Solvent Action and "Co-solvency" A Lecture Demonstration SANTI R. PALIT* Stonford Uniuersity, California
HE solubility of a substance in individual solvents may be low and yet, in a mixture of these same solvents, be very great. For example, soap has been shown (1) to be very soluble in a mixture of a glycol .with a hydrocarbon or a chlorinated hydrocarbon or an alcohol. This is termed "co-solvency." Such a system is suited for lecture demonstration, because after preliminary swelling of the soap the dissolution occurs as fast as the soap can be mixed with the solvents-as quickly as if one liquid were being dissolved in another. The demonstration is best made by using propylene glycol as one solvent with chloroform as the co-solvent, a n d taking dry powdered sodium oleate as the soap. In each of two test tubes is placed 1 g. of sodium oleate powder (small compact masses, if any, should be first broken down to powder) ; into one is poured about 5 g. (5 ml.) of propylene glycol, and into the other 5 g. (3.5 ml.) of chloroform. Each test tube is now well shaken. Plenty of undissolved soap will be found to remain in both. The contents of the two tubes are now mixed together and shaken. Immediately all the sodium oleate goes into solution in the mixed solvent, and the whole is absolutely clear. Alternatively, about 3 g. of finely powdered sodium oleate is taken in a small conical flask and shaken with 10 ml. of propylene glycol to moisten i t fully to a rather thick paste. Chloroform is now added in 1-ml. portions and the mixture is well shaken after each addition. Chloroform is almost a perfect nonsolvent for soap, the solubility being of the order of one part per thousand; however, the solution gradually becomes thinner and clearer with addition of chloroform, and on adding 3 to 4 ml. of the nonsolvent, complete solution occurs, resulting in a clear mobile fluid. This will clearly dem.onstrate how by the addition of a nonsolvent the solvent power of a liquid is profoundly increased.
*
Bristol-Myers Compsny Pmt Doctorate Fellow in Chemistry.
Though propylene glycol and chloroform were chosen here, this experiment can be successfnlly done with any of the following pairs of solvents: ethylene glycol, propylene glycol, or diethylene glycol with bntyl alcohol, amyl alcohol, chloroform, or methylene chloride; propylene or diethylene glycol with carbon tetrachloride; propylene glycol with benzene. Although the last three liquid palrs are only partially miscible a t room temperature, they can well be used in these experiments since they become completely miscible in the presence of sodium oleate. With suitable change of proportion of the amount of soap used, easily found by trial, the same experiment can be successfully demonstrated with any other alkali metal soap such as laurate, myristate, palmitate, or stearate, or even with commercial soaps if they do not contain inorganic builders. Also, metallic soaps can be successfully used for these experiments. One combination tested for use in the f i s t kind of experiment is 0.20 g. of finely powdered calcium laurate in each of the two test tubes with 5 ml. of chloroformand3ml.of propyleneglycol,respectively. The calcium laurate is readily prepared by metathesis of dilute solutions of sodium or potassium laurate and excess calcium chloride, followed by washing the precipitated soap free from chloride, and drying. A discussion of the mechanism of co-solvency which involves molecular solution, and its close relation to solubilization which in other solvents involves colloidal particles, is advanced elsewhere ( 2 ) . The author is thankful to Prof. James W. McBain for suggesting the apt term "co-solvency" in the sense used herein and for his interest in this work.
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LITERATURE CITED
(1) PALIT,S. R., J.Indian Ckem. Soc., 19,271 (1942). (2) PALIT,S. R., AND J. W. MCBAIN,communicated t o Ind. Eng. Chem., I d . Ed., December, 1946.
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