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Molecules 2020, 25, 5572 4 of 26 • β form: it is easily detected as it forms needle-like crystals with the molecules locked in a definite position, and the hydrocarbon chains tilted at an angle of circa 65 ◦C to the end-group planes. For simple triglycerides, this corresponds to triclinic subcells [23]. It is the most thermodynamically stable form and as such will be favoured in slow heating-cooling conditions. The β phase shows alternation between odd and even number of carbon atoms in a series, and more than one form for given compound are reported [15,24]. Due to their highly ordered state, β crystals cannot be spontaneously formed from an α, β’ or β melt in the absence of thermal history, but will rather grow from existing α or β’ crystals, or directly from a β melt if a thermal history or seed crystals are present. Figure 3 shows the relation between the different polymorphic phases. This has first been theorized by Malkin [11] from both thermal and powder diffraction data of a series of saturated di- and triglycerides (namely 1,2-dipalmitin, 1,2-dilaurin, tripalmitin, 2-myristodipalmitin, 2-palmitodimyristin, 2-oleodistearin). This has been later assumed to be valid for all triglycerides [23]. To summarize, since the α phase is kinetically stable, α crystals can reversibly form from the melt (liq.) in fast heating-cooling conditions, whereas β’ and β crystals can only form irreversibly from pre-existing crystals [25], or in extremely slow heating-cooling conditions from the corresponding melt. Thus, one option to maintain the β form constant is to include seeds in the melt to promote nucleation [25]. Alternatively, the possibility of creating the β’ and β form directly from solvents is also discussed in the literature [17,23]. Interestingly, once formed, due to the triglycerides’ ability to maintain a thermal “memory” of past crystalline forms, it is theorized the β crystals will not revert to less stable phases upon cycling unless the thermal history is erased via high temperature treatments [26–28]. The β’ phase is the hardest to characterize as it is metastable, thus, it tends to transform directly to β over time whenever possible [10,13,14]. Additionally, the passage from α directly to β can occur without the β’ taking place. However, it must be noted that the β’ phase occurs more often and is most stable in the presence of unsaturation or asymmetrical molecules. Figure 3. Scheme summarizing the relation between each polymorphic form of triglycerides. Each phase can be reversibly formed from the melt, however the transition from α → β’ → β or from α → β is irreversible. Image reproduced from Hernqvist [23]. Based on the data obtained from the analysis of long and short d-spacings in powder XRD patterns, first Malkin [11] and Lutton [15], then later Hagemann [18] and Hernqvist [23], described the structural arrangements of the molecules in the lattices which are still widely accepted nowadays. In particular, given that the long d-spacings are too large to be representative of the length of one single acid chain, the so-called “tuning fork” structure was proposed, which explains why the values match twice the length of the acid chain (Figure 4a). In general, the molecules are thought to assume an arrangement “head to tail” as shown in Figure 4b. However, depending on the crystalline structure, different degrees of tilting occur, which cause changes in the long d-spacings. Lutton [20] reported the long d-spacing of tristearin and observed a progressive decrease in their values from 50.6 to 46.8 and 45.2 Å corresponding to the α-, β’- and β-phases, respectively. This reflects an increasing degree of tilting of the hydrocarbon chains plane compared to the end groups axis (Figure 5a,b).PDF Image | Triglycerides as Novel Phase-Change Materials
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