Surface Charge Layer of Amorphous Solid Water with Adsorbed Acid or Base: Asymmetric Depth Distributions of H⁺ and OH^– Ions

Abstract

The charge at the surface of water and the resultant surface voltage play an important role in many natural phenomena and technological applications. However, the relationship between surface charge and the interfacial distribution of H + and OH − ions remains unclear. We measured the surface voltage produced by an ionized acid or a base at the surface of amorphous solid water (ASW) using a Kelvin work-function probe and studied the depth distributions of H + and OH − ions. H + ions were distributed over a thicker region from the surface than OH − ions, although both ions reside preferentially at the surface. This difference led to the formation of opposite surface charges in the presence of the acid or base. The deeper penetration of H + ions is attributed to efficient proton transport dynamics in the lattice and the resultant dynamic delocalization of protons. The study demonstrates that the asymmetric H + and OH − distributions may be important to understand the electrical and acid−base properties of ASW and crystalline ice surfaces and, possibly, those of the liquid water surface as well.

Tunneling Diffusion of Excess Protons in Amorphous Solid Water at 10 and 80 K

“…However, the hydronium ions could undergo thermal diffusion and accumulate at the surface of a growing ASW film at high temperature because of their thermodynamic affinity for the ice surface. ,, For this reason, the possibility of upward diffusion of hydronium ions at 80 K was worth investigating. The necessary information could be obtained from the measurement of the surface voltages of HCl-doped ASW films, as described by Lee et al An analysis of the changes in the film voltage with the growth of the ASW overlayer on the HCl-adsorbed ASW surface at 95 K (Figure 5 in ref ) showed that the average penetration depth of excess protons into the ASW overlayer was 3.5 ML when the HCl adsorbate coverage was 0.04 ML. This penetration depth decreased with an increase in HCl coverage, probably as a result of electrostatic interactions between H + and Cl – ions.…”

“…The mobility of excess protons (positive protonic defects) in ice is intimately related to the diverse physical , and chemical properties of ice . The general concept of proton transport in ice was originally formulated by Jaccard , and Onsager and is now well accepted through extensive experimental verification. − The excess protons in ice move along its hydrogen-bonded network via an efficient hopping relay (Grotthuss) mechanism. , The proton passage along the hydrogen-bonded water chain polarizes the direction of the hydrogen bonds, which blocks the passage of the subsequent protons hopping along the same path. Therefore, for a continuous flow of protons, reorientation of water molecules (Bjerrum defect motion) must take place to depolarize the hydrogen-bond direction.…”

“…An interesting question is whether proton hopping can occur in ice at extremely low temperatures, i.e., when the lattice thermal vibration is minimized and not readily available to assist in the proton transfer. The proton conductivity in ice is closely related to various ice phenomena at low temperatures, including proton order/disorder phase transitions, charge distributions at the ice surface and interior, and proton transfer reactions in ice . However, to our best knowledge, direct evidence has not been reported as yet for the mobility of excess protons in cryogenic ice near 0 K. There are some observations that indirectly support proton mobility under these conditions. ,− For example, infrared spectra of acid-doped amorphous solid water (ASW) samples showed an intense absorption of the Zundel continuum, which could be related to concerted proton transfers in the lattice. ,, The surface voltage measurement of ice films with adsorbed acids showed that the positive protonic charge was spread over a considerable depth from the surface, which was possibly due to fluctuating excess protons near the ice surface .…”

“…The proton conductivity in ice is closely related to various ice phenomena at low temperatures, including proton order/disorder phase transitions, charge distributions at the ice surface and interior, and proton transfer reactions in ice . However, to our best knowledge, direct evidence has not been reported as yet for the mobility of excess protons in cryogenic ice near 0 K. There are some observations that indirectly support proton mobility under these conditions. ,− For example, infrared spectra of acid-doped amorphous solid water (ASW) samples showed an intense absorption of the Zundel continuum, which could be related to concerted proton transfers in the lattice. ,, The surface voltage measurement of ice films with adsorbed acids showed that the positive protonic charge was spread over a considerable depth from the surface, which was possibly due to fluctuating excess protons near the ice surface . Spontaneous dissociation of halogenated acetic acids has been observed in ASW at 10–140 K, indicating efficient proton injection and migration in the lattice. , Notably, although slightly different from the transport of excess protons, concerted motion of multiple protons has been observed in pure ice samples at 5 K .…”

“…29 An interesting question is whether proton hopping can occur in ice at extremely low temperatures, i.e., when the lattice thermal vibration is minimized and not readily available to assist in the proton transfer. The proton conductivity in ice is closely related to various ice phenomena at low temperatures, including proton order/disorder phase transitions, 30 charge distributions at the ice surface and interior, 22 and proton transfer reactions in ice. 3 However, to our best knowledge, direct evidence has not been reported as yet for the mobility of excess protons in cryogenic ice near 0 K. There are some observations that indirectly support proton mobility under these conditions.…”