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Interstellar complex organic molecules (COMs) are commonly observed duringstar formation, and are proposed to form through radical chemistry in icy grainmantles. Reactions between ions and neutral molecules in ices may provide analternative cold channel to complexity, as ion-neutral reactions are thought tohave low or even no energy barriers. Here we present a study of a the kineticsand mechanisms of a potential ion-generating acid-base reaction betweenNH$_{3}$ and HCOOH to form the salt NH$_{4}^{+}$HCOO$^{-}$. We observe saltgrowth at temperatures as low as 15K, indicating that this reaction is feasiblein cold environments. The kinetics of salt growth are best fit by a two-stepmodel involving a slow "pre-reaction" step followed by a fast reaction step.The reaction energy barrier is determined to be 70 $\pm$ 30K with apre-exponential factor 1.4 $\pm$ 0.4 x 10$^{-3}$ s$^{-1}$. The pre-reactionrate varies under different experimental conditions and likely represents acombination of diffusion and orientation of reactant molecules. For adiffusion-limited case, the pre-reaction barrier is 770 $\pm$ 110K with apre-exponential factor of $\sim$7.6 x 10$^{-3}$ s$^{-1}$. Acid-base chemistryof common ice constituents is thus a potential cold pathway to generating ionsin interstellar ices.

Kinetics and mechanisms of the acid-base reaction between NH$_3$ and HCOOH in interstellar ice analogs