Effective shape and phase behavior of short charged rods

Abstract

We explicitly calculate the orientation-dependent second virial coefficient of short charged rods in an electrolytic solvent, assuming the rod-rod interactions to be a pairwise sum of hard-core and segmental screened-Coulomb repulsions. From the parallel and isotropically averaged second virial coefficient, we calculate the effective length and diameter of the rods, for charges and screening lengths that vary over several orders of magnitude. Using these effective dimensions, we determine the phase diagram, where we distinguish a low-charge and strong-screening regime with a liquid crystalline nematic and smectic phase, and a high-charge and weak-screening regime with a plastic crystal phase in the phase diagram.

Phase diagrams of charged colloidal rods: Can a uniaxial charge distribution break chiral symmetry?

“…The sphere-sphere interaction is described by a (truncated) hard-core Yukawa potential , so N s is simply a parameter that can be varied until convergence to the continuum limit is reached. As previously shown, 51 this model with N s ≥ 13 is in excellent agreement with analytic results for the excluded volume of finite aspect-ratio rods with an effective linear charge distribution. Accordingly, we choose N s = 15, which should guarantee a good agreement between the discrete-sphere and the linear-charge model.…”

“…In analogy with Ref. [51], the colloids are modeled as hard spherocylinders (HSC) of diameter D and length L. The total charge on the rods Z is fixed by embedding N s spheres interacting via a hard-core Yukawa potential (HY). The N s spheres (with N s odd) are evenly distributed along the backbone of the rod: they are separated by a distance δ = L Ns−1 such that two spheres are always at the extremities of the cylindrical part of the spherocylinder.…”

Steric asymmetry vs charge asymmetry in dilute solution containing large weakly charged ions

“…This concentration corresponds to the reduced number density r * ¼ 0.1 but to substantially different Table 1 The geometry of cations, see packing fractions in systems with cations of different size. Systems of rodlike particles exhibit transition to plastic crystal or liquid crystalline phase above certain concentration [3,11,17]. For particles studied in our work, such transitions are expected to occur beyond the concentration interval considered here [3,6,17]; this agrees with our simulation data that do not show such transitions.…”

“…In principle the electrostatic and steric interactions must be coupled. For rigid cylindrical particles, theories have been developed where coupling with the steric interactions has been taken into account on the second virial coefficient level of approximation [11]. [12] The problem with our systems is that even at high dilution the second virial EOS describes rather poorly the steric interactions.…”

“…A comprehensive overview of experiment, theory and computer simulation for such fluids is given in Refs. [10] and [11]. The majority of these works study the effect of excluded volume on phase transitions and consider the domain of relatively high concentrations; the influence of electrostatic interactions is taken into account via screened Coulomb potentials and renormalized second virial coefficients [8,11,12].…”