288 Acta Chim. Slov. 2005, 52, 288–291 Scientific Paper Application of the New Solvation Theory to Reproduce the Enthalpies of TVansfer of LiBr, Tetrabutylammonium Bromide and Tetrapentylammonium Bromide from Water to Aqueous Acetonitrile at 298 K G. Rezaei Behbehani Department of Chemistry, Imam Khomeini International University Qazvin Iran, E-mail: rezaeib@ikiu.ac.ir Received 11-02-2005 Abstract The enthalpies of transfer, AH6,, of LiBr, tetrabutylammonium bromide, w-Bu4NBr, and tetrapentylamonium bromide, w-Pen4NBr, from water to aqueous acetonitrile solvent system are reported and analyzed in terms of the new developed solvation theory. The solvation parameters obtained from the analyses indicate that the net effect of tetraalkylammonium bromides on solvent structure is a breaking of solvent-solvent bonds and thus tetraalkylammonium bromides is preferentialh/ solvated by acetonitrile. Preferential solvation of tetraalkylammonium bromides by acetonitrile is consistent with hydrophobisity of these compounds. LiBr is preferentially solvated by water. Key words: preferential solvation, variable (ari + /iN), solvent-solvent bonds Introduction The solute-solvent interaction can be investigated by calorimetric measurement of the enthalpies of solution of a solute in different solvents. The thermodynamic parameters for transfer of a solute from pure solvent to mixed solvent show a number of different complex variations with the solvent composition. The form of the transfer parameter against solvent composition profiles, are sensitive to changes in both the solute and the solvent components of the mkture. Thus, for example, the enthalpies of transfer of LiCl pass through a sharp minimum in acetonitrile-water mixtures and through a broad maximum in methanol-water mixtures, while those of tetraphenvlarsonium chloride pass through a sharp maximum in acetonitrile-water mixtures.1-8 In this paper we present enthalpies of transfer for LiBr, ra-Bu4NBr and ra-Pen4NBr from water to aqueous acetonitrile. The improved method including variable (an + /3N) values, has been used to reproduce the enthalpies transfer data. Experimental and Results The solvents were purified as described previously.912 The enthalpies of transfer of the solutes were calculated from their enthalpies of solution, AHS, into the different solvent systems. In ali cases the enthalpies of solution were measured at ten solute concentrations (0.002-0.1 mol dnr3) and then the data were extrapolated to infinite dilution. The enthalpies of solution were measured using a 4 channel commercial microcalorimetric system, Thermal Activity monitor 2277, Thermometric, Sweden. The heat of each injection was calculated by the “Thermometric Digitam 3” softvvare program. Enthalpies of transfer have been reported in kJ mol-1. The precisions of enthalpies of solution at the infinite dilution of the solute were determined as the 95 % confidence limits of intercepts of plots AHS against solute concentration. Typically values of uncertainties were found to be around 0.03 kJ moL1 or better. The estimated precisions for enthalpies of transfer are determined by the absolute precisions of the infinite dilution enthalpies of solution in water and mixed solvent and were about 0.05 kJ mol-1, or better. The enthalpies of transfer for LiBr, ra-Bu4NBr andra-Pen4NBr from water to aqueous acetonitrile are listed in Table 1. Table 1. Enthalpies of transfer of the solutes from water to aqueous acetonitrile mixtures at 25 °C in kJ mol-1. xB LiBr «-Bu4NBr «-Peii4NBr 0 0 0 0 0.1 -2.05 12.40 16.13 0.2 -4.00 24.70 30.13 0.3 -7.40 24.40 26.03 0.4 -10.81 23.67 25.63 0.5 -15.46 22.40 25.18 0.6 -19.47 21.13 24.75 0.7 -22.70 19.40 23.64 0.8 -27.03 17.70 22.53 0.9 -28.15 17.21 22.83 0.95 -23.76 20.50 25.48 1 8.65 23.80 28.13 Behbehani Application of the New Solvation Theory Acta Chim. Slov. 2005, 52, 288–291 289 Discussion Extension of the simple coordination model taking into account the effects of changes in solvent-solvent interactions is relatively straightfonvard and has been described in detail elsewhere.13 Briefly: a solute occupies a cavity in the solvent structure, surrounded by its coordination sphere of n solvent molecules. When this cavity is formed, each of these n molecules must break some of its solvent-solvent bonds, giving rise to an increase in enthalpy, -naAH*° where a is the fraction of the molar enthalpy of solvent-solvent bonding, AH*°, associated with the broken bonds. Additionally there may be a modification of solvent-solvent bonds around the coordination sphere, affecting N (note N>n) solvent molecules. By postulating that the resulting enthalpy changes is proportional to AH*° we can set it equal to an enthalpy change, -nj3AH*°, where /3 is the average proportionality constant for the modified bonds and is negative if the bonds are strengthened (leading to an exothermic contribution to the enthalpy of solution). Finally the solute may be supposed to interact with the modified solvent giving rise to an enthalpy change AAH6^. This model leads to equation 1 for the enthalpy of transfer, AH8,, of the solute from pure solvent A to mixtures of A and a second solvent B. In equation 1, LA and LB are the relative partial molar enthalpies, AA//*° is the difference between the A-A and B-B interactions in the two pure solvents, AH°A—AH°B , and is taken as the difference betvveen the enthalpies of condensation of the pure components. The superscript 9 in aH cases refers to the quantities in infinite dilution of the solute. AH? = ( pxB xA + pxB )[AAHn+{an+pN)AAir\ (an + jW) pxB (xALA+pxBLB) (1) In mixed solvent systems, preferential solvation is accounted for by: Ni n ^^L = pJL Where x4 and xB represent the mole fractions of the components, A and B, of the mixed solvent and nA and nB, NA and NB are the number of A and B components which are the nearest neighbors of the solute. p is an index of preferential solvation. p<1 or p>\ indicate a preference for solvent^ or B respectively;/> = 1 indicates random solvation. AAHS12 is the difference betvveen the enthalpies of interaction of the solute with the two pure solvent A and B. The parameter (?n + ßN) reflects the net effect of the solute on the solvent-solvent bonding and it is positive if there is a net breaking or weakening of solvent-solvent bonds and is negative if the net effect of the solute is to cause a strengthening of these bonds. LA and LB are the relative partial molar enthalpies for a binary mixtures of A and B components calculated from mixing enthalpies of solvent A and B, ?HE, as follow: L = AHE +x dAH L = AH E E dAH V dXB J In the čase of random solvation (p = l), equation 1 simplifies to: AH? = xB [AAHel2 +{an + j3N)AAH°* ] -(an + j3N)AHE (2) where AHE represents the excess enthalpy of the mixed solvent. The enthalpy of transfer from pure solvent^ A -> B to pure solvent B, A trt , is simply: AABHet = [AAH?2 + (an + j3N)AAH°* ] (3) So that equation 2 rearranges to: A->B AH< ~Xb f H< ={an+BN) (4) AHE As {an + /3N) is not constant over the range of solvent composition, it is possible to change to: AH' X\A H' =(an+BN) mtc (5) AHE H If the solvation is random, it is possible to define the net effect of the solute on solvent-solvent bonds in mixture, (an + fiN)™*, as a combination of these values in water-rich domain, (an + j!N)eA, and alcohol-rich domain, (an + j!N)eB, which can be written: (an + /3N)mtl =(an + /3N)eAxA+(an + /3N)eBxB (6) A^B AH°-xB AH? We substitute -----'----------------'— ratio instead of AHE (an + J3N)"7" in equation 6, A^>B AH< ~Xb f H< =(an+BN) AHE [(an + pN)eB-(an + pN)eA\cB (7) x A Behbehani Application of the New Solvation Theory 290 Acta Chim. Slov. 2005, 52, 288–291 After reorganizing, leads to: A->B AH0t = A H?xB+(an + fiN)AAHE -[(an + j3N)0B-(an + j3NfA}cBAHE (8) AH E for non-random solvation is x'ALA + x'BLB where x'A and x'B are the local mole fractions of the solvent A and B respectively. If we apply non-random conditions to equation 8, AH6, = A H0xB +(an + pM)eA [xALA + xBLB ] - [(an + j3N)eB - (on + J3N)A}xB [xALA +x'BLB] (9) solvent composition, which is a good support for this equation. where x'A 1 pxB xA + pxB xA + pxB AH°t values were fitted to equation 9 over the solvent compositions. In the procedure the only adjustable parameter (p) was changed until the best agreement between the experimental enthalpies transfer and calculated data was approached over the whole range of solvent composition. (an + /!N)eA and (an + /lN)eB are the net effects of the solute on solvent-solvent bonds in water-rich region and cosolvent-rich region respectively which are recovered from the coefficients of the second and third terms of equation 9. The enthalpy of transfer from pure solvents to pure solvent B, A trt which is the coetticient of the tirst term in equation 9 is as follow: A-^B A i! \(AHl2)B-(AHl2)A\ + (an + j3NfBAH;-(an + j3N)AAHA (10) where [(AHU )B — (AHU )A ] is the relative strengths of solute-solvent bonds in the pure solvents including intramolecular contribution. For simplification it is writ-ten as AAHe12 and if it is positive the solute has weaker interaction with solvent B and the negative value of this parameter indicates stronger interaction of the solute with solvent B. AH°* and AH°* are the enthalpies of condensation for pure solvent A and B respectiveh/. Applying equal value for (an + jlN)eA and (an + /lN)eB in equation 10 leads to: A-^B Het = AAHel2 + (an + j3N)(AHA -AH°B*) (11) which is equation 3. If (an + /!N)eA = (an + /5N)eB = (an + /3N), equation 9 reduces to equation 1. Using equation 9 reproducing the enthalpies of transfer LiBr, n-Bu4NBr and n-Pen4NBr from water to aqueous MeCN shows excellent agreement betvveen the experimental and calculated data (Figure 1) over the whole range of Figure 1. Comparison of the experimental (symbols) and calculated (lines) enthalpies of transfer for LiBr (•), «-Bu4NBr (?) and «-Pen4NBr (A) via equation 9. Solvation parameters obtained by the help of equation 9 were reported in Table 2. (an + /3N) values for n-Bu4NBr and n-Pen4NBr are positive, which indicates disruption of the solvent-solvent bonds by these solutes. These values for LiBr are negative which means that LiBr strengthens solvent-solvent bonds in aqueous acetonitrile. It could be seen that (an + /!N)eA and (an + /lN)eB values for LiBr in aqueous MeCN are -18.56 and -13.53 respectivelv, very close together thus and therefore a conclusion could be made that LiBr residues are surrounded by water structure over the whole range of solvent composition. These interpretations are consistent with preferential solvation of LiBr by water as p0 indicates stronger interaction of the solutes with water. Solutes LiBr n-Bu4NBr n-Peti4NBr P 0.3 1.85 3.00 (on + pN)eA -18.56 33.00 29.52 {on + pN)% -13.23 73.75 64.16 AAHe12 (kJ/mol) 352.09 905.51 751.78 Conclusion The large tetraalkylammonium ions are hydrophobic. Since the hydrophobic property of tetraalkylammonium ions eventualh/ should vanish with addition of MeCN to water, it has been expected that ra-Bu4NBr and ra-Pen4NBr prefer to leave water structure in aqueous MeCN. p values obtained from equation 9 are more than unity, p>l, for ra-Bu4NBr and ra-Pen4NBr which means that these solutes are preferentially solvated by MeCN. These results are in consistence with known hydrophobicity of these solutes and actually a good confirmation of the new developed solvation theory. The enthalpies of transfer LiBr, n-Bu4NBr and ra-Pen4NBr from water to aqueous MeCN obtained from equation 9 are in excellent agreement with the experimental data (Figure 1) over the whole range of solvent composition. References 1. G. Rezaei Behbehani, M. Dillon, J. Smyth, W. E. Waghorne,/. Solution Chem. 2002, 31, 811-822. 2. G. Rezaei Behbehani, D. Dunnion, P. Falvev, K. Hickev, M. Meade, Y. McCarthy, C. R. Symon, W. E. Waghorne, /. Solution Chem. 2000, 29, 521-539. 3. 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Waghorne,/. Chem. Soc, Faraday Trans. 1985, 81, 2702-2709. 14. E. V. Goldammer, H. G. Hertz, /. Phys. Chem. 1970, 74, 3734—3755. 15. J. A. Ripmeester, D. W. Davidson, /. Mol. Struct. 1981, 75, 67-76. 16. D. Feakins, J. Mullally, W. E. Waghorne /. Chem. Soc, Faraday Trans. 1991, 87, 89-91. Povzetek Določili smo entalpije prenosa, ?H?t, za LiBr, tertabutilamonijev bromid, n-Bu4NBr, in tetrapentilamonijev bromid, n-Pen4NBr, iz vode v mešanice acetonitrila in vode. Izmerjene vrednosti smo obravnavali s pomočjo nove solvatacijske teorije. Dobljeni parametri solvatacije kažejo, da n-Bu4NBr ruši medmolekulske vezi topilo-topilo in da je preferenčno solavtiran z acetonitrilom, kar je v skladu z znano hidrofobnostjo spojine. LiBr je preferenčno solvatiran z vodo. Behbehani Application of the New Solvation Theory