Acta Chim. Slov. 2006, 53, 363–366 363 Scientific Paper Using the Extension Coordination Model to reproduce the Enthalpies of Transfer of Tetraethylurea From Water to Aqueous Ethanol, Propan-1-ol and Acetonitrile at 298 K. G. Rezaei Behbehani,1,2 E. Tazikeh 2 and A. A. Saboury 3 1 Department of Chemistry, Islamic Azad University, Takastan, Iran 2 Department of Chemistry, Imam Khomeini International University Qazvin Iran, E-mail:rezaeib@ikiu.ac.ir, grb402003@yahoo.com 2 Institute of Biochemistry and Biophysics, University of Tehran, Tehran, Iran. Received 23-11-2005 Abstract The enthalpies of transfer of tetraethylurea, TEU, from water to aqueous ethanol, EtOH, propan-1-ol, PrOH, and acetonitrile, MeCN, are reported and analysed in terms of the Extension Coordination Model. The analyses show that the solvation of TEU is random in the considered solvent mixtures. It is also found that TEU interact more strongly with EtOH or PrOH than water. Key words: titration calorimeter; preferential solvation; tetraethylurea; variable (an+ @N) 1. Introduction The Extension of the simple Coordination Model to take account of the effects of changes in solvent-solvent interactions is relatively straightforward and has been described in detail elsewhere.1-2 Briefly a solute occupies a cavity in the solvent structure, surrounded by its coordination sphere of n solvent molecules. The solvation in mixed solvents is analogous to complexation, with the better solvent taking the role of the ligand. In order to complex to the solute each of these n molecules will have broken some fraction, a, of their bonds to other solvent molecules, resulting in endothermic enthalpy change of - naAAH"" where AAH°* 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 AAff" we can set it equal to an enthalpy change, - nfiAAH", where /5 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 AAH12. After introducing the approximation that values of a and /5 are constant over a range of solvent compositions, and some manipulation this leads to: A#f = ( PXB \\ xA + pxB jan + pN) xA + pxB AAH[2 + (an + /8N)AAH (xALA +pxBLB) where AH? is enthalpy of transfer of the solute from pure solvent A to mixtures of A and a second solvent B. xA 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 neighbours of the solute. 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, AHE, as follow: L = AH + x dAH1 dxB i 7) A J-f (1) *l dxB AAH°" is the difference between the A-A and B-B interactions in the two pure solvents and is taken as the difference between the enthalpies of condensation of the pure components. AAH? is the difference between the solute-B and solute-A interactions in the pure solvents, 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. The parameter (an+pN) reflects the net effect of the solute on the solvent-solvent bonding with an resulting from the formation of a cavity wherein n solvent molecules become the nearest neighbours of the solute and /N reflecting the enthalpy change from strengthening or weakening of solvent-solvent bonds of N solvent molecules (N?n) around the cavity (/3<0 indicates a net strengthening of solvent-solvent bonds). The superscript 6 in all cases refers to the quantities in E L = AH - x B Behbehani et al. Using the Extension Coordination Model to Reproduce.... 364 Acta Chim. Slov. 2006, 53, 363–366 infinite dilution of the solute. Preferential solvation is accounted for by p, which is defined by: Table 1. Enthalpies of transfer of TEU from water to aqueous ° EtOH, PrOH and MeCN mixtures at 25 C in kJ mol-1. NA N ;=; ------- zz pxB p<1 or p>1 indicate a preference for solvent A or B respectively; p =1 indicates random solvation. Analyses of AH? for 2-methylpropane-2-ol, TBA and a series of teteraalkylammonium halides in mixed aqueous solvents revealed that AH? could not be reproduced by equation 1 over the whole range of solvent compositions.3-6 2. Experimental and Results The solvents710 were purified as described previously. The enthalpies of transfer of the solutes were calculated from their enthalpies of solution, AHS, into the different solvent systems. In all cases the enthalpies of solution were measured to ten solute concentrations (0.002-0.1 mol dm-3) and the data 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” software program. The microcalorimeter was frequently calibrated electrically during the course of the study. Enthalpies of transfer have been reported as 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 these were around 0.05 kJ mol-1, or better with repeat abilities at the same level. 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 vary accordingly; thus, the precisions for enthalpies of transfer were about 0.07 kJ mol-1, or better. The enthalpies of transfer of TEU from water to aqueous EtOH, PrOH and MeCN mixtures are listed in Table 1. 3. Discussion The significant reason for the failure of equation 1, is the approximation of constant value for (an + /N) over the entire range of solvent compositions. The sensitivity of (an + /N) to the solvent compositions and the unsuitable assumption of the constant value for (an + /N) in equation 1 led us to extend this equation. One goal of the development of the previous solvation model, is the prediction of the thermodynamics consequences of changes in the solvent system. A second approach is to use the extended equation analytically, to obtain information about the fundamental solvation process. EtOH PrOH MeCN 0 0 0 0 0.12 0.04 0.05 0.02 0.14 0.08 0.09 0.04 0.15 0.13 0.13 0.06 0.16 0.18 0.18 0.08 0.18 0.23 0.22 0.11 0.21 0.29 0.27 0.14 0.24 0.34 0.32 0.17 0.28 0.39 0.37 0.20 0.35 0.44 0.41 0.24 0.44 0.49 0.45 0.27 0.61 0.54 0.49 0.31 0.68 0.58 0.53 0.34 1.00 0.00 1.42 -0.1 In the case of random solvation ( p =1), equation 1 simplifies to: AH? = xB [aAH?2 +(an + J3N)AAH°* J -(an + J3N)AHE (2) Where AHE represents the excess enthalpy of the mixed solvent. The enthalpy of transfer from pure solvent A to pure solvent A->B B, a ff o, is simply: A H? = [AAH?2 +(an + fiN)AAH" ] (3) So that equation 2 rearranges to: ^ = (an + J3N) (4) AH As (?n + ßN) is not constant over the range of solvent composition, it is possible to change to: AH xB A H, AH (oax + pNY (5) If the solvation is random, it is possible to define the net effect of the solute on solvent-solvent bonds in mix mixture, (an + pN) , as a combination of these values in water-rich domain, (an + /3N)A, and alcohol-rich domain, (an + J3N)B, which can be written: (an + pN)m,x =(an + pN)AxA+(an + pN)eBxB (6) n x X R Behbehani et al. Using the Extension Coordination Model to Reproduce.... Acta Chim. Slov. 2006, 53, 363–366 365 Comparing equations 5 and 6 leads to: AH? = A H?xB+(an + /8N)AAHE -[(an + pN)eB-(an + pN)A\cBAHE (8) A//L for non-random solvation is x'ALA + x^Zj, where x^ and x'B are the local mole fractions of the solvent A and B respectively. If we apply non-random conditions to equation 8, A->B AH? = A H?xB+(an + j3N)A[xALA+xBLB] -[(an + fM)f'B-(an + fM)A\c'B[x'ALA+x'BLB\ (9) Where pxB pxB pxB AH? 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 + pN)eA and (an + j3N/B 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 solvent A to pure solvent B, \ h6 which is the coefficient of the first t term in equation 9 is as follow: A^B A H? = [ (AH?2)B-(AH?2)A ] + (an + pN)eBAH"B -(an + pN)AAH°A (10) Where [(AH?2)B - (AH?2)A \ is the relative strengths of solute-solvent bonds in the pure solvents including intramolecular contribution. For simplification it is written as AAH?2 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. AHA and AHB are the enthalpies of condensation for pure solvent A and B respectively. Applying equal value for (an + pN)A and (an + j3N/B in equation 10 leads to: A H? = AAH?2 +(an + /3N)(AHA - AHB ) (11) This is equation 3. If (an + pN)eA = (an + PN)B (an + /N), equation 9 reduces to equation 1. 14- 12 10 0.8 0.6 0.4- 0.2 0.0 -0.2 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Figure 1. Comparison of the experimental (symbols) and calculated (lines) enthalpies of transfer of TEU from water to aqueous EtOH (•), PrOH (?) and MeCN(?) via equation 9. Equation 9 has been shown to reproduce AH? for both electrolytes and non-electrolytes in a wide range of mixed aqueous and non-aqueous solvents.10-11 Using equation 9 reproducing the enthalpies of transfer shows excellent agreement between the experimental and calculated data (Figure 1) over the whole range of solvent compositions for TEU in aqueous EtOH, PrOH and MeCN. Solvation parameters recovered via equation 9 were reported in Table 2. In the all cases, (an + pN)eA values are negative, indicating that the net effect of TEU is a strengthening of solvent-solvent bonds in water-rich domains. At low concentration, the shorter-chain alcohols, and other small organic molecules, enhance the structure of water.11-15 In aqueous alcohols (an + j3N)eA values decrease from aqueous ethanol to propan-1-ol, indicating that the enhancement of water structure by propan-1-ol is more than that of by ethanol. Thus we can conclude that at very low alcohol concentrations alcohol monomers are present in an aqueous structure which has been rigidified by interactions with alcohol alkyl residue. This is supported by results from NMR measurements of relaxation times of water and organic molecules in the mixtures,13 Xe NMR chemical shifts,14 neutron scattering,15 and dielectric relaxation.16 The positive values of (an + j3N/B indicates disruption of the alcoholic structure by TEU in the alcohol-rich region. The (an + pN)eB value for aqueous acetonitrile is negative, indicating that urea strengthen the acetonitrile structure. Both (an + /3N)A and (an + pN)eB values for water-acetonirile mixtures is negative, indicating that the mobilities of both components decreases in 1 Behbehani et al. Using the Extension Coordination Model to Reproduce.... 366 Acta Chim. Slov. 2006, 53, 363–366 the mixtures. This is in agreement with the diffusion coefficients of water and acetonitrile in water-acetonirile mixtures.17 p value for TEU in aqueous EtOH , PrOH and MeCN is one, indicating that solvation of TEU in these solvent mixtures is random. ??H1?2<0 indicates weaker interaction of TEU with water. Table 2 Solvation parameters for TEU in mixtures of water with EtOH, PrOH and MeCN via equation 9. ??H?12 >0 indicates stronger interaction of TEU with water. Solvent system H20-EtOH H20-PrOH H20-MeCN p 1 1 1 (m+/N)A -0.38 -0.59 -0.10 (an + j3N)"B 3.06 2.50 -0.56 ??H1?2 (kJ / mol) 134.06 130.09 -13.00 4. Conclusion Using equation 9 reproducing the enthalpies of transfer of TEU from water to aqueous ethanol, propan-1-ol and acetonitrile shows excellent agreement between the experimental and calculated data (Figure 1) over the whole range of solvent compositions which is a good support for this equation. It is clear, from these few examples and the previous published cases,10-11 that analysis of the enthalpies of transfer in this way can give remarkable insights into solvation in mixed solvents. Comparing these with the results of studies using other techniques supports this predictive theory. 5. References 1. G. Rezaei Behbehani, M. Dillon, J. Smyth and W. E. Waghorne, J. Solution Chem. 2002, 31, 811–822. 2. G. Rezaei Behbehani, D. Dunnion, P. Falvey, K. Hickey, M. Meade, Y. McCarthy, C. R. Symon, and W. E. Waghorne, J. Solution Chem. 2000, 29, 521–539 3. E. de Valera, D. Feakin and W. E. Waghorne, J. Chem. Soc. Faraday Trans. 1983, 79, 1061–1066. 4. E. de Valera, D. Feakins and W. E. 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E. Waghorne J. Chem. Soc. Faraday Trans. 1991, 87, 89–91. 16. F. Franks, J. Ravenhill, P. A. Egelstaf, D. I. Page, Proc. R. Soc. London, Ser. A 1970, 319, 189–192. 17. A. Easteal, Aust. J. Chem. 1980, 33, 1667–1675. Povzetek I Določili smo entalpije prenosa, AHt, tetraetiluree (TEU) iz vode v različne vodne mešanice etanola, 1-propanola I in acetonitrila. Rezultate smo analizirali s pomočjo razširjenega koordinacijskega modela solvatacije. Ugotovili smo, da je v obravnavanih mešanih topilih solvatacija TEU sicer naključna, da pa je interakcija TEU z etanolom I I in 1-propanolom močnejša kot z vodo. Behbehani et al. Using the Extension Coordination Model to Reproduce....