Entropy models for the description of the solid–liquid regime of deep eutectic solutions

A necessary prerequisite for applying deep eutectic solutions (DESs) is to understand the phase behavior and to be able to quantify the liquid window of these mixtures. The non-ideality of the phase behavior is determined by the contributions of excess entropy and enthalpy. While the total Gibbs energy of mixing can be inferred from the solid–liquid phase behavior, the entropic and enthalpic contributions can not be distinguished. Hence, by assuming ideal mixing entropy, all excess free energy is captured as an enthalpic contribution. The ideal mixing entropy provides a reasonable description when the components are similar in size and shape. This is not always the case for the components typically used in DESs. Here, the suitability of two non-ideal entropy models is investigated, aiming to describe the phase behavior of DESs more accurately. First, by using Flory–Huggins entropy accounting for the different molar volumes of the components, we show that ideal entropy of mixing underestimates the entropic contribution for mixtures of components often used for DESs. The value of molar volume employed has a significant influence on the resulting entropy of mixing and thus on the resulting enthalpy. Second, correcting for the molar area as well, using the Staverman–Guggenheim entropy, appears to have negligible impact for the compounds considered. Both the use of a non-ideal mixing entropy and the specific choice of the molar volume significantly affect the obtained enthalpy of mixing and will thus alter the interaction parameters, obtained using a Redlich–Kister-like mixing enthalpy, as compared to models based on ideal mixing entropy.