S. Y. LAM AND R. L. BENOIT. Introduction

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1 Some Thermodynamic Properties of the Dimethylsulfoxide-Water and Propylene Carbonate-Water Systems at 25 OC S. Y. LAM AND R. L. BENOIT D&partet?letlt rle rhitnie, UtliversitP de Mot~trPal, Mot1trPnl101, QlrPbec Received August 23, 1973 Molar excess free energies of the systems dimethylsulfoxide-water and propylene carbonate - water have been calculated from static vapor pressure measurements at 25 "C. Enthalpies of mixing at low water concentrations have also been determined. Possible association interactions in these systems are discussed. Les Cnergies libres molaires d'exces des systemes dimcthylsulfoxyde-eau et carbonate de propyltne - eau ont it6 calculees partir de mesures statiques de pressions de vapeur a 25 "C. Les enthalpies de melange ont aussi CtC dcterminces pour de faibles concentrations d'eau. On examine les possibilites d'interactions par association dans ces systtmes. Can. J. Chem., 52,718(1974) Introduction This research is related to our study of hydration of ions at low water concentrations in some dipolar aprotic solvents (1). The determination of the thermodynamic properties of the binary svstems water-dimethvlsulfoxide (DMSO) and water - propylene carbonate (PC) is mandatory for comparing ionic hydration in these media as well as being a step in understanding the nature of the interactions between the com~onents. As no vapor pressure data could be foind for the PC-H,O system and those for the DMSO-H20 system directly determined (2) are values at 70 "C, vapor pressure measurements have been carried out at 25 "C over the whole range of compositions. Heats of mixing water with both solvents have also been measured at 25 "C and low water concentrations. Experimental Chemicals The water used was conductivity water. Spectroscopic grade dimethylsulfoxide from Fisher was used without further purification. Propylene carbonate from Eastman Kodak was doubly distilled at a temperature below 100 C and pressure under 10 mm Hg. The middle portion of the distillate, about 80%, was collected. Water contents in DMSO and PC determined by Karl Fisher titrations were 0.069% and z, respectively. Vapor Pressure Vapor pressures were measured with a Texas Instruments quartz spiral gauge calibrated into millibars. To avoid condensation of vapor, the gauge was heated at 58 "C, and the tubing leading to it was kept at 40 "C with heating tapes. The bath temperature was calibrated by vapor pressure determinations on pure water against a new standard mm Hg at "C given by Stimson (3). Binary mixtures were prepared by weighing known quantities of the components. Degassing was performed by repeated freezing and evacuation. Actual measurements were taken after equilibrium was attained. Compositions of the liquid were checked by reweighing; whenever correction was applicable, the loss in weight was assumed to be that of the more volatile component :;rdure Cfllorftnetry b e ~ ~ : ~ ~ ~ for ~ measuring ~ e ~ heats & have ~ ~ Results and Discussion The experimental total pressures for DMSO- (1)-H20(2) and for the partially miscible binary PC(1)-H20(2) are given in Table 1 and Fig. 1. The values are believed to be accurate to within mm Hg for pressures higher than 5 mm, mm for pure propylene carbonate, and mm for other pressures. The vapor pressure for pure DMSO, mm Hg, was obtained by measuring the pressures of wet DMSO with concentrations of water below about 0.1 M, as determined by Karl Fisher method, and extrapolating the results to zero water concentration. This value agrees well with that of Douglas, mm Hg (5). The vapor pressure for pure propylene carbonate was obtained in the same manner. The miscibility gap for PC(1)-H,0(2) was found to extend from x2 = to as determined experimentally by mixing known masses of PC and water at 25 "C. These results agree with those quoted by Catherall and Williamson (6). The prevailing pressure over the two coexisting phases was found to be mm Hg. Although DMSO and PC are dipolar aprotic solvents, both binary solvent-water systems exhibit contrasting non-

2 LAM AND BENOIT: ON SOME THERMODYNAMIC PROPERTIES 719 TABLE 1. Total vapor pressures of DMSO-H20 and PC-H20 Systems at 25 "C DMSO(1)-HzO(2) PC(1)-HzO(2) xz P(mm Hg) x2 P(mm Hg) '27 MM Hg OOO OOO O ideal behavior: DMSO-H20 gives negative X deviation and PC-H20 gives positive 1. Total vapor pressures of the systems DMSOfrom Raoult's law. Hz0 and PC-H20 at 25 "C. Different methods (7-9) have been devised to resolve total pressures into partial pressures on the basis of the Gibbs-Duhem equation and on the Gibbs-Duhem the assumption that the gaseous mixtures were x ideal. The following procedure was adopted for In P, = In PI0 - A d In P2 x2=0 X1 our calculations. Partial pressures of water were S'" evaluated by subtracting the Raoult's law con- For PC-H20 the first integration gave final tributions of PC and DMSO from the total values of pressures; for DMSO-H,O, the second pressures. These water pressures were used to sufficed to yield converging results. A model calculate better values of partial pressures of calculation for x, = 0.05 for PC-H,O is given other components by graphical integration of below: Table 2 shows the partial pressures and the free energy functions for these systems. The excess Gibbs free energies, accurate to within 2 calories, are plotted in Fig. 2. Activity coefficients as calculated by means of the usual expression are > 1 for PC-H20 and < 1 for DMSO-H20. The assumption of ideal gas behavior was examined by using Schiebel nomograph (lo), which amounts to evaluating an activity coefficient correction factor Z that corrects the calculated activity coefficient for the departure from the ideal gas law. From the critical data 2 Q 24 available either in the literature or by estimation from the boiling points and the parachors, the correction factors obtained from the nomograph 'were between 1 to for water, to 1 for DMSO, and to 1 for PC, and were therefore neglected. The excess Gibbs free energies of mixing were fitted into the equation

3 720 CAN. J. CHEM. VOL. 52, 1974 with parameters B, C, and D, by employing a H20(2) with small x, the expression used was: procedure given by Redlich and Kister (11). It was found that D values were very small and the terms associated with D could be absorbed into [2] AGE = 2.303RT(Bx1 + CxI2) the preceding terms without loss of accuracy in The parameters chosen for these systems and the representing the free energies. For that short standard deviations of the calculated values from range of excess quantities of mixing PC(1) and the experimental values are as follows Parameter values System B C (3 xl > cal/mol PC(I)-H~O(~) 1 < lcal/mol Eq. 2 For the DMSO-H,O system, the excess free energies AGE are negative. Using Boissonnas' method for partial pressures, Kenttamaa and Lindberg (2) have calculated AGE values at 25 "C on the basis of nine total pressure measurements at 70 "C (0.09 d x, < 0.92) and of heats of mixing at 70 and 25 "C (12). Our AGE results are somewhat less negative and by some 11 cal/mol for x, d The heat of mixing per mole of added water with DMSO as deduced from eight measurements, at increasing water concentrations < x, < 0.10, was found to be kcal/mol and to be independent of the concentration, as already noted by some authors (12, 13) for x, > 0.1, but not by others (14, 15). The AGE and AH values suggest strong interactions between DMSO and water, in line with the known proton acceptor properties of DMSO. Structural interpretations of ASE values have been advanced (12). For the PC-H,O system, the partial pressures and the activity coefficients level off when concentrations approach the miscibility gap. The excess free energies are posi- tive. The average heat of mixing AgWm for three experiments at water concentrations < x, < is kcal per mol of added water. On the basis of ARWw, (kcal/mol), PC and DMSO can be compared with other dipolar aprotic solvents: nitromethane (2.4) < acetonitrile PC (1.9) < sulfolane (1.6) < acetone (1.05) < DMSO (- 1.29) (16). This reflects a "basicity" order among these solvents and suggests that high levels of acidity could be obtained in PC. It is interesting to try to account for deviations of thermodynamic properties from ideality in terms of chemical equilibria (1 7). Recently the results of calorimetric studies of the systems CHCl, (A) + DMSO (B) (18) and CHC1, (A) + (CH,),N(B) (17) were thus successfully interpreted in terms of species A, B, AB, and A,B in the first case and A, B, and AB in the second one. However, when one considers binary systems FIG. 2. Excess free energies of mixing for the systems DMSO-H20 and PC-H20 at 25 "C.

4 LAM AND BENOIT: ON SOIME THERMODYNAMIC PROPERTIES TABLE 2. Partial pressures, activity coefficieqts, and excess Gibbs free energies of the DMSO-H,O and PC-H20 systems at 25 "C x2 P(mm Hg) P,(mm Hg) P2(mm Hg) Y 1 YZ AGE(cal/mol) DMSO(1)-HzO(2) OO PC(])H,0(2) * l.oo p-.p-p-p-.. - 'A value mm HE for propylene carbonate was used. with water as a component, it is easier to restrict oneself to the water-poor mixtures because of water self-association. Thus, with PC-H,O system, assuming that the non-linearity of our water partial pressure data, above 0.1 m water concentration, is due to water self-association, from the water monomer concentration given by the limiting slope of the water vapor pressure us. water concentration, a water dimerization constant of is calculated below a 0.7 M (x, Q 0.06) water concentration; above this concentration the dimerization constant increases (0.36 for x, = 0.1) suggesting more complex associations. Cogley et al. (19) interpreted their n.m.r. and infrared spectroscopic studies in terms of water monomer-dimer equilibria at moderately low water concentrations, with a dimer formation constant per mol at 25 "C. The agreement is surprisingly good. For the DMSO-H,O system,' deviations from 'Three recent papers (20-22) quote numerous references dealing with both the nature of these widely studied mixtures and of DMSO itself.

5 722 CAN J. CHEM. VOL. 52, 1974 ideality of many properties have been attributed 5. T. B. DOUGLAS. J. Am. Chern. Soc. 70,2001 (1948). by some authors to association interactions. Our 6. N. F. CATHERALL and A. G. WILLIAMsON. J. Chern. Eng. Data, 16, 335 (1971). pressure data that the water partial 7. R. T. LATTEY. J. Chern. Sot. 91-2, 1959(1907). pressures of aqueous DMSO vary linearly with 8. CH.-G. BOISSONNAS. Helv. Chim. Acta, 22, 541 the water concentration up to 7 M (x, ), (1939). in sharp contrast to the behavior of aqueous PC 9. F. Monatsh. Chern. g2) l3 10. E. G. SCHIEBEL. Ind. Eng.Chern.41, 1076(1949). The linear dependence of the water REDLICH and A. KISTER. Ind. Eng. Chern. 40,345 partial pressure and the constancy of the heat of (1948). mixing of water both with the water concentra- 12. J. KENTTAMAA and J. J. LINDBERG. Suorn. Kemistil. tion, strongly suggest the presence of only one B33> 32 (1960). 13. W. DRINKARD and D. KIVELSON. J. Phys Chem. 62, water species, in this concentration range, likely a 1494 (1958), monomer, either ofthe 1 : 1 or1 :2(water:DMSO) 14. J. M. G. COWIE and P. M. TOPOROWSKI. Can. J. type. The observed near constancy of the water Chern. 39,2240 (1961). proton chemical shift with the water concentra H. L. CLEVER and S. P. PIGOTT. J. Chern. Therrnothat water h~drogen-bonding to dyn. 3,221 (1971). 16. R, L, BENOIT and G, CHoux, Can. J. Chern. 46, 3215 DMSO dominates the interaction, rather than (1968). hydrogen bonded self-association. 17. L. G. HEPLER and D. V. FENBY. J. Chem. Therrnodyn. 5,471 (1973). ~h~ authors to express their gratitude to the 18. R. PHILIPPE and P. CL~CHET. J. Chern. Therrnodyn. National Research Council of Canada for a grant. 5,430(1973). 19. D. R. COGLEY, M. FALK, J. N. BUTLER, and E. I. R. L. B~NolTand S. Y. LAM. 56th Canadian Chern~cal GRUNDWALD. J. Phys. Chern. 76,855 (1972). Conference, Montreal, June 44, Paper R. Fuc~s and C. P. HAGAN. J. Phys. Chern. 77, J. KENTTAMAA and J. J. LINDBERG. Suom. Kernistil. (1973). B33,98 (1960). 21. T. K. COOPER, D. C. WALKER, H. A. GILLIS, and N. 3. H. F. STIMSON. J. Res. Natl. Bur. Stand. 73A, 493 V. KLASSEN. Can. J. Chern. 51,2195 (1973). (1969). 22. J. B. KINSINGER, M. T. TANNAHILL, M. S. GREEN- 4. R. L. BENOIT, M. RINFRET, and R. DOMAIN. Inorg. BERG, and A. I. Po~ov. J. Phys. Chern. 77, 2444 Chern (1972). (1973).

Chapter 1 The Atomic Nature of Matter

Chapter 1 The Atomic Nature of Matter 1-1 Chemistry: Science of Change 1-2 The Composition of Matter 1-3 The Atomic Theory of Matter 1-4 Chemical Formulas and Relative Atomic Masses 1-5 The Building Blocks