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The Journal of Physical Chemlsfry, Vol. 82, No. 11, 1978
Y. Marcus and L.
E. Asher
Extraction of Alkali Halides from Their Aqueous Solutions by Crown Ethers Y. Marcus" and L. E. Asher Depatfment of Inorganic and Analytical Chemistry, The Hebrew University, Jerusalem, Israel (Received October 28, 1977; Revised Manuscrlpt Received January 16, 1978) Publication costs assisted by Israel Chemicals, Limited
Potassium and sodium halides can be extracted from their aqueous solutions by dibenzo-18-crown-6 and dicyclohexo-18-crown-6dissolved in protic solvents, which solvate the anions effectively. Both separate ions and ion pairs are extracted, the order of extractability among the halides being fluoride > iodide > bromide > chloride, apparently reflecting the solvation of the anions. The solubility of the crown ethers in a series of solvents are correlated with the solvent properties. The results are summarized in a prescription for selecting the best crown ether/solvent combination for effective extraction of the alkali halides.
Introduction Although the alkali halides are soluble to some extent in nonaqueous solvents, their Gibbs free energies of hydration are so negative that it is difficult to extract them from water into alternative solvents. The challenge is to find liquids or liquid solutions which can provide for the solvation requirements of the alkali and halide ions as well as, or better than, water does, and still be reasonably immiscible with water. Some practical considerations' prescribed a solution to the problem whereby the aqueous phase remains essentially unchanged, with the obvious exception of the loss of the alkali halide, and, possibly, some water. Cyclic polyethers (crown compounds) are selective solvating agents for the alkali metal ions2p3and they have in fact been used for their extraction from aqueous solutions. According to Pedersen and F r e n ~ d o r f fwhile ,~~ complexing the cation is an obvious prerequisite, extraction is efficient only if the anion is large and highly polarizable, for instance p i ~ r a t e , tetraphenylborate,lO ~-~ or dipicrylaminate.ll Large inorganic anions have also been utilized, such as permanganate: perchlorate,ll iodide,ll and thiocyanate.12 Even chloride13found application but only in a nonpolar membrane, where its concentration is very low. The solvents for the crown ethers were usually hydrocarbons or chlorinated hydro~arbons,~-l~ or in some The cases dipolar aprotic solvents such as nitr~benzene.~Jl distribution ratios, in the cases of extraction of the alkali cations together with inorganic anions, were rather low. Thus, a combination of a crown ether with a nonpolar solvent or with the dipolar aprotic nitrobenzene proved to be unsuitable for the extraction of the alkali halides from their aqueous solutions. This was caused apparently by the fact that the halide anions were not sufficiently hydrophobic to be extracted into these solvents,ll and were not effectively solvated by them. It is significant, however, that a crown ether in a solvent consisting of a volume fraction of 0.30 of l-butanol in toluene, which is a model for a biological membrane, extracts the alkali metal thiocyanates quite efficiently.12 Furthermore, the ability of 0.05 M dicyclohexo-18-crown-6(eicosahydrodibenzo2,5,8,15,18,21-hexaoxacyclooctadecin,DCC) to solubilize sodium or potassium halides is manyfold greater in benzene containing 0.25 M methanol than in pure benzene.3a Inspection of the data reveals, however, that addition of methanol to such solvents as chloroform or methylene chloride is not necessary if large anions such as iodide are used as counterions in the complexes. Obviously, addition of methanol is related to the solvation of the anion and not to the solvation of the cation.14 0022-3654/78/2082-1246$01 .OO/O
The essential point of the present research is to pursue this reasoning a step further, by providing for the crown ether a water-immiscible solvent which is capable of solvating the halide anions as well as possible. Attention is focused on the anion, although it is recognized that the complexing of the cation also depends on the solvent. However, the structure of the cyclic polyether complexes with the cation in the center of the ring does not require the stripping of the entire solvation shell of the cation. Solvent contacts are still possible in the direction perpendicular to the plane of the ring.15 When the cation is transferred from an aqueous phase, the solvent in this perpendicular axis will be water molecules. Solvent molecules from the organic phase into which the cations is transferred will play only a secondary role, and the transfer of the cation will be rather insensitive to the solvent.16 Since the transfer of the anions becomes the primary problem, as small as possible Gibbs free energy of transfer of the anion from water into the solvent is the prime condition for the choice of the solvent, Several extrathermodynamic assumptions have been applied to the problem of splitting the Gibbs free energy of transfer of an electrolyte to the ionic contributions. The most successful at present seems to be the tetraphenylarsonium tetraphenylborate assumption, according to which the Gibbs free energy of transfer of this electrolyte between any two solvents at any temperature is exactly twice the contributions of its constituent ions, which are equal.17 This is the basis for deriving the Gibbs free energy of transfer of chloride ions from water to solvents S, AGk(C1-, H20 S),l*which is known to be well correlated with independently measurable properties of these solv e n t ~ . ~ ~Hence, - ~ ' the Gibbs free energy of transfer of chloride ions is known for many more solvents than those for which it was determined directly with electrolytes containing chloride anions. A proper combination of crown ether and solvent should make the extraction of alkali halides from aqueous solutions possible. This study reports the results obtained for the extraction of sodium and potassium chlorides and potassium fluoride, bromide, and iodide with dicyclohexyl-18-crown-6 (DCC) and with dibenzo-18-crown-6 (octahydrodibenzo-2,5,8,15,18,21-hexaoxacyclooctadecin, DBC).
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Experimental Section Materials. Crown ethers were obtained from Aldrich and Fluka companies and some preliminary experiments were made with samples provided by Dr. H. K. FrensdorffeZ3The DCC consisted of a mixture of isomers which 0 1978 American Chemical Society
Extraction of Aqueous Alkali Halides by Crown Ethers
TABLE I: Molar Solubility of DBC, s, , in Aromatic and Acidic Solvents at 25.0 "C Solvent Acetophenone Aniline Benzaldehyde Benzonitrile Benzyl alcohol 2-Bromobutyric acid a 2-Chloroanilinea m-Cresol Nitrobenzene Phenethyl alcohol 2,2,2-Trichloroethanol 2,4-Xylenol a These determinations were made to whom thanks are due.
Sa,M 0.072 i 0.005 0.222 t 0.007 0.073 t 0.008 0.120 t 0.008 0.075 f 0.010 0.290 i 0.006 0.163 i 0.003 0.314 f 0.004 0.194 f 0.003 0.025 f 0.003 0.710 f 0.007 0.367 f 0.003 by Miss E. Goldstein
was not separated. The DBC was recrystallized from acetone. Inorganic salts were of analytical reagent grade, and were not further purified. Triply distilled water was utilized. The organic solvents, from Fluka, Frutarom, and BDH, were of pure grade and were utilized as received. Procedures. The solvents and water were preequilibrated with each other, in order to minimize volume changes during extractions. Known volumes of solvent and aqueous solutions were mixed in a thermostatic bath at 15.0, 25.0, or 40.0 "C (f0.5 "C) during times varying from 1min to 24 h, with no influence of the duration of shaking or direction of approaching equilibrium. The phases were allowed to settle overnight in the bath, and aliquot samples were removed from each phase for analysis. Survey equilibrations were made by vortexing thermostated samples for 2-3 min in test tubes at room temperature (20 f 2 "C). Solubilities of the crown ethers were determined as follows. An excess of the crown ether was shaken with the solvent at 25 "C for at least 48 h, or alternatively a saturated warm solution was cooled to 25.0 "C. The suspension of the crown ether in the solvent was allowed to settle and an aliquot of the saturated solution was taken. The solvent was evaporated from it in a vacuum oven,22 and the crystalline crown ether remaining was weighed. Similar solubilities were found for the two directions of saturation. The density of solid DBC was measured by weighing a certain amount in a 10.0-mL volumetric flask, adding water till the mark at the calibration temperature, and reweighing. The small solubility of DBC in water does not interfere with the determination. Sodium and potassium were determined by flame photometry at 589 and 766 nm, respectively. Organic solutions were diluted appropriately by reagent grade methanol prior to determination. Linear calibration curves were obtained, and accuracy and precision were generally within 2 % . Duplicate equilibrations, and duplicate analysis from each phase in each equilibration were generally made, and material balance was within the stated error.
Results Solubilities. The solubility of DBC in organic solvents is quite limited, and only formic acid, pyridine, chloroform, and methylene chloride were known2to effect solubilities above ca. 0.1 M, and only the last two are water immiscible. The additional solubilities at 25 "C determined in this study are shown in Table I. There are therefore several aromatic solvents which show appreciable solubilities of DBC. The solubility of DBC in water-saturated rn-cresol is somewhat lower than in dry rn-cresol, being 0.285 f 0.03 M at 25.0 "C. The solubility of DCC in benzene was found to be very large: 2.37 f 0.03 M at 25.0 "C.
The Journal of Physical Chemjstty, Vol. 82, No. 11, 1978
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The density of solid DBC was found to be 1.23 g cm-3 at 25 "C. With the molar weight of 360.4 g mol-l this yields a molar volume of 293 cm3mol-l for the solid. The liquid at the same temperature should have a similar molar volume, since the volume gained on heating the solid to and fusion at the melting point is lost when the liquid is (hypothetically) supercooled to room temperature. In fact, if group contributions to the molar volume of liquids" are used (see below), the estimated molar volume is 296 cm3 mol-l. The relevance of this to the solubilities is discussed further on. Solvent Efficiencies. Extractions were carried out from 1.00 M aqueous potassium chloride solutions at 25 "C with 0.100 M solutions of DCC in many solvents and of DBC in those solvents which afforded sufficient solubility. The results are expressed as D, the distribution ratio of potassium between the two phases. This is the ratio of the molar concentration of potassium in the organic phase containing the crown ether at a total concentration Ccw= 0.100 M in the solvent (the bar above the symbol designates the concentration in the organic phase), and the molar concentration of potassium in the aqueous phase in equilibrium with it. The volumes of the two phases remained equal after equilibration as they were before, since they were presaturated with each other in respect to solvent and water. No more potassium can be extracted than will saturate the crown ether (see below) so that EK(-) = 0.100 M and hence c K ( ~ = ~ ~0.900 ) M in the aqueous phase. The maximal value of D under these conditions is therefore 0.11, even for the most efficient solvents. For these, therefore, extractions were made from more dilute aqueous solutions, 0.100 M in potassium chloride initially. The results are shown in Table I1 as values of log D for the solvents surveyed. Extraction Isotherms. Because of the appreciable aqueous solubility2 of DCC (0.036 M at 26 "C) compared with that of DBC (9 X M) which leads to a loss of the DCC crown ether in extraction experiments which may be aggravated in the presence of potassium chloride in the aqueous solution, extraction isotherms were measured only for DBC. Data were obtained at 15,25, and 40 "C for 0.100 M DBC in m-cresol with potassium chloride and sodium chloride solutions, and for 0.100 M DBC in benzyl alcohol and for 0.230 M DBC in rn-cresol with potassium chloride solutions at 25 "C. The results are shown in Figures 1and 2, as the ratio of the extracted alkali metal chloride concentration CMcl to the crown ether concentration ecw in the organic phase, vs. the equilibrium aqueous alkali metal chloride concentrations CMCI. It is seen that whereas potassium chloride practically saturates the DBC in mcresol at 25 "C at cKcl= 0.6 M, irrespective of the DBC concentration, sodium chloride requires much higher aqueous concentrations to saturate the crown ether. With c y a c l = 3.26 M only 97.0% saturation is achieved, and only with ca. CNaCl = 3.8 M is complete saturation virtually attained. However, whereas the extraction of sodium chloride with DBC in m-cresol at 40 "C is not much different from that at 15 "C, the extraction of potassium chloride is appreciably less. The extraction with DBC dissolved in benzyl alcohol is less than with DBC dissolved in rn-cresol, but the solutions of 0.100 M DBC in benzyl alcohol are supersaturated, see Table I. Extraction of Salts Other T h a n Chlorides. A solution of 0.100 M DBC in rn-cresol was used at 25 "C to extract potassium salts from an aqueous solution initially 0.100 M in the salt. The distribution ratios found are shown in Table 111. The order of extractability is seen to be sulfate < chloride < bromide < iodide < nitrate < acetate
