David W. James and Ray L. Frost
1754
References and Notes R. G. Ktepi~r,J. Chem. Phys., 39,3528 (1963). A. J, Epstein, eta/., Solidstate Commun., 9, 1803 (1971). A. J. Epstein, etid., Phys. Rev. 6,5, 952 (1972). R, Foster and P. Hanson, Biochim. Biophys. Acta, 112,482 (1966). (5).Y . Mats,uniiga, Neiv. Phys. Acta, 36, 800 (1963). (6) M. L.itt inndJ. Surnmers,J. Polym. Sci., Part A-I, 11, 1359 (1973). (7) "Beilstctins Mandbuch der Organischen Chernie," Vol. 27,86,I 243, i i 51. (8) C. Lagercrantz, Acta Chem. Scand., 15, 1545 (1961).
(9) J. M. Lhoste and F. Tannard, J. Chim. Phys. 63, 678 (1966). (IO) J. P. Biilen, J . Chim. Phys., 61, 374 (1964). (11) A. Many, E. Harnik, and D. Gerlich, J , Chem. Phys., 23, 1733 (1955). (12) J. W. Stout and R. C. Chisholm, J. Chem. Phys., 36,979 (1962). (13) Y. Sato, M. Kinoshita, et a/., Bull Chem. SOC. Jap., 40, 2539 (1967). (14) R. M. Lynden-Bell and H. M. McConneli, J. Chem. Phys., 37, 794 (1962). (15)S. Hirama, H. Kuroda, and H. Akamota, Bull. Chem. SOC.Jap., 44, 3 (1971). (16) Z.G.Soos and D. J. Klein, d. Chem. Phys., 55, 3284 (1971).
Structure of Aqueous Solutions. Structure Making and Structure Breaking in Solutions
of Sucrose and Urea avid W. James" Chemistry Department, University of Queensiand, St. Lucia, Australia
end, Ray L. Frost Chemistry Department, Queensiand lnstitute of Technology, Brisbane, Australia
(Received Aprii 8, 1974)
The effect of sucrose and urea on the librational band of water has been studied by infrared spectroscopy. Sucrose is found to produce little change in the band while all band characteristics are changed by the addition of urea. Comparison with changes produced by electrolytes indicates that the structural change produced by urea is different from that observed in electrolyte solutions.
The terms structure making and structure breaking have been used to describe aqueous solutions containing various solutes. From various measurements it was concluded that urea was a structure breaker while sucrose was a structure maker.1,2 The region of the vibrational spectrum which corresponds to the librational energy of water molecules has been little studied although it may be expected to yield significant information on the hydrogen bonded structure in solutions. We present here a preliminary report of a study of the librational spectrum of water in solutions of sucrose and urea using a thin film infrared technique. The spectra were obtained using a Perkin-Elmer 457 spectrometer and a thin film transmission technique in which a film of -7 p thickness was held between KRS-5 plates. Appropriate corrections were made for reflection losses and variations in film thickness and reproducibility of better than 1%in band intensity could be maintained from run to run. Band intensities are adjusted to unit concentration of water and all intensities are quoted relative to that of pure water at 20". The technique is reported in detail e l s e ~ h e r eThe . ~ collected results for urea and sucrose at various concentrations and temperatures are collected in Trible I. Also shown, for comparison, are results for selected ionic salt^.^.^ Three of the four band characteristics reported in Table X show different behavior for urea and sucrose. The shift in band maximum with added solute shows a small decrease in energy wiith either sucrose or urea. This decrease is similar to that noted for a number of ionic salts. The The Journal of Physicai Chemistry, Vol. 78, No. 17, 1974
band intensity increases much faster with added urea than it does for added sucrose. In this respect urea is similar to tetraethylammonium nitrate and potassium iodide; sucrose on the other hand resembles potassium nitrate. The decrease of intensity with temperature rise noted for urea is unique among the solutes studied; pure water and all other solutions examined show an intensity increase. The band asymmetry as measured by the asymmetry index5 is markedly different for urea for which the asymmetry goes from negative for pure water to appreciably positive. The only other solutes for which positive asymmetry indices have been measured are LiN03, NaC104, LiC1, BaBra, NaI, and KI a t very high salt concentrations (e.g., for LiN03 concentration of 10 m the asymmetry index is +8). Examination of the spectrum of water as the temperature is raised indicates that a gradual disruption of the water structure produces a small increase in band intensity, a marked decrease in the band maximum, and little effect on the band asymmetry. The salts NH4C1, N H a r , and to a lesser extent NHdNOs and N&C104 show concentration-dependent behavior which resembles this. The behavior of all other solutes is more complex. Intensity increase with added solute is frequently large; the largest increases are noted when there is a great size difference between anion and cation (e.g., LiI) or when the cation has aliphatic side chains (tetraalkylammonium cation). The former may be ascribed to the structure disruption produced by the dissimilar ion sizes while the latter may generally be ascribed to hydrophobic bonding. In
Behavior of Sucrose and Urea in Solution
1755
TABLE I: Chsiracteristics of the Librational Band of Water with Added Urea and Sucrose
______-.-
H2O
Pure
IJrea
2 5 9 0.5 1 2 2 2 2 2 21’
Sucrose Et4NNOP KNOP KCla KBra KIn
1.00 1.10 1.23 1.44 1.01 1.01 1.03 1.12 1.01 1.05 1.03 1.08
690 670 640 595 685 680 675 676 690 650 645 640
445 440 432 425 440 452 418 422 485 505 503 518
-20 0 $20 +20 -20 -20 -20 -25 -40 -40 -30 -40
1.03
660
433
-25
1.05
625
420
-30
1.15 1.25 1.02 1.02 1.03
605 585 655 650 640
430 425 425 425 430
+20 +20 -30 -28 -25
1.10 1.19 1.00 1.03 1.05
600 580 628 620 610
420 415 415 420 420
+22 +20 -24 -20 -18
a Intensitios increase with temperature rise. Vnf decreases with temperature rise (small); W/Z increases with temperature rise (small). At high concentrations K I has a positive ,,A%. C I,,Iis the integrated intensity relative to that of pure water.
any case the iiitensity increase can be associated with a disruption of water structure. In the case of urea there is a marked intensity increase with added urea but neither disparate ion size nor aliphatic side chains can be responsible. If the urea acted to disrupt the hydrogen bonded network there would be increased librational freedom which would result in greater amplitude of the librational motion. This would result in an increase in band intensit y . The changes noted in the energy of the band maximum may also be attributed to a decrease in the hydrogen bonded forces holding; the water molecules in the mean librational position. The asymmetry of the librational band shows considerable variation as solutes are added to water. For mosl electrolytes (except ammonium salts) the asymmetry decreases (and may change sign) as the electrolyte concentration increases. This decrease in asymmetry is most pronounced for iodides where the large anion is expected to contribute to the absorption of water structure. The change i n band symmetry is more pronounced for urea than for any other solute studied. We feel that it gives a clear indication that the average environment of water moleculesl has been drastically altered by the addition of urea. T%is change is more pronounced than that produced by other solutes studied. The spectroscopic changes induced by addition of sucrose to water sre, in general, small. They indicate that the forces acting on water molecules do not show great change as the EUCI’OSE’ is added. Thus the intermolecular water-water anteracticins appear to be similar to the intermolecular water -sucrose interactions.
The use of the terms “structure making” and “structure breaking” to describe our results does not appear to be appropriate. It is obvious that urea produces pronounced changes in the forces acting on water molecules but whether this should be termed structure breaking is not obvious. This work is currently being extended to a range of substituted ureas and thioureas in an attempt to clarify the factors contributing to changes in the intermolecular forces influencing water in solution.
Note Added in Proof. It has been suggested that a more extensive discussion of the band variations observed, together with a comparison with findings from other techniques, would be useful. The factors influencing the librational band are not adequately illustrated by sucrose and urea. However, the findings from a study of solutes structurally related to urea has enabled a much more satisfactory discussion to be developed and this will be published shortly. References and Notes D. V. Beauregard and R. E. Barrett, J. Chem. Phys., 49, 5241 (1968); G . A. Vidulich, J. R. Andrade, P. 6. 13lanchette, and T . J. Gilligan, J. Phys. Chem.. 73, 1621 (1969); V. I . Kharnova, A. M. Ponomareva, and K. P. Mishchenko, Russ. J. Ph,ys. Chem., 40, 748 (1966); H S. Frank and F. Franks, J. Chem. Phys., 48, 4746 (1968); G. Barone. E. Rizzo, and V. Vitagliano, J. Phys. Chem.. 74, 2230 ( 1970). G. E. Walrafen. J. Chem. Ph.ys., 44, 3726 (1966). R . F. Armishaw and D. W. James, submitted for publication; R . F. Armishaw, Ph.D. Thesis, University of Queensland. St. Lucia, 1972. D. W. James and R . F. Armishaw, to be submitted for publication. The asymmetry index is defined as the difference in wave numbers between the band maximum and the middle of the isoabsorbance chord at half-band height.
The Journal of Physical Chemistry, Vol. 78, No. 17, 7974
