J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 855-858 Transfer Gibbs Energies for ClOi, BrOi, lo;, ClOi and 10: Anions for Water-Acetonitrile and Water-fed-Butyl Alcohol Mixtures JBn Benko and Olga Vollarova Department of Physical Chemistry, Faculty of Science, Comenius University, 842 75 Bratislava, Slovakia The transfer Gibbs energies, AtrSGe, of KCIO, , KBrO, , KIO,, KCIO,, KIO, and the corresponding caesium salts have been obtained through the gravimetric measurement of solubility in aqueous mixtures with acetonitrile (AN) and tert-butyl alcohol (ButOH). Single-ion values of Atr,Ge have been calculated using the TATB assump-tion. The trends observed for Atr,Ge are discussed in terms of specific ion-solvent interactions and the structur- al effect of the solvent mixtures.Single-ion thermodynamic transfer functions of solutes from one solvent (often water) to another find applications in many areas of chemistry. Much work has been done on a great variety of electrolytes in water mixtures with aceto- nitrile as well as with tert-butyl alcohol, but little attention has been paid to oxyanions of halogens except 10; and ClO, 'in H20-ButOH and ClO, in H,O-AN., Oxyanions generally show stronger specific interactions in aqueous mix- tures; this might be one reason why ClO, 'or NO, differ from monatomic anions such as halide anions5 in H,O-AN mixtures. We have recently reported the kinetics of 10; oxi-dation of some Co"' complex in water-organic mix- tures and as a by-product of these investigations we have obtained A,,G* for 10, in H,O-Bu'OH, H20-(CH3)2C0 and H20-CH30Hmixtures. We report now AwsGe for 10, in H'O-AN and other oxyanions of halogens in aqueous mixtures with AN and Bu'OH.The data for selected K+ and Cs' salts provide a basis for analysis of the extrathermo- dynamic assumption [AtrsG*(Ph,As +) = A,, G*(BPh;)] wed in the calculation of single-ion thermodynamic proper- ties. Experimental Materials The compounds NaBrO, ( >99.8%), CsNO, ( >99.0%) Lachema Brno and HJO, (>99.5%), HClO,, HIO, (>97%), KClO, (>99.5%), KI04 (>99.8%), KBrO, (>!MI%),KI03 (>99.5%), Bu'OH (>99.5%, H20 < 0.1%) and AN (>99.8%, H20c0.05%) Merck (stated % purities in parentheses) were of analytical grade.Caesium salts (CsIO,, CSIO, and CsClO,) were prepared from CsN0, and the appropriate acid. Addition of CsNO, to saturated solu- tions of NaClO, and NaBrO, resulted in precipitation of CsClO, and CsBrO,, respectively. All salts were re-crystallized from water prior to use and their purity was checked by elemental analysis (halogens titrimetrically and alkali metals using flame photometry). All solvents were redistilled before use. Solubilities Solubilities of salts in water and in aqueous-organic mixtures were determined by agitating an excess of the solid salt with the solvent at 298.2 K. The concentrations were determined gravimetrically. Evaporation of the solvent was performed carefully and slowly under an IR lamp to prevent any loss in salt weight. Solubility values were averages of three indepen- dent measurements.The standard error in the solubility determinations was & 1%. Results Measured solubilities of potassium and caesium salts in H20 and in aqueous-organic mixtures containing Bu'OH and AN are given in Tables 1 and 2. The solubilities of the salts in water S, and in the solvent mixtures S, are related to the transfer Gibbs energy of the salt by Atn Ge = ~RTIn(S,y,f/S,y:) (1) The solubilities were corrected to infinite dilution using the activity coefficients, y *, calculated from the Davies equa- tion:' log y* = -A[I"'/(l + Ill') + 0.3a (2) where A is the Debye-Huckel parameter and I is the ionic strength. A may be calculated from the known relative per- mittivities of H,O-AN and H,O-Bu'OH mixtures.* Transfer Gibbs energies of anions (Tables 3 and 4) can be calculated using published estimates for the corresponding K+and Cs+ ions in aqueous Bu'OH and aqueous ACN.Table 1 Solubilities of the salts investigated in water and H,O-Bu*OH mixtures at 298.2 K S/10-, mol dm-, ~(Bu'OH) KClO, KIO, KClO, 0 15.1 2.26 68.5 0 15.08" 2.26" 70.17" 0.010 12.6 1.91 57.1 0.02 1 11.2 1.70 48.0 0.033 9.91 1.52 41.3 0.046 9.15 1.45 36.1 0.076 7.88 1.39 27.8 0.113 7.10 1.32 22.6 x2 CsClO, CsIO, CsClO, ~ 0 8.89 5.67 35.5 0,010 7.52 4.86 30.3 0.021 6.77 4.37 25.9 0.033 6.15 4.04 22.7 0.046 5.85 3.85 20.0 0.076 5.16 3.75 15.8 0.113 4.56 3.56 13.0 " Values according to ref.11. KBrO, KIO, 47.9 42.6 47.3" 42.6" 38.3 30.8 31.0 22.7 25.6 16.9 20.9 12.7 14.6 7.66 11.0 5.23 CsBrO, CsIO, ~ ~~ 14.1 8.25 11.6 6.37 9.5 4.74 7.8 3.64 6.5 2.80 4.6 1.73 3.5 1.18 Table 2 Solubilities of the salts investigated in H,O-AN mixtures at 298.2 K S/lO-, mol dm-, x(AN) KC10, KIO, KC10, KBrO, KIO, 0.017 15.5 2.32 64.2 43.5 34.0 0.036 16.4 2.37 61.5 39.3 27.6 0.057 17.7 2.61 59.3 36.5 23.1 0.077 19.1 2.69 56.2 32.1 18.1 0.126 21.9 2.99 50.4 25.7 11.5 0.182 23.7 3.09 42.5 18.4 7.0 x2 CsClO, CsIO, CsClO, CsBrO, CsIO, 0.017 9.45 6.09 34.5 13.2 7.04 0.036 10.3 6.14 33.4 12.3 5.94 0.057 11.2 6.85 32.8 11.5 5.09 0.077 12.5 7.22 31.6 10.4 4.13 0.126 14.6 8.2 1 29.0 8.3 2.80 0.182 16.1 8.66 24.6 6.2 1.75 Discussion The results in Table 1 show that all the 1 :1 electrolytes investigated are more soluble in water than in H,O-Bu'OH mixtures. KClO,, KIO,, CsCIO, and CsIO, are, however, more soluble in H20-ACN than in H20 (Table 2).Solu-bilities of all potassium salts in water are in close agreement with those in the literature" (Table 1). The solubilities of KC104 (Table 2) are somewhat higher than those published3 but the trend with increasing AN concentration is the same. The solubility decreases with increasing molecular weight of the salts while KIO,, CsClO, and KClO, deviate from this trend in H20.In H20-AN (40% V), the effect of electrolyte size is less significant than in water, but somewhat greater than in the H,O-Bu'OH (40%V) system. Those observations are related to changes in the solvent structure due to solvent mixing. Introduction of the solutes into the solvent causes a change in the solvent structure which is greater with increas- ingly large solutes. Cs', C1-, Br-, I- and ClO, act as net structure breakers in water and K+ is a borderline case, but all the ions mentioned above are structure makers in non- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 aqueous solvents such as MeOH, DMSO and ACN.12 The different solubility behaviour of salts cannot be explained simply by structure-breaking or structure-making mecha-nisms.More informative, however, is the way in which the Gibbs energies vary with solvent composition in the mixtures. The transfer of all anions from H20 to H,O-Bu'OH mixtures is non-spontaneous, as indicated by the positive Gibbs energy of transfer (Table 3). Positive AimGe values have been inter- preted in terms of a stronger interaction of the anions with water than with Bu'OH. Preferential solvation by water is also expected to be reduced by the strong interaction of the water molecules with Bu'OH. The irregularities in AtrsG* us. composition around x2 x 0.05 (Fig. 1) are smaller than the experimental uncertainty and their existence is therefore doubtful. Nevertheless, these irregularities occur at the cosolvent concentration where the cosolvent reinforces the structure of water and this position does not depend on the nature of the ion but reflects a property of the mixture.AlrsGe values (Fig. 1) become increasingly positive in the order At, G*(I-) < At= G*(Br -) < A', Ge(Cl-)' and A,,, G*(IO;) < AtmG*(C106) but for halate anions the reverse order Air, G*(ClO;) < Alrs G*(BrO,) < A,, Ge(IO,) is observed. We compare the order of AlrsGe values with partial molal volumes, V'&, which have frequently been used as probes of ion-solvent interaction. In water, the V6 values of halide and halate anions13 are in the same order as the AtrsGe values. The partial molal volumes of C1-and Br- ions14 go through a shallow minimum and then increase as the Bu'OH content increases.This change of partial molal volume coincides with the well documented change of solvent structure occurring in H,O-Bu'OH mixtures.' 5,1 At higher cosolvent concentrations, the A,rs Ge values are somewhat more sensitive to the nature of the anion than those in the water-rich region. The ion size, the distribution of charge on the surface of the ions and the geometry are important factors in A,,Ge values. The shape of ions probably plays a role during their accommodation in the solvent cavity. The symmetrical charge distribution on the surface of X-and XO, ions in contrast to XO, and the shape of XO, being similar to the spherical symmetry of X-may explain the observed order of Airs Ge.The change in A,, G* of the anions Table 3 Transfer Gibbs energies for investigated anions evaluated from potassium and caesium salts in H,O-Bu'OH mixtures corrected to infinite dilution at 298.2 K Atn G*/kJ mol -' x(Bu'0H) ClO;(K+) ClO;(Cs+) IO;(K+) IO;(Cs+) ClO;(K+) ClO;(Cs+) BrO;(K+) BrO;(Cs+) IO;(K+) IO;(Cs+) 0.010 0.18 0.07 0.08 0.00 0.17 0.10 0.47 0.22 0.02 1 -0.17 -0.16 -0.31 -0.48 0.35 0.13 0.64 0.39 0.033 0.05 0.04 -0.13 -0.13 0.82 0.60 1.25 1-10 0.046 0.56 0.56 0.25 0.37 1.80 1.53 2.5 1 2.2 1 0.076 2.18 1.94 1.13 1.28 3.97 3.5 1 5.02 4.57 0.113 3.51 2.90 2.04 1.90 5.95 4.94 7.19 6.17 Table 4 Transfer Gibbs energies for investigated anions evaluated from potassium and caesium salts in H,O-AN dilution at 298.2 K At" G*/kJ mol-' 0.96 0.49 1.54 1.08 2.56 2.08 4.18 3.53 7.27 6.47 9.88 8.48 mixtures corrected to infinite x(AN) ClO;(K+) ClO;(Cs+) IO;(K+) IO;(Cs+) ClO;(K+) ClO;(Cs+) BrO;(K+) BrO;(Cs+) IO;(K+) IO;(Cs+) 0.017 -0.27 -0.36 -0.27 -0.40 0.21 0.07 0.36 0.23 0.99 0.65 0.036 -0.60 -0.90 -0.46 -0.50 0.38 0.10 0.80 0.43 1.94 1.29 0.057 -1.31 -1.50 -1.32 -1.39 0.13 0.04 0.78 0.49 2.50 1.75 0.077 -2.03 -2.17 -1.82 -1.74 0.13 0.01 1.06 0.79 3.19 2.53 0.126 -2.43 -2.22 -2.13 -1.96 0.98 0.80 2.26 2.18 5.47 4.65 0.182 -2.24 -1.89 -1.81 -1.17 2.37 2.65 4.47 4.5 1 8.22 7.79 CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 I 1 1 0.05 0.10 x( Bu'OH) Fig. 1 Gibbs energles of transfer to water-Bu'OH mixtures at 298.2 K. Data for Cl-, Br-and I-ions were recalculated from the published A,rsGe values of potassium salts.' (a) IO,, (b)BrO; , (c) CIO;, (4CI-, (e)Br-, (f) I-, (9)ClO,, (h)10; with cosolvent composition is largely a reflection of the H,O-Bu'OH interaction which is modified, of course, from that in pure binary mixtures by the solute ions. AN is considerably different from Bu'OH in its interaction with water in view of its very different size, shape and elec- tronic characteristics. In particular, the shape and size of AY us. Bu'OH will result in a considerably smaller hydrophobic effect. The course of the Atr,G* us.AN concentration plot 1s less dramatic than is observed in H,O-Bu'OH. The variation and order of AtrsGe values in AN-H,O are analogous to those in H,O-Bu'OH for halide and halate anions but for perhalate anions negative AIrsGe values, in contrast to the positive ones in H,O-Bu'OH, were found (compare Fig. 1 with Fig. 2). The anions studied are preferentially solvated by I 'I 6 -k4 E 73u2 d 0 -2 0.1 0.2 x(AN) Fig. 2 Gibbs energies of transfer to water-AN mixtures at 298.2 K. Data for C1-, Br-and I-ions are from published (0) lo;, (b)BrO;, (c) C1-, (d)Br-, (e)ClO;, (f)I-, (9)IO,, (h)ClO,. 857 I I I 1 0.1 0.2 x(org) Fig. 3 Transfer Gibbs energies for ClO; (0)and 10, (0)to water-rich binary solvent mixtures at 298.2 K. Data for 10, and for ClO, 'in H,O-MeOH and H,O-Me,CO are published values. (a)Bu'OH, (b)Me,CO, (c) MeOH, (d)AN.water in both water-organic cosolvent mixtures and the anomalous transfer Gibbs energies for perhalates on going from water to a variety of AN mixtures is a result of a specific effect of the anion on the solvent-solvent interaction. Sodium perchlorate' 'sl * is almost completely dissociated in AN solvent mixtures of composition up to x2 = 0.305. The irregu- larities around x2 = 0.05 in A,,,G* us. AN concentration are a reflection of changes in the solvent-solvent interactions. '' At this AN concentration the extrema in A,,, V+ for halide ions were also observed.,' Fig. 3 shows that addition of AN actually results in stabili- sation of perhalate anions but the behaviour of Bu'OH and Me,CO (acetone) is different from that of AN, MeOH adopts an intermediate position.This cosolvent difference can be understood in terms of the different Raoult's law and water activity behaviour for the respective solvent mixtures. The results obtained for AtrsGe of the anions investigated calculated from solubilities of K+ and Cs' salts (Tables 3 and 4) suggest that the TATB method in both investigated mixtures is in this case suitable for evaluation of the proper- ties of individual ions in solution at low cosolvent concentra- tions. 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