Environment of Ca²⁺ ions in aqueous solvent

Abstract

The ion Ca²⁺ is common in nature and has an important role in many biochemical processes¹. However, the way in which this ion behaves in aqueous solution is the subject of much controversy. There are two structural parameters which are necessary for a fuller understanding of Ca²⁺ in solution: the number of water molecules to which Ca²⁺ coordinates in solution, n̄, and the configuration which a water molecule adopts relative to it (defined by Φ, the angle between the plane of the water molecule and the Ca–O axis and r_CaO, the Ca–O separation). The technique of neutron diffraction in conjunction with isotopic substitution has been successful in giving unambiguous information regarding, n̄, Φ and r for a variety of aqueous solutions². Studies have been carried out on Li⁺, Ni²⁺ and Cl⁻ at several concentrations³ and Ca²⁺ at one concentration⁴. We report here the results of a study of the structure of the Ca²⁺–D₂O conformation as a function of concentration.