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

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.

PDADMAC/PSS Oligoelectrolyte Multilayers: Internal Structure and Hydration Properties at Early Growth Stages from Atomistic Simulations

“…A good agreement between experiments and simulations was observed [41]. Stimulated by this agreement, very recently we presented a first analysis of several microscopic overall properties of such system [42] that in most cases remain inaccessible to experimental observations. Oligoelectrolyte density profiles demonstrated the development of three structural regions, as predicted qualitatively in different previous works [3,43,44]: a strongly layered region next to the substrate, a dense central region and an outermost region of smoothly decaying density.…”

“…A thickness of ∆h = 1 nm is taken for these slices, which are placed at distances to the substrate ranging from h = 0.3 nm to h = 5.8 nm, with intervals of 0.5 nm. The upper limit corresponds approximately to the external border of the central region of the film [42]. The RDF of atoms of group β around a reference group α located inside the slice h is defined as…”

“…Besides providing information on different microscopic properties [42], our most recent study on the four layers system raised several important questions that could not be addressed by means of the conventional overall analysis of the simulation data that we employed there. Such questions, which are the main subject of this work, are the following.…”

“…Such questions, which are the main subject of this work, are the following. First, the accumulation of water and ions next to the substrate revealed by the density profiles [42] suggests that intrinsic charge compensation could be much weaker in such region, but a direct proof of the existence and extent of such effect requires a local correlation analysis. In the same way one can determine whether the smoother but also significant increase of water and ions in the outermost region of the multilayer that was also observed has a similar impact.…”

pH-Induced Changes in Polypeptide Conformation: Force-Field Comparison with Experimental Validation

“… 30 Specifically, the molecular dynamics (MD) method in atomistic detail has been frequently used for modeling polyelectrolytes 31 − 34 and charged polypeptides 8 , 16 , 31 , 35 , 36 in bulk solution as well as polyelectrolyte complexes 37 − 42 and multilayers. 43 − 45 Most importantly, these studies have charted intermolecular interactions, complexation, and solution characteristics. To our knowledge, charge state dependency of PP secondary structure has not been studied computationally.…”