3.2. Synthesis of Methyl and Decyl Esters
Following the optimization of the synthesis procedure, esters derived from fatty carboxylic acids coupled with MeOH and fatty alcohol 1-decanol were produced, as reported previously. After confirming an acceptable degree of purity (≥89%), the thermal properties of the different esters were determined and subsequently analyzed and compared, in order to identify possible trends between chemical structure and thermal behavior, and to gain a broader overview of the melting points of different esters.
Concerning ATR-IR characterization, the appearance of the ester characteristic peak at 1735 cm−1
, the progressive disappearance of the broad alcohol peak at 3100–3600 cm−1
, and of the acid at 1705 cm−1
over time proved the efficiency of the reaction and the formation of the desired product. For all 1-decanol esters, however, a ratio of about 2:1 between the ester peak and the acid peak, respectively, was observed after drying with Rotavapor. This, together with the presence of a peak in the alcohol region, proved that not all reagents had been consumed during the reaction time. Therefore, crystallization in MeOH was performed until there was no visible residual alcohol peak and acid peak in the final spectra. Figure 9
shows the difference between unpurified decyl palmitate (DEPA) and DEPA crystallized twice in MeOH with a ratio of 1:10 for DEPA:MeOH. Generally, as reported previously, one or two crystallizations were sufficient to obtain degrees of purity over 89%.
A clear gain in the ester band was achieved, confirming the efficacy of the purification process. However, besides purity control and synthesis control, the IR could not confirm the true identity of the esters created since no clear differences were visible in terms of the ester peak and alkane chain peaks at 2920 and 2850 cm−1. Therefore, the structure of the produced esters had to be recognized through NMR and GC-MS analysis.
The mass spectra recorded for methyl esters showed the same peaks reported for MEPA, confirming the cleavage of a methyl group with peak at 239 m/z. Esters derived from different fatty acids could be recognized by the longer or shorter fragmentation of the alkane chain by progressive loss of –CH2 (14 m/z) and by the molecular ion. On the other hand, spectra for decyl esters showed a different fragmentation pattern, especially regarding the cleavage of the alcohol chain. For instance, for DEPA (molecular peak at 396 m/z), a strong signal was recorded due to the loss of the decyl alkane chain (loss of 141 m/z, peak at 255 m/z), thus forming a carboxylate group. The decyl aliphatic chain is then thought to form a decene through resonance to give the observed mass loss of 139 m/z at corresponding peak 257 m/z. The alcohol cleavage with loss of 1-decanol (loss of 157 m/z) was observed at 239 m/z, which confirms that the ester analysed was indeed a decyl ester. Additionally, the usual progressive fragmentation of the alkyl chain from the acid side is observed through steps of 14 m/z loss each.
Similarly, the structures of the different esters could be easily identified through 1
H NMR analysis, as well as detection and quantification of impurities from residual alcohol or acid (Figure 10
). All the 1
H NMR spectra of methyl esters showed a high degree of purity and no residual alcohol, therefore their purity was estimated at ≥98%. The same main peaks as the ones observed in the spectra of replicated MEPA were observed, namely the singlet at 3.66 ppm from the deshielded methyl group –CH3
directly attached to the carbonyl group, the triplet at 2.28–2.32 ppm from the –CH2
in α-position to the carbonyl, the multiplet from the –CH2
in β-position at 1.58–1.65 ppm, the peak from the alkane chain around 1.26 ppm and the triplet at 0.86–0.90 ppm from the shielded –CH3
group at the end of the aliphatic chain. On the other hand, some variances in the spectra for decyl esters allowed differentiation between the diverse groups of esters. Here, a triplet with integral value of 2 instead of a singlet appeared at 4.05–4.07 ppm (a) from the –CH2
in α-position to the oxygen of the ester group. This is due to the fact that in 1-decanol, unlike in methanol, the aliphatic chain continues, therefore the –CH2
in α-position feels the influence of the neighboring –CH2.
The triplet at 2.28–2.32 ppm (c) from the –CH2
in α-position to the carbonyl group remained the same, while the multiplet at 1.58–1.65 ppm (d) from the –CH2
groups in β-position had an integral equal to 4 hydrogens instead of 2 due to the presence of the additional –CH2
in β-position to the oxygen from the alcohol side. Similarly, the peak from the alkane chain at 1.26 ppm (e) had an integral value of about 38 hydrogens, and the triplet at 0.86–0.90 ppm (f) corresponded to the 6 hydrogens from the shielded –CH3
at the end of the aliphatic chains from the acid and the alcohol sides. Small alcohol impurities, when present, were visible in the form of a triplet at 3.62–3.66 ppm (b) from the –CH2
in α-position to the hydroxy group of the free alcohol molecules.
The ratio between its integral value and the one from the ester signal at 4.05–4.07 ppm allowed to quantify the degree of purity of the sample. As mentioned previously, in the absence of any additional peaks from unreacted reagents, the purity was estimated to be ≥98%, like in the case of methyl esters.
Impurities still present in the decyl esters could also be observed through DSC analysis. In the presence of residual alcohol, a second broad peak appeared between 2–10 °C (Tm of 1-Decanol: 6.4 °C), and lower ΔH of about 150–160 J/g were registered. Upon further crystallizations and purifications though, such peak decreased until it was barely visible over the baseline noise of the instrument, and ΔH for the ester peak increased again to values over 190 J/g.
In general, the results obtained concerning thermal properties of synthesized esters are summarized in Table 1
. As mentioned beforehand, the esters names have been abbreviated as follows: methyl myristate (MEMY, C1–14), decy myristate (DEMY, C10-C14), methyl palmitate (MEPA, C1-C16), decyl palmitate (DEPA, C10-C16), methyl stearate (MESA, C1-C18), decyl stearate (DESA, C10-C18), methyl behenate (MEBE, C1-C22), decyl behenate (DEBE, C10-C22).
A trend can be observed in the degradation temperatures, with the longer chained esters being the most stable and starting to degrade at much higher temperatures compared to shorter ones. This can be explained using literature reports, with longer chain esters being able to create tighter crystal structures and therefore stronger intermolecular bonding, thus requiring higher temperature to break the bonds [15
]. A similar behavior would be expected regarding melting temperatures Tm
. However, as can be clearly observed, MEBE was the ester showing higher melting temperature and not DEBE, as would have been expected.
Upon reversing the order of the esters shown above, a trend is revealed between molecular structure and melting points (Table 2
). The melting points are seen increasing based on both the fatty acid carbon length and the alcohol length, but those from decyl esters follow a parallel trend to methyl ester, and do not show the same behavior (Figure 11
). What reported above is found to be true for both Tm
for all samples.
This could be due to the odd-even numbered effect explained by Noël et al. [17
] and Yang et al. [18
], according to which different but parallel trends can be observed between odd-numbered and even-numbered carbon chain series. In addition, the length of the fatty acid carbon chain seems to have a higher impact on the resulting phase transition temperature of the ester, compared to the alcohol.
The molar enthalpies of fusion (ΔH in KJ/moL) could be seen increasing with increasing chain length and molecular weight; however, no clear trends could be observed for even and odd-numbered esters as for Tm. Nevertheless, all values measured were above 190 J/g with uncertainties around 1.5%, confirming the suitability of fatty esters as PCM. All of the samples proved to be stable to short cycles (three times), with variations lower than 0.5 °C. Additionally, supercooling lower than 7 °C were observed in all cases at a rate of 10 °C/min.