Interaction between like-charged polyelectrolyte-colloid complexes in electrolyte solutions: A Monte Carlo simulation study in the Debye–Hückel approximation

Abstract

We study the effective interaction between differently charged polyelectrolyte-colloid complexes in electrolyte solutions via Monte Carlo simulations. These complexes are formed when short and flexible polyelectrolyte chains adsorb onto oppositely charged colloidal spheres, dispersed in an electrolyte solution. In our simulations the bending energy between adjacent monomers is small compared to the electrostatic energy, and the chains, once adsorbed, do not exchange with the solution, although they rearrange on the particles surface to accomodate further adsorbing chains or due to the electrostatic interaction with neighbor complexes. Rather unexpectedly, when two interacting particles approach each others, the rearrangement of the surface charge distribution invariably produces anti-parallel dipolar doublets, that invert their orientation at the isoelectric point. These findings clearly rule out a contribution of dipole-dipole interactions to the observed attractive interaction between the complexes, pointing out that such suspensions can not be considered dipolar fluids. On varying the ionic strength of the electrolyte, we find that a screening length κ −1 , short compared with the size of the colloidal particles, is required in order to observe the attraction between like charged complexes due to the non-uniform distribution of the electric charge on their surface ('patch attraction'). On the other hand, by changing the polyelectrolyte/particle charge ratio, ξ s , the interaction between like-charged polyelectrolyte-decorated (pd) particles, at short separations, evolves from purely repulsive to strongly attractive. Hence, the effective interaction between the complexes is characterized by a potential barrier, whose height depends on the net charge and on the non-uniformity of their surface charge distribution.2

Salt-induced reentrant stability of polyion-decorated particles with tunable surface charge density

“…(2) is always attractive (negative) also for like-charged particles. The presence of this attractive term is consistent with the results of Poisson-Boltzmann equation approaches and simulations [19,54,55], pointing out the existence of a short range attraction between polyelectrolyte-decorated macroions. The term "short range" is well justified since the decaying length of the attractive term (2Ä) −1 is one half of the Debye screening length that sets the scale for the screened electrostatic interactions.…”

“…The relevance of bridging and/or the presence of bridging events in a variety of colloid/polyelectrolyte mixtures has been characterized experimentally by Borkovec et al [34] who pointed out that, in systems with small polymer/colloid size ratio, individual bridging events could be only rarely identified from the retraction part of the force profiles measured by AFM, concluding that bridging forces are unimportant in these systems. Monte Carlo simulations [19] with colloids having the same nominal surface charge density of pure DOTAP liposomes and fully ionized polyelectrolyte chains have shown that the non-DLVO attractions are dominated by charge patch attraction rather than bridging, whose onset has been detected only under conditions of high overcharging of the particles, i.e. where the adsorbed polyelectrolyte-layers are characterized by several protruding chains and dangling ends.…”

“…In fact in the presence of an oppositely charged polyelectrolyte many colloids, characterized by different structures and chemical compositions, like bare [28] and lipid-coated latex particles [29], ferric oxide particles [30], dendrimers [31] and micelles [32], show the "reentrant condensation" phenomenon. Within this scenario the clusters formed by pd-particles in absence of added salt are electrostatically stabilized (out-of-equilibrium) and an increase of temperature has a destabilizing effect on the system [26,33,19]. …”

Fluid of hard spheres with dipolar-like patch interaction and effect of adding an isotropic adhesion

“…Surprisingly enough, those authors found that "the eventual aggregation of the two complexes occurs with the antiparallel orientation of the dipoles" and remark that "this antiparallel orientation of the dipole doublet signals the absence of a longrange dipole-dipole attractive contribution to the mean-force interparticle potential [32]." All this qualitatively agrees with our work .…”

“…At the end of the present work, it has been published an interesting paper on a realistic system which has strong similarities with our model and thus provides it with more physical plausibility. It deals with a fluid of charged colloidal spheres, whose surface is decorated with a thin and nonuniform layer of adsorbed polyelectrolytes [32]. On varying the ionic strength of the electrolyte solution, the screening length κ −1 D can become short compared to the size of the colloidal particles.…”