J . Chem. SOC., Faraday Trans. I, 1982, 78, 37-42 Solute-Solvent Interactions in Water + t-Butyl Alcohol Mixtures Part 1 1 .-Enthalpies of Transfer of Ammonium and Tetra-alkylammonium Salts BY JEAN JUILLARD Laboratoire d’Etude des Interactions Solutis-Solvants, Universite de Clermont 11, BP 45, 63170 Aubiere, France Received 29th September, 1980 Enthalpies of solution of tetra-alkylammonium halides (from ammonium to tetrapentylammonium) in Enthalpies of transfer thus obtained are discussed in terms of solvent structure and solute-solvent mixtures of water and t-butyl alcohol (from 0 to 40% by weight) are reported. interactions. There have been many studies of solute properties in water + t-butyl alcohol (TBA) mixtures. As the largest alcohol which is entirely miscible with water, TBA added to water induces some striking features.Most of them have been attributed to a strong enhancement of the water network when the first amounts of TBA are added. It is accepted by many authors that this increase reaches a maximum and that the so-called ‘ structuration maximum ’ corresponds to extrema in the curves of the thermodynamic properties of the solutes. This whole question has been amply discussed in previous papers of this series.’ As far as enthalpies of transfer are concerned many types of solute have been studied. All of these enthalpies, except those corresponding to tetrabutylammonium bromide, studied by Alhuwalia,2 go through a maximum when successive fractions of TBA are added to water. This unusual behaviour prompted us to study the whole series of the tetra-alkylammonium bromides.EXPERIMENTAL t-Butyl alcohol and water were purified as previously stated.l Ammonium chloride and tetra-alkylammonium bromides were pure commercial products (Fluka puriss) used as received. Tetrapentylammonium bromide was kindly provided by G. Perron and J. E. Desnoyers. Heats of solution were obtained using an LKB solution calorimeter following a procedure already described. RESULTS The standard molal enthalpies of solution for ammonium chloride, tetramethyl-, tetraethyl-, tetrapropyl-, tetrabutyl- and tetrapentyl-ammonium bromides are reported in table 1. From the previously reported4 values of pK, of the ammonium ion it can be ascertained that there is no significant hydrolysis of the ammonium chloride under the experimental conditions of the calorimetric measurements.Data supplied by Alhuwalia et aZ.2+ were available for tetrabutyl- and tetrapentyl-ammonium bromides; for these two salts our investigations were restricted first to four mixtures in order to compare both our methods and results with theirs. There was excellent agreement between our results and those of Alhuwalia et al. for NBu,Br. The same was not true 3738 INTERACTIONS IN WATER -I- t-BUOH MIXTURES TABLE 1 .-ENTHALPIES OF SOLUTION OF TETRA-ALKYLAMMONIUM BROMIDES AND AMMONIUM CHLORIDE weight ma AHsb AHFC AH? % TBA mol kg-' /kJ mol-l /kJ mo1-I /kJ mo1-I 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 0 10 20 30 40 0 5 10 15 20 25 30 40 tetramethylammonium bromide 1.1 24.53 f 0.05 24.4 1.3 25.96 f 0.04 25.8 1.3 27.53 k0.04 27.3 1.3 28.82 f 0.02 28.5 1.2 29.61 f 0.03 29.3 1.2 29.87 k0.03 29.5 1.4 29.86 f 0.04 29.3 1.3 29.73 & 0.05 29.1 1.3 29.45 f 0.04 28.7 tetraethylammonium bromide 1 .o 6.30 f 0.01 6.1 1 .o 8.53 k0.03 8.3 1.2 1 1.32 f 0.01 11.1 1.1 14.04 f 0.03 13.8 1 .o 15.73 fO.01 15.4 1 .o 16.47 f 0.02 16.1 1.2 16.97 & 0.02 16.5 1.2 17.25 & 0.01 16.7 1.1 17.49 & 0.02 16.8 tetrapropylammonium bromide 1.1 - 4.29 f 0.01 -4.5 1.1 0.00 & 0.02 - 0.2 1.1 5.61 fO.01 5.4 1.1 12.21 fO.01 11.9 1.2 16.95 f 0.04 16.6 1.1 19.08 f0.02 18.7 1.2 20.30 f 0.03 19.8 1.2 21.14 & 0.03 20.6 1.2 22.20 f 0.02 21.5 tetrabutylammonium bromide 0.75 8.32 f 0.02 8.1 0.70 3 1.36 f 0.03 31.1 0.74 35.35 f 0.03 34.9 0.8 1 35.95 f 0.02 35.3 tetrapentylammonium bromide 0.55 3.83 +O.Ol 3.8 0.53 14.57 k0.03 14.4 0.53 3 1.02 f 0.02 30.8 0.50 52.26 f 0.06 52.0 0.50 68.78 f 0.16 68.5 0.47 68.30 f 0.05 68.0 0.53 64.63 & 0.09 64.3 0.53 59.20 0.10 58.7 0.85 -8.31 fO.O1 - 8.5 1.4 2.9 4.1 4.9 5.1 4.9 4.7 4.3 2.2 5.0 7.7 9.3 10.0 10.4 10.6 10.7 4.3 9.9 16.4 21.1 23.2 24.3 25.1 26.0 16.6 39.6 43.4 43.8 10.6 27.0 48.2 64.7 64.2 60.5 54.9J.JUILLARD 39 TABLE 1. (cont.) ~~ ~~ weight ma AHsb AH?' AH? % TBA mol kg-l /kJ mol-l /kJ mol-l /kJ mol-1 0 5 10 15 20 25 30 35 40 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .O 1 .o ammonium chloride 15.01 15.89 16.85 17.93 19.06 19.24 19.01 18.57 17.94 14.85 15.69 0.84 16.61 1.76 17.65 2.8 1 18.73 3.88 18.85 4.05 18.54 3.70 18.02 3.17 17.28 2.44 a Approximate mean concentration of the various measurements; * mean value of 2 or 3 maximum uncertainty 0.1 (except for NH,Cl, measurements with maximum uncertainty ; 0.02).I 8o t I 1 I I 0.05 0.10 0.15 X FIG. 1 .-Standard molal enthalpies of solution of tetrabutylammonium bromide (lower curve) and tetra-n-pentylammonium bromide (upper curve) plotted against mole fraction of TBA in the water +TBA mixtures ( x , our data; 0, data of Alhuwalia et U I . ~ . ~ ) ) .40 INTER ACTIONS I N WATER + t-BUOH MIXTURES " I --- 0 OlO 5 0:10 X FIG. 2.-Standard molal enthalpies of transfer of 0, ammonium; 0, tetramethylammonium; ., tetraethylammonium ; A, tetrapropylammonium; V, tetrabutylammonium; and 0, tetrapentylammonium bromides from water to water + TBA mixtures plotted against TBA mole fraction.for NPen,Br as is shown in fig. 1, and other data have therefore been recorded in order to confirm our results. The standard molal enthalpies AH? are obtained from the molal enthalpies AHs at finite concentration (also given in table 1) corrected for the heat of dilution. This last term is calculated using the Debye-Hiickel extended law as applied to enthalpies in the form we have already proposed.6 From these values of standard molal enthalpies of solution, enthalpies of transfer of the various salts from water to the mixtures have been obtained. In order to facilitate a comparison of the ammonium ion with the tetra-alkylammonium ion the values for NH,Br were calculated from the corresponding enthalpies of transfer of NH,Cl, KC16 and KBr.la In fig.2 the enthalpies of transfer for all the bromides are plotted against the mole fraction of t-butyl alcohol in the mixtures. Tetra-alkylammonium bromide-t-butyl alcohol interactions in water have been previously studied by Perron et at.' Such studies require either measurements taken in solution of NR,Br in t-butyl alcohol+water mixtures rich in water (up to mTBA = 1) or measurements taken in solutions of t-butyl alcohol in NR,Br+water mixtures rich in water. Both measurements are supposed to give, as the first term in a series, the same electrolyte-non-electrolyte pair-interaction parameter in water. Here the enthalpy of transfer is related to the pair- and triplet-interaction parameters (hNE and h"E, respectively) by7 AHP/rnN = 4hNE +6h"~ mN where m is the molality and where the subscript E stands for the electrolyte and N for the non-electrolyte.Although it was not the purpose of the present study to determine these parameters,J. JUILLARD 41 as they were already known, the coefficient h,, can be obtained from the slope of the tangent at the origin of the curve AH,e =f(m,). The values thus obtained are in good agreement with the values reported by Perron et al.:7 for tetramethyl- ammonium bromide, 500 as compared with 480 ; for tetraethylammonium bromide, 800 as compared with 808 ; and for tetrapropylammonium bromide, 1500 as compared with 1420 (all coefficients in J mo1-2 kg). Such a determination is not possible for tetrabutylammonium and tetra- pentylammonium bromides owing to the rapid deviation from the limiting slope which is observed for these two electrolytes; however, the results of Perron et ai., h,, = 2340 and 3280 J mo1-2 kg, respectively, are compatible with our data.In addition, the value 290 J moF2 kg is obtained here for the parameter h,, corresponding to the value for ammonium chloride with an acceptable accuracy (k 5). This value is slightly less than the value reported for LiC17 (302-3 12). DISCUSSION Fig. 2 shows that the enthalpy of transfer is positive for all the electrolytes studied here and increases with the size of the cation. In media rich in water enthalpic pair-interaction parameters h, are representative of the interactions between both ions (bromide and tetra-alkylammonium) and t-butyl alcohol in water. Their meaning and interpretation have been amply discussed by Perron et Most of the thermodynamic interaction parameters of the species studied here can be predicted using the scaled-particle theory (s.p.t.) in so far as the term corresponding to the formation of the cavity, calculated according to this theory, is the leading one.In the case of both g N E and h,, the interaction terms are frequently important. In any case, for such large ions as the tetra-alkylammonium cation it may be expected that most of the heat of transfer should be due to the change in the heat of cavity formation. Such calculations have been attempted by Desrosiers and Desnoyers for Bu,NBr.s They give a correct estimation of the trend of the process, at least in water-rich media. The increase observed here in the hNE parameters with the size of the cation is also in agreement with such a picture. Calculations have been made for all these cations.They show that results are very sensitive to the choice of hard-sphere diameter for t-butyl alcohol since unfortunately this diameter is not known for certain8 There is at least a qualitative agreement between the variations in calculated and experimental h,, parameters and it can be ascertained that much of the enthalpic effect observed in the transfer of tetra-alkyl ions from water to t-butyl alcohol mixtures is related to the cavity effect and thus increases with size. One question remains, namely whether the description of water + TBA media as mixtures of hard spheres is realistic, even if the parameters used in the calculations (density and expansibility) are those of the bulk.These mixtures are frequently depicted as highly structured and in the previous papers of this series, we have favoured an interpretation of our former data using the 'fluctuating-cage In this model it is supposed that the water + t-butyl alcohol mixtures retain something of the structure of the solid clathrate. Recent investigations by Iwasaki and Fujiyama'O using light-scattering techniques support the picture of the mixtures as given by this model. As far as our data on enthalpies of transfer of solutes3* 6 + l 1 l 3 are concerned, a maximum is always observed in the curves of the enthalpies plotted against the concentration of t-butyl alcohol; the location as well as the amplitude of the maximum could generally be explained within the scope of the 'fluctuating-cage model ' .1 1 9 l3 Here, except for the two extremes in size, ammonium and tetrapentylammonium42 INTERACTIONS I N WATER+t-BUOH MIXTURES bromide, there is no maximum in the AH, = f ( x ) curves. However, it may be that tetra-alkylammonium ions are as able as t-butyl alcohol to form clathrate-like structures with water and that there is some sort of competition between the destruction of the water clathrate of TBA, which is supposed to be endothermic, and the build-up of the water clathrate of NR:, which can be supposed exothermic. If such assumptions are retained there is no way of predicting apriuri the shape of the curves which represent the variation of the enthalpy of transfer with the TBA concentration.At most it can be noted that for NPen,Br the first effect would be the dominant one. Thus, arguments in favour of or against the quasi-clathrate model do not arise from these results but such a model can be reconciled with our present findings. A more thorough discussion would involve distinguishing between the effects of the two ions, tetra-alkylammonium and bromide, in the enthalpy of transfer of these electrolytes; this will be presented in the following paper. (a) Part 9, Y. Pointud and J. Juillard, J. Chem. SOC., Faraday Trans. I , 1977, 73, 1907. (b) Part 10, N. Dollet, J. Juillard and R. Zana, J. Solution Chem., 1980, 9, 827. R. K. Mohanty, T. S. Sarma, S. Subramanian and J. C. Ahluwalia, Trans. Furaday Soc., 1971, 67, 305. L. Avedikian, J. Juillard, J-P. Morel and M. Ducros, Thermochim. Acta, 1973, 6, 283. Y. Pointud, H. Gillet and J. Juillard, Tulunta, 1973, 23, 741. R. K. Mohanty, S. Sunder and J. C. Ahluwalia, J. Phys. Chem., 1973, 76, 577. Y. Pointud, J. Juillard, L. Avedikian, J-P. Morel and M. Ducros, Thermochim. Acta, 1974, 8, 423. ' G. Perron, D. Joly, J-E. Desnoyers, L. Avedikian and J-P. Morel, Can. J. Chem., 1978, 56, 552. N. Desrosiers and J. E. Desnoyers, Can. J. Chem., 1976, 54, 3800. * E. K. Baumgartner and G. Atkinson, J. Phys. Chem., 1971, 75, 2236. lo K. Iwasaki and T. Fujiyama, J. Phys. Chem., 1977, 81, 1908. l2 Y. Pointud, J-P. Morel and J. Juillard, J. Phys. Chem., 1976, 80, 381. l3 Y. Pointud and J. Juillard, J. Chem. Soc., Furaday Trans. I , 1977, 73, 1907. N. Dollet and J. Juillard, J. Solution Chem., 1976, 5, 77. (PAPER O / 1482)