Ab initio metadynamics study on hydronium ion dynamics at acid-functionalized interfaces: effect of surface group density

Abstract

This article presents an ab initio metadynamics study of elementary hydronium ion transitions at dense arrays of surface groups with sulfonic acid head groups. Calculations simulate minimally hydrated conditions of the interfacial ionic system. The specific focus is on the influence of the surface group density on hydronium ion transport. Results reveal a high sensitivity of the activation free energy of hydronium translocations to the surface group density. A spontaneous concerted transition with low activation barrier is found at a surface group separation of 6.8 Å. When hydroniums translocate concertedly, the activation barrier of the transition drops by more than a factor of two to the value of 0.25 eV. An approach is presented to determine interaction constants of hydronium ions and anionic surface groups as well as the surface group flexibility from the analysis of frequency spectra. These properties are discussed in the context of a recently developed soliton theory of interfacial proton transport.

Acid-Group-Content-Dependent Proton Conductivity Mechanisms at the Interlayer of Poly(N-dodecylacrylamide-co-acrylic acid) Copolymer Multilayer Nanosheet Films

“…The l AA values for n ≥ 0.32 (6.4–8.9 Å) are close to the optimal distance, and it can thus be concluded that the advanced 2D hydrogen-bonding network was formed at the interlayer of the multilayer film. Indeed, the E a values for proton conduction (∼0.30 eV) are similar to the theoretically calculated value (0.25 eV), which experimentally supports that the protons move via the Grotthuss mechanism using the advanced 2D hydrogen-bonding network. The unique temperature dependence of the proton conductivity for n = 0.19 can be explained by taking into account the following considerations: at low temperatures, the advanced 2D hydrogen-bond network forms among water molecules and the carboxylic acid moieties.…”

“…Several groups have reported an optimal acidic group distance for high proton conduction in 2D nanospaces. For example, the ideal center-to-center distance ( l cc ) of sulfonic acid groups in 2D interfaces has been calculated by Spohr and Ilhan ( l cc = 6.0- 8.5 Å) and Eikerling and co-workers ( l cc = 6.5–6.8 Å). − Oliveira et al have used stearic acid Langmuir films to study the relationship between the l cc of the stearic acid and the proton conduction, and they observed the highest proton conductivity for l cc = 7.0 Å . These groups concluded that the high proton conductivity at l cc is a result of the formation of an advanced 2D hydrogen-bonding network between the acid groups and water (Figure ).…”

Theory of collective proton motion at interfaces with densely packed protogenic surface groups

“…The activation energy for these concerted hydronium ion translocations is therefore significantly reduced. In explicit ab initio molecular dynamics studies, [27], we found an activation energy of 0.3 eV for the concerted proton transition. The concertedness of hydronium ion translocations depends on the strength of HB.…”

“…The system is depicted in figure 1. It consists of an array of surface groups (SG) of the type R-SO 3 H, where R represents a residual group or chain of fluorocarbon or hydrocarbon molecules [24][25][26][27]. Without restricting the generality of the model, we will often refer to the specific system with R = CF 3 .…”

“…Creation of such HB defects is energetically expensive. This transition was studied explicitly in [27] using the Metadynamics method for performing molecular dynamics simulations [28,29]. The barrier energy for creation of this defect was found in the range of ∼0.6 eV, whereas the barrier for the reverse transfer to the perfectly ordered state is 0.1 eV.…”