Proton transfer in liquid water confined inside graphene slabs

Abstract

The microscopic structure and dynamics of an excess proton in water constrained in narrow graphene slabs between 0.7 and 3.1 nm wide has been studied by means of a series of molecular dynamics simulations. Interaction of water and carbon with the proton species was modelled using a multistate empirical valence bond Hamiltonian model. The analysis of the effects of confinement on proton solvation structure and on its dynamical properties has been considered for varying densities. The system is organized in one interfacial and a bulk-like region, both of variable size.In the widest interplate separations, the lone proton shows a marked tendency to place itself in the bulk phase of the system, due to the repulsive interaction with the carbon atoms. However, as the system is compressed and the proton is forced to move to the vicinity of graphene walls it moves closer to the interface, producing a neat enhancement of the local structure. We found a marked slowdown of proton transfer when the separation of the two graphene plates is reduced. In the case of lowest distances between graphene plates (0.7 and 0.9 nm), only one-two water layers persist and the two-dimensional character of water structure becomes evident. By means of spectroscopical analysis, we observed the persistence of Zundel and Eigen structures in all cases, although at low interplate separations a signature frequency band around 2500 cm −1 suffers a blue-shift and moves to characteristical values of asymmetric hydronium ion vibrations, indicating some unstability of the typical Zundel-Eigen moieties and their eventual conversion to a single hydronium species solvated by water.

Potentials of mean force in acidic proton transfer reactions in constrained geometries

“…This fact has been recently related to three-body interactions by Wiedemair et al [40]. Further, this feature was also previously observed in our simulations [43] and it suggests the equivalence of the present MS-EVB simulations with the MD calculations with the dissipative potential [42] regarding the meaning of local proton structures. In our case, this first maximum of the RDF is essentially invariable, regardless of T or d. This fact gives an indication of the strength of the proton to create a first solvation shell which remains virtually invariable at all conditions.…”

“…First of all, this feature is located at distances closer than those reported elsewhere [41,42] including the observation (see [43]) of the compression of the local coordination of the proton (the "inward shift" by Lockwood and Garofalini [42]). Second, at the LDA state the maximum is centered at 4.2Å in all cases but moves to 3.5Å when d = 0.7 nm.…”

“…The implementation of such kind of methodology has been widely described in the literature [44][45][46][47][48][49][50][51][52][53][54][55][56][57][58], so that here I will restrict myself to give a fair but schematic description of the main features of the method. Full details can be found in a previous work [43] and in references therein.…”

“…The calculation of the PT rates (see Ref. [43]) delivered rates of 1.2 ps −1 for the unconstrained system, whereas this rate decreased to ∼ 0.01 ps −1 when d = 0.7 nm. Now the reason for such drop is clear that has to be charged to the large size of the energy barrier.…”

“…Although there is plenty of information on microscopic properties of lone protons in constrained water, the influence of a variety of factors on them, from variations in thermal energy to hydrophobicity effects makes the problem very difficult to handle. Based on a previous work of computer simulations of excess protons confined in narrow nanometric channels at room conditions [43], I will report in the present paper the potentials of mean force between the instantaneous protonated water molecule, i.e. the water molecule carrying the proton, and its closest neighbors in an attempt to evaluate the influence of temperature and confinement together on the likeliness of the transfer episodes and on the characteristics of the energy barriers associated with acidic PT.…”