Proton Traffic Jam: Effect of Nanoconfinement and Acid Concentration on Proton Hopping Mechanism

Abstract

The properties of the water network in concentrated HCl acid pools in nanometer-sized reverse nonionic micelles were probed with TeraHertz absorption, dielectric relaxation spectroscopy, and reactive force field simulations capable of describing proton hopping mechanisms. We identify that only at a critical micelle size of W₀ =9 do solvated proton complexes form in the water pool, accompanied by a change in mechanism from Grotthuss forward shuttling to one that favors local oscillatory hopping. This is due to a preference for H⁺ and Cl^- ions to adsorb to the micelle interface, together with an acid concentration effect that causes a "traffic jam" in which the short-circuiting of the hydrogen-bonding motif of the hydronium ion decreases the forward hopping rate throughout the water interior even as the micelle size increases. These findings have implications for atmospheric chemistry, biochemical and biophysical environments, and energy materials, as transport of protons vital to these processes can be suppressed due to confinement, aggregation, and/or concentration.

Keywords: Grotthuss mechanism; THz spectroscopy; confined water; micelles; proton hopping.

Figure 1

Structure of the nonionic surfactant IGEPAL CO‐520. a) The reverse micelle surfactant is a polyether with a polar alcoholic group head group and a nonpolar chain incorporating an aromatic unit. b) The reverse micelle system with the pure water or HCl acidic pools encapsulated by the surfactants (in gray) and solvated in cyclohexane (in light green).

Figure 2

THz‐TDS and FTIR absorption spectra for pure water and HCl solutions as a function of reverse micelle size. The difference absorption spectra $\Delta {\alpha }_X\left(v,c\right)$ for RMs filled with a) water and b) 1 M HCl after subtraction of W₀=0. Below 100 cm⁻¹ spectra were collected with THz‐TDS with an average error of 0.6 cm⁻¹. Above 100 cm⁻¹ spectra were collected with THz‐FTIR with an average error of 5 cm⁻¹. c) The double difference absorption spectra, $\Delta \Delta \alpha \left(v,c\right)$ , for 1 M HCl acid solutions, determined by subtracting the reverse micelles of the same size filled with pure water. The reverse micelles exhibit vibrational bands corresponding to hydration water (120–150 cm⁻¹), the solvated Eigen‐complex (360 cm⁻¹) and the vibrational band corresponding to the Cl⁻ rattling mode in bulk 1 M HCl solution (180 cm⁻¹). Since the 180 cm⁻¹ mode is missing in the acidic RMs, it indicates the Cl⁻ ions are adsorbed at the micelle surface.

Figure 3

Real and imaginary components of the dielectric permittivity of reverse micelles containing water and 1 M HCl acid solution. The real part ϵ′ for RMs containing a) pure water and b) 1 M HCl. The imaginary part ϵ′′ for c) pure water and d) 1 M HCl.

Figure 4

Rotational relaxation time constants and conductance determined for reverse micelles containing water and 1 M HCl acid solution. Simultaneous fitting of the real and imaginary parts of the dielectric response yield Debye relaxation times for a) interfacial water (${\tau }_1$ ), b) slow core water (${\tau }_2$ ), c) fast core water (${\tau }_3$ ), and d) conductance (σ_DC). Further details are provided in Methods and the numerical results of the fits can be found in Supplementary Tables S1, S2 and S3.

Figure 5

Molecular dynamics simulation of the pure and HCl water pools in IGEPAL reverse micelles using ReaxFF/C‐GeM. a) The cross‐section depiction of the HCl acid water pool showing that the Cl ions (gold), Eigen‐like hydronium in green, Zundel‐like hydronium in blue are largely seen to be distributed at the RM interface. b) Number of forward proton hops (without oscillatory motion) calculated from an MD trajectory from each RM system and a bulk water system with 1 M HCl solution. c) Proton hopping rates per hydronium (blue dot) and ratio of oscillatory to non‐oscillatory hops (red square) calculated from an MD trajectory from each 1 M HCl reverse micelle system. d) Proton hopping rates per hydronium (blue dot) and ratio of oscillatory to non‐oscillatory hops (red square) calculated from an MD trajectory from W₀=9 reverse micelle system of varying HCl concentration. The full results can be found in Supplementary Figure S5 and Table S4.

Figure 6

Alterations in the 1^st shell solvation structure of the hydronium ion in 1 M HCl reverse micelles induces an increase in the Zundel motif. The Eigen complex with a central hydronium oxygen (blue) pairs with not only other water molecules (pink) but shows increases in substitutions with surfactant oxygens (red), chloride (gold), and other hydronium ions. The pairings are defined by the 1^st solvation shell defined by the radial distribution function. As the substituted pairings increase, we see an increase in Zundel structures (δ<0.2 for Zundel and otherwise for Eigen ^[67] ) with the other water molecules.