How to engineer the boiling temperature of a liquid

When you mix different metals together and create a new metal you create an alloy. That alloy will have different properties from the ingredient metals. If you take copper with a melting point of 1084.62 °C and mix it 88 to 12 with tin which has a melting point of 231.93 °C, then you create a bronze that melts at about 950 °C. The exact temperature varies with the percentage of copper and tin.

This is the example to follow when you want to create a liquid with a new boiling temperature. With metals, not all mixes become one metal with a new melting point. An example is copper and cobalt. With liquids there is a like phenomena, some mixes can still be separated by boiling. This is what a still does. Water and ethanol are miscible, they can be mixed in any proportion, and they dissolve in each other. But with a still, the ethanol which is more volatile can be boiled out of the mixture leaving a higher proportion of water. But the boiling off is not pure, some water vapor also boils off. The more alcohol in the liquid, the more alcohol that is in the vapour. As the ratio changes in the liquid, the ratio also changes in the vapor. At an ethanol to water ratio of 96.4 %, the vapor has an identical composition. Further boiling does not concentrate the liquid any further.

This liquid is called an azeotrope. A no change after boiling liquid. Azeotropes are pressure dependent. By changing the pressure you can change the ratio of components in the vapor. This fact can be used to jump over an azeotrope barrier. This is pressure swing distillation. Once the concentration changes enough, the pressure can return to atmospheric, there is no longer an azeotrope. The concentration can be strengthened further with continued distillation. Note however, that the other component liquid may now be the more volatile one.

For further reading on azeotropes I recommend Norwegian engineering student Eva-Katrine Hilmen’s thesis.

Now to engineer boiling temperature, we want to create azeotropes. The mixture of multiple liquids together in varying ratios is where we must search for our azeotropes. The following description of many such constant boiling admixtures will give us some hope that more can be found. As we saw with the ethanol and water mixture, just boiling off the volatile vapors pushes the concentration toward the azeotrope ratio. So starting with varying ingredients and trial and error of boiling can be the method of search.

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