J. Chem. SOC., Faraday Trans. I , 1985,81, 1555-1561 Thermodynamic Functions for the Transfer of 1 -Naphthoic Acid from Water to Mixed Aqueous Solvents at 298 K BY M. CARMEN PEREZ-CAMINO, ENRIQUE SANCHEZ, MANUEL BALON AND ALFREDO MAESTRE* Departamento de Fisicoquimica Aplicada, Facultad de Farmacia, Universidad de Sevilla, 41012 Sevilla, Spain Received 26th June, 1984 Values of free energies, AG?, enthalpies, AH?, and entropies, A F , of transfer of 1-naphthoic acid from water to water + ethanol, water + t-butyl alcohol and water + dimethyl sulphoxide (DMSO) mixtures at various mole fractions have been determined from solubility measurements at different temperatures. Variations of AG? with xcosolvent in the water-rich alcoholic mixtures are governed by the sign of the entropy term.For water + DMSO mixtures AGP is negative, but the variation of AH? with xDMSO shows different behaviour from that of water + alcohol mixtures. We have calculated molecular pair-interaction parameters (g,,, h,, and Ts,,) in water for the three systems studied; h,, > 0 for water+alcohol mixtures and h,, < 0 for water + DMSO mixtures. The results arediscussed in terms of solute-cosolvent and solute-water interactions. In the last few years, several studies, using different experimental techniques, have been made of mixed aqueous solvents in order to gain a better understanding of the structural properties of water. However, there is still disagreement as to the nature of the interactions responsible for certain thermodynamic functions, some of which have been studied using ele~trolytesl-~ and non-electrolytes6-10 as probes (transfer functions) and others using mixed aqueous solvents (excess functions).It has been observed that adding small quantities of an organic solvent to water (water-rich region) can reduce, increase or scarcely affect its three-dimensional structure, according to the properties of the cosolvent. It is known that alcohols enhance the structure of water," dimethylformamide (DMF) and dioxane scarcely affect it12 and sulpholane and acetonitrile reduce it.13 The concentration dependence of the transfer functions in mixed aqueous solvents is a measure of the perturbation of the water and cosolvent components in the vicinity of solute transferred as the concentration of the cosolvent is changed.Depending on the regions upon which the study is focused, different interactions will predominate.12 The purpose of this paper is to report thermodynamic transfer functions for un-ionized 1 -naphthoic acid by measuring the temperature dependence of its relative solubilities in water and three mixed solvents (water + ethanol, water + t-butyl alcohol and water + dimethyl sulphoxide) at various concentrations. Molecular pair- interaction parameters, solute-cosolvent, in water have been determined in order to obtain a more detailed description of the aqueous mixtures studied. 15551556 THERMODYNAMICS OF TRANSFER FROM WATER TO MIXED SOLVENTS EXPERIMENTAL The solvents ethanol (EtOH), t-butyl alcohol (ButOH), and dimethyl sulphoxide (DMSO) were all reagent-grade chemicals (Merck) and were used without purification.Water was distilled twice using an all-glass apparatus. 1-Naphthoic acid (Fluka, purum grade, > 97%) was recrystallized twice from water + EtOH, dried at 105 "C and stored in a vacuum desiccator over silica gel. The solvents were prepared by mixing weighted quantities of water and cosolvent. For the solubility measurements, an excess of 1-naphthoic acid was added to water or the mixed solvent (10-80 wt%) in order to obtain saturated solutions, which were then agitated by a magnetic stirrer for 24 h at 55-60 "C, transferred to a thermostatted bath at seven different temperatures (1040 "C) and left for at least 6 days to attain equilibrium. Two aliquots of the same sample and four samples for each experiment were removed and analysed by two different methods.(a) The total solubility of the 1-naphthoic acid was quantified by titrating aliquots with standard NaOH solution and phenolphthalein indicator for water and water + alcohol (when the cosolvent was DMSO we used either thymol blue or cresol red indicators because these exhibit a behaviour similar to that seen in pure water14). (b) A spectrophotometric technique using a Perkin-Elmer Lambda 5 ultraviolet-visible spectrophotometer was used to measure the total solubility of 1 -naphthoic acid (AH + A-) and that of the un-ionized form (AH). The two methods agree to within 3%. The precision in two runs was 3% at the lower concentration of cosolvent and pure water and 1 % at higher concentrations. It was considered that 1-naphthoic acid was un-ionized when the concentration of the cosolvent was 50 wt% EtOH, 50 wt% DMSO and 30 wt% ButOH.RESULTS The solubilities of the 1-naphthoic acid in water and water+cosolvent in terms of log S (table 1) were fitted by a non-linear regression analysis15 to the expression l o g s = A-B/T+ClnT (1) where S is the solubility in mol dm-3, T is the temperature in K and A , B and C are adjustable parameters. The derived values of AGP, A@ and AH? were calculated from the equations AGP = - 2.302 RT log (SJS,) (2) AGP = (AL-A&) T+(Bi-Bk)+(Ci-C&) Tln T (3) AH? = (BL-B&)+(Ck-CL) T (4) ASP = (Ak-AL)+(Ck-Ci)+(Ck-Ci) In T ( 5 ) where the subscripts w and s represent water and mixed solvents, respectively, A' = -2.303RA, B' = 2.303RB and C' = -2.303RC.The values of A , B and C in water and mixed solvent are given in table 2 for un-ionized 1-naphthoic acid along with AGP, A@ and AH? at 298.15 K. The maximum uncertainties in the transfer- function values were calculated from standard deviations associated with A , B and C and were f 5% in AGP and AH? and 10% in ASP. We have assumed that the ratio of activity coefficients of un-ionized 1 -naphthoic acid in the saturated solutions (water and mixed solvents) is unity. This may not be correct in the case of higher concentrations of DMSO (> 70 wt%), where the solubility of the acid is high.M. C. PEREZ-CAMINO, E. SANCHEZ, M. BALON AND A. MAESTRE 1557 Table 1. Values of -log (S/mol dm-3) for un-ionized 1 -naphthoic acid in various mixed aqueous solvents at different temperatures - mole T/"C fraction cosolvent (wt%) 10 15 20 25 30 35 40 - ethanol 10 20 30 50 70 20 30 50 70 dimethyl sulphoxide 10 20 30 50 65 70 75 80 t-butyl alcohol 10 water 3.62 3.47 3.34 3.19 3.04 2.93 2.82 3.19 3.08 2.91 2.81 2.62 2.47 2.32 2.53 2.40 2.27 2.09 2.01 1.87 1.75 1.34 1.21 1.15 1.05 0.95 0.86 0.78 0.80 0.70 0.64 0.57 0.50 0.42 0.33 3.62 3.47 3.31 3.14 3.08 2.92 2.73 2.87 2.65 2.41 2.26 2.15 1.92 1.79 1.73 1.60 1.52 1.41 1.28 1.17 1.08 1.06 0.97 0.85 0.81 0.73 0.66 0.57 0.75 0.66 0.55 0.51 0.42 0.35 0.29 3.59 3.46 3.31 3.20 3.12 2.99 2.87 3.19 3.08 2.97 2.86 2.67 2.56 2.49 2.83 2.74 2.62 2.49 2.34 2.23 2.11 1.77 1.68 1.59 1.48 1.35 1.21 1.11 0.82 0.71 0.59 0.50 0.40 0.25 0.14 0.40 0.29 0.17 0.11 -0.03 -0.05 -0.14 -0.05 -0.10 -0.13 -0.16 -0.22 -0.28 -0.33 -0.38 -0.40 -0.42 -0.43 -0.45 -0.46 -0.47 3.97 3.81 3.70 3.59 3.43 3.33 3.26 Table 2.Coefficients of eqn (1) and thermodynamic functions for transfer from water to water + cosolvent at 298 K mole fraction AG? As? AH? cosolvent (wt%) A BIK C /kJ mol-l /J K-l mol-l /kJ mol-l ethanol 10 20 30 50 70 t - but yl 10 alcohol 20 30 50 70 sulphoxide 20 30 50 65 70 75 80 dimethyl 10 water 63.741 13.596 -271.97 -11.661 - 80.630 - 89.134 201.74 - 77.556 41.563 86.230 33.266 95.503 - 79.875 - 280.72 - 177.32 178.50 - 145.05 34.597 131.1 1 5014.2 -8.7951 -9748.3 41.5070 2672.4 - 1.1856 918.9 2.4044 2422.1 12.626 - 1668.8 14.104 1 1 746.3 - 28.890 - 1742.0 12.341 3 075.4 - 5.6245 5005.0 - 12.275 3 395.9 - 1571.1 6 242.1 - 10728.0 -6 164.7 9288.4 1745.6 - 5 749.6 - 4.4029 12.598 42.697 27.406 22.104 - 13.520 - 25.876 - 4.9684 7 800.2 - 19.046 - 2.28 - 4.45 - 8.56 - 14.50 - 17.24 - 2.57 - 7.59 - 12.45 - 15.87 - 17.58 - 2.23 -4.17 - 6.28 - 12.05 - 17.64 - 19.87 -21.41 - 22.95 24 47 40 17 7 34 90 30 7 9 4 18 27 32 51 31 -11 - 43 5.03 9.66 3.43 - 9.43 - 15.07 7.55 19.16 - 3.62 - 13.94 - 14.92 - 0.95 1.22 1.74 - 2.53 - 2.34 - 10.52 -24.53 - 35.701558 THERMODYNAMICS OF TRANSFER FROM WATER TO MIXED SOLVENTS 4 52 Fig.1. Variation of AGf) and T A P for the transfer of 1-naphthoic acid from water to water + EtOH (O), water + BdOH (A) and water + DMSO (0) at 298 K. DISCUSSION TRANSFER FROM WATER TO WATER 4- ALCOHOL (EtOH OR BUtOH) The decrease in AG? with the mole fraction (fig. 1) indicates preferential solvation of the 1-naphthoic acid molecule by alcohol.In the water-rich region the rise in AH? is clearly compensated by a still larger contribution from - A p . Therefore, the entropy term governs the sign of AGP in this region. Both A@ and AH? reach maxima that have frequently been related to an enhancement of the lo, l6, l7 In general, the position, height and sharpness of these maxima depend on the nature and size of the solute transferred. For other non-electrolytesBv lo* the size seems to be the controlling factor for a given water+alcohol mixture; i.e. the larger the hydrophobic group of the non-electrolyte, the more the maximum is shifted towards lower mole fraction of alcohol. In addition, an increase in the hydrophobic nature of the alcohol (e.g. methyl to t-butyl) produces the same effect.M.C. PEREZ-CAMINO, E. SANCHEZ, M. BALON AND A. MAESTRE Table 3. Molecular pair-interaction parameters (in J kg m o P ) in water at 298 K 1559 ~ X Y ref. 1-naphthoic acid EtOH ButOH DMSO benzoic acid MeOH EtOH PriOH p-nitroaniline EtOH Pr’OH ButOH naphthalene EtOH - 478 -719 - 843 - 198 - 250 - 308 - 96 - 538 -116 - 409 1430 2776 - 755 419 733 1640 736 3060 553 1 1972 1908 3495 88 61 7 983 1948 832 3598 5617 2384 this work this work this work 9 9 9 10 10 10 26 s,,/J kg mol-I Fig. 2. Enthalpy+xtropy compensation plot for the transfer of naphthalene (A), 1 -naphthoic acid (O), benzoic acid (a) and p-nitroaniline (A), from water to water+EtOH mixtures. We have calculated the molecular pair-interaction parameters, 1 -naphthoic acid- cosolvent (J,,,f= g , h, Ts), in water (table 3) on the basis of the Gibbs free energies and enthalpies of transfer for 1-naphthoic acid from water to water +cosolvent mixtures.The Ts,, were derived from g,, and h,, using the expression Ts,, = h,,-g,,. For the sake of comparison, table 3 also shows f,, of other non-electrolytes. The compensating effect between AH? and - T A* is also shown by h,, and Ts,,. In water+alcohol mixtures h,, are positive and increase with the alkyl group of the alcohol. These positive values mean unfavourable interactions with the alcohol. The introduction of a 1-naphthoic acid molecule to a water-rich alcohol mixture can produce a breakdown of hydrogen bonds between the water and the1560 THERMODYNAMICS OF TRANSFER FROM WATER TO MIXED SOLVENTS alcohol (hZy increases) and dispersion interactions and dipole-dipole interactions should occur between the 1 -naphthoic acid molecule and the alcohol (hXy decreases).Evidently, the former effect is stronger than the latter, resulting in h,, > 0. The greater value of h,, for ButOH with respect to that for EtOH is a consequence of stronger hydrogen bonds. The same pattern is shown by other hydrophobic non-electrolytes with alcohols (table 3). Ts,, values are positive, which seems to be connected with the size of the alcohol and may be due to the competitive effect (alcohol-water against alcohol-solute interactions) previously mentioned for h,,. This type of enthalpy- entropy compensation behaviour is shown by plotting h,, against szy for various non-electrolytes in water + EtOH mixtures (fig.2). TRANSFER FROM WATER TO WATER + DMSO In the case of water + DMSO mixtures, AGP decreases as the proportion of DMSO in the solvent increases (fig. 1). However, whereas AS? passes through a maximum (xIIMSO = 0.30-0.35, fig. l), AH? behaves differently from water + alcohol mixtures. This sort of behaviour of AH? has been observed for the transfer of un-ionized o-methoxybenzoic acidlg and p-hydroxyaniline20 from water to water + DMSO. Several studies using different techniques on the effect of DMSO on the water structure have proved 22 maintains that DMSO is a typical non-aqueous non-electrolyte because its aqueous mixtures are in fact characterized by IAHEI > TIAS”\. However, it seems that in the water-rich region strong water-DMSO interactions occur,24 possibly of the dipole-dipole type.25 We have calculated the molecular pair-interaction parameters, f,,, in water (table 3) and they show a pattern that is different from that obtained for water+alcohol mixtures; i.e.although g,, is negative, h,, and Ts,, take negative and small positive values, respectively. A value of h,, < 0 for the 1-naphthoic acid-DMSO pair means that a favourable interaction occurs. If the interactions between water and DMSO are preferentially of the dipole-dipole type in the water-rich region, then adding 1-naphthoic acid to a water-rich DMSO mixture could conceivably produce a com- petitive effect on the DMSO. The h,, < 0 value may be explained by supposing that dipoledipole and dispersion interactions between DMSO and 1 -naphthoic acid are stronger than water-DMSO interactions.A small positive value of Ts,, would be in accordance with the above interpretation. CONCLUSIONS In the transfer of non-electrolytes from water to aqueous mixtures, AG? accounts for solute-medium interactions only. Therefore it is necessary to determine the entropy and enthalpy terms. This is supported by analysis of molecular pair-interaction parameters, since these parameters (hx, and Ts,,) represent not only solute-cosolvent interactions, but also cosolvent-water interactions. We thank Dr Sanchez-Burgos, University of Seville, for helpful discussions. K. Bose, K. Das, A. K. Das and K. K. Kundu, J . Chem. SOC., Faraday Trans. I, 1978,74, 1051 and references therein.0. Popovych and R. P. T. Tomkins, Nonaqueous Solution Chemistry (Wiley, New York, 1981), chap. 4. K. K. Kundu and A. J . Parker, J . Solution Chem., 198 1, 10, 847. M. H. Abraham, T. Hill, H. Chiong Ling, R. A. Schulz and R. A. C. Watt, J . Chem. SOC., Faraday Trans. I , 1984, 80, 489.M. C. PEREZ-CAMINO, E. SANCHEZ, M. BALON AND A. MAESTRE 1561 F. Rodante and M. G. Bonicelli, Thermochim. Acta, 1983, 66, 225. S. Murakami, R. Tamaka and R. Fujishiro, J. Solution Chem., 1974, 3, 71. M. Roseman and W. P. Jenks, J. Am. Chem. SOC., 1975,97, 631. H. Gillet, L. Avedikian and J. P. Morel, Can. J. Chem., 1975, 53, 455. K. Bose and K. Kundu, Can. J. Chem., 1977,553961. lo K. Das, A. K. Das, K. Bose and K. K. Kundu, J. Phys. Chem., 1978,82, 1242. l 1 F. Franks and D. J. G. Ives, Q. Rev. Chem. Soc., 1966, 20, 1. l2 C. Visser, G. Perron and J. E. Desnoyers, J. Am. Chem. SOC., 1977,99, 5894. l 3 G. Petrella and M. Petrella, Electrochim. Acta, 1982, 27, 1733. l4 M. Georgiera, P. Zokolov and 0. Budevsky, Anal. Chim. Acta, 1980, 115, 41 1. l5 K. J. Johnson, Numerical Methods in Chemistry (Marcel Dekker, New York, 1980), chap. 5. l6 N. Dollet and J. Juillard, J. Solution Chem., 1976, 77, 5 . l 7 M. C. R. Symons and M. J. Blandamer, in Hydrogen-bonded Solvent Systems, ed. A. K. Covington and P. Jones (Taylor & Francis, London, 1968), p. 21 1. ** N. R. Choudhury and J. C. Ahluwalia, J. Solution Chem., 1982, 11, 189. lQ F. Rodante, G. Ceccaron and F. Fantauzzi, Thermochim. Acta, 1983, 67, 45. 2o F. Rodante and M. G. Bonicelli, Thermochim. Acta, 1983, 66, 225. 2 1 G. Petrella, M. Petrella, M. Castagnolo, A. Dell’Atti and A. De Giglio, J. Solution Chem., 1981, 10, 22 G. Petrella and M. Petrella, Electrochim. Acta, 1982, 27, 1733. 23 F. Franks, in Hydrogen-bonded Solvent Systems, ed. A. K. Covington and P. Jones (Taylor & Francis, 24 M. F. Fox and K. P. Whittingan, J. Chem. Soc., Faraday Trans. 1, 1975,71, 1407. 25 R. K. Wolford, J. Phys. Chem., 1964,68, 3392. 26 D. Bennet and W. J. Canady, J. Am. Chem. SOC., 1984, 106,910. 129. London, 1968), p. 34. (PAPER 4/1098)