Since the publication of the previous volumes many new aspects of the
physical and life sciences have been developed in which the properties
of water play a dominant role. Although, according to its preface,
Volume 5 was to be the last one of the treatise, these recent
developments have led to a revision of that statement. The present
volume and its companion, still in preparation, deal with topics that
were already mentioned in the preface to Volume 5 as gaining in
importance. The recent development of X-ray and, more particularly,
neutron scattering techniques have led to studies of "structure" in
aqueous solutions of electrolytes on the one hand, and to the role of
water in protein structure and function on the other. Both these topics
have reached a stage where reviews of the present state of knowledge are
useful. The application of ab initio methods to calculations of
hydration and conformation of small molecules has a longer history, but
here again a critical summary is timely. The role of solvent effects in
reaction kinetics and mechanisms should have had a place in Volume 2 of
this treatise, but, as sometimes happens, the author who had taken on
this task failed tQ live up to his promise. However, since 1972 the
physical chemistry of mixed aqueous solvents has made considerable
strides, so that the belated discussion of this topic (by a new author)
is built on evidence that was not available at the time of publication
of Volume 2.