Depletion-induced forces and crowding in polymer-nanoparticle mixtures: Role of polymer shape fluctuations and penetrability

Abstract

Depletion forces and macromolecular crowding govern the structure and function of biopolymers in biological cells and the properties of polymer nanocomposite materials. To isolate and analyze the influence of polymer shape fluctuations and penetrability on depletion-induced interactions and crowding by nanoparticles, we model polymers as effective penetrable ellipsoids, whose shapes fluctuate according to the probability distributions of the eigenvalues of the gyration tensor of an ideal random walk. Within this model, we apply Monte Carlo simulation methods to compute the depletion-induced potential of mean force between hard nanospheres and crowding-induced shape distributions of polymers in the protein limit, in which polymer coils can be easily penetrated by smaller nanospheres. By comparing depletion potentials from simulations of ellipsoidal and spherical polymer models with predictions of polymer field theory and free-volume theory, we show that polymer depletion-induced interactions and crowding depend sensitively on polymer shapes and penetrability, with important implications for bulk thermodynamic phase behavior.

Theoretical predictions of structures in dispersions containing charged colloidal particles and non-adsorbing polymers

“…Nevertheless, for our system the AO polymer model has several drawbacks. Treating polymers as spherical objects is by itself a crude approximation, as is a neglect of polymer shape fluctuations [9][10][11]51 . We also note that water is a good solvent for PEG, i.e., even the ideal polymer model that the AO aims to simplify, is inappropriate for our system.…”

“…A colloidal dispersion with non-adsorbing polymers form an especially useful model system. Various theoretical models of particle-polymer mixtures have been utilized in previous work, aiming to quantify the various interactions and to establish how they affect the macroscopic properties [5][6][7][8][9][10][11] . Several experimental methods are available to elucidate the structure of particlepolymer mixtures, which can then be compared with the model predictions [12][13][14][15][16][17][18][19][20] .…”

Influence of Solvent Quality on Depletion Potentials in Colloid-Polymer Mixtures

“…The spherical polymer approximation, while incorporating an important length scale, ignores aspherical conformations and fluctuations in conformation, both of which can significantly affect depletion-induced forces. As in our earlier work on polymer crowding [71,[116][117][118], we extend the AO model by representing the polymers as effective ellipsoids that fluctuate in size and shape according to random-walk statistics. In the current study, we consider only the colloid limit, in which the colloids are larger than the polymer coils, such that penetration of polymer by colloids is negligible.…”

“…For a system in chemical equilibrium with a polymer reservoir, the PMF is defined as the change in grand potential Ω(r). In earlier work [116], we applied an alternative (but equivalent) definition, that is more appropriate in the nanoparticle limit, in which polymer coils are significantly larger than and penetrable by the colloids (nanoparticles).…”

“…Previous studies have investigated depletion forces induced by aspherical depletants (e.g., rods, ellipsoids) that are fixed in size and shape [58,74,114,115]. Recently, we explored polymer crowding and depletion forces in models of colloid-polymer mixtures, with polymers modeled as fluctuating, penetrable ellipsoids, in both θ solvents [71,116,117] and good solvents [118], distinguished by whether polymer segments are effectively ideal (noninteracting) or nonideal, excluding volume to one another. The purpose of the present paper is to assess the interconnected influences of polymer shape and solvent quality on depletion interactions in colloid-polymer mixtures.…”