Atomistic Molecular Dynamics Simulations of Charged Latex Particle Surfaces in Aqueous Solution

Abstract

Charged particles in aqueous suspension form an electrical double layer at their surfaces, which plays a key role in suspension properties. For example, binder particles in latex paint remain suspended in the can because of repulsive forces between overlapping double layers. Existing models of the double layer assume sharp interfaces bearing fixed uniform charge, and so cannot describe aqueous binder particle surfaces, which are soft and diffuse, and bear mobile charge from ionic surfactants as well as grafted multivalent oligomers. To treat this industrially important system, we use atomistic molecular dynamics simulations to investigate a structurally realistic model of commercial binder particle surfaces, informed by extensive characterization of particle synthesis and surface properties. We determine the interfacial profiles of polymer, water, bound and free ions, from which the charge density and electrostatic potential can be calculated. We extend the traditional definitions of the inner and outer Helmholtz planes to our diffuse interfaces. Beyond the Stern layer, the simulated electrostatic potential is well described by the Poisson-Boltzmann equation. The potential at the outer Helmholtz plane compares well to the experimental zeta potential. We compare particle surfaces bearing two types of charge groups, ionic surfactant and multivalent oligomers, with and without added salt. Although the bare charge density of a surface bearing multivalent oligomers is much higher than that of a surfactant-bearing surface at realistic coverage, greater counterion condensation leads to similar zeta potentials for the two systems.

Surfactant Binding to Polymer–Water Interfaces in Atomistic Simulations

“…12,13 The polymer slab consists of 36 acrylate chains, each of which are random copolymers of take the area per SDS as 2.632 nm 2 (0.38 SDS per nm 2 ). We refer to this system as "full coverage" -not because the surface is saturated with SDS, but because this is as much SDS as we expect to see on commercial latex particles.…”

“…These parameters have been shown in our previous work to produce reasonable bulk and interfacial properties. 12,13 Potentials for the aqueous ions (Na + and Cl ? ) are likewise taken from OPLS, with Na + and Cl ?…”

“…We adjust the partial charges of the terminal methyl so that the overall charge of the anion is -1, as described in previous work. 12 To create equilibrated initial configurations for pulling these SDS fragments, we start with corresponding configurations in which an entire SDS molecule to be pulled is present, delete either the tail or the headgroup, and add a hydrogen to terminate the remaining fragment. Then we equilibrate, so that surrounding material can fill in and relax the holes left behind by the deleted atoms.…”

“…Corresponding electrostatic potentials are plotted against the right-hand vertical axis, for SDS full coverage (solid) and half coverage (dashed). The right-hand scale is adjusted so that differences in SDS headgroup electrostatic energy and PMF free energy can be directly compared.ing to our estimate of the typical SDS coverage for commercial latex coating formulations (0.38 per nm 2 ),12 and "half coverage" (0.19 per nm 2 ). The resulting PMFs are displayed inFig.…”

Intraparticulate speciation analysis of soft nanoparticulate metal complexes. The impact of electric condensation on the binding of Cd2+/Pb2+/Cu2+by humic acids

“…15 For the case with a 1-1 background electrolyte, the outcome is that divalent counterions may accumulate to quite high local concentrations in the DL zone. 21 Since the associative reactions of Ca 2+ with negatively charged ligands are dominated by electrostatics, 22,23 the binding features of Ca 2+ may be adopted as an approximate basis for discriminating between electrostatic and covalent contributions to binding of transition metal ions such as Cd 2+ , Pb 2+ , and Cu 2+ . the counterion condensation characteristics of polyelectrolytes such as DNA, 17 and soft nanoparticles such as dendrimers, [18][19][20] as well as the observation of strong counterion condensation in core-shell nanoparticles with a 3D structural charge in the shell.…”