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The shapes of bacteria can vary widely; they may, for instance, be spherical, rod-like, string-like, or curved. In general, bacilli are highly anisotropic. For research and (bio)technological purposes, it can be useful to concentrate bacteria, which is possible by adding nonadsorbing polymers. The induced phase separation originates from a polymer-mediated depletion interaction, first understood by Asakura and Oosawa. Here, it is shown that free volume theory (FVT) can semi-quantitatively describe the phase transitions observed when adding sodium polystyrene sulfonate polymers to E. coli bacteria [Schwarz-Linek et al., Soft Matter 6, 4540 (2010)] at high ionic strength. The E. coli bacteria are described as short, hard spherocylinders. FVT predicts that the phase transitions of the mixtures result from a fluid-ABC crystal solid phase coexistence of a hard spherocylinder-polymer mixture.

We present the phase diagram of a colloid-polymer mixture in which the radius a of the colloidal spheres is approximately the same as the radius R of a polymer coil (q=R/a≈1). A three-phase coexistence region is experimentally observed, previously only reported for colloid-polymer mixtures with smaller polymer chains (q≲0.6). A recently developed generalized free-volume theory (GFVT) for mixtures of hard spheres and non-adsorbing excluded-volume polymer chains gives a quantitative description of the phase diagram. Monte Carlo simulations also agree well with experiment. Copyright © EPLA, 2008.

We study the phase behaviour of mixtures of colloidal spheres and polymers that have an excluded-volume interaction dispersed in a (background) solvent using the concept of free volume theory. The depletion layer thickness is calculated from the negative adsorption of polymer segments around a sphere. The correlation length and thermodynamic properties of the excluded-volume interacting polymer chains in solution are taken into account by using results from the renormalization group theory. For small polymer–colloid size ratios the difference from an ideal description of the polymers is small, while for larger size ratios the gas–liquid coexistence region shifts in the direction of higher polymer concentrations and at the same time the liquid–crystal coexistence region becomes more extended. Both the gas–liquid region and the gas– liquid–crystal region become less extended. These features are compared to experiment.