This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution 4.0 International License. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz. Ion-Solvent and Solvent-Solvent Interactions. X-ray Study of Aqueous Alkali Chloride Solutions G. Pálinkás, T. Radnai, and F. Hajdu Central Research Institute for Chemistry of Hungarian Academy of Sciences, Budapest Z. Naturforsch. 35a, 107-114 (1980); received November 19, 1979 The first neighbour model, FNM, used frequently for the description of hydration has been modified by dropping the assumption that the structure of the "free" solvent is identical with that of the pure solvent. The modified model, FNM2, reproduces the experimental X-ray structure functions of alkali chloride solutions quite well and enables the study of perturbed solvent- solvent interactions. The refined structural parameters reasonably indicate the dependence of the hydration of the alkali cations on their radius and concentration. The hydration of Cl~ has been found to be almost independent of both concentration and the type of counter cation. The model assumes regular symmetry for the first neighbour coordination sphere, but the high rms deviations of the water-water distances within the shells of the aggregates indicate con- siderable individual asymmetries. A further modification of the FNM will be discussed in a following paper. Introduction One way to describe the structure of multi- component liquids is their characterization by atom pair correlation functions. Since 1-1 aqueous electrolyte solutions can be considered as four component liquids (0, H, C + , A - ), the total struc- ture functions obtained from scattering experi- ments have to be regarded as weighted sums of ten partial structure functions. The treatment can be simplified by taking the solvent molecule (s) as a unit thus reducing the number of components to three (S, C + , A - ) and the number of terms in the total structure function to six. Also, the contribu- tions of the ion-ion interactions to the diffraction pattern of electrolyte solutions can often be neglected [1], In the past decade numerous attempts have been made to extract the contributions of ion-solvent interactions from the measured total structure functions, either approximately or exactly. For this purpose first neighbour models (FNM) w r ere intro- duced and difference methods such as isotopic sub- stitution and isomorphic replacement [2, 3, 4, 5] have been developed. The main object of the present work was to test a certain version of the first neighbour model for describing the ionic hydration in concentrated aqueous alkali chloride solutions. The experimental basis for this investigation was provided by the X-ray diffraction studies of the authors. Reprint request to Dr. G. Pälinkäs, P.O.B. 156, H-1431 Budapest, 8, Ungarn. First Neighbour Models (FNM) The first version of this model (FNM1) was introduced by Narten, Vaslow, and Levy [2] in their extensive X-ray and neutron diffraction studies on aqueous lithium chloride solutions. The basic assumptions of FNM 1 are the following: a) Two states of water are distinguished: water bound in the hydration shells and water outside the hydration shells ("free" water), b) hydration shells comprise only nearest neigh- bours, c) hydration shells are imbedded in "free" water, d) hydration shells exhibit the highest degree of symmetry (water molecules occupying the vertices of regular polihedra), e) the structure of "free" water is identical with that of pure water. In terms of FNM1 the structure function of a solution can be written as H(k) = Xc^c(k)+XAJfA(k) + Xt^(k), (1) where A c , X A and Xt are the mole fractions of the aggregates and "free water", respectively. J^c(k), J^A (k) are structure functions of ion-water aggre- gates and (k) is the structure function of pure water, where k denotes the scattering variable k = (47tß) sin(#/2). X is the wavelength of the primary and coherently scattered radiation, and & the scattering angle. 0340-4811 / 80 / 0100-0107 $ 01.00/0. - Please order a reprint rather than making your own copy.