlike what has been recently claimed. The formalism allows us to compare the efficiency of colloids vs polymers as condensing agents.