In this work, the solid–liquid
phase equilibrium of N,N′-diethylthiourea
(DETU) was
investigated in different solvent systems from both solid-state structure
and solution thermodynamics. The single-crystal structure of DETU
was determined for the first time, and the intermolecular hydrogen
bonding N–H···S interactions are dominant in
the crystal packing. The thermodynamic equilibrium solubility was
determined by the gravimetric method in 12 solvents (methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butyl alcohol, n-pentanol, acetone,
butanone, methyl acetate, ethyl acetate, and acetonitrile) from 283.15
to 323.15 K. It was found that the solubility in alcohols decreases
with increasing the length of the carbon chain and follows the rule
of “like dissolves like”, whereas in non-alcohol solvents,
the rule does not apply. The experimental data were further correlated
by the modified Apelblat equation, λh equation,
and NRTL model. All the correlation derivations are lower than 5%,
and the NRTL model receives the best fitting performance. Furthermore,
the thermodynamic functions of mixing including enthalpy, entropy,
and Gibbs energy were derived, and the results expectedly suggest
a spontaneous and entropy-driven mixing process. All the crystallographic
and thermodynamic data reported in this study will provide the fundamental
data for designing and optimizing the crystallization of DETU.