J . Phys. Chem. 1985,89, 1687-1690
1687
Fluorescence Probing Study of the Association of Brie Salts In Aqueous Solutions R. Zana* and D. Guveli C.R.M. and Greco Microemulsions, CNRS, 67000 Strasbourg, France (Received: July 6, 1984;
In Final Form: November 19, 1984)
The fluorescence spectra and the fluorescence decay of pyrene solubilized in aqueous micellar solutions of sodium cholate, deoxychlolate,and taurochenodeoxycholatehave been studied. The results show that the micellar environment of the solubilized pyrene is much more apolar in the bile salt than in classical surfactant micelles. They also indicate that the bile salt micelles are made of small primary micelles containing about 11 bile salt molecules. Because the migration of pyrene from one primary micelle to a neighboring one in the secondary micelles is very slow, it is concluded that the bonding regions between primary micelles may be of hydrophobic nature but with a narrow cross section and/or may include H bonds which would give them a partly hydrophilic character. These results support the mechanism of association in aqueous bile salt solutions proposed by Small and co-workers.
Introduction The bile salts are among the most important biological detergent-like molecules, and their physicochemical properties have been extensively studied both in water and aqueous sodium chloride ~olutions.~-~ Bile salts in solution are known to associate to form aggregates which will be referred to as micelles in the following, even though they differ from the micelles found in solutions of classical detergents, owing to the very different chemical structure of the bile salts. Contrary to the molecules of classical detergents ‘where the hydrophilic and lipophilic moieties are clearly separated, bile salt molecules have a lipophilic surface which is the convex side of the steroid nucleus and a hydrophilic surface which is the polyhydroxylated concave side of this nucleus.ls2 The mechanism of bile salt aggregation and the structure of aggregates in solution are still a matter of discussion. Most authors agree that aggregation proceeds in two stages. First, small primary micelles are formed and then the primary micelles associate to form larger secondary micelles, at higher concentration of bile salt and/or of added sodium chloride. According to Small and c o - ~ o r k e r s $ ~ ~ the primary micelles are stabilized through hydrophobic interactions, whereas the formation of secondary micelles involves hydrogen bonding between the hydroxylic groups at the micelle surfaces. A different view was presented by Oakenfull and Fisher: who claimed hydrogen bonding to be essential in the formation of the primary micelles. This conclusion was strongly opposed in several studies.’-I0 In a recent detailed investigation of the thermodynamics of bile salt association, Mazer et al.” concluded that primary micelles form owing to hydrophobic interactions and that, a t high bile salt and/or NaCl concentrations, primary micelles polymerized in a linear fashion to form secondary, rodlike micelles, stabilized through hydrophobic interactions between the surfaces of the primary micelles. Another very important problem when dealing with bile salt solutions concerns the size of the micelles. Some of the controversies existing in the literature have been dealt with by Kratohvil et al.3J2913 These authors have conclusively shown that, at low (1) D. M. Small, Adu. Chem. Ser., No.84, 31 (1968). (2) D. M. Small in ‘The Bile Acids”, Vol. 1, P. P.Nair, and D. Kritchevsky, Eds., Plenum Press, New York, 1971, Chapter 8, p 249. (3) J. Kratohvil, Hepatology (N.Y.),4, 85 (1984), and references therein. (4) M. C. Carey and D. M. Small, Am. J . Med., 49, 590 (1970). (5) D. M. Small, S.A. Penkett, and D. Chapman, Biochim. Biophys. Acta, 176, 178 (1969). (6) D. G. Oakenfull and L. R. Fisher, J . Phys. Chem., 81, 1838 (1977); 82, 2443 (1978). (7) M. Vadnere, R. Natarajan, and S. Lindenbaum, J. Phys. Chem., 84, 1900 (1980). (8) R. Zana, J . Phys. Chem., 82, 2440 (1978). (9) A. Djavanbakht, K. M. Kale, and R. Zana, J. Colloid Interface Sci., 59, 139 (1977). (10) P.Carpenter and S. Lindenbaum, J . Solution Chem., 8,347 (1979). (1 1) N. A. Mazer, M. C. Carey, R. F. Kuvasnick, and G.B. Benedek, Biochemistry, 18, 3064 (1979). (12) J. Kratohvil, W. P.Hsu, M. Jacobs, T. Aminabhavi, and Y. Mukunoki, Colloid Polym. Sci., 261, 781 (1983).
0022-3654/85/2089-1687$01.50/0
bile salt concentration, the purity of the samples as well as the method used for the analysis of the data can result in sizable differences of apparent aggregation numbers. In an attempt to find new information which would help clarify the above situation and particularly the mechanism of association, we have undertaken a study of aqueous solutions of bile salts (mainly sodium deoxycholate, SDOC) by means of the fluorescence probing technique^.'^-'^ Recall that fluorescence probes such as pyrene, which is nearly insoluble in water, are solubilized by micelles. Then by studying the emission spectrum of the micelle-solubilized probe as well as its fluorescence decay, one can obtain information on the micelles. In particular, the analysis of the probe fluorescence decay allows the determination of the average micelle aggregation number N as a function of the surfactant or NaCl concentration without requiring any extrapolation to the cmc. This method is completely insensitive to intermicellar interactions and preferential absorption phenomena which greatly complicate the determination of micellar weights by means of classical methods such as light scattering. Two studies of bile salt solutions by means of fluorescence probing have been These studies which focused on the fluidity of the micelle interior dealt with neither the association mechanism nor the nature and size of the aggregates as is the purpose of the present work. In the following we present the information gained in probing SDOC in aqueous NaCl by means of pyrene. For the purpose of comparison some measurements were also performed with sodium cholate (SC), which is known to be less extensively associated than SDOC in aqueous solution, and also with sodium taurochenodeoxycholate (STCDOC) where the chain bearing the ionic group is much longer than in either SC or SDOC. The measurements involved fairly concentrated solutions (3-1 0% w/v) at fairly high NaCl concentration (0.2-0.6 M) as we were mostly interested in the range where the transition from primary to secondary micelles occurs.
Experimental Procedure Materials. Sodium deoxycholate was prepared by adding a slight excess of carbonate-free NaOH solution to deoxycholic acid (Fluka, puriss grade), redissolving with 100 mL of distilled methanol, evaporating, twice recrystallizing from acetone(13) J. Kratohvil, T. Aminabhavi, W. Hsu, S. Fujime, A. Patkowski, F. Chen, and B. Chu, Croat. Chem. Acta, 56, 786 (1983). (14) J. K. Thomas, Chem. Reu., 80, 283 (1980). (15) A. Yekta, M. Aikawa, and N. J. Turro, Chem. Phys. Lett., 63, 543 11979). (16) S. Atik, M. Nam, and L. Singer, Chem. Phys. Lett., 67.75 (1979). (17) P. Infelta, Chem. Phys. Lett., 61, 88 (1979). (18) M. Tachiya, Chem. Phys. Lett., 33,289 (1975); J. Chem. Phys., 76, 340 (1982). (19) P. Liana, M. Dinh-Cao, J. Lang, and R. Zana, J. Chim. Phys.-Chim. Biol., 78, 497 (1981). (20) L. R. Fisher and D. G. Oakenfull, Aust. J. Chem., 32, 31 (1979). (21) M. Chen, M. Gratzel, and J. K. Thomas, J. Am. Chem. Soc.,97,2052 (1975).
.
0 1985 American Chemical Society
1688 The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 TABLE I: Monomeric Pyrene Fluorescence Lifetime and Values of 1,/13in Various EnvironmentsaP medium T, O C 1091,'s 11/139c 3% SDOC-0.6 M NaCl 28 408 0.67 45.3 376 347 0.67 60 4% SDOC-0.2 M NaCl 28 410 0.66 5% SC-0.4 M NaCl 28 40 1 0.7 1 65.4 321 0.74 5% STCDOC-0.2 M NaCI 25 37 1 0.83 sodium laurate 25 336 0.99
sodium.dodecyl sulfate sodium octylbenzenesulfonate
28 28
323 323
1.04 1.12
These measurements were performed at pyrene concentrations of about M, with oxygen-free and argon-saturated solutions. bThe values of 11/13determined in the present study were found to be about 10% lower than those reported by other workers (K. Kalyanasundaram and J. Thomas, J . A m . Chem. Soc., 99,2039 (1 977); D. Dong and M. Winnick, Photochem. Photobiol., 35, 17 (1982)). However, since we are only comparing 11/13values in different media, such a systematic difference does not affect the conclusions inferred from the above data. CExperimentaluncertainty on T and 11/13f 2 % . methanol, and drying in vacuo. A sodium cholate solution was prepared by stoichiometric neutralization of cholic acid (Fluka, puriss grade) by aqueous sodium hydroxide. Sodium taurochenodeoxycholate was a gift from Pr. B. Brun (University of Montpellier, France). Sodium chloride (Fluka,puriss) and pyrene (99% pure, Ega-Chemie, West Germany) were used as received. Water was deionized and twice distilled from all-glass still. Solutions. A stock of pyrene in distilled methanol was transferred into dry flasks and methanol evaporated. Bile salt solutions prepared on a w/v basis having different NaCl concentrations, adjusted to pH 8.2-9.2 with 1 M NaOH, were added, and solutions were stirred for 3-4 at 80-90 OC. When solubilization of pyrene was complete, the solutions were transferred to a water bath thermostated at 28 f 0.02 OC, and they were left to equilibrate for several hours. The error on the pyrene concentration was of about 5 X 10" M for the most concentrated solutions. This introduced a negligible error on the fluorescence studies. It was observed that pyrene is less soluble in S C solutions than in either SDOC or STCDOC solutions at a given temperature, concentration, and NaCl content. Fluorescence Measurements. Fluorescence spectra and fluorescence decay curves were obtained by means of the same equipment as in previous s t ~ d i e s . ~The ~ ? spectra ~~ were used to determine the ratio Z,/Z3 of the intensities of the first and third vibronic peaks of monomeric pyrene solubilized within bile salt aggregates. This ratio provides information on the polarity of the probe m i c r o e n ~ i r o n m e n t . ' ~ ,The ~ ~ decay curves (fluorescence intensity Z(t) vs. time t ) of micelle solubilized pyrene were computer-fitted to the e q ~ a t i o n ' ~ . ' ~ I(?) = Z(0) exp{-(t/T) - R [ 1 - exp(-kEt)]) by using a nonlinear least-squares weighted procedure. This equation describes the behavior of fluorescence probes solubilized within micelles, where they can form excimer upon excitation. The computer analysis yields the following: (i) the fluorescence lifetime T of pyrene in its microenvironment; (ii) the ratio R = C,/[M] of the molar concentrations of micelle solubilized pyrene, C,, and micelle, [MI (expressed in moles of micelle per liter). This ratio is related to the aggregation number N through the relationship N = R(C - cmc)/C,, where C is the concentration of the bile salt and cmc its critical micellization concentration; (iii) the rate constant kE for intramolecular excimer formation. Recall that theoretical treatments indicate that kE is inversely related to the micelle volume, that is to the micelle aggregation number.2s (22) P. Lianos, J. Lang, C. Strazielle, and R. Zana, J . Phys. Chem., 86, 1019 (1982). (23) P. Lianos and R. Zana, J. Colloid Inrerface Sci., 84, 100 (1981). (24) P. Lianos and S. Georghiou, Photochem. Photobiol., 30,355 (1979). ( 2 5 ) M.Van der Auweraer, J. Dederen, E. Gelade, and F. De Schryver, J . Chern. Phys., 74, 11 10 (1980).
Zana and Guveli Results and Discussion Table I lists the values of Zl/13 and T for pyrene in micellar solutions of bile salts and of three typical surfactants. Recall that low values of Z1/13and large values for T are found in hydrocarbons, whereas the opposite trends are observed in polar solv e n t ~ . ' ~We , ~ thus ~ found Il/Z3 = 0.54 and 1.70 in cyclohexane and water, with the corresponding 7 values of 420 and 200 ns. The results of Table I clearly show that the bile salts are able to provide a very apolar microenvironment to pyrene and likely to other hydrocarbons, where they are very effectively shielded from water. In fact, the values of Z1/Z3 found for the three bile salts are the lowest ones and the values of 7 the largest ones observed for pyrene solubilized in micelles formed in aqueous solutions, in the absence of additives such as alcohols and oils. The listed values of Zl/Z3 and T for the three classical surfactants are seen to be respectively larger and smaller than for the bile salts. The values of Zl/Z3 and 7 in Table I also show that the apolar character of the pyrene microenvironment decreases as one goes from SDOC to SC and STCDOC. The difference between SDOC and SC is obviously due to the more hydrophilic character of the latter which contains three OH groups, against two for SDOC. Concerning STCDOC, its peculiar behavior is probably due to its longer side chain bearing the ionic group than that of the other two salts. The nature of the ionic group (SO3-vs. C02-) may also play a role. Thus, it can be seen that Zl/Z3 for sodium laurate is lower than for sodium octylbenzenesulfonate. The low values of 11/13and large values of 7 characterizing bile salts have some implication concerning the nature of the interior of the bile salt micelles, irrespective of their size and shape. Indeed they indicate that the micelle interior where pyrene is solubilized is strongly apolar and thus probably devoid of any OH groups and hydrogen bonding. Therefore, our results do not support models which assume bile salt micelles to be made up of bile salt dimers stabilized through hydrogen bonding between OH groups. We note that Fisher and OakenfullZohave observed that the fluorescence of N-phenylnaphthalen- 1-amine is much larger in bile salt micelles than in classical surfactant micelles, revealing a less polar microenvironment in the former than in the latter systems, in agreement with our findings. However, the authorsZo inferred no conclusion from this observation. It should also be pointed that in micelles of classical surfactants the first site of solubilization of aromatic hydrocarbons such as pyrene is the micelle palisade layer, because these compounds are slightly surface a c t i ~ e . ~ ~Had * ~ 'bile salt micelles been structured as micelles of classical surfactants, one would have expected pyrene to dissolve in the palisade layerI4 and to show Z l / I 3 and T values close to those, say, of sodium laurate. Those very different values found suggest the absence of a palisade layer and, thus, an organization of the molecules in bile salt micelles different from that in classical surfactant micelles. This, undoubtedly, arises from the difference between the chemical structures of bile salts and classical surfactant molecules. A pyrene molecule solubilized in a bile salt micelle is probably squeezed between the lipophilic sides of several steroid nuclei which shield it very effectively against water. The palisade layer, in the sense given to it in classical surfactant micelles, would not exist in bile salt micelles, at least with S C and SDOC, because the bile salts may be lying flat at the micelle surface, just as they do at the air-water interface. With STCDOC, however, the longer side chain may result in a partial solubilization of pyrene between the micelle interior and the terminal ionic group, explaining its distinctly larger Il/Z3 value. Whereas the Z,/Z3 and T values for bile salt strongly differ from those found for classical surfactants, the effect of temperature on both Z1/Z3 and 7 is very similar for the two types of compounds. Thus, 1,/13is rather insensitive to temperature T whereas 7 decreases upon increasing T. The experimental activation energy2* E,* obtained from the In T vs. 1 / T plots for SDOC and SC was (26) P. Mukerjee and J. R. Cardinal, J . Phys. Chem., 82, 1620 (1978). (27) J. R. Cardinal and P. Mukerjee, J . Phys. Chem., 82, 1614 (1978). (28) S. Gladstone, K. Laidler, and E. Erying, "The Theory of Rate Processes", McGraw-Hill, New York, 1941, p 97.
The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 1689
Bile Salts in Aqueous Solutions
and residence time. Consider now a secondary micelle which is characterized by a size and aggregation number much larger than a primary micelle. If its structure included a continuous hydrophobic interior with a cross-sectional area comparable to that of the interior of a primary micelle, pyrene would have moved freely through the interior of the whole secondary micelle and the pyrene excimer formation process would have probed the whole secondary micelle and yielded Nand kEvalues much larger and smaller (vide supra), respectively, than in the low SDOC concentration range where primary micelles predominate. This is contrary to the experimental results which indicate that pyrene probes only a part In these measurements the pyrene concentration was fairly high, of the secondary micelle which is identical with a primary micelle. such that C,/[M] was around 0.6. For the SDOC in H20-0.6 M This leads to the conclusion that the secondary micelles must be NaCl the primary to secondary micelle transition occurs at about 4.5% made of primary micelles connected by regions through which SDOC.3' The impossibility to dissolve enough pyrcne in the 4% SCpyrene diffuses very slowly, in a time much larger than the 0.6 M NaCl solution prevented accurate determination of N for this fluorescence lifetime or the time required for excimer formation. system. The maximum value of C /[MI which was achieved for this system was about 0.2 at 28 OC ancfyielded an aggregation number of If these regions are of hydrophobic nature," then their cross5. This value is as expected lower than for the two dihydroxy bile salts sectional area must be much smaller than that of pyrene in order investigated and probably represents a lower bound, because of the to explain the slow diffusion of pyrene through them. The diffusion difficulty associated with the solubilizationof pyrene. Nevertheless, it of pyrene between adjacent primary micelles in a secondary micelle is very close to the value obtained by light scattering and equilibrium would be also very slow if the connecting regions contained many ultracentrifugation.' Owing to the limited solubility of pyrene in bile H bonds which would confer to them a somewhat hydrophilic salt micelles, the assumption of a Poisson distribution of pyrene in the character. Indeed the diffusion through these regions would then micelles underlying the method used in this work may not be strictly a involve a process similar to the exit of pyrene into a water-like valid, and thus the N values may be affected by a systematic error. Nevertheless, this error must be small. Indeed, determination of N medium, which is very slow (see above). values performed using lower (3-fold) pyrene concentration, where the We are thus led to two conclusions. First, the large secondary error due to the limited solubility of pyrene would be much smaller, micelles appear to be made of small primary micelles containing yielded the same N values, within the experimental error. bValue at about 11 bile salt molecules. This number is only slightly larger 63.5 OC. than that inferred from molecular models, on a purely geometric basis.' At this point we recall that the aggregation numbers of found to be 1.Of 0.2 and 1.2 f 0.2 kcal/mol, respectively. These values are very close to those found for classical a m p h i p h i l e ~ , ~ ~ micelles of classical surfactants are, at low concentration, often only slightly larger than those calculated by using the oil-drop as well as for pyrene in organic solvents.30 model, which is also a geometric Our results therefore We now examine the aggregation numbers N obtained from give more evidence of the importance of geometric factors in the analysis of the fluorescence decay curves in the case where determining the micelle size and shape. Second, the primary the C , / [ M ] concentration ratio was near unity. The N values micelles constituting the larger secondary micelles appear to be are listed in Table I1 for various concentrations of SDOC and connected by regions through which the diffusion of pyrene is very NaCl and for one STCDOC-NaCl system. Within the experislow. These regions may be of hydrophobic nature with a narrow mental error, the values of N are independent of the bile salt and cross section and/or may include some H bonds, which would give NaCl concentrations, temperature, and even nearly independent them a partially hydrophilic character. It is worthwhile pointing of the nature of the bile salt. It is first to be noted that the values out that the N values obtained in the present study do not conof N are close to the value of the aggregation number obtained tradict the results obtained in other studies by means of other by Small,' on the basis of Stuart-Briegleb molecular models. techniques which usually indicate much larger aggregation numThus,Small found that a maximum of 10 bile salts could associate ber~."-'~Indeed these techniques are sensitive to the secondary with their lipophilic surface facing the micelle interior and their micelles whereas fluorescence decay apparently is sensitive to the hydrophilic surface in contact with the solvent. According to smaller primary micelles constituting the secondary micelles. unpublished viscosity measurements," the concentration range The rate constants for excimer formation in primary bile salt for the transition from primary to secondary micelles in H 2 W . 6 micelles (see Table 11) are smaller than that found with micelles M NaCl lies around 4% SDOC. Only primary micelles would of a classical surfactant such as sodium dodecyl sulfate (about thus be present in solutions of 4% SDOC or below, at NaCl 2.2 X lo7 s-' at 25OCZ9).The ratio qBs/qsDs of the microviscosity concentration below 0.6 M. On the contrary, in 10% SDOC-0.6 experienced by pyrene in its motion in bile salt (BS) and sodium M NaCl the micelles would be mainly secondary micelles with dodecyl sulfate (SDS) micelles, respectively, can be approximated much larger aggregation numbers. The fact that N remains as the ratio DSDs/DBs of the diffusion coefficients of pyrene in constant in going from primary to secondary micelles strongly these micelles. This ratio in turn is equal to the ratio of the rate supports a model somewhat similar to that proposed by Small,' constants for excimer formation within these micelles, corrected where the secondary micelles are made of polymerized primary by that part of the volume of the micelle in which pyrene diffuses, micelles. Our results however do not permit us to conclude on the assumption that excimer formation is diffusion controlled whether the driving force for polymerization is H bonding or (~E,SDS~SDS)/ within the micelle.25 This results in ~BS/VSDS hydrophobic interactions. The reasonsing is as follows. The (ICE,BSVBS) where the Vs refer to the volumes. Using the value residence time of pyrene in classical surfactant micelles is about NBS = 11, a volume per steroid core of 600 A3,2assuming that 500 ~ ( 1 . 9and ~ ~ is expected to be of the same order in bile salt only the outer half of the volume of the SDS micelle of radius micelles. Thus, when two pyrene molecules are present in a given 21 A is accessible to pyrene, in accordance with its preferential primary micelle, a pyrene excimer will form in every instance solubilization in the micelle palisade layer, and taking qSDs = 20 where one of the two probes is excited by the excitation light, since c P , we ~ ~obtained qBS = 90 cP. This large value of the microthe average time required for excimer formation (Le. l / k E 70 viscosity of the pyrene environment in bile salt micelle is in line ns) is much smaller than both the fluorescence lifetime (400 ns) with the conclusion that pyrene is solubilized in the micelle interior between steroid nuclei rather than at the micelle surface, as in (29) A. Malliaris, J. Sturm, J. Le Moigne, and R. Zana, submitted for classical surfactant micelles. It should be emphasized that, in the publication. fluorescence probing study of bile salt micelles previously re(30) J. Birks, D. Dyson, and I. Munro, Proc. R.Soc. London, Ser. A , 275, 575 (1963). ported,21the microviscosity evaluated from fluorescence depo(31) D. Guveli, manuscript in prepamtion. (32) M. Almgren, F. Grieser, and J. Thomas, J . Am. Chem. Soc., 101,279 TABLE II: Aggregation Number Nand Rate Constant kE for Intramicellar Pyrene Excimer Formation at 28 OC4 system N 10-7kE,s-I 4% SDOC-0.4 M NaCl 11 f 2 1.44 f 0.2 11 f 2 1.44 f 0.2 4% SDOC-0.5 M NaCl 10 f 2 1.34 f 0.2 4% SDOC-0.6 M NaCl 4% SDOC-0.6 M NaCIb 2 f0.2 12 f 2 12 f 2 8% SDOC-0.6 M NaCl 1.26 f 0.2 13 f 2 10% SDOC-0.6 M NaCl 1.26 f 0.2 13 f 2 0.85 f 0.2 5% STCDOC-0.2 M NaCl
-
(1979).
(33) K. Zachariasse, Chem. Phys. Left., 57, 429 (1978).
J . Phys. Chem. 1985,89, 1690-1692
1690
larization measurements was found to be extremely high (up to 670 cP21). The difference between this result and the estimation made in the present work illustrates once more how cautious one should be in measuring microviscosities of organized assemblies. Different probes and/or different methods of measurement can yield results differing by 1 order of magnitude. Conclusions The results reported in this paper show that primary bile salt micelles are made of about 1 1 bile salt molecules. When bile salt or NaCl concentration is increased, these micelles associate probably through H bonds and/or hydrophobic bonding to form secondary micelles. The primary micelles provide to solubilized aromatic hydrocarbons a very effective shielding to water, much
more effective than micelles of classical surfactants.
Note Added in Proof. Since this paper was submitted for publication a similar study of sodium taurocholate at high NaCl concentration was reported by Hashimoto and Thomas.34 The main results of this study-low polarity of the micelle interior and of the values of the micelle agregation numbers-agree with our conclusions. R w t r y NO. SDOC, 302-95-4; SC, 361-09-1; STCDOC, 6009-98-9; sodium laurate, 629-25-4;sodium dodecyl sulfate, 15 1-21-3;pyrene, 129-00-0;sodium octylbenzenesulfonate, 28675-11-8. (34) S.Hashimoto and J. K. Thomas, J . Colloid. Znterfoce Sci., 102,152 (1984).
Redeterdnation of the Thermodynamics of the Reactions (NH,),*S02(s)
c nNH,(g)
4-
S02(9) Ronald Landretb, Rosa G. de Pena, and Julian Heicklen* Departments of Chemistry and Meteorology and Center for Air Environment Studies, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: August 27, 1984)
The reactions between NH3 and SO2 have been reexamined at temperatures between 5 and 45 OC. Previous values of the standard enthalpies and entropies of decomposition of the products have now been corrected. The reactions are (1) ("3)z'SO2(s) * 2"3(d + so2(g) ",+Q(s)
*
+ SOz(d
(2)
The thermodynamic parameters are AHl r* 33 kcal/mol; hsl N 87 cal/(mol K);AH, = 18.4 kcal/mol; hs2= 45.1 cal/(mol K). The uncertainty in the parameters is f10% for reaction 2 and at least f 2 0 % for reaction 1.
Introduction The reaction between NH3 and SO2has been studied by several inve~tigatorsl-~ including use4 Above 10 OC, the reaction between NH, and excess SO, produces a 1 :1 adduct which is a yellow solid. With excess NH3 or below 10 O C an additional white solid is produced which is the adduct of two molecules of NH, and one molecule of SO2. Both reactions are reversible, and the solid products will sublime back to NH3 and SO2 when the pressure is reduced. The thermodynamics of the adducts has been a subject for debate. Scott et ai.' measured the vapor pressures of the solids between -10 and -70 OC. They estimated the enthalpies of sublimation to be 1 and 15 kcal/mol, respectively, for the 1:l and 2:l adducts. Later Scott and Lamb2 calculated AH and AS of sublimation to be 32.2 kcal/mol and 84.8 cal/(mol K) respectively for the 1:l adduct and 62.2 kcal/mol and 174.8 cal/(mol K) for the 2:l adduct. They assumed the products formed from direct gaseous reactions of N H 3 and SO2at 10 OC were two mutually soluble solids with an NH3:S02ratio of 1:l for one and 2:l for the other. Next we4 determined equilibrium pressures of SO,and NH, when the solid adduct is just formed at temperatures from 5 to 45 OC. We demonstrated (from thermodynamic arguments) that a 1 :1 NH, and SO2adduct was directly formed at all temperatures except at 5 and 15 OC at high [NH3]:[S0,] ratio of reactant gases. Using the van't Hoff equation we obtained the values of standard
-
- -
-
(1) Scott, W. D.; Lamb, D.; Duffy. D. J. Atmos. Sci. 1969, 26, 727. (2) Scott, W.D.; Lamb, D. J. Am. Chem. SOC.1970, 92, 3943. (3) McLaren, E.;Yencha, A. J.; Kushnir, J. M.; Mohnen, V. A. Tellus 1974, 26, 29 1. (4) Landreth, R.;de Pena, R. G.; Heicklen, J. J . Phys. Chem. 1974, 78, 1378.
0022-3654/85/2089-1690$01.50/0
enthalpy and entropy. The values were AH = 9.5 kcal/mol and AS = 15.3 cal/(mol K)for the 1:l adduct and AH = 23 kcal/mol and A S = 82 cal/(mol K) for the 2:l adduct. Finally McLaren et al.3 used a special calorimeter (capable of rapid injection of gases) with a magnetically driven stirrer and a fast-reacting thermocouple to determine enthalpies. They injected a small amount of SO2 into 750 torr of N H 3 or N H 3 and O2 and found that the gases reacted with a 30 kcal/mol loss in enthalpy in both cases. N o complete product determination was made. Since the previous thermodynamic values are not in any sort of agreement, this work was undertaken to reexamine our earlier work! In the previous study, the equilibrium pressures were taken as the condensation pressures. Since supersaturation is needed for condensation, in this study we use the evaporation pressures. Also in the previous study, the reaction cell windows were not temperature controlled. The temperature control bath was enlarged to completely enclose the reaction cell, including the windows. Finally more rigorous stirring and temperature control was used in the temperature-control water bath to ensure uniform precise temperatures. Thermodynamic functions for both 1:1 and 2:l NH, and SO2 adducts are reported. Experimental Section The experimental apparatus and procedure were the same as for the previous work4 except for two improvements. The reaction vessel in our previous work4 was not completely immersed in the constant-temperature water bath. In this work the reaction vessel was completely immersed in a constant-temperature water bath with better stirring, thereby eliminating possible cold spots. The second improvement in the experiments was in the procedure for determining equilibrium pressures. As before, either SO2or NH3 of known pressure was added to the empty reaction vessel. After 0 1985 American Chemical Society
