J . Phys. Chem. 1990, 94, 381-387
38 1
Ternary Water in Oil Microemulsions Made of Cationic Surfactants, Water, and Aromatic Solvents. 1. Water Solubility Studies A. Jada, J. Lang, and R. Zana* Institut Charles Sadron (CRM-EAHP), CNRS- ULP Strasbourg, 6 Rue Boussingault, 67000 Strasbourg, France (Received: March 2, 1989)
Two series of cationic surfactants [(i) n-C,H2m+,!n-C,Hzn+l)N+(CH3)2Xwith m = 12 or 16, n = 1-8, and X- = CI- or Br-; (11) n-C,H2,+l(C6H5(CH2)p)N+(cH3)2xwith m = 10, 12, 14, 16, and 18, p = 0-2, and X- = CI-or Br-] have been synthesized and used to prepare water in oil microemulsions, with various aromatic solvents. The water solubility in these microemulsions and the phase behavior of the systems when the water content slightly exceeded the water solubility have been investigated as a function of surfactant chain length (variation of m ) , size of the surfactant head group (variation of n or p ) , nature of the counterion (substitution of Br- by Cl-), the molar volume, and the type of aromatic solvent. Mixtures of surfactant homologues have also been investigated: for instance, C16H33(C6H5)N+(CH3)2c~and C16H33(C6H5CH2CH2)N+(CHS)2clor C16H33(C6H5CH2)N+(CH3)2Cland Br-. The results concerning the effect of the surfactant chain length and the nature of the counterion and of the solvent have been interpreted in terms of the Hou and Shah model (Langmuir, 1987, 3, 1086) for the solubility of water in water in oil microemulsions. This model is based on the two main effects that determine the stability of these systems, namely, curvature of the surfactant film separating the oil and water and interactions between water droplets. The effect of the surfactant head-group size was found to be opposite to the prediction of this model. This is probably due to the fact that part of the head group of the synthesized surfactants passed from the water side to the oil side as its size was increased and, thus, acted as a cosurfactant. Our results become consistent with the Hou and Shah model if this cosurfactant effect of the head group is assumed to dominate the behavior of the systems.
Introduction Water in oil microemulsions (w/o) have been the topic of many studies aimed at determining the size of the water droplets present in these systems and understanding the interactions between Also, electrical conductivity studies have shown the
existence in some w/o microemulsions of percolation phenomena whereby above a certain volume fraction of the dispersed phase (water surfactant) or temperature the conductivity increased by several orders of Finally, other studies of these systems have revealed the existence of "sticky" collisions between droplet^:''*^^*^^-^^ upon collision, droplets can merge
( I ) Eicke, H. F.; Rehak, J. Helv. Chim.Acta 1976, 59, 2883. (2) Dvolaitzky, M.; Guyot, M.; Lagiies, M.; Le Pesant, J. P.; Ober, R.; Sauterey, C.; Taupin, C. J. Chem. Phys. 1978, 69, 3279. (3) Robinson, B. H.; Steyler, D. C.; Tack, R. D. J . Chem. SOC.,Faraday Trans. I 1979, 75, 481. (4) Zulauf, M.; Eicke, H. F. J . Phys. Chem. 1979, 83, 480. ( 5 ) Sein, E.; Lalanne, J. R.; Buchert, J.; Kielich, S.J . Colloid Interface Sci. 1979, 72. 363. (6) Day, R. A.; Robinson, B. H.; Clarke, J. H. R.; Doherty, J. V. J . Chem. SOC..Faraday Trans. I 1979, 75, 132. (7) Cabos, C.; Delord, P. J . App. Crystallogr. 1979, 12, 502. (8) Gulari, E.; Bedwell, B.; Alkhafaji, S. J . Colloid Interface Sci. 1980, 77. 202. (9) Ober, R.; Taupin, C. J . Phys. Chem. 1980, 84, 2418. (IO) Cazabat, A. M.; Langevin, D. J . Chem. Phys. 1981, 74, 3148. ( 1 1 ) Atik, S. S.; Thomas, J. K. J . Am. Chem. SOC.1981, 103, 3543. ( I 2) Assih, T.; Larch&,F.; Delord, P. J. Colloid Interface Sei. 1982, 89, 35. (13) Kotlarchyk, M.; Chen, S. H.; Huang, J. S. J . Phys. Chem. 1982,86, 3273. (14) Bridge, N. J.; Fletcher, P. D. 1. J . Chem. SOC.,Faraday Trans. I 1983, 79, 2161. ( 1 5 ) Robinson, B. H.; Toprakcioglu, C.; Dore, J. C.; Chieux, P. J . Chem. SOC.,Faraday Trans. I 1984, 80, 13. (16) Toprakcioglu, C.; Dore, J. C.; Robinson, B. H.; Howe, A. J. Chem. SOC.,Faraday Trans. I 1984, 80, 413. (17) Kotlarchyk, M.; Chen, S. H.; Huang, J. S.; Kim, M. W. Phys. Reu. A 1984, 29, 2054. (18) Kotlarchyk, M.; Huang, J. S.; Chen, S. H. J . Phys. Chem. 1985,89, 4382. (19) Clarke, J. H. R.; Nicholson, J. D.; Regan, K. N. J. J. Chem. SOC., Faraday Trans. I 1985, 81, 1173. (20) Cabos, C.; Marignan, J. J . Phys. Letr. 1985, 46, L-267. (21) Pileni, M. P.; Zemb, T.; Petit, C. Chem. Phys. Left. 1985, 118, 414. (22) Chatenay, D.: Urbach, W.; Cazabat, A. M.; Langevin, D. Phys. Reu. Lett. 1985, 54, 2253.
(23) Ganz, A. M.; Boegner, B. E. J. Colloid Interface Sci. 1986,109,504. (24) Lang, J.; Jada, A.; Malliaris, A. J . Phys. Chem. 1988, 92, 1946. (25) Huang, J. S.; Safran, S. A.; Kim, M. W.; Crest, G. S.; Kotlarchyk, M.; Quirke, N. Phys. Rev.Lett. 1984, 53, 592. (26) Howe, A. M.; Toprakcioglu, C.; Dore, J. C.; Robinson, B. H. J . Chem. SOC.,Faraday Trans. 1 1986,82, 241 1. (27) Kim, M. W.; Huang, J. S. Phys. Rev. B: Condens. Matter 1982,26, 2703. (28) Huang, J. S. J . Chem. Phys. 1985,82,480. (29) Roux, D.; Bellocq, A. M. Phys. Reu. Lett. 1984, 52, 1895. (30) Honorat, P.; Roux,D.; Bellocq, A. M. J . Phys. Lett. 1984,45, L-961. (31) Lemaire, B.; Bothorel, P.; Roux, D. J. Phys. Chem. 1983,87, 1023. (32) Brunetti, S.; Roux, D.; Bellocq, A. M.; Fourche, G.;Bothorel, P. J. Phys. Chem. 1983,87, 1028. (33) Roux, D.; Bellocq, A. M.; Bothorel, P. frog. Colloid Polym. Sci. 1984, 69, I . (34) Hou, M. J.; Kim, M.; Shah, D. 0. J. Colloid InterfaceSci. 1988,123, 398. (35) Lagiies, M.; Ober, R.; Taupin, C. J. Phys. Letr. 1978, 39, L-487. Lagiies, M.; Sauterey, C. J . Phys. Chem. 1980, 84, 3503. (36) Lagues, M. J . Phys. Lett. 1979, 40, L-331. (37) Cazabat, A. M.; Chatenay, D.; Meunier, J.; Pouchelon, A. J . Phys. Lett. 1982,43, L-89. Cazabat, A. M.; Chatenay, D.; Guering, P.; Langevin, D.; Meunier, J.; Sorba, 0.; Lang, J.; Zana, R.; Paillette, M. In Surfactants in Solution; Mittal, K., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 3, p 1737. (38) Eicke, H. F.; Hilfiker, R.; Holz, M. Helu. Chim.Acta 1984, 67, 361. (39) Eicke, H. F.; Kubik, R.; Hasse, R.; Zschokke, I. In Surfactanrs in Solution; Mittal, K., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 3, p 1523. (40) Hilfiker, R.; Eicke, H. F.; Geiger, S.; Furler, G. J. Colloid Inferface Sei. 1985, 105, 378. (41) Geiger, S.; Eicke, H. F. J . Colloid Inferface Sci. 1986, 110, 181. (42) Van Dijk, M. A.; Casteleijn, G.; Joosten, J. G. H.; Levine, Y. K. J . Chem. Phys. 1986,85,626. (43) Borkovec, M.; Eicke, H. F.; Hammerich, H.; Gupta, B. D. J . Phys. Chem. 1988, 92, 206. (44) Mathew, C.; Patanjali, P. K.: Nabi, A,; Maitra, A. Colloids Surf. 1988, 30, 253.
0022-3654/90/2094-038 1 $02.50/0
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0 1990 American Chemical Societv
382 The Journal of Physical Chemistry, Vol. 94, No. I , 1990 temporarily and exchange material during the time they remain merged (see Figure 1 in the following paper in this issue53). The reported results mostly concerned w/o microemulsions based on the surfactant sodium bis(ethylhexy1)sulfosuccinate (AOT) and alkanes as the oil. These microemulsions are usually ternary surfactant-oil-water systems, thereby avoiding the complexity associated with the presence of a cosurfactant. The results clearly showed that the magnitude of interdroplet interactions increases with droplet size and that a decrease of alkane chain length results in a decrease of both droplet size and interdroplet interaction^.^^,^^,^^ A recent has shown that the overall bimolecular rate constant for droplet collision with exchange of material increases very much when the oil chain length is increased, all other parameters remaining unchanged. Finally, scattered results indicated that the percolation threshold is decreased when the oil chain length is i n ~ r e a s e d . ~ ~ ~In' ~other "~'~ recent s t u d i e ~ ,the ~ ~solubility ,~~ of water in these microemulsions was investigated both theoretically and experimentally and its relationship to the spontaneous radius of curvature of the surfactant layer and to interdroplet interactions discussed. Thus, the effect of the oil chain length on the size and dynamics of water droplets in w/o microemulsions appears to be well investigated. On the contrary, the effect of the surfactant chain length and head-group size and of the nature of the counterion on the properties discussed above has been investigated only a little, owing to difficulties associated with the synthesis of homologous series of anionic surfactants where one of the above parameters is changed at a time. However, such studies are much needed for a full check of some predictions of existing theories of the stability of microemulsions. This led us to look for new ternary w/o microemulsions that would permit the study of the effect of the above parameters on microemulsion properties. We turned to cationic surfactants because the synthesis of homologous surfactants of the quaternary ammonium type of selected chemical structure is comparatively easier than of anionic s ~ r f a c t a n t s . Two ~ ~ surfactant series were selected that allowed us to perform the desired studies: series I, alkyl(alky1')dimethylammonium halides, CmH2,+'(C,H2ntl)N+(CH3)2X-;series 11, alkyl(phenyla1kyl)dimethylammonium halides, CmHZm+, [C6HS(CH2),]N+(CH3),X-. Indeed, it has been shown that the series I homologue with m = 12, n = 4, and X- = Br- can form water in chlorobenzene microemulsions with a [water]/[surfactant] molar concentration ratio up to 80.57.ss On the other hand, the cetylbenzyldimethylammonium chloride of series I1 is known to form ternary , ~ ~ ,aromatic ~ solvents water in benzene m i c r o e m ~ l s i o n s . ~ ~Other such as toluene, bromobenzene, etc., can also be used, and
(45) Eicke, H. F.; Shepherd, J. C. W.; Steinemann, A. J. Colloid Interface Sei. 1976, 56, 168. (46) Atik, S. S.; Thomas, J. K. Cfiem. Pfiys. Lett. 1981, 79, 351. (47) Fletcher, P. D. 1.; Robinson, B. H. Ber. Bunsen-Ges. Pfiys. Cfiem. 1981, 85, 863. (48) Fletcher, P. D. 1.; Howe, A. M.; Perrins, N. M.; Robinson, B. H.; Toprakcioglu, C.; Dore, J. C. In Surfactants in Solution; Mittal, K., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 3, p 1745. (49) Brochette. P.; Pileni, M. P. Nouu. J. Cfiim. 1985, 9, 551. Pileni, M. P.; Furois, J. M.; Hickel, B. In Surfactants in Solution; Mittal, K.. Lindman, B., Eds.; Plenum: New York, 1984; Vol. 3, p 1471. (50) Fletcher, P. D. 1.; Howe, A. M.; Robinson, B. H. J. Cfiem. SOC., Faraday Trans. I 1987, 83, 985. ( 5 1 ) Howe, A. M.; McDonald, J. A,; Robinson, B. H. J. Cfiem. SOC., Faraday Trans I 1987,83, 1007. (52) Dutkiewicz, E.; Robinson, B. H. J. Electroanal. Cfiem., in press. (53) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J . Pfiys. Cfiem., following paper in this issue. (54) Hou, M. J.; Shah, D. 0.Langmuir 1987, 3, 1086, and references cited therein. (55) Leung, R.;Shah, D. 0. J. Colloid Interface Sei. 1987, 120, 320 and 330. (56) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (57) Kubota, K.; Tominaga, Y.; Kon-no, K.; Kitahara, A. Rep. Prog. Polym. Pfiys. Jpn. 1985, 28, 453. (58) Verrall, R.; Milioto, S.; Zana, R. J. Pfiys. Cfiem. 1988, 92, 3939. (59) Miller, D.; Klein, A.; Hauser, M. Z . Naturforscfi. 1977, 32A, 1030. (60) McNeil. R.; Thomas. J. K. J. Colloid Interface Sei. 1981. 83, 57.
Jada et al. therefore the investigationof the effect of the oil dielectric constant becomes possible. Indeed, the dielectric constants of aromatic solvents stretch over a large range, depending on the chemical group bound to the aromatic moiety. The aim of this paper is to present the results of a systematic study of the solubility of water in binary systems made of surfactants of series I and I1 and aromatic solvents. These results are discussed on the basis of the recent approach of Shah et a1.54,55 for the stability of w/o microemulsions. In the following paper in this issue,53 we report and discuss results concerning water droplet sizes, interactions between droplets, occurrence of electrical conductivity percolation, and rates of exchange of material between droplets through interdroplet collisions, for selected surfactants of series I and 11.
Materials and Methods 1 . Materials. The surfactants have been synthesized by quaternization of amines by appropriate alkyl bromides or alkylphenyl bromides. Thus, the surfactants of series I, referred to as N m , n , l , l , X , have been obtained from the reaction ru.
with m = 16, n = 4, 6, 8, and X- = Br-. The preparation of the surfactants with m = 12, n = 1-6, 8, and 10, and X - = CI- or Br- has been described p r e v i o u ~ l y . ~ ~ The bromide surfactants of series I1 referred to as Nm,p+,l,I ,Br were prepared according to reaction 2a for the homologues with p = 0, m = 16 a n d p = 1, m = 10and 18 and according toreaction 2b for the homologues w i t h p = 1, m = 12 and p = 2, m = 16.
Br-
+ The quaternization reactions were performed in anhydrous ethanol at about 70 OC for 36-48 h under reflux. After reaction, ethanol and unreacted reactants were removed as thoroughly as possible by rotatory evaporation under vacuum. The surfactant was then under the form of a solid or a paste. It was recrystallized three to four times in pure ethyl acetate, or ethyl acetate ethanol or ether, depending on the surfactant, and dried under vacuum in the presence of calcium chloride, at 40 OC. The chloride surfactants of series I1 were prepared from the corresponding bromides by ion exchange, using a Merck 111 resin in the chloride form. The aqueous solutions of the chloride surfactants were evaporated under vacuum, after checking that bromide ions had been quantitatively exchanged by chloride ions. The remaining solid was recrystallized twice or thrice from ethyl acetate or ethyl acetate + ethanol or ether and dried under vacuum in the presence of calcium chloride, at 40 "C. Two homologues of series 11, Nl4,ld,l,l,Cl and Nl6,l$~,l,l,Cl, were purchased from Fluka (Switzerland) and BDH (England), respectively. They were recrystallizedas above. The corresponding bromides were obtained through ion exchange as described above. The impure surfactant N 10,1d,l,I,Cl could not be recrystallized. It was therefore purified by chromatography with a column of silica gel 60 (Merck, 70-230 mesh ASTM) and eluting with ethyl acetate-ethanol mixtures of increasing ethanol content. The surfactant purity was checked by elemental analysis (C, H, N, X ) . The residual water content was determined by Karl-Fisher titration. It never exceeded 4% and was taken into account in water solubilization studies. The water used in the measurements was deionized and distilled. The solvents, benzene, toluene, xylene, ethylbenzene, 1,3,5-tri-
+
Water Solubility of Water in Oil Microemulsions
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990 383
methylbenzene, styrene, chlorobenzene, bromobenzene, and 1bromonaphthalene, were all distilled before use. Their water content was checked prior to the solubility measurements. 2. Methods. In the following, the water present in the system is expressed as the molar concentration ratio w
-
= [H,O] / [surfactant]
The solubility of water refers to the value w, of this ratio above which the system becomes polyphasic. The water solubilities were determined by visual inspection as follows. The surfactant concentration was kept constant close to about 0.1 M for series I, as in a previous study,5sand 0.27 M for series 11. To a sample of 2-4 cm3 of surfactant solution in a given solvent, placed in a thermostated cell (30 OC for series I and 20 OC for series 11), water was added dropwise under stirring from syringes accurate to within 0.002 cm3 until phase separation took place. In many instances, the turbidity of the system greatly increased before phase separation and the last additions were performed more slowly. Stirring was stopped for the observation of phase separation. For those systems where phase separation took place very slowly, a series of tightly closed tubes containing the same ternary system with increasing U-values were prepared, shaken for a while, and allowed to equilibrate in a thermostated bath. They were examined at regular time intervals to check the range of w corresponding to phase separation. The overall error on the w, values is estimated to be f 2 for wc ranging between 40 and 100, f 1 for wc ranging between 10 and 40, *OS for wc ranging between 4 and IO, and f0.2for wc ranging between 1 and 4. At w > wc, the systems were usually biphasic, occasionally triphasic. To determine the nature of the phases in equilibrium, we used two dyes:54 methylene blue (MB), which is selectively soluble in water, and 4-(dimethy1amino)benzene (DAB), which is selectively soluble in oil. Thus, an equilibrium between excess water and a water in chlorobenzene microemulsion as a lower phase will give an intensely blue upper phase and a slightly bluish lower phase upon solubilization of MB or a colorless upper phase and a yellow lower phase upon solubilization of DAB. An equilibrium between two water in chlorobenzene microemulsions will give two blue-colored phases, the upper being more blue than the lower phase upon solubilization of MB, or two yellow phases upon solubilization of DAB. In the case of water in benzene microemulsions, the positions of the phases are reversed owing to the lower density of benzene with respect to water. Notice that with long-chain surfactants ( m = 16 or 18) the phase separation led sometimes to an ill-defined phase: creamy and emulsion-like, generally in small amount, in addition to the w/o microemulsion phase. Several weeks were then necessary to reach final equilibrium. The presence of liquid crystals was not tested for systematically.
Solubility of Water in w/o Microemulsions: Theoretical Reminder In recent papers, Shah et a1.54.55presented a model for the qualitative interpretation of the fast changes of water solubility in w/o microemulsions with various parameters. This model is based on the theory of the stability of microemulsions (see ref 54 for comprehensive referencing). Indeed the water solubility w, corresponds to the situation where the one-phase microemulsion system becomes unstable and phase-separates in two or more phases upon addition of a slight excess of water. Shah et a1.54J5considered two limiting situations as regards microemulsion stability. In the first one, the stability is determined by the curvature of the interfacial film separating oil from water. This situation corresponds to systems with low interfacial tension between oil and water. The interfacial film (a surfactant layer, essentially, in the case of the ternary systems investigated here) in the presence of water and a given oil is characterized by its spontaneous (or natural) radius of curvature R O . At low water content ( w ) . however, the droplet radius R is smaller than R O . As w is increased, R increased (note that an increase of R can also be induced by a change of other parameters at constant w ) .
OIL WLAP VOLUPE
c S I Z E 3F S U F P C T A N T HEAD GROUP S A L I N I T Y . S U R F A C T A U ChAlN
c
LENtTP
COSURFhCTANT ClrAIN LEIISTP
-
Figure 1. Schematic representation of the variation of the solubility of water in w/o microemulsions with various parameters, according to Shah et a1.54355
Safran and Turkevich61have shown that when R tends to become larger than RO,the microemulsion phase-separates in two phases: a w/o microemulsion and an excess of water (the so-called Winsor I1 systems). The value of wc thus corresponds to droplets having a radius equal to the natural radius of curvature of the surfactant layer. The second limiting situation corresponds to w/o microemulsions where the stability is determined by interdroplet interact i o n ~ . Indeed, ~ ~ , ~ ~it is known that the strength of these interactions (measured for instance by the second virial coefficient of the scattered intensity of light or diffusion coefficient) increases with droplet size, for a given system. In systems where the spontaneous radius of curvature is very large, it may happen that upon increasing the droplet radius, by changing an appropriate parameter, interdroplet interactions increase so much as to bring about phase separation when the droplet radius reaches the critical value Rf. In this case, the system separates in two microemulsion phases, differing by the droplet concentration. Also, the system generally displays a critical behavior. Of course, the limiting value of Rf corresponds to the solubility of water in the system. Shah et a1.54,55 have pointed out that Ro and RE vary in opposite fashions with any of the parameters that control the solubility of water in w/o microemulsions: cosurfactant and surfactant chain lengths, surfactant head-group size, salinity, oil chain length (or molar volume), etc. This model therefore predicts maxima in the change of water solubility in w/o microemulsions with all of the above parameters. We have schematically represented in Figure 1 the change of the water solubility with the various parameters of interest in the present study. Shah et al.”955 did check a number of predictions of their model (oil and cosurfactant chain lengths, [cosurfactant]/ [surfactant] ratio, salinity, etc.) In the present study, the synthesized cationic surfactants will permit us to check the effect of additional parameters relating to the nature of the surfactant: surfactant chain length, nature of the counterion, and head-group size. The above model allowed us to locate our systems in the representation of Figure 1 and to know whether their stability is determined by interdroplet interactions or curvature effects. It will be seen however that difficulties arose in the interpretation of the effect of the size of the surfactant head group, because the change of this parameter apparently also resulted in the change of another parameter governing the phase behavior. Some of the conclusions reached in the present paper will be confirmed by results presented in the following one in this issue.
Solubility of Water in w/o Microemulsions: Experimental Results and Discussion The results will be presented in the form of tables and figures. Some indications on the phase equilibria at w just above wc will be given by using some of symbols adopted by Leung and Shah,s5 namely, “w” for an equilibrium between excess water and a w/o microemulsion (systems on the Ro branch), “0” for an equilibrium between two w/o microemulsions (systems on the RE branch), “p” for an equilibrium between a w/o microemulsion and some creamy (61) Safran, S . ; Turkevich, L. Phys. Reu. Lett. 1983, 50, 1930.
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The Journal of Physical Chemistry, Vol. 94, No. I , I990
TABLE I: Solubility of Water (w,) in Solutions of Surfactants Nm,l@,l,l,Cl (Line A ) and Nm,l@,l,l,Br (Line B) in Chlorobenzene at 20 OC m 10 12 14 16 18
90 (p)" b 4 (w) 'At w > w,, the system gave two bhases after equilibration for 24 h. The lower phase was opalescent with the light still going througb it. The upper phase was viscous, creamlike, and opaque. After 3 months, however, the upper phase gave two opalescent phases not yet fully separated. When the system was investigated at 24 "C rather than at 20 OC, it showed two phases: a lower one slightly opalescent and an upper one strongly opalescent. *This surfactant was insoluble in water in the absence of H20.A clear solution was obtained upon heating that precipitated after 2 h upon cooling. At 20 OC, the surfactant remained partly insoluble even at ci = 20. A
5
(0)
B
25 ( 0 )
65 ( 0 )
4 (w)
4 (w)
t----4
-I
20 0
10
I
20
I
30
I
10
T"C
-
TABLE 11: Solubility of Water (w,) in Solutions of Nm,4,1,1,Br in Aromatic Solvents of Various Dielectric Constants (6) and Molar Volumes (V J wr solvent tb VmC N12,4,1,1,Br N16,4,1,1,Br benzene 2.28 88.7 8 (0) 83 (w) toluene 2.38 106.3 6 (0) 70 (w) 2.5 (0) 10 ( 0 ) o-xylene 2.57 120.6 2.5 (0) 7.5 (0) m-xylene 2.37 122.8 1.3 (t) 6 (0) p-xylene 2.27 123.3 ethylbenzene 2.41 122.4 4 (0) od 138.9 0 (t)' w 1,3,5-trimethylbenzene 2.28 2.43 114.8 IO (0) 50 (w) styrene 80 (0) 20 (w) chlorobenzene 5.7 101.7 bromobenzene 5.4 104.7 50 (0) I-bromonaphthalene 4.8' 139.7 12 ( 0 )
"Values of w, are at 30 OC and for a surfactant concentration close to 0.1 values are at 20 'C except for toluene and styrene at 25 "C. From: Handbook of Chemistry and Physics, 44th ed.; US.Rubber Chem. Co., Cleveland, OH, 1963. 'Calculated from molecular weights and densities listed in the reference in footnote b. dThe surfactant was insoluble up to w = 12. Above this value, the system was monophasic up to w = 67 and then phase-separated in two w/o microemulsions. 'The surfactant was insoluble in the absence of water. At w = 4, the system gave rise to three liquid phases with the lower one being birefringent. 'The surfactant was insoluble in the absence of water. Addition of water gave rise to a gellike, birefringent lower phase in equilibrium with an upper clear phase. The volume of the lower phase was much larger than that of the surfactant + water, indicating extensive absorption of oil., g From: Table of Dielectric Constants of Pure Liquids; Maryott, A., Smith, E., Eds.; NBSC 514; US. Department of Commerce: Washington, DC, 1951. M. "11
' Uc
I
50
60
UC
Figure 2. Solubility of water in solutions of Nm,l@,l,l,Cl in chlorobenzene at 20 "C and [surfactant] = 0.27 M, for m = 12 ( O ) , m = 14 (A),and m = 16 (a).
or emulsion-like phase or surfactant precipitate swollen by water, and "t" for a three-phase equilibrium. 1 . Effect of the Surfactant Chain Length and Nature ofthe Soloent. Table 1 lists the solubility of water in chlorobenzene solutions of the series I1 surfactants N m , l & l , l , C l and Nm,l +,I , I ,Br. The solubilities are small with the bromide surfactants and show no measurable dependence on surfactant chain length, within the experimental error. On the contrary, a large increase of wc with m is seen with the chloride surfactants for m = 10-16. Figure 2 shows that the same increase is observed in the whole T range investigated from 10 to 60 O C . At w slightly above wcr the two phases in equilibrium were identified as w/o microemulsions for the Nm, 14,1,1,Cl-containing systems, for m = 10,12,14, and 16 (see footnotes in Table I for m = 16 and 18) and as a w/o microemulsion in equilibrium with a small excess of water for the Nm,ld,l , I ,Br-containing systems. Thus, the change of w, with m for the chloride surfactant systems and their phase behavior agree with the prediction of the schematic representation of Figure 1. The m = 10, 12, and 14 systems are located on the RE branch. The ambiguous phase behavior of the m = 16 system suggests that it may be located just at the crossover between the RC branch and the Ro branch. As for the system containing N 18,1@,1,1,CI, its peculiar behavior may due to the high Krafft temperature of the surfactant: 37.5 f 0.6 O C (determined as part of this work). A preliminary test performed at 35 "C showed this surfactant to be then soluble in chlorobenzene and its solution to be able to solubilize water up to w, = 92. The effect of the chain length of series I surfactants on water solubilization was investigated for only two homologues, N12,4,1,1,Br and N16,4,1,1,Br, but in a variety of aromatic solvents. The results, listed in Table 11, indicate a very complex behavior with a dependence of wc on the surfactant chain length as well as on the solvent molar volume (compare the results for benzene, toluene, xylene, and trimethylbenzene), the solvent dielectric constant (compare the results for toluene and bromobenzene or trimethylbenzene and bromonaphthalene), and the shape of the solvent molecule (compare the results for ethylbenzene
10
50
0
0 SOLVENT MOLAR VOLUME ( cm3/mol)
Figure 3. Effect of the solvent molar volume on the solubility of water in systems containing N12,4,1,1,Br (0)and N16,4,1,1,Br (e): full line,
F branch; broken line, Ro branch.
and m-xylene or for the three xylenes). Most of these results can nevertheless be reasonably understood on the basis of the predictions summarized in Figure 1 and by consideration of the phases in equilibrium, which are given in Table 11. For the three xylene isomers, the systems at wc lie on the RC branch with both surfactants. There, as predicted by Figure 1, the water solubility increases with surfactant chain length, as with the series I1 surfactant solutions in chlorobenzene, and decreases upon increasing molar volume V, of the solvent. The latter effect is also clearly seen for N12,4,1,1,Br in halogenated aromatic solvents (see below). Consider now the nonhalogenated aromatic solvents. All of the N12,4,1,1,Br systems fall on the RE branch and as expected the w,-values, although small, decrease upon increasing solvent molar volume as seen in Figure 3. The result for styrene is, however, off the line going through the wc-V, data in Figure 3. With the N16,4,1,1,Br surfactant, the increase of V,,, results in a passage from the Ro branch (solvents of small V, such as benzene, toluene, styrene) to the RE branch (solvents of large V, such as xylenes) again in general agreement with the prediction of Figure 1 . The results in Figure 3 indicate that the increase of surfactant chain length shifts to a larger value the solvent molar volume where the crossover from the Ro branch to the RE branch takes place and increases the value of wc. These two results conform to the schematic representation of Figure 1 when it is
Water Solubility of Water in Oil Microemulsions 1
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The Journal of Physical Chemistry, Vol. 94, No. I , 1990 385
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~~ 20
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Figure 4. Schematic representationof the variation of the water solubility with the Nm,4,1,1 ,Br surfactant chain length in chlorobenzene ( O ) , benzene (+), and styrene ( 0 ) : full line, RC branch; broken line, Ro branch.
noticed that surfactant chain length and oil molar volume have opposite effects on water solubilization. Nevertheless, Figure 3 and Table 11 show that, for the N16,4,1,1,Br solutions in benzene, toluene, and styrene, the water solubility decreases upon increasing V,,, contrary to the prediction of Figure 1 for these systems that are located on the Ro branch. The explanation for this behavior probably lies in the fact that the increase of V,,, in going from benzene to toluene for instance is obtained by adding to benzene an aliphatic group. Aromatic and aliphatic groups are not likely to have the same effect as regards water solubility and phase behavior. In fact, the Nm,4,1,1,Br surfactants are readily soluble in strongly aromatic solvents, become progressively less soluble in aromatic solvents increasingly substituted by aliphatic groups, and are insoluble in n-alkanes in the absence of water. Thus, N 16,4,1,1,Br is.insoluble in trimethylbenzene and ethylbenzene. Another evidence of the importance of the chemical nature of the group bound to the aromatic ring is provided by the comparison of the results found with ethylbenzene and styrene. The double bond in the side group in the styrene molecule dramatically effects water solubility. As part of this work, we have observed that a small addition of water to a suspension of N16,4,1,l,Br in nheptane corresponding to w = 8, brought about the solubilization of the surfactant and a phase separation in two clear liquid phases. The lower water-rich phase had a volume about a 100-fold larger than that of the added water, indicating the presence of a considerable amount of oil. The same observation was made for N12,4,1,1,Br in n-heptane or n-hexane, but the two phases were already obtained at w = 1.5. Also, N12,4,1,1,Br was soluble in 1-bromohexane, and this solution solubilized water up to wc = 1.5. Above this value, two phases were obtained with the water-rich upper phase having a volume fraction of about 0.1 at o = 4.5. These results indicate that aliphatic solvents tend to generate systems on the RC branch, with a lower water solubility. The inversion noted above for the o,values in benzene and toluene is readily understood if the methyl group of toluene is assumed to have the same qualitative effect as aliphatic solvents on the wc value. We now turn to halogenated aromatic solvents. They give larger water solubilities when associated with N12,4,1,1,Br than nonhalogenated solvents, and as to be expected for systems on the Rc branch, w c decreases rapidly upon increasing solvent molar volume, from chlorobenzene to bromobenzene and to l-bromonaphthalene. This decrease gives a true measure of the effect of the solvent molar volume on w,since the three solvents have about the same dielectric constant and contain no aliphatic group. For N16,4,1 ,I?Br,the results show that the water solubility is smaller in an halogenated aromatic solvent than in a nonhalogenated one having the same molar volume. The effect of halogen substituents is to increase the solvent dielectric constant. This in turn should somehow decrease the electrostatic repulsions between head groups, which is equivalent to an increase of salinity. Thus, according to Figure 1, going from nonhalogenated to halogenated aromatic solvents should increase w, for systems on the RE branch and decrease wc for systems on the R" branch. Our observations agree with these predictions since the N 12,4, I , 1,Br systems are
0.2 0.L 0.6 0.8
1
Figure 5. Solubility of water in solutions of mixtures of N 16, I@, I , 1,Cl and N l 6 , l @ , l , l , B rin chlorobenzene as a function of the Nl6,lq!~,l,l,Cl mole fraction at 20 O C : total surfactant concentration, 0.1 M; temperature, 20 "C.
on the RCbranch and the N16,4,1,1,Br systems are on the Ro branch. In Figure 4, we give a schematic representation of the variations of w, with surfactant chain length for the two Nm,4,1,1,Br homologues in three systems, in order to illustrate the above discussion. Finally, in the course of this study, some indications were obtained concerning the temperature dependence of w,. Thus, Figure 2 shows that w, decreases slightly upon increasing T for the series I1 surfactants in chlorobenzene. A similar result was reported for the solubility of water in solutions of N12,4,1,1,Br in chlor~benzene.~~ However, this decrease of wc upon increasing T is far from general. For instance, wc increases with T for N12,4,1,1,Br and N16,4,1,1,Br in both benzene and o-xylene. It thus appears as if the T dependence of wc depends mainly on the nature of the solvent and not on whether the system belongs to the Ro or Rc branch. Indeed, the N 12,4,1,1,Br-chlorobenzene, -benzene, or -0-xylene systems are on the REbranch and so is the N16,4,1,1,Br-o-xylene systems whereas the N16,4,1,1,Brchlorobenzene or -benzene systems are on the Ro branch. Note that the literature does not discuss much the effect of T o n water solubilization in w/o microemulsions. 2. Effect of the Nature of the Counterion. This study was performed in chlorobenzene solutions where large water solubilities have been observed with surfactants of both series I and 11. Recall that the substitution of Br- by CI- with the series I surfactants N12,n,l ,l,Br has been found to result in a shift of the value of n where w,goes through a maximum from 4 to 5.5.58 This substitution also resulted in a dramatic decrease of the w, values for the systems containing the butyl and isobutyl homologues (located on the RCbranch in Figure 1) from 80 and 40 to 4 and 4, respectively. On the contrary, this substitution resulted in an increase of the w,-values for the pentyl and hexyl homologues.58 In the present work, the substitution of Br- by CI- for the Nm,l$,l,l,Br surfactants was found to result in no less dramatic increases of o,for the m = 12, 14, and 16 homologues, from 4, 4, and 4 to 25, 65, and 90, respectively. We have determined the solubility of water in chlorobenzene solutions of mixtures of Nl6,l$,l,l,Cl and Nl6,l$,l,l,Br. The results plotted in Figure 5 as a function of the chloride surfactant mole fraction Xcl show a maximum at Xc, E 0.9 and then w, decreases rapidly with Xcl. The above results can be easily explained on the basis of the representation of Figure 1 and by making use of the similarity between the effects of salinity and of counterion substitution on water solubility. Recall that an increase of salinity always results in a decrease of molecular area per surfactant head group and thus in a decrease of the radius of spontaneous curvature of the surfactant layer in w/o microemulsions. An increased salinity thus decreases the water solubility for systems on the Ro branch but increases it for systems on the RE branch, as indicated in Figure 1. Chloride ions are known to be less bound than bromide ions to quaternary ammonium head groups of the surfactant layer, irrespectively of the sign of the layer c u r ~ a t u r e . ~ ~Lesser *~*
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Se, Se, Figure 6. Schematic representation of the effect of the B r - 4 - substitution on the solubility of water in chlorobenzenesolutions of N12,butyl or isobutyl,l,l,X (A) and Nnz,l@,l,l,X (B). The solubility of water is plotted as a function of the equivalent salinity .Sw, in arbitrary units. The curves in solid lines represent Rc branches and those in broken lines Ro branches. Key: ( 0 ) bromide data; ( X ) chloride data. counterion binding, as is the case upon substitution of Br- by CI-, will result in stronger electrostatic repulsions between head groups and is therefore equivalent to a decrease of salinity. In the N 12,4,I,l,Br- and N12,isobutyl,l,l,Br-containingsystems being located on the Rf branch, the substitution of Br- by CI- (reduced equivalent salinity) should thus result in a decrease of w,, as is indeed observed. This is schematically represented in Figure 6A where the water solubility for each surfactant system is plotted as a function of some equivalent salinity, S The Nm, 1$,1,1 ,Br-containing systems w i a m = 12, 14, and 16 are located on the Ro branch in Figure 1 . The decrease of equivalent salinity brought about by the Br--CI- substitution should therefore result in increased water solubilities, as is observed. Since the Nm,l$,l,l,Cl systems have been found to be located on the Rc branch, we are led to the schematic representation of Figure 6B for the effect of the Br--CI- substitution on the a,-values. I n fact, the solubility results in the N l 6 , l $ , l , l , B r N 16,14,1,I ,CI mixture provide an experimental basis to the above interpretation, and the wc vs XC, curve of Figure 5 becomes identical with those of Figure 6A,B if Xci is replaced by the equivalent salinity S . The decrease of S , upon progressive substitution of Br- by for the N16,14,1,1,Br-containing system located on the Ro branch results in an increase of w, that extends up to Xc, N 0.9. At this point, the mixed system becomes located on the RCbranch. A further increase of X,,,that is a decrease of S,, brings about a decrease of w,. 3. Effect of the Size of the Surfactant Head Group. In principle, this effect can be conveniently investigated by increasing n at constant m ( 1 2 or 16) for series I or by increasingp at constant m for series 11. The results for series 1 with m = 12 and X- = Br- in several aromatic solvents have been reported recently.58 It was found that wc goes through a maximum for a value of n that depends on the solvent. This maximum was found to occur at n = 4 with solvents of intermediate dielectric constant (e N 5-10) such as chlorobenzene, bromobenzene, and dichloroben~ene.~~ In these solvents, wC was rather large (40-80). In other aromatic solvents, of lower or larger dielectric constant, the position of the maximum of w, was found to be shifted to values of n larger or lower, respectively, whereas the values of wc were always smaller, around 20-30. Finally, the solubility of water in mixtures of N12,3,1,1,Br and N I2,5,1,1 ,Br was found to go through a sharp maximum in the nearly equimolar mixture of these two surfactants, Le., for an average value of n close to 4, with wc N 80.58 As part of this work, we have investigated the nature of the phases in equilibrium at w just above w, for the series I surfactants NI 2,n,l,l,Br in chlorobenzene. We thus found that homologues with n 5 4 give rise to an equilibrium between two w/o microemulsions (systems on the RC branch in Figure 1) whereas the
21-
( 6 2 ) Fabre, H.; Kamenka, N.; Khan, A,; Lindblom, G.; Lindmann, B.; Tiddy, G. J. J . Phys. Chem. 1980, 84, 3428.
Jada et al. homologues with n > 4, including the N 12,isopentyl,I , 1,Br gave rise to an equilibrium between a w/o microemulsion and excess water (systems on the Ro branch in Figure 1). Therefore, these results indicate that the systems go from the o-type to the w-type upon increasing size of the surfactant head group, contrary to the prediction of Figure 1. A phase behavior and a water solubility change opposite to the prediction of Figure 1 were also found for the series I surfactants N 16,n, 1,1,Br in chlorobenzene. Thus, w, decreased from 20 to 5 in going from n = 4 to n = 8, and both systems were of the w-type. An abnormal behavior was thus found in the changes of both w, and the phase behavior upon increasing the size of the surfactant head group with the systems containing surfactants of series I . This suggests that the increase of the surfactant head-group size, as it was performed in the present study, probably also induces the change of another parameter that effects water solubility and phase behavior. At this stage, it must be recalled that data on cmc in aqueous solutions and surface area per head group at the air-water interface strongly suggested that for the N12,n,l , I ,Br surfactants with n 5 3 the variable chain C,HMl is mostly located in the aqueous phase, whereas for n 2 5 the variable chain is mostly located in the micelle hydrophobic core or in the air.58 The homologue with n = 4 is in an intermediate situation. Translated to the case of N12,n,l,l,Br-containing w/o microemulsions, the variable chain will thus have an increasing tendency to go from the water side to the oil side as n increases. The variable chain thus behaves like a cosurfactant, as pointed out p r e v i ~ u s l y .If~ ~ we accept that the main effect of increasing n is to increase the length of this pseudo-cosurfactant,then Figure 1 correctly accounts for our experimental observations. Indeed, as the cosurfactant chain length increases, the model of Shah et a1.54s55predicts a passage from the RE branch to the Ro branch with the water solubility first increasing and then decreasing. The peak in the wc vs n curve for N 12,n,l,l,Br surfactants in chlorobenzene is very narrow because the transfer of the variable chain from the water side to the oil side essentially takes place between n = 3 and 5 . It is interesting to note that a recent paper dealing with the effect of the cosurfactant chain length on water solubility reports a dramatic change of w, when the cosurfactant (alcohol) chain grows from n-propyl to n - ~ e n t y l . ~ ~ This interpretation of the results concerning the effect of head-group size on the basis of a cosurfactant effect is likely to hold for the results for surfactants N16,n,l,l,Br of series I as well as the mixture of N 12,3,I , 1 ,Br-N 12,5,1,1,Br studied p r e v i ~ u s l y . ~ ~ Thus, the procedure adopted in the present study to investigate the effect of the head-group size appears not to be adequate. Owing to the transfer of the variable chain from the water side to the oil side upon increasing chain length, the effect of the head-group size is overshadowed by the cosurfactant effect. A suggestion for future work is to use surfactants where the variable chain would be terminated by a hydroxylic group. This will probably greatly delay its transfer from the water side to the oil side and thus permit the study of the effect of the head-group size on the water solubility and phase behavior.
Summary and Conclusions This paper reports extensive water solubility measurements in w/o microemulsions made of quaternary ammonium surfactants and a variety of aromatic solvents, as well as on the phase behavior of these systems when the water solubility is exceeded. The surfactants synthesized allowed the study of the effect of parameters related to the surfactant structure, chain length, nature of the counterion, and size of the head group, which had been investigated little thus far. The effect of the nature of the solvent was also investigated. The results were interpreted on the basis of the model recently proposed by Shah et al. We showed that this model correctly predicts the phase behavior and water solubility changes with the surfactant chain length and nature of the counterion as well as with the nature of the solvent. Concerning the latter, the surprising changes observed when going from nonhalogenated to halogenated aromatic solvents have been in-
J . Phys. Chem. 1990, 94, 381-395 terpreted on the basis of the effect of the solvent dielectric constant on interactions between head groups. The effect of head-group size was found to be opposite to the predictions of the Shah et a]. model. This is probably due to the fact that part o f t h e head group Of the 'ynthesized surfactant played the role Of a cosurfactant and passed from the Water side to the Oil side as the head-group Size Was increased. The effect Of the WSUrfaCknt Size overshadowed that of the head-group size, making our observations consistent with the Shah et al. model. Registry No. N l6,4,l,l,Br, 96018-76-7; N16,6,1,1,Br, 73458-93-2;
387
N16,8,1,1,Br, 107004-19-3;Nl6,O@,l,l,Br,17695-00-0;NlO,l@,l,l,Br, 32014-84-9; Nl8,l@,l,l,Br, 22546-65-2; Nl2,l@,l,l,Br, 7281-04-1; Nl6,
[email protected],l,Br,122699-34-7;N16S)@~l~l~CL 26038-94-8; NIO,l@*l~I~CL 965-32-2; NI8,14t1.I>CIt 1 2 2 - 1 9 4 N 1 2 * I ' b ~ l , I ~ 139-07-1; cl~ Nl6,2@,l,l,Cl,122699-33-6;Nl4,l@,l,l,Cl,139-08-2;Nl6,1@,1, I ,CI, 122-18-9; Nl2,4,1,1,Br(Bu), 29481-60-5;Nl2,4,1,1,Br(i-Bu), 11453279-5; PhMe2N, 121-69-7; (phCH2)Me2N,103-83-3; H20, 7732-18-5; hexadecyldimethylamine, 112-694; dodecyldimethylamine, 112- 18-5; benzene, 71 -43-2; toluene, 108-88-3;xylene, 1330-20-7;ethylbenzene, 100-41-4; 1,3,5-trimethylbenzene,108-67-8;styrene, 100-42-5;chlorobenzene, 108-90-7;bromobenzene, 108-86-1; 1-bromonaphthalene,9011-9.
Ternary Water in Oil Microemulsions Made of Cationic Surfactants, Water, and Aromatic Solvents. 2. Droplet Sizes and Interactions and Exchange of Material between Droplets A. Jada, J. Lang,* R. Zana, Institut Charles Sadron (CRM- EAHP), CNRS- ULP Strasbourg, 6 Rue Boussingault. 67000 Strasbourg, France
R. Makhloufi, E. Hirsch, and S. J. Candau Laboratoire de Spectrometrie et d'lmagerie Ultrasonores, 4 Rue Blaise Pascal, 67000 Strasbourg, France (Received: March 2, 1989)
Ternary water in oil microemulsionsmade of cationic surfactants, water, and aromatic solvents have been investigated by means of time-resolved fluorescence quenching, quasi-elasticlight scattering, and electrical conductivity in order to determine the surfactant aggregation number N per water droplet, the rate constant k, for the exchange of material between droplets through collisions with temporary merging, the droplet diffusion coefficient D, and the coefficient of interaction between droplets (Y and to study the Occurrence of electrical percolation as a function of the surfactant chain length, head-group size, and water content of system (expressed as the molar concentration ratio w = [water]/[surfactant]). Most measurements were performed with chlorobenzene as solvent. In one instance, chlorobenzenewas substituted by benzene in order to investigate the effect of the nature of the solvent. For a given surfactant, Nand k, increased with w and upon substituting chlorobenzene by benzene. Also, at a given w, N a n d k, increased when the surfactant chain length was decreased. The increases of k, were always extremely large. The droplet hydrodynamic radii from quasi-elastic light scattering were found to agree with the droplet sizes calculated with the N values from fluorescence quenching. The droplet interaction coefficient a became more negative as the surfactant chain length decreased, indicating increasingly attractive interdroplet interactions. Finally, electrical percolation was found to occur in all systems were interdroplet interactions were sufficiently attractive. The percolation threshold w-values increased with surfactant chain length. Our results clearly showed that, under fixed experimental conditions, a decrease of surfactant chain length can result in a moderate increase of N, an increase of the magnitude of attractive interdroplet interactions, a very large increase of k,, and a decrease of the percolation-threshold value. From a more quantitative viewpoint, it was noted that in all instances, including numerous other studies where conductivity data and k, values are available, the percolation threshold corresponds to k, values of about (1-2) X lo9 M-' s-I. This result led us to attribute the electrical conductivity of water in oil microemulsionsabove the percolation threshold to the motion of surfactant counterions within transient water channels arising in droplet clusters upon opening of surfactant layers separating adjacent water droplets.
Introduction The first part of this work,' reported the results of a systematic study of the solubility of water in binary systems made of cationic surfactants and aromatic solvents. The aim of this study was to investigate the effect of surfactant chain length, counterion and head-group size, and the oil nature on the water solubility in these binary systems. The results were discussed in terms of the two main effects that govern the stability of water in oil (w/o) microemulsions, namely, the curvature of the surfactant film separating the oil and water and the attractive interactions between water droplet^.^,^
In this second part of our work, we report results concerning the size of water droplets, interactions between droplets, and the rate of the exchange of material between droplets through collisions with temporary merging (sticky collisions: see Figure 1) and measurements of electrical conductivity, for some of the systems studied in part 1' and selected on the basis of their large water solubility. Two types of cationic surfactants have been studied: dodecylbutyldimethylammonium bromide: C12H25(C4H9)N+(CH3)2Br- referred to as N12,4,1,1 ,Br; alkyl(phenylalkyl)dimethylammonium chlorides: C m H W l[C6H~(CH2),]N+(CH3)2C1-
( I ) Jada, A,; Lang, J.; Zana, R. J . Phys. Chem.,the preceding paper in this issue. (2) Hou, M. J.; Shah, D. 0.Longmuir 1987, 3, 1086, and references cited
(3) Leung, R.; Shah, D. 0. J . Colloid Interface Sci. 1987, 120, 320 and 330. (4) Fletcher, P. D. I.; Robinson, B. H. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 863.
therein.
0 1990 American Chemical Society
