Vaporization enthalpy measurement

What is vaporization?

The vaporization of a sample corresponds to its passage from the liquid state to the gaseous state. This change of state requires a large amount of heat in order to separate the molecules from each other.

Vaporization of a compound can take place in two ways: by evaporation and by boiling. Evaporation takes place more or less rapidly at any temperature at the gas/liquid or gas/solid interface. The higher the temperature, the faster the evaporation. When the temperature becomes too high, boiling can take place within the sample: this is a sudden change of state during which bubbles of compound vapor are produced and the temperature no longer changes until to complete evaporation.

At Calnesis Lab, vaporization enthalpies are measured by DSC over a wide temperature range.

Florian Rodrigues, R&D Project Manager, Calnesis

How to measure the enthalpy of vaporization?

This measurement is carried out by calorimetry, and more particularly by differential scanning calorimetry (DSC). About ten milligrams of sample are introduced into a pierced crucible. The latter then undergoes a controlled temperature rise. Without change of state, the heat flux measured corresponds only to the heat necessary to heat the liquid contained in the crucible (thermal/calorific capacity). When the boiling point is reached, a very significant endothermic heat flow is then measured (see figure opposite). The integration of the endothermic peak obtained makes it possible to determine the enthalpy of vaporization. The same type of measurement also makes it possible to determine the boiling point.

Nevertheless, it frequently happens that complex mixtures, containing multiple compounds, never make it possible to obtain boiling in DSC. The more volatile compounds simply evaporate one after another during the temperature ramp. In this case, the measured thermogram does not show usable boiling peaks, but globally endothermic heat flow variations. An overall enthalpy can in some cases be determined as an indication, as well as the characteristic temperatures at the start and end of vaporization.