WATER
`
`A COMPREHENSIVE TREATISE
`
`Edited by Felix Franks
`Unilever Research Laboratory
`Sharnbrook, Bedford, England
`
`Volume 1
`The Physics and
`Physical Chemistry of Water
`
`c.±?PLENUM PRESS • NEW YORK-LONDON •1972
`
`ETHICON EXHIBIT 1019
`
`

`

`Library of Congress Catalog Card Number 78-165694
`ISBN 0-306-37181-2
`
`© 1972 Plenum Press, New York
`A Division of Plenum Publishing Corporation
`227 West 17th Street, New York, N.Y. 10011
`
`United Kingdom edition published by Plenum Press, London
`A Division of Plenum Publishing Company, Ltd.
`Davis House (4th Floor), 8 Scrubs Lane, Harlesden, London, NWIO 6SE, England
`
`All rights reserved
`
`No part of this publication may be reproduced in any
`form without written permission from the publisher
`
`Printed in the United States of America
`
`

`

`Contents
`
`Chapter 1
`
`Introduction-Water, the Unique Chemical
`F. Franks
`
`Introduction
`1.
`The Occurrence and Distribution of Water on the Earth
`2.
`3. Water and Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4.
`The Scientific Study of Water-A Short History . . . . . . . .
`5.
`The Place of Water among Liquids . . . . . . . . . . . . . . . . . . . .
`
`2
`4
`8
`13
`
`Chapter 2
`
`The Water Molecule
`C. W. Kern and M. Karplus
`
`I.
`2.
`
`3.
`
`4.
`5.
`
`6.
`
`Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`Principles of Structure and Spectra: The Born-Oppenheimer
`Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`The Electronic Motion
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`3.1. The Ground Electronic State of Water...... ......
`3.2. The Excited Electronic States of Water . . . . . . . . . . .
`The Nuclear Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`External-Field Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`5.1. Perturbed Hartree-Fock Method..... .. ........ ...
`5.2. Perturbed Configuration Interaction Method . . . . . . .
`Conclusion
`
`21
`
`22
`26
`31
`50
`52
`70
`74
`78
`80
`
`vii
`
`

`

`CHAPTER I
`
`Introduction-Water, The Unique Chemical
`
`F. Franks
`Unilever Research Laboratory Colworth/Welwyn
`Co/worth House
`Shambrook, Bedford, England
`
`1. INTRODUCTION
`
`The physical nature of water, its role in life processes, and its occurrence,
`distribution, and turnover present a wide spectrum of problems to physical
`and life scientists and engineers. Although each specialist is normally con(cid:173)
`versant with such issues as can be formulated in the particular terminology
`of his own discipline, few people have a full appreciation of the influence
`which water has had, and will have, on ecological, sociological, and in(cid:173)
`dustrial developments.
`It is not the function of this volume to survey in depth subjects such
`as water conservation, purification, or pollution, or technologies based on
`changing the state of water in a variety of natural and synthetic products.
`Rather, it is our aim to review the present state of knowledge concerning
`the nature of the molecular interactions in the liquid, and to suggest areas
`where more information is yet required. At the same time it must be borne
`in mind that there are vast areas where it is hoped that any success achieved
`by physicists and physical chemists can be translated into applications and
`it is mainly for this reason that this introductory chapter contains a brief
`summary of the manner in which our total water supplies ate distributed,
`utilized, and turned over. Similarly, a short factual account of those life
`processes in which water plays an important role suggests that a better
`understanding of the nature of water-solute interactions at the molecular
`level may prove to be of considerable help in the elucidation of problems
`
`1
`
`

`

`2
`
`Chapter 1
`
`such as enzyme catalysis, biological mass transport, and the formation of
`biological structures from molecules and molecular aggregates.
`Probably because of the ubiquity of water in our environment, the
`general realization that this liquid had many unique features worth closer
`study came relatively recently and this is highlighted in Section 4 dealing
`with the history of the scientific study of water. However much interest is
`now shown in problems of water structure, we can only hope to advance
`our knowledge of this subject at the same rate at which progress is made
`in the general understanding of the liquid state. The final section of this
`chapter therefore discusses briefly the concepts and techniques which have
`been applied to the characterization of the liquid state in terms of bulk
`properties and molecular correlations and potentials. A comparison of
`the thermodynamic and transport properties of water in the solid and liquid
`states with those of a number of simple substances suggests that, in many
`ways, the behavior of water is more or less what might be expected from an
`assembly of H20 molecules. Differences between water and simple fluids
`are often subtle, but all the same they have far-reaching implications in
`regard to the conditioning of our terrestrial environment.
`
`2. THE OCCURRENCE AND DISTRIBUTION OF WATER ON THE
`EARTH
`
`The available water is distributed over the face of the earth in a very
`uneven manner. Thus, less than 0.027% of the total water is fresh and
`immediately available. In fact, most of the fresh water is locked up in the
`Arctic and Antarctic ice caps and, with the world demand for fresh water
`constantly increasing, it has been suggested that the towing of icebergs to
`the temperate zones is quite feasible and, in terms of economics, compares
`favorably with all presently known desalination processes. Thus, it has been
`calculated120> that of an iceberg measuring 2700 x 2700 x 250m, towed at
`a speed of half a knot from the Amery ice shelf to Australia, 30% would
`arrive intact. The water would be worth $ 5.5 million, i.e., about 10%
`of the cost of a similar quantity of desalinated water, compared with a
`towing cost of $ I million.
`Ninety seven percent of the water available on the earth is found in
`the large oceans. Thus, the oceans cover an area of 3.6 x 108 km2 and
`contain 13 x 108 km3 of water. Compared to this, the lower seven miles of
`the earth's atmosphere only contain 13 x 103 km3, i.e., 0.000053% of the
`total water. The main process of the hydrological cycle consists of evapora-
`
`

`

`Introduction-Water, The Unique Chemical
`
`3
`
`tion from the oceans and subsequent precipitation and runoff back into
`the oceans. The annual turnover of water amounts to 3.5 x 105 km3 (3.5
`x I014 tons). The Antarctic ice cap, which covers 1.5 x 107 km 2, makes up
`the largest volume of fresh water (2.5-2.9 x 107 km 3). If melted, it could
`supply all the earth's rivers for 830 years. By comparison, the Greenland
`ice cap is quite insignificant; it contains 2.6 x 106 km3 of water. However,
`in terms of the available fresh water this is still a very large volume since,
`if it were melted, it could supply river systems such as the Amazon or
`Mississippi for 4000-5000 years. The total water locked up in glaciers
`amounts to only 2.I x I05 km3• Just as glaciers can be compared with rivers,
`so the ice caps can be compared with lakes, in that they cover the landscape
`and flow radially outward.
`The world's great lakes contribute a minor but important amount of
`the available surface fresh water, totaling 1.2 x I 05 km3• More than half
`of this volume comes from the four largest lakes: Baikal (26,000), Tanga(cid:173)
`nyika (20,000), Nyasa (13,000), and Superior (12,000). A volume of water
`equal to that in the fresh-water lakes also exists in saline lakes, of which
`the Caspian Sea is by far the most important (78,000 km3). On the con(cid:173)
`tinents of Asia, America, and Africa, 75% of the fresh surface water is
`accounted for by lakes. The corresponding figure for Europe is only 2%.
`This is one of the reasons why, in spite of the high density of population,
`the problems connected with pollution and the adequate supply of fresh
`water are not as pressing in Europe as they are in America.
`Although the amount of fresh water present in all the rivers of the world
`is only 1200 km 3, the annual runoff into the oceans amounts to 34,400 km3•
`Of this, the three largest river systems, the Amazon, Congo, and Mississippi,
`respectively, contribute 5IOO km3• The fact that the rivers of the United
`States discharge fresh water into the sea at the rate of 4.9 x I04 m3 sec-1
`dramatically illustrates the large turnover of water.
`The underground reservoirs provide further supplies of fresh water.
`The topmost layer is a saturated zone in which the liquid is held by capillary
`forces. The upper boundary is referred to as the water table and this may
`lie at the land surface, e.g., in swamps, or several hundred meters below,
`as in deserts. Further down is an unsaturated zone from which water per(cid:173)
`colates to the water table. This zone contains about 41,000 km3 of water,
`which is not extractable but which recharges the ground-water reservoirs
`from which the water can be extracted. Altogether, 4.I x I 06 km3 of fresh
`ground water extends down to a depth of I km. Further down, large res(cid:173)
`ervoirs of highly mineralized water exist, but the extraction of these is
`not economically feasible.
`
`

`

`4
`
`Chapter 1
`
`The period for which water remains underground varies from a few
`hours to several hundred years, and at very great depths the water may
`remain for up to ten thousand years. The amount of water in the top layer
`of the earth's crust is equivalent to 4000 times the water in all the earth's
`rivers. Underground water is, however, not wholly self-renewing, and it is
`for this reason that water conservation is now of great importance. It has
`been calculated that for irrigation schemes in arid regions the level of the
`underground water supply may be reduced through pumping by 60 em per
`year, whereas it is only replenished at the rate of 0.5 em per year.
`Finally, probably the most important source of fresh water is rain,
`the distribution of which over the earth is quite nonuniform. As a result
`of rainfall and percolation from the water table to the topsoil, the total
`moisture content of the soil in the world is 25,000 km3• Plants normally
`grow on what is considered to be "dry" land, and it is not generally realized
`that even "dry" dust contains up to 15% of water. It appears that plant
`growth requires extractable water. Thus, an ordinary tree withdraws and
`transpires about 190 liters per day.
`The average annual rainfall in the United States is 75 em, 53 em of
`which is returned to the atmosphere and only 7.5 em is used by man. The
`remainder goes to replenish the underground water reservoirs. Many
`attempts have been made to increase man's control over the rate of precipita(cid:173)
`tion and to retard losses by evaporation, since the problems associated with
`the supply of fresh water are becoming more urgent all the time. Urbaniza(cid:173)
`tion always leads to increases in water usage which far outstrip the rate at
`which the population increases. In some countries, the use and supply of
`water are already subject to government legislation, and as problems
`associated with water pollution become more urgent and receive wider
`publicity, so more economical methods of cleaning waste water and of
`using second-rate water in industrial processes will be developed.
`
`3. WATER AND LIFE
`
`Even a superficial study of liquid water, and to some extent of ice,
`must suggest that life on this planet has been conditioned by its abnormal
`properties, since water was present on this planet long before the evolution
`of life. It is well known that water forms a necessary constituent of the cells
`of all animal and plant tissues and that life cannot exist, even for a limited
`period, in the absence of water, so that we have the somewhat strange
`position that the only naturally occurring inorganic liquid is essential for
`
`

`

`Introduction-Water, The Unique Chemical
`
`5
`
`the maintenance of organic life. Bearing in mind also that natural processes
`are characterized by the economy with which energy (matter) is utilized,
`it seems permissible to conclude that in organisms which consist of up to
`95% water, this liquid fulfills a function other than that of an inert substrate.
`It is, of course, very much harder to elucidate the exact role of water in
`life processes, u043 , although biochemical and medical studies have yielded
`some useful data. Apart from acting as a proton-exchange medium, water
`moves through living organisms and functions as a lubricant in the form
`of surface films and viscous juices, e.g., dilute solutions of mucopoly(cid:173)
`saccharides. Nothing is yet known about the manner in which water acts in
`the formation of organized biological structures at the subcellular, cellular,
`and multicellular levels, and, at the molecular level, the role of water in the
`stabilization of native conformations of biopolymers has only recently been
`receiving some attention. (248•748•1043, The almost complete disregard of the
`role of the solvent in tertiary and quaternary structure phenomena is an
`interesting example of how established experimental findings are sometimes
`ignored because they cannot be reconciled with existing concepts. Thus,
`some time before X-ray techniques were successfully applied to establish
`the structure of DNA, it was well known that the polymer required some
`30% of water to maintain its native conformation in the crystalline state,
`and that partial dehydration led to denaturation. Available X-ray techniques
`cannot "see" the water in biopolymers because of its relatively high mobility
`and therefore when the double helix structure was confirmed it was claimed
`that it owed its stability to intramolecular hydrogen bonds, van der Waals(cid:173)
`type interactions between purine and pyrimidine bases, and electrostatic in(cid:173)
`teractions between sugar phosphate groups. Clearly this cannot be the whole
`story, and further studies will reveal the function of water in the stabiliza(cid:173)
`tion of the double helix. At present the most useful method for observing
`small displacements of molecules or segments of molecules in solution is
`undoubtedly NMR spectroscopy, and the application of this technique to
`the study of water in biopolymer systems shows some promise. (l84, 372•394l
`Although the contribution of water to life processes at a molecular
`level is almost completely unexplored, a considerable amount of data
`exists on the distribution, synthesis, and turnover of water at a more complex
`leveL( 64•343•582l Thus, the water content of living organisms varies between
`the extremes of 96-97% in some marine invertebrates to less than 50%
`in bacterial spores. The adult human has a water content of 65-70%, but
`the water is unevenly distributed; nervous tissue contains 84%, liver 73%,
`muscle 77%, skin 71%, connective tissue 60%, and adipose tissue 30%.
`The water content of biological fluids such as plasma, saliva, and gastric
`
`

`

`6
`
`Chapter 1
`
`JUices is between 90-99.5%. Approximately 45-50% of the organism is
`made up of intracellular water, 5% of plasma water, 30-35% of non(cid:173)
`aqueous matter, and the remainder can be termed interstitial or extracellular
`water. The hydration of an organism changes during its development. A
`human embryo during its first month has a water content of 93% and, as a
`child develops to maturity, so the intracellular water content increases at
`the cost of the extracellular liquid. These levels are maintained constant
`until old age, when the process is reversed.
`Water is the solvent which promotes biological hydrolysis (digestion)
`in which proteins and carbohydrates are broken down; lipids, although not
`actually modified chemically, are solubilized in the aqueous medium. On
`the other hand, biosynthesis of water results from condensation polymeriza(cid:173)
`tion, examples being the production of glycogen from glucose and the
`formation of proteins from amino acids. Thus, the energy required for
`biosynthesis derives partially from the energy of formation of water.
`The study of the properties and functions of water in biological systems
`is complicated by the nature of the medium, which contains polymers and
`colloidally dispersed particles or may be in a gel state. Important fields of
`study include the origin of resistance toward freezing and dehydration shown
`by most animal and plant tissues. This raises the question of the nature of
`1164
`.. bound" water which is currently receiving some attention( 22•148•231 •958•
`l
`and can be characterized by such diverse techniques as osmometry, spec(cid:173)
`troscopy, and adsorption measurements. Another important function of
`water is the thermal regulation of living organisms; its large heat capacity
`coupled with the high water content (45 kg in an adult human) are responsi(cid:173)
`ble for maintaining isothermal conditions and the high thermal conductivity
`of water prevents serious local temperature fluctuations. The high latent
`heat of evaporation permits large losses of heat: the average adult eliminates
`water at a daily rate of 300-400 g by respiration and 600-800 g by cutaneous
`evaporation, with an associated loss of heat amounting to 20% of the total
`heat produced in a day.
`It is well known that living organisms cannot survive without a mini(cid:173)
`mum supply of water, although the tolerance toward dehydration varies
`widely throughout the animal and plant kingdoms. The average daily
`intake of water by an adult is 2.5 liters in the form of drink and solid food.
`Table I shows that of an average daily intake of 1.5 kg of "solid" food,
`57% is actually water. In addition, the average adult produces water en(cid:173)
`dogenously by the combustion of food at a daily rate of 350 g, accompanied
`by a heat liberation of 1.31 kcal. This endogenous production of water is
`surprisingly constant under different physiological conditions and always
`
`

`

`Introduction-Water, The Unique Chemical
`
`7
`
`TABLE I. Average Daily Intake of Food and Its Water Content
`
`Weight of food,
`g
`
`Corresponding water intake,
`g
`
`Bread
`Milk
`Lean meat
`Potatoes
`Vegetable
`Fruit
`Cheese
`Fish
`Meat products (sausage, etc.)
`Fat
`Sugar
`
`300
`200
`100
`300
`150
`50
`60
`60
`80
`40
`40
`
`100
`175
`76
`225
`133
`40
`21
`49
`9.5
`0
`0
`
`takes place via a series of coupled reactions involving cytochrome and
`cytochrome oxidase in which eventually hydrogen and oxygen become avail(cid:173)
`able in a form in which they can combine to yield water.
`Most of the water present in living organisms acts as an irrigant, dis(cid:173)
`tributing nutrients and removing waste products. The internal circulation
`of water proceeds via intestinal absorption, hemodynamic flow, and diuresis.
`Most diseases connected with water result from irregularity in the rate of
`blood flow, the composition of the interstitial and cellular aqueous media,
`and partial dehydration or hyperhydration. The uptake and endogenous
`production of water constitute one part of the biological water cycle which is
`completed by the processes which involve loss of water, namely surface
`transport, excretion, and vapor loss by respiration; the last of these also
`regulates the supply of carbon dioxide. The water vapor thus lost enters the
`hydrological cycle via the atmosphere, whereas liquid waste water eventually
`replenishes the natural water reservoir. The hydrological cycles of the bio(cid:173)
`sphere are shown in Fig. I. The subcycle relating to plants is completed
`by the process of photosynthesis, in which water vapor and carbon dioxide
`are assimilated. Isotope tracer studies have shown that the oxygen liberated
`is eventually reconverted to water. The importance of this subcycle is
`demonstrated by the annual turnover of 6.5 x 1011 tons of water by photo(cid:173)
`synthesis of green plants and marine organisms.
`The above short summary of the different ways in which water is bound
`up with life processes indicates clearly that water, acting as solvent and
`dispersing and lubricating medium, is also a versatile reactant and that
`
`

`

`8
`
`Chapter 1
`
`WATER VAPOUR ~ \\/'\
`/
`J~B
`)
`
`IPLANTSI
`
`SUB-SURFACE WATER
`
`Fig. 1. The hydrological cycle, including the biological subcycles.
`
`morphologically and functionally, life and water are inseparable. It is there(cid:173)
`fore hardly surprising that living organisms are sensitively attuned to the
`properties of water. This can be demonstrated by changing these properties
`"slightly," e.g., by raising the temperature. It appears that for each species
`there is a definite temperature above which it can exist for only very limited
`periods; for mammals this temperature is 40°C. Another method for altering
`the properties of water is by changing its isotopic composition. Although
`from a chemical standpoint H20 and D20 closely resemble each other,
`the rates of many chemical, and especially of biochemical processes are
`extremely sensitive to the nature of the aqueous substrate, and life processes
`can be retarded by replacing H20 in the body fluids by D20. Until recently
`it was believed that D20 could not support life, but Katz< 5761 has shown
`that algae, bacteria, yeasts, fungi, and protozoa can be induced to grow
`in 99.8% D20, and _higher organisms in lower concentrations. In fact,
`completely deuterated biopolymers have thus been biosynthesized. How
`such adaptation is brought about and what are the effects of deuteration
`on the functional properties of biopolymers is not yet fully understood, but
`investigations in this field clearly indicate that much can be learned by
`tracing the path of hydrogen, rather than carbon, in biological processes.
`
`4. THE SCIENTIFIC STUDY OF WATER-A SHORT HISTORY
`
`In order to place the subject matter contained in this volume in its
`right perspective, it is helpful to outline briefly the historical development
`of the scientific study of water in all its various aspects. The realization that
`
`

`

`Introduction-Water, The Unique Chemical
`
`9
`
`water is a unique substance goes back a long way in history, and already at
`the time of the famous Greek philosophers it was realized that water
`played an important role in supporting life. Aristotle included water among
`the four elements, alongside earth, air, and fire. Not until the 18th century
`was it established that earth and air were mixtures, and that fire was simply
`the manifestation of chemical change. In 1781 Priestley synthesized the
`last remaining one of Aristotle's elements, and shortly afterward Lavoisier
`and Cavendish succeeded in decomposing liquid water into "ordinary air"
`(oxygen) and "inflammable air" (hydrogen). Although these classical
`experiments established the fact that water cannot be regarded as an
`element (in the sense now accepted), it nevertheless plays a unique role
`among chemical compounds. It is therefore somewhat surprising that some
`of the famous 19th century scientists did not enquire more deeply into its
`nature. The explanation may lie in the fact that on this planet water is
`ubiquitous and has, for a long time, been taken for granted. Looking back
`now at the volume of experimental data on aqueous solutions of electrolytes
`and nonelectrolytes assembled over the past hundred years, it is easy to
`see that physical and organic chemists would not have encountered so
`many difficulties if the most readily available solvent had not been water.
`Thus, the observed phenomena associated with equilibrium and kinetic
`processes were usually interpreted without allowing for the fact that the
`solvent itself was a highly complex one. Probably one of the best examples
`is provided by the development of the Debye-Htickel theory of electrolytes,
`based almost completely on observations derived from studies on aqueous
`solutions. <914 > Yet the only physical property ascribed to the solvent was
`a dielectric constant, and although in its limiting-law form the theory is of
`great value, the various extensions which have, from time to time, been
`proposed to account for the concentration dependence of activity coefficients
`are, at best, semiempirical.
`Thus, in the main, scientists did not pay much attention to any possible
`functions of the solvent in physicochemical processes, although an in(cid:173)
`creasing number of the properties of water were accurately measured and
`found to differ from those of other liquids or chemically related compounds.
`Probably the earliest suggestion that liquid water might contain "solid
`particles" was made in 1884, and in 189\ Vernonmosi postulated the aggrega(cid:173)
`tion of water molecules in order to account for the phenomenon of maxi(cid:173)
`mum density. A year later Rontgen<910> qualitatively explained several of
`the apparently anomalous properties of water. However, none of these
`early attempts to draw attention to the peculiar nature of water was followed
`up. This is also true for the now famous monograph by Dorsey, <209 > ironically
`
`

`

`10
`
`Chapter 1
`
`entitled "Properties of Ordinary Water Substance," which contains many
`examples of the physical anomalies which had been observed over the
`previous 50 years. Even the classic paper by Bernal and Fowler, 1711 in
`which they advanced the first plausible model for liquid water, failed to
`make an immediate impact, although it has since been realized that it laid
`the foundation for subsequent work, and we are even now still in the era
`of Bernal and Fowler. Later in the 1930's came the first X-ray17611 and
`infrared< 7201 studies on liquid water, and Butler<t68•1591 and Eley13041
`in(cid:173)
`vestigated the origin of negative partial entropies exhibited by a variety of
`simple solutes in aqueous solution. These studies culminated in the famous
`paper by Frank and Evans, <3591 which introduced the concept of "icebergs"
`induced in water by solute molecules. At the same time Samoilov10401 was
`investigating the nature of the interactions between water and ions in solu(cid:173)
`tion. In 1948 Hall, 14441 studying acoustic relaxation in water, advanced the
`first detailed mixture model, i.e., a model based on the concept of water
`as a mixture of two distinguishable species. Shortly afterward Wang11144- 11401
`published his extensive studies on the self-diffusion behavior of water of
`different isotopic compositions. The next major step forward was made
`by Pople, <8551 resulting from his earlier studies into the properties of the
`hydrogen bond. Basing his calculations on the earlier model of Bernal
`and Fowler, he suggested the possibility of hydrogen bond bending, rather
`than breaking, to account for the temperature dependence of the dielectric
`constant and some other properties of water.
`In the 1950's physicochemical studies of water and its interactions with
`solutes gradually gathered momentum and several molecular models were
`proposed, but the subject of water structure only attained the status of
`scientific respectability in the 1960's. Frank's reminder that the existence
`of long-lived structures in liquid water was most unlike1y and that a useful
`description might involve "flickering clusters"<301 > of hydrogen-bonded
`molecules was very timely, whilst Kauzmann's outstanding discussion of
`the possible role of water in determining protein conformation and dena(cid:173)
`turation behavior<5791 helped to make biochemists aware of the peculiar
`nature of the solvent medium in which life processes take place. In fact,
`this review was to prove instrumental in promoting a veritable flood of
`publications describing various aspects of the phenomenon of "hydrophobic
`bonding" in biopolymer systems.
`In 1962 Nemethy and Scheraga published a series of papers 1787- 789 >
`in which they attempted to apply statistical mechanics to the development
`of a molecular description of water and of aqueous solutions of apolar
`solutes and proteins. Although the mathematical treatment has subsequently
`
`

`

`Introduction-Water, The Unique Chemical
`
`11
`
`been criticized, <1082, and the numerical parameters employed may well be
`unrealistic, nevertheless these papers constitute the first attempt to put on
`a quantitative basis what had, up to that time, been based on empiricism,
`and as such they constitute a significant development.
`Comparisons of various bulk and microscopic transport processes
`(e.g., viscosity, self-diffusion, and dielectric and NMR spin-lattice relaxa(cid:173)
`tion) in water had shown that they could all be associated with an identical
`energy of activation, and this led to speculations regarding the nature of
`molecular motions in water. High-precision Raman studies< 1134 , of the
`intermolecular (hydrogen bonding) modes in liquid water followed, and
`recent infrared and laser Raman techniques have been applied to the
`investigation of the uncoupled intramolecular OH and OD stretching
`modes in H20 and 0 20. <902•1138•1140, The spectral contours have been care(cid:173)
`fully analyzed in an effort to adduce evidence for or against models which
`treat water as a mixture. At the time of writing, however, this question is
`still unresolved, although mixture (on a suitable time scale) models are at
`present the more favored ones. In fact, the observed effects of solutes on
`the physical properties of water cannot readily be accounted for by a con(cid:173)
`tinuum model.
`Until recently all the model calculations had to be based on experi(cid:173)
`mental results covering only the normal liquid range, but during the last
`five years some of the physical properties of water have been studied up to
`and beyond the critical point. <354 •703 , so that much more rigorous tests
`can now be applied to the molecular models.
`The development of sophisticated computing techniques has led to
`attempts to study water-water interactions by ab initio quantum mechanical
`methods. Thus, water dimers have received attention< 250 •704 , and work is
`currently in progress to extend these techniques to trimers and larger aggre(cid:173)
`gates. <251 , It is hoped that this approach will provide detailed information
`about the nature of the hydrogen bond in liquid water and, in particular,
`whether hydrogen-bonding processes in liquid water involve a cooperative
`element. Computer simulation is also being applied to the study of water,
`although experiments reported so far have been confined to Monte Carlo
`calculations of thermodynamic properties based on a rather simple pair
`potential. <stl
`Although a rigorous treatment of simple liquids, let alone water, in
`terms of realistic potential functions may not become available for some
`time (see Chapter 3), nevertheless, the manner in which solute molecules
`interact with water and with one another in aqueous solutions can be
`profitably studied, and the last five years have seen some useful progress
`
`

`

`12
`
`Chapter 1
`
`362
`
`633> There is, however, an urgent need for reliable
`in this field. <06 •
`•303 •
`experimental data on very dilute aqueous solutions of a large range of
`solutes, and with very few notable exceptions, such data are only now finding
`their way into the literature. Even precise experimental data on aqueous
`nonelectrolytes, covering the whole composition range, are still scarce. The
`present position is put in perspective by reference to standard texts on liquids
`and solutions, <505 •873 •928> in which little space is devoted to aqueous solutions.
`One interesting issue which still awaits resolution concerns the existence,
`or nonexistence, of thermal anomalies (also referred to as "kinks" and "ther(cid:173)
`mal discontinuities") in the physical properties of water. The chief protag(cid:173)
`onist of their existence has been Drost-Hansen, <278> whose claims have been
`supported by some, and hotly contested by others. Essentially the suggestion
`is that higher-order structural transitions occur in water at, or near, 15,
`30, 45, and 60° and that these transitions give rise to discontinuities in the
`second or higher temperature derivatives of physical properties. Although
`at one time Drost-Hansen believed that such transitions occurred in pure
`water, <277> subsequent experimental measurements at closely spaced tem(cid:173)
`perature intervals and analyses of previous work have led him and others
`to cast doubt on this hypothesis. However, in some biological systems,
`especially those involving membrane transport phenomena, <1063 •1064> these
`discontinuities have been clearly established and even in relatively si