Atomistic simulation of PDADMAC/PSS oligoelectrolyte multilayers: overall comparison of tri- and tetra-layer systems

TL;DR

This study uses large-scale molecular dynamics simulations to analyze the microscopic properties and growth mechanisms of PDADMAC/PSS oligoelectrolyte multilayers, comparing tri- and tetra-layer systems on silica substrates in saline solutions.

Contribution

It provides detailed atomistic insights into the early growth stages and structural differences of oligoelectrolyte multilayers, which are difficult to observe experimentally.

Findings

Strong oligoelectrolyte entanglement observed

Fuzzy layering of the multilayer structure

Main charge compensation is intrinsic, with some extrinsic compensation in the last layer

Abstract

By employing large-scale molecular dynamics simulations of atomistically resolved oligoelectrolytes in aqueous solutions, we study in detail the first four layer-by-layer deposition cycles of an oligoelectrolyte multilayer made of poly(diallyl dimethyl ammonium chloride)/poly(styrene sulfonate sodium salt) (PDADMAC/PSS). The multilayers are grown on a silica substrate in 0.1M {NaCl} electrolyte solutions and the swollen structures are then subsequently exposed to varying added salt concentration. We investigated the microscopic properties of the films, analyzing in detail the differences between three- and four-layers systems. Our simulations provide insights on the early stages of growth of a multilayer, which are particularly challenging for experimental observations. We found a rather strong entanglement of the oligoelectrolytes, with a fuzzy layering of the film structure. The main charge compensation mechanism is for all cases intrinsic, whereas extrinsic compensation is relatively ehanced for the layer of the last deposition cycle. In addition, we quantified other fundamental observables of these systems, as the film thickness, water uptake, and overcharge fractions for each deposition layer.