Isopropyl Alcohol/ Toluene/ Water: A Thermomorphic Ternary Azeotrope for Extractive Purification

 

There are several references to this Ternary System: Isopropyl Alcohol, Toluene, and Water at 25°C.

E. Roger Washburn and Albert E. Beguin, J. Am. Chem. Soc. 1940, 62, 3, 579–581; J. Am. Chem. Soc. 61,1694 (1939); 54, 4217 (1932); 56, 361 (1934)

It is well established that isopropyl alcohol, toluene, and water mixtures upon distillation pass over into the distillate as a tertiary azeotrope that boils at 76.3 C and has the weight composition 38.2% IPA, 48.7% toluene and 13.1% water. When it separates into two liquid phases the upper layer is 92% by volume and the lower 8% by volume. The composition of the upper layer is 38.2% IPA, 53.3% toluene and 8.5% water and has specific gravity 0.845. By contrast the small lower layers 38.0% IPA1.0% toluene and 61.0% water has specific gravity 0.930. This behaviour of splitting into two layers on cooling is called thermomorphic.

What is not apparent from my examination of what literature I can access is the temperature at which these two phases merge into a single phase. That is: What is this mixture’s upper critical solution temperature (UCST)?

To be useful in the application I am contemplating there needs to be a manageable difference between the lowest temperature needed to get a homogeneous solution and the azeotropic boiling temperature. That is, specifically the UCST needs to be less than say 50 C.

My idea is to use portions of the lower phase of the separated ternary azeotropic composition (with a composition of 38.0% isopropanol, 1.0% toluene and 61.0% water)  in the volume proportion of 8 volume % versus the 92 volume % of the upper phase to sequentially wash the upper toluene rich phase, so as to remove somewhat preferentially more polar constituents from a mixture of substrates initially dissolved in the homogeneous single phase ternary azeotropic combination of isopropyl, toluene and water.

This, it is hoped, will produce a result similarly to what is called ‘swish’ trituration. In ‘swish’ trituration a solid mixture of substrates is repeatedly triturated with an anti-solvent in which the predominant component is very nearly completely insoluble but in which the impurities are meagerly soluble. Even so, they are removed because of the substantial quantities of the triturant used. If this were to work, the result would be the purification of the main component.

This is how I imagine the process would be executed.

Experimental

10 liters of the IPA, toluene, water tertiary azeotrope are prepared. The mixture is heated to a temperature conveniently above the UCST and divided into two portions, one of about 1 liter and the second about 9 liters. The 9 latter portion is allowed to cool below its UCST and the lower more polar phase separated and stored in a stoppered vessel. This phase labelled (A) will provide the multiple wash portions used to remove more-polar components of the mixture of substrates. In a separatory funnel, part of the 1 liter portion of the still warm, still a single phase tertiary azeotrope which we call (B) is used to dissolve the mixture of substrates to be separated. For about 2 grams of mixture, 100 ml of warm homogenous azeotrope is used. The test mixture must dissolve completely at the warm temperature where there is just one liquid phase and when the solution cools it is necessary that two liquid phases separate. 

It is essential that two liquid phases form even though the presence of the charge of substrates is included. It is for this reason that I am choosing to only use 2% weight to volume (substrates to solvent). If two phases do not separate, it will be necessary to increase the amount of azeotrope solvent mixture until they do. On the other hand, for reasons of the throughput of the purification, it is desirable to use as large an amount of substrate mixture as is consistent with retaining two separating liquid layers.

When the liquid in the separatory funnel has cooled to room temperature, remove the smaller volume of the lower phase into a graduated cylinder and note the volume of this phase. Transfer this, what I will call the 1st wash, to an erlenmeyer flask and save it for analysis to learn the degree to which you have concentrated the polar impurities in this 1st wash.

Now, into the separatory funnel add the exact same volume of  (A) as you have removed in the first cut. Two phases should be present since what you are adding is pretty close in solvent composition as what has been removed. Warming in the separatory funnel to above the UCST will produce one phase and cooling back will split the volume into two again. The substrates, which we seek to separate, will have again partitioned between the layers. Cut again and move that layer into a second erlenmeyer flask for analysis.

How successful the technique is for purifying a major less-polar substance will depend upon the substance being substantially more soluble in the toluene-rich layer than in the water-rich layer. The more polar impurities need not be particularly soluble in the water-rich phase so long as they are more soluble than the major component. Poorer solubility of the polar impurities only means a larger number of these small volume washes will be required. Note that the size of the washes must be the same as the volume of the first lower layer. If not the composition of the ternary azeotrope in the separators funnel will change too much and the layers may no longer separate properly. 

An advantage of working at-scale in a closed inverted reactor as the separatory vessel is that a separation temperature as low as -20 C can be used because extraneous water from the plant air cannot be condensed into the liquid medium from the air— the liquid layers are covered by inert gas!