Confinement-Induced Fractionation and Liquid-Liquid Phase Separation of Polymer Mixtures

Abstract

The formation of (bio)molecular condensates via liquid-liquid phase separation in cells has received increasing attention, as these aggregates play important functional and regulatory roles within biological systems. However, the majority of studies focused on the behavior of pure systems in bulk solutions, thus neglecting confinement effects and the interplay between the numerous molecules present in cells. To better understand the physical mechanisms driving condensation in cellular environments, we perform molecular simulations of binary polymer mixtures in spherical droplets, considering both monodisperse and polydisperse molecular weight distributions for the longer polymer species. We find that confinement induces a spatial separation of the polymers by length, with the longer ones moving to the droplet center. This partitioning causes a distinct increase in the local polymer concentration near the droplet center, which is more pronounced in polydisperse systems. Consequently, the confined systems exhibit liquid-liquid phase separation at average polymer concentrations where bulk systems are still in the one-phase regime.

Keywords: Dextran; PEG; confinement; droplet; molecular simulation; phase separation; polymer mixture; protocell.

Figure 1

Schematic representation of the model, illustrating the coarse-graining of PEG and Dextran polymers.

Figure 2

(a,b) Simulation snapshot of the bulk system at (a) $c_{\mathrm{P}}=3\mathrm{wt}\%$, $c_{\mathrm{D}}=3\mathrm{wt}\%$, and (b) $c_{\mathrm{P}}=4\mathrm{wt}\%$, $c_{\mathrm{D}}=4\mathrm{wt}\%$. Only Dextran beads shown for clarity. Snapshots rendered using Visual Molecular Dynamics (version 1.9.3) [42]. (c) Radial concentration profile of Dextran monomers $c_{\mathrm{D}}\left(r\right)$ in the largest Dextran aggregate.

Figure 3

Phase diagram of PEG–Dextran blends in bulk from (a) simulations and (b) experiments [18]. The dotted lines indicate the estimated binodal. Simulation data are generated from monodisperse systems with $M_{\mathrm{n}}^{\mathrm{P}}=6\mathrm{kg}/\mathrm{mol}$ and $M_{\mathrm{n}}^{\mathrm{D}}=500\mathrm{kg}/\mathrm{mol}$, while experimental data were gathered for blends with $M_{\mathrm{w}}^{\mathrm{P}}=6\mathrm{kg}/\mathrm{mol}$ and $M_{\mathrm{w}}^{\mathrm{D}}=500\mathrm{kg}/\mathrm{mol}$.

Figure 4

Radial monomer concentration profiles for droplets with (a) $R\approx 260\mathrm{nm}$ and (b) $R\approx 380\mathrm{nm}$. Solid and dashed lines show simulation results for monodisperse and polydisperse Dextran chains, respectively. The horizontal dashed grey line indicates the average polymer concentration of each species ($3\mathrm{wt}\%$).

Figure 5

Radial profiles of the radius of gyration of monodisperse Dextran, $R_{\mathrm{g}}$, and its normal (${〈G_{\mathrm{n}}〉}^{1/2}$) and tangential (${〈G_{\mathrm{t}}〉}^{1/2}$) components relative to the droplet surface. Data plotted against the distance between the droplet surface and the polymer’s center of mass, $r^{\prime }$. Solid and dashed lines show results for $R\approx 260\mathrm{nm}$ and $R\approx 380\mathrm{nm}$, respectively.

Figure 6

(a) Simulation snapshot of the confined systems ($R\approx 380\mathrm{nm}$, indicated by dashed circle). The top half shows the monodisperse case with Dextran chains colored in red. The bottom half shows the polydisperse case with Dextran chains colored according to their molecular weight as in panels (b,c). Water and PEG particles have been omitted for clarity. (b,c) Probability $P\left(r\right)$ to find a Dextran chain in the specified molecular weight range at center-of-mass position r in a droplet with (b) $R\approx 260\mathrm{nm}$ and (c) $R\approx 380\mathrm{nm}$. The shaded area indicates the standard deviation from three independent runs. The dashed black line shows the distribution of the monodisperse case.

Figure 7

Radial concentration profile of Dextran monomers $c_{\mathrm{D}}\left(r\right)$ in the largest Dextran aggregate in bulk and droplet systems ($R\approx 380\mathrm{nm}$) at overall average concentration $c_{\mathrm{P}}=3\mathrm{wt}\%$ and $c_{\mathrm{D}}=3\mathrm{wt}\%$.