Understanding and Analyzing Freezing-Point Transitions of Confined Fluids within Nanopores

Understanding phase transitions of fluids confined within nanopores is important for a wide variety of technological applications. It is well known that fluids confined in nanopores typically demonstrate freezing-point depressions, δT_f, described by the Gibbs-Thomson (GT) equation. Herein, we highlight and correct several thermodynamic inconsistencies in the conventional use of the GT equation, including the fact that the enthalpy of melting, δH_m, and the solid-liquid surface energy, _SL, are functions of pore diameter, complicating their prediction. We propose a theoretical analysis that employs the Turnbull coefficient, originally derived from metal nucleation theory, and show its consistency as a more reliable quantity for the prediction of δT_f. This analysis provides a straightforward method to estimate δT_f of nanoconfined organic fluids. As an example, we apply this technique to ibuprofen, an active pharmaceutical ingredient (API), and show that this theory fits well to the experimental δT_f of nanoconfined ibuprofen.