Charge and Electric Field Fluctuations in Aqueous NaCl Electrolytes

Abstract

Crystalloluminescence, the long-lived emission of visible light during the crystallization of certain salts, was first observed over 200 years ago; however, the origin of this luminescence is still not well understood. The observations suggest that the process of crystallization may not be purely classical but also involves an essential electronic structure component. Strong electric field fluctuations may play an important role in this process by providing the necessary driving force for the observed electronic structure changes. The main objective of this work is to provide a basic understanding of the fluctuations in charge, electric potentials, and electric fields for concentrated aqueous NaCl electrolytes. Our charge analysis reveals that the water molecules in the first solvation shell of the ions serve as a sink for electron density originating on Cl(-). We find that the electric fields inside aqueous electrolytes are extremely large (up to several V/Å) and thus may alter the ground and excited electronic states in the condensed phase. Furthermore, our results show that the potential and field distributions are largely independent of concentration. We also find the field component distributions to be Gaussian for the ions and non-Gaussian for the O and H sites (computed in the lab frame of reference), however, these non-Gaussian distributions are readily modeled via an orientationally averaged nonzero mean Gaussian plus a zero mean Gaussian. These calculations and analyses provide the first steps toward understanding the magnitude and fluctuations of charge, electric potentials, and fields in aqueous electrolytes and what role these fields may play in driving charge redistribution/transfer during crystalloluminescence.

One-step electric-field driven methane and formaldehyde synthesis from liquid methanol

“…As stated before, field intensities of the order or greater than 1 V Å –1 are currently found (locally) in disparate condensed systems in which simple solvated ionic species are present, 43–45 or even at the surface of clean and apolar oxides, such as the (001) MgO surface. 42 In addition, external field strengths of the order of 0.5 V Å –1 have been also employed in order to reproduce in silico the historical Miller experiment.…”

“…Indeed, the field strength of 0.30 V Å –1 represents the known methanol dissociation threshold 37 and the formation of ionic species such as CH 3 OH 2 + and CH 3 O – produces intense local fields, as is usual when dealing with solvated ions. 43–45 Moreover, the region of the free energy barrier that is significantly modified by the field action corresponds to the effectively recorded passage in our unbiased AIMD simulation ( i.e. , it is the natural reaction pathway when a strong electric field is applied).…”

“…30 Moreover, the spontaneous presence of even stronger fields in the proximity of certain clean and apolar mineral surfaces 42 and in regions surrounding solvated ionic species 43–45 is well established.…”

A matter of quantum voltages

“…The fidelity of the sampling for the aqueous NaCl electrolyte structure was ensured by comparison with experimental neutron diffraction and has been published previously. 36 One hundred uncorrelated snapshots were taken from these trajectories with equal stride in time and used for quantum mechanical (QM) resampling with DFT utilizing the PBE 37 functional. A high cutoff of 750 Rydberg was used to get high spatial resolution of the electrostatic potential grid (0.058 Å).…”

The influence of polarizability and charge transfer on specific ion effects in the dynamics of aqueous salt solutions

“…21,22 This results in ions that have charges of around ±0.7 e to ±0.9 e. 14,21,22 Treating the charges as variational parameters leads to charges of ±0.80 e. 18 These values are close to those that result from charge transfer to the solvent. [23][24][25][26][27] However, in the charge transfer models, unlike in ECC, all ions are not necessarily reduced in charge by the same amount, the charge of an ion fluctuates in time, and the water acquires transient charges. That these methods tend to agree on the charge values and the ion effects on water diffusion leads to questions about physical origins of the reduce charges.…”