J . Chrrii. Soc.. Fizr-nd~j~ Trans. I . 1989, 85(9). 2669-2674 Solubilization of some Tetramethylammonium Salts and of Ethyltrimethylammonium Bromide by their Homologues in Ch1oroform-t Jan Czapkiewicz Institute of Chemistry, Jagiellonian University, 30460 Krakow, Poland The solubilities of tetramethylammonium chloride, bromide, thiocyanate and perchlorate and of ethyltrimethylammonium bromide in chloroform at 25 f 0.2 "C have been determined. The solubilities of these salts increase markedly in the presence of a variety of higher homologues of common co- ions. It is suggested that this phenomenon involves the cooperative formation of reversed micelles. The low solubility of tetramethylammonium halides and of ethyltrimethylammonium bromide in chloroform had been noted already in 1907 by Wagner,' who also found that at an elevated temperature the latter salt separates out from the solution and forms a solute-rich liquid phase. Walden et a1.2 reported the extremely low solubility of tetramethylammonium perchlorate in this solvent.These findings and more recent solubility data published by Abraham and Danil de Namor3 for an extensive series of quaternary onium salts in 1,2- and 1,l-dichloroethanes as well as the comments and some data on the solubilities of such salts in dichl~romethane~ indicate that halogenated hydrocarbons are, as a rule, extremely poor solvents for tetramethyl and other low- molecular-weight onium salts. The solubility of homologous salts in these solvents increases markedly with growth of the hydrocarbon chain.This effect, however, should not be freely extrapolated to long-chain salts because a reversed situation may occur at some chain length as was shown for di-n-alkyldimethylammonium chlorides, the solubility of which in chloroform and carbon tetrachloride decreases in the order C,,-C,, > C,,-C,, > C,,-C,, over a broad range of temperat~res.~ A similar behaviour was found by Kertes' for normal dialkylammonium chlorides in carbon tetrachloride and benzene. The break in solubility occurred in both solvents upon passing from dioctyl to the didodecyl homologue. The present work was inspired by results of vapour-pressure osmometry studies carried out first for a series of long-chain (C16-Clo) alkyltrimethylammonium bromides in chloroform' and extended later to the short-chain (C,-C,) homologues.8 It was found that the mean aggregation number of these salts increases with the decrease in number of the carbon atoms in the alkyl chain.The concentration range covered for ethyltrimethylammonium bromide was, however, limited by its low solubility. It thus seemed interesting to explore the possibility of enhancing the solubility of this salt and of the tetramethylammonium salts, presumably possessing strongest aggregational properties, through the formation of mixed reversed micelles with freely soluble higher homologues. The results here presented should be treated as model studies on compartmentalization of sparingly soluble salts in micellar assemblies formed in chloroform. 1. Paper presented at the Third International IUPAC Symposium on Solubility Phenomena, held at the University of Surrey.23-26 August, 1988. 26692670 Solubilization of Tetramethylammonium Salts Experimental Tetramethylammonium bromide and chloride were commercial samples which were additionally recrystallized from isopropyl alcohol. The perchlorate salt was prepared by mixing aqueous solutions of the chloride and of sodium perchlorate which was an analytical-grade reagent. The precipitate was recrystallized several times from water. The tetramethylammonium thiocyanate was prepared according to the procedure described by Bekkevoll et al.'. Butyl-, propyl- and ethyl-trimethylammonium bromides were prepared by mixing a cooled (ca. - 15 "C) solution of trimethylamine in acetone (100 cm3 of amine per 1 dm3 of solvent) with a cooled acetonic solution of the appropriate alkyl bromide used in a ca.10 Yo molal excess. The mixtures were tightly stoppered and kept in the fridge for two days and then left for a few more days under the hood at room temperature. The products were than filtered off and thoroughly washed with acetone. Recrystallizations were best carried out from nitromethane, chloroform, ethyl acetate and from 1,2- dichloroethane. These and further operations were carried out in a dry box because the salts, especially the ethyl homologue, are hygroscopic. Tetrapropylammonium bromide was prepared by quaternization of tripropylamine with propyl bromide in butan-2-one in a sealed glass ampoule heated for 4 days at ca. 80 "C. Tetrabutyl-, decyltrimethyl- and decyl-tripropylammonium bromides as well as dodecyltrimethylammonium chloride and tetradecyltrimethylammonium thiocyanate were prepared in this laboratory ear lie^.^ All the samples were thoroughly dried in a heated vacuum pistol containing P,O,.Analytically pure chloroform was washed with concentrated sulphuric acid, then five times with a double volume of water. It was initially dried with CaCI,, then with type A-4 zeolites and finally distilled, the middle fraction yielding 50 %. The purified solvent was used to test for solvate formation by applying a modified procedure of de Ligny et al."' The salts were stored in evacuated closed vessels over chloroform and P,O,. Removal of air secured the bromides from turning yellow-brown in the presence of chloroform vapour.The test was negative for all tetramethyl- ammonium salts studied. The ethyltrimethylammonium bromide appears, however, to form solvates. At room temperature it absorbs from the vapour phase c w . 1.5 mol chloroform per mole of salt. For the solubility measurements in chloroform an excess amount of the salt was added to the solvent and the flasks containing the mixture were tightly stoppered and shaken for 50 h in a thermostat at 25 & 0.1 "C. In the case of tetramethylammonium bromide the crystals assumed a slight yellow-green colour after several hours of contact with chloroform and a yellow-brown colour of the solution could be detected at the end of the experiment. This side-effect was eliminated by purging chloroform with argon prior to solubility measurements.It was also observed that this bromide remained colourless in the presence of higher alkyltrimethylammonium salts in solution. Aliquots of the saturated solutions were removed, weighed and evaporated to dryness by applying slightly reduced pressure. A similar procedure was adopted for solubility measurements of the Me,NX salts in solutions of their homologues of known molal concentration in chloroform. The weight of the solution used for the solubilization was noted and checked at the end of the experiment. The residue left after the removal of solvent was dissolved in water. The halides and the thiocyanate were determined potentiometrically by titration with a standard AgNO, solution. Appropriate anion-selective electrodes and an Ag-AgCI reference electrode with an NH,NO,-agar bridge were used for this purpose. Blank experiments without added Me,NX salts were carried out, with a precision of 1 %.The perchlorate anion was determined spectrophotometrically in the form of an ion pair with methylene blue extracted quantitatively to 1,2-dichloroethane."J. Czapkiewicz 267 1 Table 1. Solubilities of Me,NX salts and of EtNMe,Br in chloroform, at 298 K" _ _ _ _ ~ . ~ - ______ _ _ _ _ ~ - salt solubility/mol kg-' Me,NCI Me,NCNS Me,NClO, ca. I x (ca. 1.5 x EtNMe,Br 2.85k0.05 x (4.7 x lo-*) -. ~- ~ _ _ _ _ _ ~ 9.7kO.l x lo-' (1.45 x 8.2kO.l x 10-5 ( 1 . 2 ~ lo-'; 8.3 x lo-, in DCM') 1.14 x lo-' in I,2DCEb) Me,NBr 3.7+0.1 x 10-4 (5.5 x 10-4) 3.58 x lo-' in I,2DCEb and 8.08 x 10-5 in I,IDCEb In parentheses are given values expressed on a molar scale and included are available data for 1,2-dichIoroethane ( I ,2DCE), 1 , l -dichloroethane ( I , 1 DCE) and dichloromethane (DCM).Taken from ref. (3). 'Taken from ref. (4). The solubility of ethylthrimethylammonium bromide was also measured by preparing a warm solution of the salt and leaving it in a thermostat for 50 h at 25 "C. Both methods gave comparable results. Since the solubilization of this salt in the presence of its homologues occurs almost instantaneously, the following procedure was used. A small flask tightly stoppered with a polyethene plug was weighed accurately and reweighed after addition of the salt. Samples weighing 10&500 mg were used. The flask was then placed in a thermostatted oil bath contained in a large glass beaker and the solubilizer of known molar concentration was injected dropwise using a glass syringe with a steel needle which was pierced through the plug.The flask was illuminated, shaken by hand and observed with the aid of a lens. After complete dissolution of the solid material or of a second liquid phase the flask was thoroughly wiped and reweighed. Such a titration procedure lasted 1-2 h and gave reasonably reproducible results. Results and Discussion The solubilities of the Me,NX salts and of the solvated EtNMe,Br in chloroform are shown in table 1. Included are the relevant data reported for other halogenated solvents. The values confirm the early on the low solubilities of Me,NX salts in chloroform. There seems, however, to be no correlation between the data for this solvent and for the two isomeric dichloroethanes as well as for dichloromethane.The chloride appears to be by an order of magnitude more soluble in chloroform than in 1,2-dichloroethane whereas a reversed situation occurs for the perchlorate. The thiocyanate is by an order of magnitude less soluble in chloroform than in dichloromethane. Since solvate formation by Me,NX salts in both dichloroethane solvents was also unobserved3 it seems that the apparent lack of consistency of the solubility data may account for the fact that the dichloroethane and dichloromethane solvents have dielectric constants which are ca. two times higher than that for chloroform (e = 4.7) and that, in consequence, ion-ion pair equilibria are most important for the latter solvents, whereas the ion pair-n-mers equilibria are the predominant processes in chloroform.In table 2 values are given for the solubility of Me,NCl in the presence of dodecyltrimethylammonium chloride. Least-squares analysis of the data for the solubility of Me,NCl in the presence of the long-chain solubilizer indicates a linear relation with a correlation coefficient of 0.999, a slope of 0.140 0.002 and the intercept of 0.1 1 1 kO.01 I x lo-' mol kg-l. It appears that this extrapolated value is slightly higher than that determined for Me,NCl in pure chloroform. This shift may well account for the defects of the surface of the solubilized crystals caused by adsorption of the higher homologue.2672 Solubilization of Tetramethylammonium Salts Table 2.Solubilization of Me,NCl by C,,H,,NMe3Cl in chloroform at 298 K concentration of solubility of C12H25NMe3C1/ Me,NCl/ lo-, mol kg-' lo-, mol kg-' 0.42 0.57 0.68 1 S O 2.03 2.60 3.64 6.0 1 8.98 9.40 0.097 0.19 0.20 0.22 0.34 0.36 0.44 0.62 0.96 1.36 1.45 The data for solubilization of Me,NBr by PrNMe,Br and by C,,H,,NMe,Br as well as for solubilization of Me,NCNS by C,,H,,NMe,CNS confirm the results obtained for the chloride. An increase in solubility of the bromide and the thiocyanate was observed systematically. However, owing to their low solubilities they contributed an amount of the anions which never exceeded 5% of the total concentration in the systems studied. Thus, the results are not so convincing and are not here included. Much more interesting are the results obtained for the solubilization of EtNMe,Br by its various homologues as shown in fig.1. A manifold increase in solubility is also observed here. The situation, however, is reversed in the sense that the solubilizate plays the role of the host in the mixed micelles. The results for the Et-Pr ternary system exhibit some pecularities. A second liquid phase separates out upon addition of the solution of PrNMe,Br and exist over a broad concentration range. Below the dotted curve on fig. 1 a single liquid phase exists. As mentioned earlier, such a phenomenon was already observed by Wagner' for warm solutions of EtNMe,Br in chloroform. This phase separation resembles the so-called cloud point observed in the case of aqueous solutions of polyoxyethylene non-ionic detergents.It is caused by dehydration of the solute at elevated temperature. Indeed, the two-phase liquid systems studied presently could be reversibly cooled down below 25 "C to form a homogeneous solution. The temperature of such transitions depends on the concentration of PrNMe,Br. The separation of a second phase is caused most probably by desolvation of the ion pairs and n-meric species. Knowledge of the composition of such solute-rich phases might perhaps give an insight into the structure of reversed micelles. Their cores may still be substantially solvated. Higher homologues, which are surface active, do not form liquid two-phase systems with Et-Me,Br as the solubilizate. Their behaviour is similar to that observed for the C,,H,,NMe,Cl-Me,NCl system as exemplified by the Pr,NBr-EtNMe,Br and C,,H,,NMe,Br-EtNMe,Br systems.In both cases the extent of solubilization grows linearly with increase in concentration of the additive and the relation satisfactorily extrapolates to the solubility values determined for EtNMe,Br in pure chloroform. There are two additional data points on fig. 1 corresponding t o Bu,NBr and C,,H,,NPr,Br used as solubilizing agents, both 0.05 mol kg-'. These were chosen to illustrate the relation between the ability of the salts to solubilize and their tendency to form micellar aggregates. In table 3 the mean aggregation number, 6, determined by vapour pressureJ. Czapkie w icz 2673 0.20 ym 0.15 24 - 8 6 3 g 0.10 1 h v F 0.05 Pr4NBr f DecNMe3 Br A Bu4NBr A DecNPr3Br J / ,A' -0- PrNMesBr 0.01 0.03 0.05 concentration of solubilizing saltlmol kg- ' Fig.1. Solubilization of EtNMe,Br by homologous salts as a function of their concentration. Table 3. Mean aggregation number, n', of quaternary ammonium salts (0.05 molal) in chloroform and equilibrium concentration, C,,,, of solubilized EtNMe,Br salt ii C,,/mol kg-' C,,H,,NMe,Br 2.50 0.175 C,oH,,NPr,Br 1.86 0. I45 Pr,NBr 2.19 0.205 Bu,NBr 1.87 0.160 __ ~~~ ~- ______________ .- _- osmometry for the four 0.05 mol kg-' salts are compared with the equilibrium concentration of solubilized EtNMe,Br. The present results support the general finding' that in the group of RNRiBr salts, where R 3 R', the mean aggregation number grows with the decrease in R and/or R'. The pairwise comparison of the solubilization abilities of the ammonium salts, Pr,NBr > Bu,NBr, C,,H,,NMe,Br > C,,H,,NPrBr and Pr,NB > C,,H,,NPr,Br, correlates well with their aggregational tendency.This is not true for the Pr,NBr > C,,H,,NMe,Br pair. These two salts, however, do not have common structural features. It may be expected that the relation PrNMe,Br > C,,H,,NMe,Br would hold if measurements were carried out below the critical temperature at which phase separation occurs. It is noteworthy that for the system which is 0.05 molal with respect to Pr,NBr and2674 So 1 u h ilizu t ion o j Te t r urne t h y lam rn on iu rn Salts 0.205 mol kg-I with respect to EtNMe,,Br, the vapour pressure osmometry yields a value of the mean aggregation number of 10.1. The results discussed here represent model studies. They suggest, however, that solubilization of scarcely soluble electrolytes through the formation of mixed assemblies in chloroform may be a potential tool in extraction and separation procedures. This work supported by grant no. R.P.1-08. References I L. Wagner, Z. Kryslullogr., 1907. 43, 148. 2 P. Walden, H. Ulich and B. Busch, 2. Phq”. Chcm., 1926, 123, 443. 3 M. H. Abraham and A. F. Danil de Namor, J. Chem. Soc., Furuduy Truns. I , 1976, 72. 955. 4 S. Bekkevoll, I. Svorstol. H. Hoiland and J . Songstad, Ac/u C ’ l i m / . Scrrntl.. See/. B, 19x3. 37. 935. 5 H. J. Harwood and P. L. Du Brow, J. Org. Chcw., 194X, 13, 186. 6 A. S. Kertes, J. Inorg. Nucl. Chem., 1965, 27, 209. 7 J. Czapkiewicz, J. Colloid Inlcr-ucc Sci., in press. 8 J. Czapkiewicz, to be published. 9 J. Czapkiweicz and B. Czapkiewicz-Tutaj, J. Chcm. Soc., Furuduy Trunr. I , 1980, 76, 1663. 10 C. L. de Ligny, D. Bax, M. Alfenaar and M. G. L. Elferink, Rrcl. Truti. Clihi. Puj-.s-Bu.s, Bdg., 1969, 88, 1183.