HCN is a molecule central to interstellar chemistry, since it is the simplest molecule containing a carbon-nitrogen bond and its solid state chemistry is rich. The aim of this work was to study the NH3 + HCN -> NH4+CN- thermal reaction in interstellar ice analogues. Laboratory experiments based on Fourier transform infrared spectroscopy and mass spectrometry were performed to characterise the NH4+CN- reaction product and its formation kinetics. This reaction is purely thermal and can occur at low temperatures in interstellar ices without requiring non-thermal processing by photons, electrons or cosmic rays. The reaction rate constant has a temperature dependence of k(T) = 0.016+0.010-0.006 s-1.exp((-2.7+-0.4 kJmol-1)/(RT)) when NH3 is much more abundant than HCN. When both reactants are diluted in water ice, the reaction is slowed down. We have estimated the CN- ion band strength to be A_CN- = 1.8+-1.5 x10-17 cm molec-1 at both 20 K and 140 K. NH4+CN- exhibits zeroth-order multilayer desorption kinetics with a rate of k_des(T) = 10^28 molecules cm-2 s-1.exp((-38.0+-1.4 kJmol-1)/(RT)). The NH3 + HCN -> NH4+CN- thermal reaction is of primary importance because (i) it decreases the amount of HCN available to be hydrogenated into CH2NH, (ii) the NH4+ and CN- ions react with species such as H2CO, or CH2NH to form complex molecules, and (iii) NH4+CN- is a reservoir of NH3 and HCN, which can be made available to a high temperature chemistry.
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