UDC 596.8 An examination of the properties of solutions and a geometric analysis of the redistribution of the contacts have been used to distinguish the concentration region in which the ordered structure of the solution is based on the water structure, and the region in which there exist polymeric forms made up of both ions and water molecules, and the boundary between them. A structural model has been proposed for the concentration rearrangement, in which the number of contacts of the second weakly conducting ionic subsystem increases more rapidly than the number of ions in the solution. This explains the molecular factors responsible for the appearance of a specific electrical conductivity maximum on the concentration scale and a number of other properties of solutions. The separation of the ranges of concentrated solutions of electrolytes at the present time is somewhat arbitrary and depends on the sensitivity of different experimental procedures or the nature of the change in the concentration dependence of a given property. Thus an important question is how it is possible, on the basis of internal molecular factors, to distinguish different concentration zones in a solution. The first of these is the water-like region, where the structure of the solution is based on the ordered structure of water [1-4]. The upper boundary of its existence for a uniform distribution of the ions in the solution can be found geometrically using modelsof the introduction of ions and complexes into the structure of water according to the data of [1-3]. This limit is determined by the number of water molecules necessary for each dissolved particle (or hydrate or ionic complex formed by it), replacing a water molecule in the sites of the water framework, to have its own environment of water molecules with preservation of the original type of packing in it. At these concentrations of the electrolyte, a water molecule in the first sphere of the ion or complex forms hydrogen bonds with three other water molecules, and this is sufficient to preserve the pattern of the ordered structure and to create a single network of bonds throughout the entire volume. In this scheme, five water molecules, joined by hydrogen bonds to form a tetrahedron, are regarded as the single residual fragment of the water structure in the solution, in which, for the central molecule, first-sphere and second-sphere correlations in the arrangement of the particles, similar to those for pure water, are preserved. Its stabilityinsolut ions with fairlyhigh concentrations can apparently be related to the specific features of the short-range hydrogen bonds between the water molecules, compared with the ion-ionbonds. The electrolyte concentrations corresponding to this geometric limit for a number of solutions are given in Table 1. Account was also taken of the water molecules in the sites if they were present in the hydrate complex, which was not required for the description of the bulk properties [1, 2] (since the values of the molar volumes in a first approximation were the same, irrespective of whether the water molecule was present in the site in the hydrate complex of the ion or not). In the case of the strongly hydrated anions SO~-, OH-, and F-, which form short hydrogen bonds with water molecules, the possible appearance of associates S(~4--H20 (with the transfer of a water molecule into a cavity), F--H20 , and OH--H20 (with a water molecule in a neighboring site) is assumed [3]. The position of the ion in the cavity is assumed to be hydrophobized to a certain extent, so that it is assumed somewhat arbitrarily that the particles in the cavities should not be taken into account in the determination of the concentration boundary being considered. Table 1 gives the corresponding geometric limits for several versions of the arrangement of the ions. For CsC1 solutions, where a high proportion of ion pairs was detected using x-rays [5], a boundary at higher concentrations, realized when association takes place, is indicated. The second upper geometric limit for the introduction of ions into the water structure is given by the concentration corresponding to the filling of all its cavities, with the arbitrary assumption that the ions which are