W0183

Structural Patterns of Water Aggregation in Heteromolecular Solids. Janusz Lipkowski, Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44, 01 224 Warszawa, Poland, e-mail: klatrat@ichf.edu.pl

The problem of interaction between organic and water moieties (neutral or ionized water molecular species) is of particular interest in chemistry in view of its implications to physicochemical behavior of chemical and biological systems. The paper will report on hydration patterns found experimentally in the systems composed of organic species and water. Interrelation between hydrophilic and hydrophobic modes of hydration will be discussed with emphasis on possible influence of hydration on complexation equilibria and transport properties in aqueous media.

In the crystal structures of supramolecular hydrates two different modes of hydration co-exist. These are: hydrophilic hydration, in which water is H-bonded either to, typically, oxygen or nitrogen atoms of the guest species and hydrophobic hydration in which water molecules enclathrate lipophilic parts of guest moieties.

Structural studies of interaction between water and hydrophobic species have revealed a fascinating variety of the guest-host architecture. Water molecules aggregate to form all possible structure types ranging from small water clusters through one-dimensional chains or two-dimensional layers to extended three-dimensional networks of zeolite or clathrate structural type. The formation of channel structures has also been found. Host component may bear electric charge and the ionic species can replace water molecules in the host skeleton. The simplest pattern of this sort may be obtained with ionized water (H₃O⁺ or OH^-) or halogenide anions substituting some of the water molecules. Anions may, in some particular situations, be located within the free space between the framework forming species, and are thus able to diffuse through the crystal lattice.

The most intriguing factor stabilizing the structures is hydrophobic hydration. It consists of multimolecular combination of weak intermolecular interactions which apparently are the structure determining factor, however identifiable with difficulty. The guest species play the important role of stabilizing a host framework, thus serving as a template in the crystal structure formation.

The interplay between solvophobic and solvophilic interactions will be exemplified by a series of supramolecular hydrates, as illustrated here by the structure of the glycylglycine/water/18-crown-6 ternary compound, depicted in Figure below. It shows two distinct types of the water-complex interaction. One is 'typical' hydrogen bonding between water molecules and the hydrophilic end groups of the peptide (i.e. the carboxylic end group, the peptide existing in the zwitterionic form in the structure). The other is hydrophobic hydration, i.e. clathration of the hydrophobic part of the complex by the supramolecular assembly of water molecules.