Néstor E. Valadez-Pérez

Phase behavior of colloids and proteins in aqueous suspensions: Theory and computer simulations

The fluid phase behavior of colloidal suspensions with short-range attractive interactions is studied by means of Monte Carlo computer simulations and two theoretical approximations, namely, the discrete perturbation theory and the so-called self-consistent Ornstein-Zernike approximation. The suspensions are modeled as hard-core attractive Yukawa (HCAY) and Asakura-Oosawa (AO) fluids. A detailed comparison of the liquid-vapor phase diagrams obtained through different routes is presented. We confirm Noro-Frenkels extended law of scaling according to which the properties of a short-ranged fluid at a given temperature and density are independent of the detailed form of the interaction, but just depend on the value of the second virial coefficient. By mapping the HCAY and AO fluids onto an equivalent square-well fluid of appropriate range at the critical point we show that the critical temperature as a function of the effective range is independent of the interaction potential, i.e., all curves fall in a master curve. Our findings are corroborated with recent experimental data for lysozyme proteins.

Extended law of corresponding states for protein solutions

The so-called extended law of corresponding states, as proposed by Noro and Frenkel [J. Chem. Phys. 113, 2941 (2000)], involves a mapping of the phase behaviors of systems with short-range attractive interactions. While it has already extensively been applied to various model potentials, here we test its applicability to protein solutions with their complex interactions. We successfully map their experimentally determined metastable gas-liquid binodals, as available in the literature, to the binodals of short-range square-well fluids, as determined by previous as well as new Monte Carlo simulations. This is achieved by representing the binodals as a function of the temperature scaled with the critical temperature (or as a function of the reduced second virial coefficient) and the concentration scaled by the cube of an effective particle diameter, where the scalings take into account the attractive and repulsive contributions to the interaction potential, respectively. The scaled binodals of the protein solutions coincide with simulation data of the adhesive hard-sphere fluid. Furthermore, once the repulsive contributions are taken into account by the effective particle diameter, the temperature dependence of the reduced second virial coefficients follows a master curve that corresponds to a linear temperature dependence of the depth of the square-well potential. We moreover demonstrate that, based on this approach and cloud-point measurements only, second virial coefficients can be estimated, which we show to agree with values determined by light scattering or by Derjaguin-Landau-Verwey-Overbeek (DLVO)-based calculations.

Percolation in colloidal systems with competing interactions: the role of long-range repulsion

Percolation in suspensions driven only by short-ranged attractions has been studied for a long-time due to its imminent relation with equilibrium and non-equilibrium processes, such as gelation and glass transition. Recently, the effects of an additional long-range repulsion have received attention as the competition between both contributions of the potential features are shown to be important to understand the phase behavior of many charged colloidal systems, such as proteins in aqueous solutions. Due to the inherent importance of the percolation in determining the structure of a fluid and its pertinent relation with dynamical arrest, here, we use Monte Carlo computer simulations to systematically study the influence of the repulsion on the percolation. The formation and geometry of clusters when a system percolates are investigated. Our results indicate that the addition of the long-ranged repulsion increases the average cluster size, resulting in the shift of the percolation threshold to lower volume fractions. We also show that the structure of small clusters is mostly affected by the attraction, while the morphology of intermediate and large size clusters is determined by the energetic balance between attraction and repulsion.

Phase behavior of the modified-Yukawa fluid and its sticky limit

Simple model systems with short-range attractive potentials have turned out to play a crucial role in determining theoretically the phase behavior of proteins or colloids. However, as pointed out by D. Gazzillo [J. Chem. Phys. 134, 124504 (2011)], one of these widely used model potentials, namely, the attractive hard-core Yukawa potential, shows an unphysical behavior when one approaches its sticky limit, since the second virial coefficient is diverging. However, it is exactly this second virial coefficient that is typically used to depict the experimental phase diagram for a large variety of complex fluids and that, in addition, plays an important role in the Noro-Frenkel scaling law [J. Chem. Phys. 113, 2941 (2000)], which is thus not applicable to the Yukawa fluid. To overcome this deficiency of the attractive Yukawa potential, D. Gazzillo has proposed the so-called modified hard-core attractive Yukawa fluid, which allows one to correctly obtain the second and third virial coefficients of adhesive hard-spheres starting from a system with an attractive logarithmic Yukawa-like interaction. In this work we present liquid-vapor coexistence curves for this system and investigate its behavior close to the sticky limit. Results have been obtained with the self-consistent Ornstein-Zernike approximation (SCOZA) for values of the reduced inverse screening length parameter up to 18. The accuracy of SCOZA has been assessed by comparison with Monte Carlo simulations.

Structure of colloidal gels at intermediate concentrations: the role of competing interactions

Colloidal gels formed by colloid-polymer mixtures with an intermediate volume fraction (ϕc ≈ 0.4) are investigated by confocal microscopy. In addition, we have performed Monte Carlo simulations based on a simple effective pair potential that includes a short-range attractive contribution representing depletion interactions, and a longer-range repulsive contribution describing the electrostatic interactions due to the presence of residual charges. Despite neglecting non-equilibrium effects, experiments and simulations yield similar gel structures, characterised by, e.g., the pair, angular and bond distribution functions. We find that the structure hardly depends on the strength of the attraction if the electrostatic contribution is fixed, but changes significantly if the electrostatic screening is changed. This delicate balance between attractions and repulsions, which we quantify by the second virial coefficient, also determines the location of the gelation boundary.

Performance enhancement of polymer-based solar cells by induced phase-separation with silica particles

Adding metallic nanoparticles into bulk-heterojunction, polymer-based solar cells has been proven an effective strategy to enhance light absorption of the active layer and device performance. However, the high-energy surfaces on the nanoparticles may also affect the morphology of the active layer by influencing phase-separation, which has not been studied in detail. Here, we show that silica particles embedded in the active layer will affect the aggregation behavior of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) in the bulk-heterojunction of poly(3-hexylthiophene) (P3HT):PCBM. Using a novel graphical technique to analyze the absolute scattering intensity of small angle neutron scattering data, we conclusively demonstrate that some PCBM will migrate away from the bulk solution to the surface of the silica upon annealing and improve the device performance. The overall effect is to decrease the device series resistance and improve the power conversion efficiency by 10 to 20% relative to the control group. In contrast to metallic nanoparticles that utilize the surface plasmon resonance, our results indicate that, even with optically inert particles, the induced phase separation of PCBM may also result in an improved device.