Ard Louis

Abstract:

The effective interactions and the second osmotic virial coefficient B2 of protein solutions incorporating the electrostatics within the "primitive" model of electrolytes was calculated. For discrete charge distributions, the interactions and related B2 vary in a nonmonotonic fashion with increasing ionic strength, while for the smeared charge model, a standard workhorse of colloidal physics, this effect was absent. These correlated-induced effects were missed within nonlinear PB theory, and similar coarse-graining techniques taken from the theory of colloids.

Abstract:

The effect of solvent quality on the effective pair potentials of the interacting linear polymers of a solution was investigated. The inversion of c.m. pair distribution function, by using the hypernetted chain closure method, was employed for the derivation of effective pair potentials. The pair potential was found to be strongly dependent on the polymer concentration and temperature.

Abstract:

Solutions of interacting linear polymers are mapped onto a system of "soft" spherical particles interacting via an effective pair potential. This coarse-graining reduces the individual monomer-level description to a problem involving only the center of mass (c.m.) of the polymer coils. The effective pair potentials are derived by inverting the c.m. pair distribution function, generated in Monte Carlo simulations, using the hypernetted chain closure. The method, previously devised for the self-avoiding walk model of polymers in good solvent, is extended to the case of polymers in solvents of variable quality by adding a finite nearest-neighbor monomer-monomer attraction to the previous model and varying the temperature. The resulting effective pair potential is found to depend strongly on temperature and polymer concentration. At low concentration the effective interaction becomes increasingly attractive as the temperature decreases, eventually violating thermodynamic stability criteria. However, as polymer concentration is increased at fixed temperature, the effective interaction reverts to mostly repulsive behavior. These issues help to illustrate some fundamental difficulties encountered when coarse-graining complex systems via effective pair potentials.