## 4 Discussion  
As can be observed in Table 3, by ordering the esters by increasing carbon number no trend is revealed between the chemical structure and the phase change temperatures, as for example PEBE has a higher T\({}_{\rm m}\) than DEAR but shorter carbon chain. Therefore, the esters have been ordered in Table 4 according to increasing phase change temperatures. In order to allow us to comprehensively compare the fatty esters and to gain a clear overview of possible trends, the esters synthesized and tested by [PERSON] et al. [14] were added to the series in the table. By doing so, some trends correlating the chemical structure and the phase change temperatures can be observed. While, as expected, the melting point is raised by the addition of carbons to the aliphatic chain, it appears that the length of the carboxylic acid chain has a higher impact on the final T\({}_{\rm m}\) than the length of the alcohol chain. For example, when considering MEPA, DEPA and MESA, the first two are derived from the same carboxylic acid (palmitic acid C\({}_{16}\)) but different alcohols (methanol C\({}_{1}\) and 1-decanol C\({}_{10}\)), while MESA is formed from the same alcohol as MEPA but with a longer carboxylic acid (stearic acid, C\({}_{18}\)). In this case, the onset melting temperature increases from MEPA (26.25 \({}^{\circ}\)C) to DEPA (29.03 \({}^{\circ}\)C) by 2.78 \({}^{\circ}\)C due to the increased length of the alcohol chain that gives rise to a firmer crystal packing. However, when increasing the carboxylic acid chain length of two carbons, the temperature increases from MEPA (26.25 \({}^{\circ}\)C) to MESA (35.63 \({}^{\circ}\)C) of 9.38 \({}^{\circ}\)C. Such effect is visible throughout the whole series, but appears to be more marked the longer the carboxylic chain. For instance, despite deriving from a shorter alcohol, MEBE has a higher T\({}_{\rm m}\) than DEBE of 3.12 \({}^{\circ}\)C. In the same way, from MEMY to DEMY the melting temperature increases by 10.01 \({}^{\circ}\)C, while from MEMY to MEPA by 11.10 \({}^{\circ}\)C. As such, it appears the alcohol chain length has a higher influence on the melting point of esters derived from shorter carboxylic acids than from longer ones. Nevertheless, it can be concluded that, in general, the carboxylic acid chain length has a stronger impact on the final T\({}_{\rm m}\) than the alcohol chain, with such trend being more marked for esters formed from longer carboxylic acids. This could be explained by referring to a theory reported by [PERSON] et al. [26], according to which structures with the ester group at the end of the chain (such as methyl esters) possess higher melting points that those that see the ester bond in the middle of the molecule (decyl esters). This is caused by the reported flexibility of the O-C=O ester bond, which is able to rotate freely. A rotation in the middle of the molecule would cause vibrations in the whole chain far greater than at the end of the chain. Such vibrations would then disrupt the crystal lattice stability thus lowering the melting point. Therefore, two opposite effects are suspected to take place with the addition of longer chained alcohols: On one hand, the longer chain generates stronger intermolecular interactions and increases the rigidity of the structure, while on the other hand it shifts the position of the ester group from the end of the molecule to the middle thus causing stronger vibrations. Hence, this would explain the reason behind the lower impact of the alcohol chain on the final melting point compared to the carboxylic chain. Another trend that emerges from Table 4 is the consistency of pentyl esters to show markedly lower phase change transitions than the corresponding methyl and decyl esters derived from the same carboxylic acid. This can be easily observed in Figure 4 below.