Neutron diffraction with isotopic substitution (NDIS) experiments were done on both natural abundance potassium and isotopically labeled 41KCl heavy water solutions to characterize the solvent structuring around the potassium ion in water. Preliminary measurements suggested that the literature value for the coherent neutron scattering length (2.69 fm) for 41K was significantly in error. This value was remeasured using a neutron powder diffractometer and found to be 2.40 fm. This revision increases significantly the contrast between the natural abundance K and 41K by about 30% (from 1.0 to 1.3 fm). The experimentally determined structure factor of the potassium ion was then compared to that calculated from molecular dynamics (MD) simulations. Previous neutron scattering measurements of potassium gave a solvation number of 5.5 (see below). In this study, the NDIS and MD results are in good agreement and allowed us to derive a coordination number of 6.1 for water molecules and 0.8 for chloride ions around each K+ ion in 4 molal aqueous KCl solution.

Department of Chemistry Georgetown University Box 571227 Washington DC 20057 United States

Department of Food Science Cornell University Ithaca New York 14853 United States

H H Wills Physics Laboratory University of Bristol BS8 1TL Bristol U K

Institut de Minéralogie de Physique des Matériaux et de Cosmochimie UMR 7590 CNRS Sorbonne Université Campus Pierre et Marie Curie 4 Place Jussieu 75005 Paris France

Institut Laue Langevin 71 Avenue des Martyrs 38042 Grenoble Cedex 9 France

Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic and Center for Biomolecules and Complex Molecular Systems 16610 Prague 6 Czech Republic

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