9 - Highly ionic hydroxides: unexpected proton conductivity in Mg(OH)₂ and homologues

Summary

Introduction

Hydrogen bonding is considered a prerequisite for proton conduction. The underlying rationale is that protons can jump with relative ease only when the energy barriers between donor and acceptor sites are low. The corollary is that, when energy barriers are high, proton conductivity is not expected to occur.

Since the idea of low energy barriers is intuitively appealing, the search for proton conductors has concentrated worldwide on systems with recognizable H-bonding, both in inorganic systems and biological membranes. This has created a bias which may have prevented the recognition of some important proton transport mechanisms that appear to be present in non H-bonded system.

In this chapter, I shall discuss highly ionic hydroxides that are probably the least likely candidates for proton conduction. However, on closer inspection, they reveal an interesting fundamental behaviour that may be important for understanding proton transport in other systems, for instance across bilayer membranes containing inner sections that are considered to be proton impermeable.

Non-hydrogen bonded systems

Inorganic hydroxides with the highest degree of ionicity are the binary hydroxides of the alkali and alkaline earth metals, Me^IOH and Me^II(OH)₂ with Me^I = Na⁺, K⁺ …, and Me^II = Mg²⁺, Ca²⁺ …, respectively. They all contain well-defined, essentially non H-bonded OH^– anions.

Most alkali hydroxides undergo a rotational transition prior to melting.