When Are Two Aqueous Salt Solutions Insoluble in Each Other? A Phase-Diagram Experiment Illustrating a Peculiarity of Micelle-Forming Systems Thomas C. Franklin and Susan 8. 8 Baylor University. Waco, TX 76796
With the growing interest in emulsions and micelles in areas such as ohase transfer (1) and micelle catalvsis 12). reactions in organized media i3j, analytical chemistry (41, sensitized reactions, and artificial photosynthesis in micelles and vesicles ( 5 , 6 )and the related area of liquid crystals (7), it is of interest to reinsert into the Dhvsical chemistrv labora. . tory some experiments illustrating the peculiarities that are observed in these systems. One of these ex~erimentsthat can be done readily in the laburatory is the determination uf the three-component phasc diagram for a system producing a concervate (8).This not only gives some exposure to the peculiarities of micelle systems, but also it gives experience in the rommon methods of determining three-cumponent phase diagrams around room temperature. As salts are dissolved in water.. they.are dissociated. There may be ion pairing, but because of the repulsion of like ions there is little expectation that massive aeereeates will be " present in the soiution. However, ionic surfactants, like the quaternary ammonium halides, upon dissolving, form relatively large particles, with hydrophobic regions. These hydrophobic regions, because of van der Waals-London forces. form hydrophobic bonds and the ionic atmosphere is spread so much that their repulsive forces cannot overcome the hydrophobic attractive forces and thus large clusters can form, to the point that they may become insoluble forming a liquid layer in the aqueous systems. This type of system-is known as a coacervate. Coacervates are formed with a number of systems. Thus surfactants, polyelectrolytes, or polymers (natural or synthetic) can be "salted out2'of an aqueous system to form aliquid layer (8).The aeereeation of the laree -molecule or ion in-the aqueous system can be affected gy addition of electrolytes, other colloids, or nonaqueous solvents. Most of the coacervate systems that are commonly investigated are systems that are somewhat difficult to investigate in a basic chemistry laboratory, since they include such things as the complex natural polymers of interest in biochemistry, polymer systems of interest in such areas as microencapsulation or the large surfactant systems used in tertiary oil recovery systems. Discussions of the properties of these more complex systems have been reviewed (9,101 and phase diagrams for a number of these svstems have been given ( l l j . However, one can illustrate'the principles by determination of the three-component phase diagram for water and the two water soluble salts, potassium carbonate and tetrabutylamonium bromide (TBABr). TBABr can be "salted out" of solution by a number of aqueous electrolytes such as alkali metal hydroxides, ammonium and alkali metal sulfates, or ammonium nitrate. T h e TBABr-K2C03 system was selected for a laboratory experiment, because (1) the two-phase system could be studied over a wide concentration range before it began to precipitate solids, (2) it was convenient to use because the equipment was simple and relatively safe to use and the chemicals were readily available commercially, (3) the bromide and carbonate could be analyzed easily, and (4) i t was felt that it would he of interest to students since the TBABr-K2COa svstem had been used in electrochemical synthesis (12, 13) and the coacervate of ~
~
~~~~~~
. ,-,. ~~~
~
"-
~
TBABr has been studied in the extraction of U022' (14). However, if desired, the experiment can he modified simply by the use of other electrolytes in place of potassium carbonate. The identical procedure can be followed with an alkali metal hydroxide. Experlrnenlal This experiment furnishes an illustration of two methods of determining phase diagrams in three-component systems. Descriptions of similar experiments on simpler compounds have been given in a number of physical chemical laboratory manuals (15). Titration Method Titration is a convenient method for determining the boundary line between the two-phase and one-phase regions. A constant amount (2.00 mL) of a concentrated stock solution of TBABr is mixed with varying amounts (0.30-2.40 mL) of a concentrated stock solution of potassium earhanate. Each sample, previously equilibrated in a constant temperature bath, is titrated with water while shaking until the two phases become one. [The stock solutions, were prepared by direct weighing of TBABr and K2C03. l1I2H20, checked by titration (Volhard determination of bromide and carbonate titration with hydrochloric acid).Within experimental error the results from weighing and titration were the same. Since it is usual to report in percent by weight, it is necessary that the densities of the solutions and water he determined.] Tie Line Methcd e nrr determined hy direct analyri~ofthe two phases. The t ~ lmes Wriglwd amounts t1.O-5U E I of 'I'I3AHr. puta\rium rnrbonote ! K , ( ' O < . 1'. H?OI, and water are plxed in 25-rill. rwaratorv i o n nels. In all experiments the sizes the samples are kept sm& in parttodevelopexperimentaltechnique but also tosave onmaterials because of the cost of TBABr. The separatory funnels are stoppered
of
11x10 SOLID .ID
LmYm WISE,
4
Threecomponsm phase dmgram at 25 C ' for the coacervating system tetrabutylammon~umbromidelpotessium carbonatelwater.
Volume 63
Number 9
September 1986
821
and equilibrated at 25.0 "C in a constant temperature bath overnight. This requires two days for the experiment or the preparation of the solutions in the preceding laboratory period. The two layers are then carefully separated and weighed. Each layer is diluted to 100 mL and analyzed for carbonate and hromide. The fact that the three points, the compositions of the synthetic mixture and the two ends, lie on a straight line is a check on the analytical technique of the student. [By separate analysis it was determined that in the two layers, within experimental error, the potassium ion concentration wasequivalent to the carbonateand theTBAt wasequivalent to the bromide. Therefore, in doing the calculations in all experiments it can be assumed that one is analyzing for potassium carbonate and TBABr.] From the total weights and the amounts of potassium carbonate and TBABr the percentages are calculated. The water percentage is determined by subtraction. Results Figure 1shows the experimentally determined phase diagram. In the region below the bottom tie line solids began to separate and the tie line became very uncertain. Within the given boundary line the system consisted of two liquid layers with the compositions given by the tie lines. The size of the circles were used to indicate the scatter of the data or the estimated uncertainty of the experimental points. After preparation of the two phases it is informative to do solubility experiments in the two-phase system. One can observe by using appropriately colored compounds that nonpolar compounds such as bromine are primarily dis-
822
Journal of Chemical Education
solved in the TBABr layer while polar compounds are dissolved in the potassium carbonate "water" layer. One can also observe the solution of insoluble substances such a s oils in this three-component "aqueous system". Acknowledgment We thank the Robert A. Welch Foundation of Houston for support of this work. Literature Cltec1 111 Starkr.Charler MI.;Liott.C."PhareTcansfer
Catalysis: Principlesand Techniques"; Academic: NeuiYark, 1978. mdler, E. J. "Catalysis in Micollar and Macromoleeulsr Systemr":
~. . senberg de Jong, H. G. In "Calloid
...,."--,
"
. ".
Science": Kruyt, H. R., Ed.: Elsevier: New
(Ill Mitt.a]. K. L.: Lindman. B., Eda."Surfacfants in Solution": Plenum: New York. 1984: Vnl 1, Pt 1. pp 1-269. 1121 Franklin, T. C.: Honda, T. In "Micellization, Solubilirafion, and Microernulsions"; Mittal. K.L.. Ed.: Plenum: New Yolk. 1977; Vol2. pp 617-25. 1181 Fmnklin. T. C.; Honds. T. E!eclrorhim. Acto 1978,23,439. 1141 Lucss, M. J. Inorg.Nuc!.Chem. 1970.32. 3692. llbi Steinbach, 0.F.: King. C. V. "Experiments in PhysicalChemiatry"; American Bwk Co.: New York, 1950:pp 12P29.
