T.S. SARMA AND J. C.AHLUWALIA
1366
Thermodynamics of Transfer of Tetrabutylammonium Bromide from Water to Aqueous Urea Solutions and the Effects on the Water Structure
by T. S. Sarma and J. C. Ahluwalia* Department of Chemistry, I n d i a n Institute of Technology, Kanpuw.16, I n d i a
(Received March 92,1971)
Publication costs borne completely by T h e Journal of Physical Chemistry
The enthalpies (at 25 and 35') and the partial molal heat capacities (at 30") of transfer of BurNBr from water to aqueous urea solutions (up to 10 m) have been determined calorimetrically. The entropies of transfer are obtained by combining the enthalpies of transfer with the free energies of transfer reported in the literature. The results show that transfer of BurNBr from water to aqueous urea solutions is accompanied by a negative free energy change, positive enthalpy and entropy changes, and a decrease in partial molal heat capacity. The results are consistent with the view that solutes containing nonpolar groups find aqueous urea less structured than pure water. The effect of urea breaking down water structure, which seems to be significant only at higher concentrations of urea, increases with the increase in concentration reaching saturation effect around 10 m. The implication of the results on the role of urea causing denaturation of proteins has been discussed.
Introduction The role of urea in affecting the structure of water has been critically analyzed recently by Holtzer and Emerson.' The pertinent references on the various studies on the urea-water binary system have been listed in our previous paper^.^^^ Our studies2g3 on the nmr and the excess partial molal heat capacities of the urea class of protein denaturants have led us to suggest that urea does not appear to alter water structure significantly, and if the water structural changes play an important role in the denaturation of proteins by urea, then it is more likely that urea modifies the water structure in the presence of proteins. It would then seem more relevant and fruitful to study the transfer behavior of model compounds Containing large nonpolar groups (structure-making solutes) from water to aqueous urea solutions containing large concentration of urea. A number of studies in this direction4+' support the contention that urea behaves as a structure breaker. The studies'opll on the transfer behavior of structure-breaking solutes like common electrolytes from water to concentrated urea solutions also lead to the same conclusion that urea breaks water structure. Recent studies on the activity coefficients of the waterurea-Bu4NBr ternary system by Wen and Chen12have led them to suggest that the lowering of the activity coefficients of urea and BudNBr in the urea-waterBuaNBr ternary system may be due to either the waterstructure-breaking effect of urea or to the formation of some complexlike aggregates between urea and R4N isalts in aqueous solution. I n this paper, we have reported the results on the enthalpies, entropies, and heat capacities of transfer of Bu4NBr from water to aqueous urea solutions (up The Journal of Physical Chemistry, Vol. 76, N o . 9, 1979
to 10.2 m), and we find that urea does behave as a structure breaker (the structure-breaking effect of urea increasing with concentration) in the ternary system of urea-water-BudNBr. The implication of this is discussed in the denaturation of proteins by urea.
Experimental Section The submarine calorimeter and the operational procedure used for the measurements of the integral heats of solution have been described previously. The temperature of the thermostat was maintained to ztO.002'. Electronic amplification was used with a thermistor as the temperature-sensing device. With this calorimeter a temperature difference of the order of 2 X 10-6' could be detected. The calorimeter was calibrated by measuring heats of solution of KC1 (1) A. Holtaer and M. F . Emerson, J. Phys. Chem., 73, 26 (1969). (2) S. Subramanian, D. Balasubremanian, and J. C. Ahluwalia, ibid., 73, 266 (1969). (3) S. Subramanian, T . S. Sarma, D. Balasubramanian, and J. C. Ahluwalia, ibid., 75, 815 (1971). (4) P. L. Whitney and C. Tanford, J . BioE. Chem., 237, PC 1735 (1962); Y. Noaaki and C . Tanford, ibid., 238, 4074 (1963). (5) D. B. Wetlaufer, S. K . Malik, L. Stoller, and R. L. Coffin, J . A m e r . Chem. SOC.,86, 508 (1964). (6) G. C. Kresheck and L. Benjamin, J . Phys. Chem., 68, 2476 (1964). (7) W. A. Hargraves and G . C. Kresheck, ibid., 73, 3249 (1969). (8) F. Franks and D. L. Clarke, ibid., 71, 1155 (1967). (9) H. S. Frank and F. Franks, J . Chem. Phys., 48, 4746 (1968). (10) J. H . Stern and D. J. Kulluck, J. Phys. Chem., 73, 2795 (1969). (11) G. A. Vidulich, J. R. Andrade, P. P. Blanchette, and T . J . Gilligan, 111, ibid., 73, 1621 (1969). (12) W. Y. Wen and C. L. Chen, ibid., 73, 2895 (1969). (13) S. Subramanian and J. C. Ahluwalia, ibid., 72, 2525 (1968). (14) T . 8. Sarma, R . K. Mohanty, and J. C. Ahluwalia, Trans. Faraday SOC.,65, 2333 (1969).
TRANSFER OF Bu4NBr FROM WATERTO UREA
1367
Table I: Enthalpies and Heat Capacities of Transfer of Bu4NBr from Water to Aqueous Urea 25.0' Urea,
a
AIL0
-2050
cal mol-1
15"
- 1319 f 29
-815 f 19 -427 zt 23 -343 i:24
Values in pure water taken from ref 14.
30.0' ACpt.9 0a1 deg-1 mol-1
AHtr
cal mol-1
m
0 2.20 4.89 8.01 10 * 20
35.0'
?
AHtr
Ai780
731 zt lgb 1235 f 25 1623 f 20 1707 i:28
-260 f 15" 2 4 6 f 18 593 rfr: 10 761 i:25 796f 18
Uncertainty (std dev) in AHtr was calculated as eAgi,, = [(eAf?.")z
in water and of THAM (tris(hydroxymethy1)aminomethane) in 0.1 N HC1. The values obtained were in good agreement (within 0.2%) with those reported by Gunn.15 BurNBr was obtained from Eastman Organic Chemicals. It was recrystallized by the method reported in the literature.la The recrystallized salt was dried in vucuo at 60-80". Urea was obtained from B D H Ltd. (AR grade, assay >99.5%) and was used as such. Calorimetric measurements were made with freshly prepared urea solutions.
Results The partial molal heats of solution A R s of BurNBr in 2.20, 4.89, 8.01, and 10.2 m aqueous urea solutions at 25 and 35" are given as supplementary material." Since the concentration of Bu4NBr in aqueous urea solution (0.0007-0.0035 m) was close to infinite dilution, any small concentration dependence of heats of solution A n s being within the experimental error, the partial molal heat of solution at infinite dilution, A R s " , was taken to be the average of a number of Ai?, measurements. The A R s O values of Bu4NBr in 2.2, 4.89, 8.01, and 10.2 m aqueous urea solutions at 25 and 35" are listed in Table I along with the values in pure water reported earlier from this 1ab0ratory.l~ The uncertainty in the A n s o values is expressed as standard deviation from the mean value. The A R s " values of Bu4NRr in aqueous urea solutions a t 25" are in fairly good agreement with those reported by Cassel and Wen.l* The enthalpies of transfer, A H t r (at 25 and 35"), and partial molal heat capacities of transfer, ACptr at 30" from water to aqueous urea solutions were derived from these data and are listed in Table I. The recently reportedI2 free energies of transfer of Bu4NBr from water to aqueous urea solutions (up to 5 m) at 25" are plotted as a function of molality of urea in Figure 1 and extrapolated to 8 m urea. The values of free energies of transfer, AGtr, thus obtained (up to 8 m urea) are listed in column 2 of Table 11. In Figure 1 are also plotted the enthalpies of transfer, A H t r , at 25 and 35" given in Table I as a function of concentration of urea up to 10.2 m, and the interpolated values at 25" are given in column 3 of Table 11. By combining these enthalpies of transfer, A H t r , at 25", with free energies
0 -22 rfr: 3 -38f 3 -60f4 -65 f 4
506 rfr: 23 855 & 18 1021 f 30 1056 f 24
+ (eAHso)2]1/2.
of transfer, AGt,, at 25", the entropies of transfer a t 25" were derived and are listed in column 4 of Table 11. The values of the partial molal heat capacity transfer, ACPtr (up to 10.2 m ) at 30" are plotted as a function of molality of urea in Figure 2, and the smoothed and interpolated values are given in column 5 of Table 11.
Table I1 : Thermodynamics of Transfer of Bu4NBr from Water to Aqueous Urea AUtr5(25'), cal mol-1
Urea, m
- 133 - 247 - 345
1 2 3 4 5 '6 8 10
-430 - 507 - 575 704
-
AStrc(25'), AHtrb(25*), oal deg-1 oal mol-1 mol-1
396 680 910 1095 1235 1300 1600 1715
1.8 3.1 4.2 5.1 5.8 6.3 7.7
ACpttd(300), oal deg-1 mol-1
-10.5 -19.5 - 27 - 34 -40.5 -46.5 -57.5 -66.5
0 aGt, values were obtained by plotting the AGt, values reported in ref 12 as a function of molality and extrapolating to 8 m urea. b AH,, values given in Table I were plotted as a function of molality of urea (seeFigure l),and smoothed out values are given here. The estimated uncertainty (standard deviation) in Derived from AGtr and A H t , AHt, values is f 2 5 cal mol-'. values given in columns 1 and 2. The estimated uncertainty Smoothed-out (standard deviation) in A& values is h O . 1 eu. values from Figure 2. The estimated uncertainty (standard deviation) in AC,,, values is h 3 cal deg-1 mol-'.
Discussion The results given in Table I1 and Figures 1 and 2 show that the transfer of the highly structure-making solute Bu4NBrlS from water to aqueous urea is accom(15) S. R. Gunn, J . Phys. Chem., 69, 2902 (1965) (16) A. K. R. Unni, L. Elias, and H. I. Schiff, ibid., 67, 1216 (1963). (17) The detailed listing of the values of partial molal heats of solution A#, will appear immediately following these pages in the microfilm edition of this volume of the journal. Single copies may be obtained from the Business Operations Office, Books and Journals Division, American Chemical Society, 1155 Sixteenth St., N.W., Washington, D . C. 20036, by referring to code number JPC-72-1366. Remit check or money order for $3.00 for photo copy or $2.00 for microfiche . (18) R. B. Cassel and W. Y . Wen, J. Phys. Chem., 7 6 , 1369 (1972).
The Journal of Physical Chemistry, Vol. 76,N o . 9,1972
1368
T. S. SARMA AND J. C. AHLUWALIA
I
0
2
I
I
I
4 6 8 Mdality of urea
I
1
r0
i Molality oi urea
Figure 1. The free energies and enthalpies of transfer of BurNBr from water to aqueous urea solutions as a function of molality of urea: (A) AGa a t 25', ( 0 )AHtr a t 25O, (0)AHtr a t 35'.
Figure 2. The partial molal heat capacity and entropies of transfer of BudNBr from water to aqueous urea solutions as a function of molality of urea: (A) A& at 2 5 O , (0)ACP,, a t 30'.
panied by a negative free energy change, positive enthalpy and entropy changes, and a decrease in partial molal heat capacity. The effect on these transfer thermodynamic functions increases with the increase in the urea concentration, reaching almost maximum effect around 10 m urea. The enthalpies of transfer of Bu4NI and Am4NI from water to 7 M aqueous urea as obtained from solubility studies by Franks and Clarke8 are also positive, though less in numerical value, but are of the same order of magnitude as our calorimetric values. The positive entropy change and negative partial molal heat capacity change (see Figure 2 ) of BudNBr from water to aqueous urea solutions indicate that Bu4NBr finds aqueous urea as less structured than pure water. The ACPtrvalues (see Table I1 and Figure 2 ) indicate that with increasing concentration of urea the structure of aqueous urea solution decreases appreciably, reaching the saturation effect around 10 m. In 10 m aqueous urea solution, the excess partial molal heat capacity and therefore the structure-making capacity of Bu4NBr is reduced by about one-third its capacity in pure water. Recently Wen and Chen12 have reported that their activity coefficient results of wTater-urea-Bu4NBr and mater-urea-Me4NBr could be explained on the basis of either (i) that urea breaks water structure or (ii) that urea forms complexes with R4N+ salts in aqueous solutions. Assuming that complex formation is accompanied by a decrease in entropy, the positive enthalpies of transfer of BueNBr from water to aqueous urea rule out the complex formation of urea with Bu4NBr in aqueous urea, thus favoring the other possibility that urea breaks water structure. The results of this study are also consistent with those of earlier studies4-8 on calorimetry, solubility and partial molal volume of solutes containing nonpolar groups such as hydrocarbons, amino acids, as well as
studies on calorimetry and viscosity of common electrolytes.'Otll The calorimetric investigations on the transfer of amino acids from water to 6 M urea by Kresheck and Benjamin6 show that the transfer (except in the case of glycine which is a structure-breaking solute) is accompanied by decrease in free energy, enthalpy, and partial molal heat capacity and increase in entropy. However, the increase in nonpolar side chain of the amino acids was found to increase the enthalpy (less negative) and entropy of transfer while further decreasing the partial molal heat capacityn6 It was shown by these workers that the effect of an increase in the nonpolar side chain in an amino acid by an isobutyl group was to increase the enthalpy of transfer by 833 cal mol-', increase the entropy of transfer by 3.5 eu, and decrease the partial molal heat capacity by 37 cal deg-' mol-I. This is the same trend as found for an R4NBr salt, indicating that the effect of the transfer of the nonpolar groups in different aliphatic compounds from water to aqueous urea solutions may be comparable and possibly involve the same mechanism. Wetlaufer, et from their solubility studies of hydrocarbons showed that the transfer of hydrocarbons from water to 7 M urea is accompanied by a negative change in free energy and a positive change in enthalpy, which is consistent with our results and supports the contention that R4N+ salts behave as soluble hydrocarbons. Frank and Franksg have recently proposed a model to account quantitatively for the results of Wetlaufer, et al., on the solubilities of hydrocarbons in water and 7 M aqueous urea solutions. They have suggested that the effect of urea on the water structure is rather subtle in the sense that it acts as a statistical structure breaker, unlike the structure-breaking action by ions of common electrolytes. They have given the details in their model as to how this is brought about.g
The Journal of Physical ChEmktTy, Vol. 76,No. 9, 1978
HEATSOF SOLUTION OF TETRAALKYL BROMIDES The statistical structure-breaking effect of urea on water seems to be consistent with our previous studies2q3 in the water-urea binary system, where it was shown that addition of small amounts of urea did not alter the water structure, as well as being consistent with the present study in the ternary system of water-ureaBuaNBr, which shows that the structure-breaking effect of urea increases appreciably with increasing concentration until reaching almost saturation limit around 10 m (-7 M ) , where the water to urea ratio is about 6 to 1. If a concentrated aqueous urea solution is less structured than pure water, then the addition of structurebreaking solutes such as common electrolytes should be accompanied by a decrease in their structure-breaking capacity. I n other words, the transfer of structurebreaking solutes from water t o aqueous urea solutions should be accompanied by a positive standard free energy change and negative standard enthalpy and entropy changes and an increase in standard partial molal heat capacity. One indeed finds evidence in support of this in the limited studies in this direction.'O
1369 The importance of the role of the effect of urea on the water structure in the denaturation of proteins cannot be stated with certainty a t this stage. It may be, as has been suggested earlier by many workers, that as a consequence of the weakening of hydrophobic forces the solvation of the denatured form of the protein is favored in aqueous urea solution. If the structurebreaking effect of urea plays an important part in denaturation, then the mechanism of its structure breaking, as has been suggested earlier in this paper, should be different from that of ions of common electrolytes. The idea of urea acting as statistical structure breaker, suggested by Frank and frank^,^ which is supported by our observation, needs to be verified by further calorimetric and other investigations on the effect of the transfer of solutes, from water to concentrated aqueous urea solutions, as well from water t o concentrated aqueous solutions of structure-breaking solutes such as common electrolytes.
Acknowledgment. We are thankful to the Council of Scientific and Industrial Research, India, for part of the financial support.
Thermodynamics of Transfer of Three Tetraalkylammonium Bromides from Water to Aqueous Urea Solutions at 25" 1 by R. Bruce Cassel and Wen-Yang Wen* Jeppson Laboratory, Chemistry Department, Clark University, Worcester, Massachusetts 01610 (Received April 91, 1971) Publication costs assisted by the National Science Foundation
The heats of solution of tetramethyl-, tetraethyl-, and tetrabutylammonium bromides have been measured in water and in aqueous urea solutions of different concentrations. The measurements were carried out over a salt concentration range of 0.001-0.1 m in order to observe the effect of urea on the apparent molal heat content and to aid in the extrapolation to infinite dilution. The enthalpies of transfer from water to aqueous urea were calculated and were combined with free energies of transfer of the tetramethyl- and tetrabutylammonium salts to give entropies of transfer. While the free energies of transfer for the two salts are both small negative, the enthalpy and entropy of transfer of tetramethylammonium bromide were found to be large negative, and those of tetrabutylammonium bromide large positive. These contrasting results are discussed in terms of changes in water structure induced by urea and by the tetraalkylammonium ions. Our results support models of urea as a water-structure breaker.
Introduction The investigation of the changes in the conformational structure of proteins induced by added solute in solution has stimulated interest in studies of electrolytes and nonelectrolytes in water.2 I n order to acquire a meaningful model for the structural changes taking
place at the protein-solvent interface and in the regions surrounding ions, it has been useful to study simple two-comPonent systems. From studies of the tetra(1) Abstracted in part from Ph.D. thesis of R. B. Cassel, Clark University, 1971. (2) W. Kauzmann, Advan. Protein Chem., 14, 1 (1969).
The Journal of Physical Chemistry, VoZ. 76, N o . 9, 1972
