Saturated Calcium Chloride Brine for Separations?

kilomentor | 11 May, 2011 15:54

Calcium chloride is very soluble in water. It is a slow, inefficient but ‘thirsty’ drying agent. In moist air the anhydrous solid is deliquescent. Quite a few solvents can be expected to be immiscible with its aqueous brine solution and these could serve as an immiscible partner phase for liquid-liquid extractions. Saturated aq. calcium chloride can be expected to behave similarly to saturated sodium chloride solution; it forms a separate phase with isopropanol or pyridine, liquids with which water alone is miscible. But what molecular structural types would interact and pass into a calcium chloride brine that would not as easily or more easily dissolve in plain water?

This question seems appropriate because it would be logical to think that the complexing power of calcium ion would be largely erased when comingled with so much of the Lewis base, water. Yet this is apparently not so, if expired patent US 3225113 is to be believed. According to this patent, calcium chloride brine forms complexes with hydrocarbon ethers, when the ratio of the number of carbons oxygens is <8:1. The best complexes are when the ratio is 2:1 as in polyethylene glycol ethers.

This complexing could be useful in work-ups that require a solvent switch where an ether or glyme solvent needs to be removed in favour of another solvent. Thus, when using soluble polyethylene glycol polymer as solvent/diluent in synthesis there may be an option besides the prior art procedure of precipitating these polymer with hexanes or diethyl ether. Aqueous saturated calcium chloride is very likely to precipitate the glyme as a filterable solid.

Patent US3225113 shows that a solution of calcium chloride in water can precipitate quantitatively glyme solvents in the presence of another hydrocarbon-like substance. Filtration and washing the solid with the non-polar hydrocarbon cleans up the glyme calcium chloride complex. The aqueous and hydrocarbon phases can be themselves separated since they are immiscible. This complexing also occurs between aqueous calcium chloride and 1,4-dioxane suggesting a way to solvent switch from dioxane.

It is thus also not a foregone conclusion that no organic species would occupy the calcium chloride/ water layer in a two phase combination with a hydrocarbon or halogenated solvent. A substance that complexes with calcium ion might enter such an aqueous salt layer if its complex is stronger than the water one. Glymes apparently meet the criterion. 1,2- or 1,3-diols or cyclic polyethers might also prefer aqueous calcium chloride more than water alone or might precipitate out when so treated. This would be a particularly propitious means to recover expensive cryptate cyclic ether ligands.

Whether a particular polyether is likely to be strongly complexed by an aqueous calcium chloride solution may be judged by a simple test described and applied in the aforementioned patent. In it, 5 cc. of a saturated calcium chloride solution was added to a test-tube containing the ether. No heat was added to the reaction zones. The temperature rise at the end of two minutes is measured. Symmetrical dioxane gave a rise from 26 to 42 C. The increase for bis[2(2methoxyethoxy)ethyl] ether was from 28 to 71 C with solid complex separation.

Aqueous calcium chloride stirred with a potentially complexing compound in an immiscible organic solvent that does not complex calcium chloride, such as a hydrocarbon or MIBK, could it seems partially precipitate some material, dissolve some in the aqueous –salt phase and dissolve some in the hydrocarbon or MIBK phase. Filtration would separate the solid and the two liquid layers could be themselves separated giving three distinct phases. Uncomplexed organic will most likely be concentrated in the hydrocarbon or MIBK solution.

This method might be applicable to the frequent problem changing from a high boiling polar aprotic solvent such as DMF, DMA, N-methylpyrrolidone or DMSO, to a low boiling hydrocarbon solvent by distilling away the dipolar aprotic solvent using a small volume of a higher boiling glyme solvent to chase it, adding the low boiling hydrocarbon phase and then precipitating the glyme complex with an aqueous calcium chloride solution. Filtration would give two phases: the low oiling hydrocarbon liquid containing the reaction mixture and the residual saturated calcium chloride brine.