Solid–Liquid Phase Equilibrium of the n-Nonane + n-Undecane System for Low-Temperature Thermal Energy Storage

The current article presents an exploration of the solid–liquid phase diagram for a binary system comprising n-alkanes with an odd number of carbon atoms, specifically n-nonane (n-C₉) and n-undecane (n-C₁₁). This binary system exhibits promising characteristics for application as a phase change material (PCM) in low-temperature thermal energy storage (TES), due to the fusion temperatures of the individual components, thereby motivating an in-depth investigation of the solid–liquid phase diagram of their mixtures. The n-nonane (n-C₉) + n-undecane (n-C₁₁) solid–liquid phase equilibrium study herein reported includes the construction of the phase diagram using Differential Scanning Calorimetry (DSC) data, complemented with Hot–Stage Microscopy (HSM) and low-temperature Raman Spectroscopy results. From the DSC analysis, both the temperature and the enthalpy of solid–solid and solid–liquid transitions were obtained. The binary system n-C₉ + n-C₁₁ has evidenced a congruent melting solid solution at low temperatures. In particular, the blend with a molar composition x_undecane = 0.10 shows to be a congruent melting solid solution with a melting point at 215.84 K and an enthalpy of fusion of 13.6 kJ·mol^–1. For this reason, this system has confirmed the initial signs to be a candidate with good potential to be applied as a PCM in low-temperature TES applications. This work aims not only to contribute to gather information on the solid–liquid phase equilibrium on the system n-C₉ + n-C₁₁, which presently are not available in the literature, but especially to obtain essential and practical information on the possibility to use this system as PCM at low temperatures. The solid–liquid phase diagram of the system n-C₉ + n-C₁₁ is being published for the first time, as far as the authors are aware.