Effective electrostatic interactions in colloid-nanoparticle mixtures

Abstract

Interparticle interactions and bulk properties of colloidal suspensions can be substantially modified by addition of nanoparticles. Extreme asymmetries in size and charge between colloidal particles and nanoparticles present severe computational challenges to molecular-scale modeling of such complex systems. We present a statistical mechanical theory of effective electrostatic interactions that can greatly ease large-scale modeling of charged colloid-nanoparticle mixtures. By applying a sequential coarse-graining procedure, we show that a multicomponent mixture of charged colloids, nanoparticles, counterions, and coions can be mapped first onto a binary mixture of colloids and nanoparticles and then onto a one-component model of colloids alone. In a linear-response approximation, the one-component model is governed by a single effective pair potential and a one-body volume energy, whose parameters depend nontrivially on nanoparticle size, charge, and concentration. To test the theory, we perform molecular dynamics simulations of the two-component and one-component models and compute structural properties. For moderate electrostatic couplings, colloid-colloid radial distribution functions and static structure factors agree closely between the two models, validating the sequential coarse-graining approach. Nanoparticles of sufficient charge and concentration enhance screening of electrostatic interactions, weakening correlations between charged colloids and destabilizing suspensions, consistent with experiments.

Screening Lengths in Ionic Fluids

“…Our theory explains recent ex-perimental findings concerning a jump of the wavelength of the structural force in ionic fluids [14], and it sheds new light on the screening in dense electrolytes and the fitting of structural forces [19]. Our results are important for the interpretation of measurements and effective interactions [19,[40][41][42], because they show that species correlation functions can be superpositions of charge contributions and density contributions of the same order of magnitude. A fit using the asymptotic form (12) hence cannot be expected to be accurate on intermediate length scales.…”

Structure and stability of charged colloid-nanoparticle mixtures

“…This comparison demonstrates that ions confined to the surfaces of charged nanoparticles have a significantly stronger screening effect, and thus overall damping effect on correlations, than an equal number of free salt ions. Furthermore, this interpretation is consistent with conclusions drawn from an effective interaction theory, recently developed by one of us [60]. This theory, which is based on linear-response and random-phase approximations, maps a charged colloid-nanoparticle mixture onto a coarse-grained, one-component model of pseudo-colloids governed by an effective electrostatic pair potential…”

“…The latter trend is consistent with amplification of the electrostatic coupling with increasing particle charge. The weakening of colloid-colloid correlations can be interpreted as an enhancement of electrostatic screening by charged nanoparticles, as predicted by a theory of effective electrostatic interactions in charged colloid-nanoparticle mixtures [60]. Our results demonstrate that nanoparticleenhanced screening tends to promote irreversible aggregation of weakly charged colloidal particles in a fluid suspension and can induce melting of colloidal crystals.…”

“…As confirmed by our simulations and the simulations of ref. [60], charged nanoparticles increase the effective screening constant much more efficiently than microions. For example, a nanoparticle of valence Z n = 10 contributes more to screening than 10 free microions.…”

Structural correlations and phase separation in binary mixtures of charged and uncharged colloids

“…While the effective interactions and structural correlations in strongly interacting colloidal fluids are by now wellunderstood in one-component or even polydisperse systems of either charged [4][5][6][7][8][9][10] or uncharged colloidal spheres [11][12][13], much less is known about binary mixtures of charged and uncharged particles. Such binary neutral-charged systems occur frequently in mixtures of colloids with nanoparticles culminating in the charged nanoparticle-halo effect around neutral colloids which provides colloidal stabilization [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. Moreover, in ordinary mixtures of charged colloids the particle charge can be tuned by the pH of the solution [29][30][31] such that one component stays charged and the other can become neutral close to the isoelectric point realizing a charged-uncharged colloidal system.…”