Charge transport by solitons in hydrogen-bonded materials

Theoretical calculation of proton mobility for collective surface proton transport

We present a theoretical study of surface proton mobility at a minimally hydrated array of protogenic surface groups. At dense packing, the array assembles into a 2D bicomponent lattice that is formed by sulfonate anions, which are only allowed to fluctuate about fixed equilibrium positions, and mobile hydronium ions. Proton transport on the lattice proceeds by collective translocations of hydronium ions. This type of motion is described within the framework of soliton theory. Our main objective in this article is to establish the relation between microscopic surface structure and effective proton mobility. To this end, we present an approach to calculate microscopic interaction parameters that determine hydronium ion motion. The developed formalism enables us to theoretically derive an expression for soliton mobility at a given surface structure and compare it with experimentally measured mobilities.

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

We present a theoretical study of collective proton transport at a 2D array of end-grafted protogenic surface groups with sulfonic acid head groups. The graft positions of the surface groups form a regular hexagonal lattice. We consider the interfacial array at a high packing density of the surface groups and under minimal hydration with one water molecule added per head group. For these conditions, the stable interfacial conformation consists of a bicomponent lattice of hexagonally ordered sulfonate anions and interstitial hydronium cations. Hydronium ion motion occurs as a travelling solitary wave. We analyse the microscopic parameters of the solitons and study the influence of different potential profiles on the proton motion created by rotation and tilting of sidechains and sulfonate anions. Using soliton solutions of the equation of motion, we establish relations between the energy and mobility of the solitons and the microscopic structural and interaction parameters of the array.

Proton relay in a one-dimensional hydrogen-bonded chain composed of water molecules and a squaric acid derivative

The crystal structure of a hydrated crystal of bis(squaryl)biphenyl (BSQB*4H2O), in which two squaric acid moieties are connected with a 4,4'-biphenyl unit, was characterized by the presence of a one-dimensional hydrogen-bonded chain composed of BSQB and water molecules. X-ray crystallographic analysis showed that BSQB exists in a dianion form and that, on average, two of the four water molecules are protonated. The enhanced temperature dependence of the thermal parameters of the oxygen atoms of the water molecules suggested dynamic disorder of the water molecules. The solid-state magic angle spinning deuterium NMR spectrum of BSQB*4D2O revealed that deuterons are exchanged between heavy water molecules and oxonium ions with an exchange rate of ca. 700 Hz around 250 K and that deuterons start to migrate in a hydrogen-bonded cluster of water molecules. Ac dielectric measurements were also used to examine the dynamic process in the hydrated crystal. The dielectric permittivity of the crystal dramatically increased above 250 K with a distinct frequency dependence (epsilon' = 4.7 x 10(4) at 340 K and 1 kHz). The frequency dependence of tan delta at 290 K exhibited a maximum at 3.0 kHz, and this maximum shifted to lower frequencies when the temperature of the crystal decreased. These experimental results suggested that in the one-dimensional hydrogen-bonded chain of BSQB*4H2O a proton relay between oxonium ions and water molecules occurred within a cluster of four water molecules and that the relay was transmitted to the adjacent cluster mediated by the modulation of the negative charge distribution of the BSQB dianion. These phenomena were interpreted as the solitonic migration of the charged domain boundaries along the one-dimensional hydrogen-bonded chain.