Premelting Phenomena in Long-Chain Fatty Acids

Abstract

Kuclear magnetic resonance spectra and the infrared absorption between 700 and 760 cm-I have been observed a t temperatures near the melting points of the even-numbered fatty acids from Cl8 to Clo. The results are interpreted as indicating breakdown of crystalline character and onset of liquid-like motion a t scattered points in the solid structure several degrees below the melting point, and increasing in extent as the melting point is approached.A proton magnetic resonance study of anhydrous stearic acid (I) gave some quantitative indication of liquid-like molecular motion occurring in part of an otherwise rigid lattice a t temperatures several degrees below the melting point. I t seemed appropriate to look for this behavior, which was considered to be a premelting phenomenon (2), in other fatty acids and by other methods. Acids with an even number of carbon atoms from palmitic (C16) to capric (Clo) have been examined by nuclear magnetic resonance, and the change with temperature of the shape of the infrared absorption band a t about 720 cm-I has been studied for the even-numbered acids from stearic (C18) to lauric (Clz).This infrared band, which is characteristic of hydrocarbon chains (CHZ), with n >, 4, is frequently a doublet below the melting points of the compounds showing it, and a singlet above the melting point or for the compound in solution. I t has been assigned to the CH2 rocking mode (3, 4). A series of higher normal modes of CH2 rocking has been identified in the spectra of n-paraffins in the range 720-1050 cm-I (4). These bands are of much lower intensity than the first mode a t 720 cm-l, and their number depends on the length of the methylene chain. Stein has used the strong absorption of the first mode in a n investigation of the crystallinity of polyethylene (5), attributing the component of the doublet a t 720 cm-I equally to crystalline and amorphous parts of the polymer and the component a t 730 cm-I exclusively to crystalline parts. The splitting of the band was explained by the perturbation of the CH2 rocking vibration by the interaction of nearestneighbor methylene chains in the crystalline array. I t was shown (6) that the crystal field shifted the frequency of the out-of-phase rocking mode only slightly to higher energy and that the frequency of the in-phase mode was moved much further to higher energy. In an amorphous substance (solid, liquid, or solution) the unperturbed band occurs a t essentially the same frequency as the out-of-phase mode in the crystalline substance. Changes with temperature in the intensity of this band have also been used to study phase transitions in waxes (7). In the work reported here, the variation with temperature of the integrated intensity of the components of the doublet has been examined as a possible indication of loss of crystalline character in fatty acids below the melting point.The acids used in this investigation were all Eastman Kodak white label grade that were further purified by repeated recrystallizations from acetone following the...

Thermal and odd–even behaviour in a homologous series of lithium n-alkanoates

“…isotropic melt, in the region of 501 K to 540 K. For the long chain length soaps, LiC 14-19 inclusive, a pre-melting transition, preceded by a small exothermic transition, is always observed. The pre-melting transition overlaps with the melting transition and has been reported for long chain monocarboxylic acids [19][20][21]. It has been ascribed to rocking motions of methylene groups, in the solid, which tend to disorder the lattice in the temperature range close to the melting temperature.…”

“…This indicates that these carbons are in a similar chemical environment. Also, no odd-even alternation was observed in the asymmetric stretch of 1.12 ⁄ LiC 10 1.08 1.07 LiC 11 1.12 ⁄ LiC 12 1.06 1.06 LiC 13 1.10 ⁄ LiC 14 1.05 1.05 LiC 15 1.10 ⁄ LiC 16 1.05 1.04 LiC 17 1.09 ⁄ LiC 18 1.04 1.04 LiC 19 1.08 ⁄ ⁄No available data, q exp ± 0.02 Á 10 3 kg Á m À3 . the methyl groups in the IR spectra [9].…”

“…Clearly, the evidence supports a rotator phase as the structure for phase II, but is by no means conclusive. [13][14][15][16][17][18][19] . A plot of phase transition temperatures versus n C is shown in figure 8.…”

Thermodynamic properties of organic compounds

“…Some works on the fusion of stearic acid exist in the literature [7][8][9]. Perron et al found a melting temperature of 69.9~ (measured by DTA methods); Mod et al evaluated the melting enthalpy from the slope of some crystallization curves and proposed the value of 16.8 kcal/mole; by NMR and 1R method,s Barr et al found a small fraction of disordered lattice until 50~ below T r.…”

Dependence of the viscosities of some organic aliphatic acids on temperature and molar volume

“…The prefusion enthalpy for stearic acid represents a value of about 10 70 of the fusion enthalpy; this value decreases from stearic acid to the acids with lower molecular weights. Such an energetic value, according to Barr et al [3 ], is involved in the rocking motions of the methylene groups in the chain.…”