Systematic Trends in the Melting Temperature and Composition of Eutectic Binary Mixtures with One Component from a Homologous Series

Materials that store a significant amount of heat in a narrow temperature range by phase change solid-liquid or solid-solid are called Phase Change Materials (PCMs). Many PCMs are members of homologous groups of materials with similar composition and properties. Often, similarities are due to a common molecular composition with a repeating unit, e.g. for n-alkanes H-(CH2)n-H. Typical is an n related trend in the melting temperature. Based on observations on solvents, the question arises if such a trend also exists in eutectic binary mixtures with one component fixed while the other, from a homologous series, is varied. For verification, literature data were collected, specifically experimental data, each set with at least three variations from a single source. Eight data sets were collected, covering eutectic binary mixtures of n-alkanes, n-alkanols, and n-alkanoic acids. With one exception, all data sets show a systematic trend in the melting temperature and the composition. It is shown that the trends can be understood from thermodynamic theories of mixtures (Schröder – van Laar equation) combined with typical trends within homologous series. The findings offer new options in PCM development as well as the selection of PCM for specific application temperatures.