Acid Dissociation Writ Small

How many water molecules does it take to dissolve one molecule of hydrochloric acid? Just four, say the scientists who created the minimalist collections of atoms at extremely cold temperatures (Science 2009, 324, 1545). The work lays bare the fundamental process of acid dissociation, unfettered by variables like extra molecules and heat. An international team led by Martina Havenith and Dominik Marx of Ruhr University, in Bochum, Germany, used protective superfluid helium droplets to encapsulate single HCl molecules with H₂O molecules at temperatures below 1 K. On the basis of an infrared spectral signature and in conjunction with ab initio calculations of H₃O⁺ in the dissociated H₃O⁺(H₂O)Cl^– ion pair, they determined that only four water molecules are needed to cause one HCl molecule to spontaneously dissociate. The results could help scientists understand, for example, how HCl dissolves on ice surfaces—a phenomenon relevant to ice-particle chemistry in the atmosphere, notes Yale University physical chemist Mark A. Johnson. "These clusters offer a way to look at what is going on in a regime where we can understand the origin of barriers at the molecular level, where theory is actually predictive," Johnson says.