Topic 2000 Gibbs Energies; Liquid Mixtures; Typically Aqueous (TA)

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1 Topic 2000 ibbs nergies; Liquid Mixtures; Typically Aqueous (TA) For any binary aqueous liquid ixtures, the pattern shown by the olar excess therodynaic paraeters is > 0; T S > H. This pattern of excess olar properties defines TA ixtures. is positive because the excess olar entropy of ixing is large in agnitude and negative in sign. In these ters ixing is doinated by the entropy change. The excess olar enthalpy of ixing is saller in agnitude than either or T S but exotheric in water-rich ixtures. The word Typically in the description stes fro observation that this pattern in therodynaic variables is rarely shown by non-aqueous systes. At the tie the classification was proposed [1], ost binary aqueous liquid ixtures seeed to follow this pattern. Aong the any exaples of this class of syste are aqueous ixtures fored by ethanol, 2-ethyl propan-2-ol and cyclic ethers including tetrahydrofuran[2]. In water-rich ixtures, a large in agnitude but negative in sign produces a large (positive) endotheric ixing produces a positive water-rich ixtures a positive T S. For ixtures rich in the apolar coponent. The key point is that in eerges fro a negative T S. With reference to voluetric properties of these syste, the partial olar volue V(ROH) for onohydric alcohols can be extrapolated to infinite dilution; i.e. liit(x 2 0)V(ROH) = V(ROH;aq) where x 2 is the ole fraction of alcohol. The difference, [V(ROH;aq) -V * (ROH; l )] is negative. In fact this pattern is observed for both TA and Typical Nonaqueous binary aqueous ixtures. xaples where this pattern is observed included aqueous ixtures fored by DMSO, H 2 O 2 and CH 3 CN. Significantly [V(ROH;aq) - V * (ROH; l )] is negative for TA ixtures, decreasing fro [V(ROH;aq) -V * (ROH; l )] with increase in ole fraction of ROH, accopanying by a tendency to iiscibility. The initial decrease in [V(ROH;aq) - V * (ROH; l )] with increase in x 2 is ore draatic the ore hydrophobic the non-aqueous coponent; for 2-ethyl

2 propan-2-ol aqueous ixtures at K and abient pressure, the iniu occurs at an alcohol ole fraction 0.04 (at K). Many explanations have been offered for the coplicated patters shown by the dependence of [V(ROH;aq) -V * (ROH; l )] on ole fraction coposition. In one odel, the negative [V(ROH;aq) -V * (ROH; l )] at low ole fractions of ROH is accounted for in ters of a liquid clathrate in which part of the hydrophobic R-group occupies a guest site in the water lattice. The decrease in [V(ROH;aq) -V * (ROH; l )] is accounted for in ters of an increasing tendency towards a clathrate structure. But with increase in x 2 there coes a point where there is insufficient water to construct a liquid clathrate water host. Hence [V(ROH;aq) -V * (ROH; l )] increases. An iportant characteristic of TA ixtures is a tendency towards and in soe cases actual decrease in liquid iscibility with increase in teperature. At abient T and p, the ixture 2-ethyl propan-2-ol + water is iscible (but only just!) in all olar proportions. The corresponding ixtures prepared using butan-1-ol and butan-2-ol are partially iscible. TA systes are therefore often characterised by a Lower Critical Solution Teperature LCST. In fact nearly all exaples quoted in the literature of systes having an LCST involve water as one coponent; e.g. LCST = 322 K for 2-butoxyethanol + water [3]. This tendency to partial iscibility is often signalled by the properties of the copletely iscible systes. Returning to the patterns shown by relative partial olar volues, [V(ROH;aq) -V * (ROH; l )], a stage is reached whereby with increase in ole fraction of the non-aqueous coponent, this property increases after a iniu. Other properties of the ixtures also change draatically including a arked increase in (α a /ν 2 ) where α a is the aplitude attenuation constant and ν is the frequency of the sound wave in the MHz range; e.g. 70 MH z. Actually the pattern is coplicated. Over the range of ixture ole fractions x 2 where [V(ROH;aq) -V * (ROH; l )] decreases with increase in x 2, the ratio (α a /ν 2 ) hardly changes although the speed of sound increases. At a ole fraction x 2 characteristic of the teperature and

3 the non-aqueous coponent, (α a /ν 2 ) increases sharply, reaching a axiu where the ixture has a strong tendency to iiscibility. This interplay between in-phase and out-of-phase coponents of the coplex isentropic copressibility when the ole fraction coposition of the ixture is changed highlights the olecular coplexity of these systes. By way of contrast the ratio (α a /ν 2 ) for DMSO + water ixtures (a TNAN syste) changes gradually when the ole fraction of DMSO is changed. For TA ixtures where (α a /ν 2 ) is a axiu [4], other evidence points to the fact these ixtures are icro-heterogeneous; cf. excess olar isobaric heat capacities. Phase separation of the ixture 2-ethyl propan- 2-ol is observed when butane gas is dissolved in the liquid ixture. The iscibility curve shows an LCST near 282 K [5]. Footnotes [1] F. Franks in Hydrogen-Bonded Solvent Systes, ed. A. K. Covington and P. Jones, Taylor and Francis, London,1968, pp [2] The following references refer to properties of TA binary liquid ixtures. (a) alcohol + water ixtures. Isobaric heat capacities. (i)h. Ogawa and S. Murakai, Therochi. Acta, 1986, 109,145. (ii). I. Makhatadze and P. L. Privalov, J. Solution Che.,1989,18,927. (iii) R. Arnaud, L. Avedikian and J.-P. Morel,J. Chi. Phys., 1972,45. (b) fluoroalkanol + water ixtures. R. Jadot and M.Fralha,J. Che.ng. Data,1988,33,237. J. Murto and A. Kiveninen, Suoen Keist. Ser. B, 1967,40,258. nthalpies M. Denda, H. Touhara and K. Nakanishi, J. Che. Therodyn., 1987, 19,539. A. Kivinen, J. Murto and A.Vhtala, Suoen Keist.1967,40,298. Volues; J. Murto, A. Kivinen, S. Kiviaa and R. Laakso, Suoen Keist., Ser. B, 1967, 40,250.

4 (c) aine + water ixtures (i) X and iscibility; J. L. Copp and D. H. verett, Discuss. Faraday Soc.,1953, 15,174. (ii) t 3 N + water; iscibility; A. Belleans, J.Che.Phys.,1953, 21, 368. (d) THF + water J. Matous, J. P. Novak, J. Sobr and J. Pick, Collect. Czech. Che.Coun.,1972,37,2653. C. Treiner, J.-F. Bocquet and M. Chela, J. Chi..Phys., 1973,70, 72. for THF + D 2 O; J. Lejcek, J. Matous, J. P. Novak and J. Pick, J. Che. Therodyn., 1975,7,927. Vapour coposition; W. Hayduk, H. Laudie and O. H. Sith, H J. Che.ng Data,1973,18,373. H. Nakayaa and K. Shinoda, J. Che. Therodyn., 1971,3,401. Activity of water K. L. Pinder, J. Che. ng. Data, 1973,18,275. Theral expansivities O. Kiyohara, P. J. D Arcy and. C. Benson, Can. J. Che., 1978,56,2803. and V R. Signer, H. Ar and H. Daeniker, Helvetica Chiica Acta, 1969, 52, Methyl propan-2-ol + water In the cheical literature, 2-ethyl propan-2-ol is often called t-butanol but as Prof. David J.. Ives often pointed out, there is no organic copound, t-butane. and H

5 Y. Koga, W. W. Y. Siu and T. Y. H. Wong, J. Phys. Che., 1990, 94,7700. nthalpies Y. Koga, Can. J.Che.,1986,64,206;1988,66,1187,3171. Volues A. Hvidt, R. Moss and. Nielsen, Acta Che. Scand.,Sect. B, 1978, B32, 274. M. Sakurai, Bull. Che. Soc. Jpn.,1987,160,1. C p and V B. de Visser,. Perron, and J.. Desnoyers, Can J. Che.,1977,55,856. X-ray scattering K. Nishikawa, Y. Kodera and T. Iijia, J. Phys. Che., 1987, 91,3694. Sound velocity H. ndo and O. Nooto,Bull. Che. Soc. Jpn., 1973, 46, Isentropic copressibilities; J. Lara and J.. Desnoyers, J. Solution Che., 1981,10,465. Propan-1-ol +water V. C. Benson and O. Kiyohara, J. Solution Che.,1980,9,791. M. I. Davis, Therochi. Acta, 1990,157,295. C. De Visser,. Perron and J.. Desnoyers, Can. J.Che.,1977,55,856. Propanone + water X references; M. J. Blandaer, N. J. Blundell, J. Burgess, H. J. Cowles and I. M. Horn, J. Che. Soc. Faraday Trans.,1990,86,283. Methyl vinyl ketone + water LCST = 301 K; UCST = 356 K; J. Vojtko and M.Cihova, J. Che. ng. Data, 1972,17,337. [3] F. lizalde, J racia and M. Costas, J Phys.Che.,1988,93,3565.

6 [4] M. J Blandaer and D. Waddington, Adv. Mol. Relax.Processes,1970,2,1. [5] R. W. Cargill and D.. MacPhee, J. Che. Soc. Faraday Trans.1, 1989, 85, 2665; an excellent observation! [5] ; M. J. Blandaer, J. Burgess, A. Cooney, H. J. Cowles, I. M. Horn, K. M. Martin, K.W. Morco and P. Warrick, J. Che. Soc. Faraday Trans., 1990, 86, 2209.