Inﬂuence of Concentration and Anion Size on Hydration of H + Ions and Water Structure † R. Mancinelli, A. Sodo, F. Bruni, and M. A. Ricci* Dipartimento di Fisica “E. Amaldi”, UniVersita ` degli Studi “Roma Tre”, Via della Vasca NaVale 84, 00146 Roma, Italy A. K. Soper ‡ ISIS Facility, Rutherford Appleton Laboratory, Harwell Science and InnoVation Campus, Didcot, Oxfordshire, OX11 0QX United Kingdom ReceiVed: June 13, 2008; ReVised Manuscript ReceiVed: July 22, 2008 Neutron diffraction experiments with hydrogen isotope substitution on aqueous solutions of HCl and HBr have been performed at concentrations ranging from 1:17 to 1:83 solute per water molecules, at ambient conditions. Data are analyzed using the empirical potential structure reﬁnement technique in order to extract information on both the ion hydration shells and the microscopic structure of the solvent. It is found that the inﬂuence of these solutes on the water structure is less concentration dependent than that of salts or hydroxides. Moreover protons readily form a strong H-bond with a water molecule upon solvation, at all proportions. The majority of them is also bonded via a longer bond to another water molecule, giving a prepeak in the g OwOw . At high solute concentration, the second water molecule may be substituted by the counterion. In particular at solute concentrations of the order of 1:17 or higher, all protons have an anion within a distance of 4.5 Å. I. Introduction The abnormal diffusion of water ions, namely H + and OH - , in water 1,2 is currently rationalized as a Grotthus process, or structural diffusion as opposed to the mass diffusion process, or Stokes diffusion, undergone by all other ions in solution. This process is governed by thermally assisted proton transfer between two water molecules along an hydrogen bond (H-bond); consequently the local structure around the ion and how well this ﬁts into the extended H-bond network is one of the key issues which may promote or limit the process. Concerning this point we have seen an intense debate in the literature on whether hydronium ions (H 3 O + ) preferentially coordinate into Eigen (H 9 O 4 + ) or Zundel (H 5 O 2 + ) complexes. 3-16 On the other hand, recent ab initio simulations of an excess proton in water 7,16 have shown that both complexes are needed in order to describe the Grotthus process generated within the simulation, as the proton transfer proceeds via a structural ﬂuctuation from an Eigen- like complex, through a Zundel-like one, to eventually form an Eigen-like complex with another water molecule. Recent analysis of neutron diffraction data from a concen- trated HCl aqueous solution 17 has highlighted the difﬁculty of making a clear distinction between Eigen and Zundel complexes, due to the continuous random network of H-bonds formed between water molecules and hydrated protons. In that work the experimental data have been analyzed through the empirical potential structure reﬁnement code, (EPSR) 19,20 that is a Monte Carlo (MC) simulation, which equilibrates an ensemble of molecules in conﬁgurations compatible with the experimental data. In doing so three distinct hypothesis on the proton solvation have been considered: ﬁrst that all protons form hydronium complexes, second that all of them form instead Zundel complexes, and third that the H + ions have been left free to equilibrate within the simulation box toward their preferred hydration shell. All three hypothesis allow ﬁtting of the experimental data and give consistent results, but interestingly, it is not unusual to ﬁnd Zundel-like complexes in the ﬁrst simulation or Eigen-like in the second. Moreover the third simulation reveals that all protons readily form an H-bond with a water molecule, and a large amount of them are bonded to two water molecules: whether this hydrated proton could be called an Eigen or a Zundel complex is largely a question of deﬁnition, also in view of the large distortions of the structure of these complexes shown in the simulation and the high solute concentration considered at that time. On the other hand experimental studies of proton hydration can only be performed by solvation of acids and consequently the counterion of an H + in these studies may be Cl - , as in refs 11 and 17 or any other anion, but OH - . This raises the question of the inﬂuence of the counterion on the hydronium solvation shell and on the solvent structure, as a function of the solute concentration. The latter issue, in particular, has been studied in a variety of aqueous solutions, 11,21-25 and the effect as- similated to that of an external pressure applied to pure water: a similarity that has been recently conﬁrmed by molecular dynamics (MD) simulations as far as the dynamical properties of water in the presence of ions are concerned. 26 Finally a study performed at different solute concentrations can elucidate the microscopic mechanism which limitates the equivalent conductance of these solutions at high concentra- tion. 27,28 Here we address the issues outlined above, by studying the microscopic structure of aqueous solutions of HCl and HBr as a function of solute concentration. † Part of the special section “Aqueous Solutions and Their Interfaces”. * To whom correspondence should be addressed. E-mail: riccim@ ﬁs.uniroma3.it. ‡ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom. J. Phys. Chem. B 2009, 113, 4075–4081 4075 10.1021/jp805220j CCC: $40.75  2009 American Chemical Society Published on Web 09/23/2008 Downloaded by INIST TITAN SCIENCES on August 11, 2009 Published on September 23, 2008 on http://pubs.acs.org | doi: 10.1021/jp805220j