Polymorphic transformations in sn-1, 3-distearoyl-2-ricinoleyl-glycerol

Polymorphic transformations of sn-1,3-distearoyl-2-ricinoleyl-glycerol (SRS) have been studied with differential scanning calorimetry, X-ray powder diffraction (XRD), synchrotron radiation X-ray diffraction, and Fourier transform infrared spectroscopy (FTIR) techniques by using a 99.8% pure sample. Four polymorphs, α, γ, β′₂, and β′₁, were isolated. The thermal behavior of the four forms showed that the fusion of α at 25.8°C was followed by the crystallization of γ which melts at 40.6°C, and β′₂ and β′₁ revealed melting peaks at 44.3 and 48.0°C, respectively. No β form was observed, even when the two β′ forms were annealed around their melting points over one week. The XRD long spacing indicates that α packs into a double chain-length structure; however, γ and the two β′ phases pack into a triple chain-length structure. The polarized and nonpolarized FTIR spectra in methylene scissoring and methylene rocking regions indicated a parallel subcell packing in γ, and a mixture of orthorhombic perpendicular and parallel or hexagonal subcells in the β′₂ and β′₁ phases. Consequently, SRS exhibits quite a unique polymorphic behavior, compared to tristearoyl glycerol and sn-1,3-distearoyl-2-oleoyl-glycerol.     

Binary phase behavior of 1,3-distearoyl-2-oleoyl-sn-glycerol (SOS) and 1,3-distearoyl-2-linoleoyl-sn-glycerol (SLS)

The binary phase behavior of SOS (1,3-distearoyl-2-oleoyl-sn-glycerol) and SLS (1,3-distearoyl-2-linoleoyl-sn-glycerol) was examined by using DSC and conventional and synchrotron radiation X-ray diffraction. The solid-solution phases were observed in the metastable α and γ forms in all concentration ranges. Results indicated that the miscible γ form did not transform to the β′ form when the mixtures were subjected to simple cooling from a high-temperature liquid to a low-temperature solid phase. However, and α-melt-mediated transformation into β′ and β₂ resulted in the formation of immiscible phases in concentration ranges of SLS below 30%. By contrast, at SLS concentration ranges above 30%, the α-melt-mediated transformation caused crystallization of only the γ form, and β′ and β₂ crystals did not appear. These results show that the specific interactions between SOS and SLS are operative in the phase behavior of the mixture states of SOS and SLS.     

Modulated temperature differential scanning calorimetry for examination of tristearin polymorphism: I. Effect of operational parameters

The transformations of tristearin were examined by modulated temperature differential scanning calorimetry (MTDSC) in order to study the effect of operational parameters on the nature of information obtained from this technique. Tristearin has been used as a model polymorphic system showing metastable phases and complicated transformation routes occurring at relatively slow rates. The parameters examined were underlying heating/cooling rates and the amplitude of modulation. The first conclusion is that MTDSC enables overlapping α-melting and β-crystallization events to be separated, thus increasing the information obtained compared to normal thermal analysis. Other general conclusions are that observation of reversible processes is strongly influenced by the underlying heating rate; low to moderate heating rates are recommended. Amplitude of modulation has a complicated effect on the phenomenon being studied; when studying systems that exhibit metastable or polymorphic transitions, it is recommended that a range of amplitudes be tested to enable confirmation of whether an observed ‘recrystallization’ effect is a new phase or the same phase as the one melting. Cooling with modulation disturbs the crystallization process, possibly leading to smaller or imperfect crystals; however, the phases obtained are not different compared to normal DSC. The usefulness of MTDSC in analyzing these types of complicated systems is primarily qualitative at the moment. Some recommendations have been made as to the combinations of parameters for studying such systems.     

Specific heats of the solid-state phases of trimargarin and tristearin

Differential scanning calorimetry is used to obtain specific heats of the α, β′₂, β′₁ and β phases of trimargarin and tristearin in the temperature range from 190–350 K. Unequal specific heats are observed for β′ phases of the 2 lipids in contrast to nearly coincident values for their respective α and β phases. These results are discussed on the basis of odd vs even chain length triglycerides.     

Thermal and structural properties of sn-1,3-dipalmitoyl-2-oleoylglycerol and sn-1,3-dioleoyl-2-palmitoylglycerol binary mixtures examined with synchrotron radiation X-ray diffraction

Thermodynamic and polymorphic behavior of POP (sn-1,3-dipalmitoyl-2-oleoylglycerol) and OPO (sn-1,3-dioleoyl-2-palmitoylglycerol) binary mixtures was examined using differential scanning calorimetry and conventional and synchrotron radiation X-ray diffraction. A molecular compound, β_C, was formed at the 1:1 (w/w) concentration ratio of POP and OPO, giving rise to two monotectic phases of POP/compound and compound/OPO in juxtaposition. β_C has a long-spacing value of 4.2 nm with a double chainlength structure and the melting point of 31.9°C. A structural model of the POP-OPO compound is proposed, involving the separation of palmitoyl and oleoyl chain leaflets in the double chainlength structure. In the polymorphic occurrence of the POP-OPO mixtures, the POP fraction transformed from α to β′ with no passage through γ, then transformed to β. The presence of OPO in POP promoted the β′-β transformation of POP during the melt-mediated crystallization.     

Polymorphism of POP and SOS. I. occurrence and polymorphic transformation

The polymorphic modifications of POP and SOS were identified with X-ray diffraction (XRD), DSC and Raman spectroscopy by using pure samples (99.9%). In POP, six polymorphs, α,γ, pseudo-β′₂, pseudo-β′₁, β′₂ and β′₁, were obtained, whereas five polymorphs, α, γ, pseudo-β′, β₂ and β₁, were isolated in SOS. Thermodynamic stability increased from α to β₁ straightforwardly both in POP and SOS, because the polymorphic transformation went monotropically in the order described above. Additionally, the 99.2% sample of POP crystallized another form, δ, but the 99.9% sample did not, implying subtle influences of the impurity. The four forms, α, γ, β₂ and β₁, of POP, revealed XRD and DSC patterns identical to the four forms of SOS designated by the same symbols. The chain length structure was double inα and triple in the other three forms in both POP and SOS. Peculiarity of POP was revealed partly in the chain length structure of pseudo-β′₂ and pseudo-β′₁ which were double, whereas pseudo-β′ of SOS was triple. This apparently showed contrast to the facts that the three forms revealed rather similar XRD short spacing patterns. Another peculiarity of POP was revealed in enthalpy value of the melt crystallization of α: ΔH_c (α) = 68.1 kJ/mol which was much larger than that of SOS (47.7 kJ/mol), and also than AOA and BOB. These peculiarities mean that the double chain length structures of POP are more stabilized than the others. Raman bands of CH₂ scissoring mode of SOS indicated parallel packing in γ, β₂ and β₁, and orthorhombic perpendicular packing in pseudo-β′. The polymorphic transformation mechanisms were discussed based on the proposed polymorphic structure models. Presented at the AOCS annual meetings in New Orleans, Louisiana in May 1987 and Phoenix, Arizona in May 1988.     

The polymorphism of odd and even saturated single acid triglycerides,C8–C22

The polymorphism of single fatty acid odd triglycerides, C₁₁ through C₁₇ has been reinvestigated, with extension of the study to C₉, C₁₉ and C₂₁. With study of the even glycerides C₈, C₂₀ and C₂₂ it has been possible to review the whole series (odd and even) C₈ through C₂₂. The odd glycerides resemble the even in showing three distinct melting levels. Lowest melting forms are α. Stable forms are β except for C₉ and C₁₁ which show a different structure type. Intermediate melting β′ forms of odd glycerides are substantially more stable than their even counterparts as well illustrated by differential thermal analysis. There are no vitreous forms. Alternation in mp (between odd and even) is confirmed for stable phases and nonalternation for α and (within experimental error) for β′. Both β′ and β long spacings show alternation but not α. Alternation is evident in the short spacings of β′ and β forms. While short spacings of β′ forms of even triglycerides are much alike especially for C₁₄ through C₂₂, those of odd glycerides show a fortuitous 4-carbon cycle. This appears to involve no significant structural variation as chain length increases but simply an approximate 4-carbon cycle of variation in diffraction details derived from the presumed unvarying 0 ⊥ type subcell structure.     

Structural investigations of β′ triacylglycerols: An x-ray diffraction and microscopic study of twinned β′ crystals

To reveal the structure ofβ′ triacylglycerols in detail, LML (C₁₂C₁₄C₁₂) was purified by a zone-melting procedure, and twinned crystals ofβ′ stable LML were obtained from a melt,β′ LML crystallizes in the monoclinic space group C₂, with eight molecules in the unit cell. A powder X-ray diffraction study of solid compounds of 1:1 mixtures of selected triacylglycerols led to the conclusion that the triacylglycerol molecules in theβ modification have a 1,2 chair-conformation (i.e., the fatty acid chains on glycerol positions 1 and 2 are adjacent, with the chain on the 3-position forming the back rest of the chair). Packing studies and the positions of two-fold axes and two-fold screw axes in the unit cell require that the molecules are bent at the glycerol site. The fatty acid chains make an angle of 25° with the long axis of the unit cell. Electron micrographs and precession photographs indicate that the twinning results from the stacking of a large number of thin crystalline platelets in two distinct orientations.     

Polymorphism of stabilized and nonstabilized tristearin,pure and in the presence of food emulsifiers

The polymorphism of tristearin (SSS) was studied by means of differential scanning calorimetry and powder X-ray diffraction. The influence of 5% in weight of different food emulsifiers—i.e., 1-monostearin, sorbitan tristearate, and sugar monostearate—was also studied. Because polymorphism is sensitive to thermal treatment, two thermal conditionings were applied. According to the dynamic process (melting, quenching, and heating at 5°C/min), SSS showed three polymorphic forms: α, β′, and β₁. The presence of the emulsifiers hindered the β′→β transformation, and a destructured β₂ form was recorded. According to the stabilization process (stabilization at 57°C for various periods of time), SSS showed two β′ forms: β ₂ ^′ and β ₁ ^′ . Three hours of stabilization were necessary to recover the whole triglyceride under the β form. The emulsifiers slowed down the polymorphic transition rates. Indeed, after six hours of stabilization, mixtures of β′ and β were observed. Sugar monostearate seemed to have the most powerful effect on the transition kinetics because large amounts of α form were detected. Presented at the 86th AOCS Annual Meeting & Expo, in San Antonio, Texas, in May 1995.     

Crystallization kinetics of palm stearin in blends with sesame seed oil

This study investigates the crystallization kinetics of palm stearin (PS), a palm oil fraction, in blends with sesame seed oil. The results indicate that the crystallization behavior of PS in sesame oil is mainly associated with the crystallization of tripalmitin. Therefore, crystallization of blends of 26, 42, 60, and 80% (wt/vol) PS in sesame oil was described by equations developed for simpler systems (e.g., Fisher and Turnbull equation). The isothermal crystallization, melting profile, and fitting of the kinetics of nucleation to the Fisher and Turnbull equation showed that the 26, 42, and 60% PS/sesame oil blends crystallized mainly in the β₁′ polymorph state. In contrast, the 80% blend crystallized in two different polymorph states (i.e., β₁′ at T⪯307.6 K and β₁ at T≽308.2 K). The data indicated that, in spite of the higher concentration of PS in the 80% PS/sesame oil system, crystallization in the β₁ state required more free energy for nucleation (δG _c ) than β₁′ crystallization in the 26, 42, and 60% PS/sesame oil. At the low cooling rate used (1 K/min) it was observed that, for a particular PS blend, the higher the effective supercooling the higher the viscosity of the oil phase and the smaller the induction time of crystallization (T_i). Additionally, the β₁′ crystals from PS, developed at the highest effective supercooling investigated, were smaller than the β₁ crystals obtained at lower effective supercooling.     

Polymorphism of POP and SOS. II. kinetics of melt crystallization

Melt crystallization of the polymorphs of SOS, α,γ, pseudo-β′, β₂ and β₁. and of POP, α,γ, pseudo-β′₂ pseudo-β ₁, β₂andβ ₁was examined using pure samples (99.9%). Induction time τ for newly occurring crystals in the melt phase was measured with a polarizing microscope equipped with a temperature-controlled growth cell. Rate of crystallization, 1/τ, was obtained for each polymorph of POP and SOS whose identification was done with X-ray diffraction and differential scanning calorimetry (DSC). Two modes of crystallization, melt-cooling and melt-mediation, yielded approximately the same results for POP and SOS: (a) The rates of crystallization were always higher in less stable than in more stable forms,β ₂only crystallized via a γ-melt mediation, butβ ₁did not occur by the melt crystallization; (b) the rate of meltmediated crystallization was always higher than the simple melt-cooling as examined at the same crystallization temperature; (c) the occurrence behavior of the polymorphs differed between the simple-cooling and meltmediation. The results were related to the solidification behavior of the polymorphs of cocoa butter. Presented at the AOCS Annual Meeting in Phoenix, Arizona in May 1988.     

Crystal packing of a homologous series β′-stable triacylglycerols

From a homologous series of C _nC_n+2C_n (n = even) triacylglycerols which are β′-stable, Guinier X-ray powder diffraction photographs were used to determine cell parameters and space groups. The powder diffraction patterns are consistent with a pseudo-orthorhombic unit cell with space group lc2a. Only one axis is changing as a function of chain lengthening. The experimental results were used to propose a β′-crystal packing for triacylglycerols. In contrast to earlier proposed β′-crystal structures, the acyl chains of this structure are not tilted with respect to the methyl-end group plane. Furthermore, with only one molecule in the asymmetric unit, overall orthogonal chain packing is obtained when the intramolecular acyl zigzag planes are parallel.     

Polymorphic transformation of 1,3-distearoyl-sn-2-linoleoyl-glycerol

Polymorphic transformation behavior of sub-α₁, sub-α₂, α, and γ in 1,3-distearoyl-sn-2-linoleoyl-glycerol (SLS) has been studied with X-ray diffraction, differential scanning caloremetry, and Fourier-transform infrared spectroscopy. Synchrotron radiation X-ray beam was employed to observe rapid transformation processes from the sub-α and α forms to the γ form. The chain length structures were double in sub-α₁, sub-α₂, and α, whereas γ was of triple chain-length structure. The subcell packing was pseudohexagonal for the two sub-α forms, hexagonal for the α form, and parallel type for the γ form. In comparison with 1,3-distearoyl-sn-2-oleoyl-glycerol (SOS), the occurrence behavior of sub-α, α, and γ of SLS was the same as that of SOS. However, the absence of β′ and β was unique for SLS. The chain-chain interactions between the linoleoyl moieties may stabilize the γ form, prohibiting the transformation into β′ and β forms. The presence of two cis double bonds may cause this stabilization, revealing the disordered chain conformation of the unsaturated chains.     

Polymorphism of pos. I. occurrence and polymorphic transformation

Polymorphic behavior of 1,3-rac-palmitoyl-stearoy 1-2-oleoylglycerol, 99.9% purity (POS) was examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), solubility measurements and optical microscopy in comparison with 1,3-dipalmitoyl-2-oleoylglycerol (POP) and 1,3-distearoyl-2-oleoylglycerol (SOS). Melt crystallization and solvent crystallization were examined for the occurrence of metastable and stable polymorphs. The number of independent polymorphs was four; α,δ, pseudoβ′ andβ. The lowest melting form, α, was identical to that commonly observed in POP and SOS lowest melting forms. As to the highest melting form,β, the XRD shortspacing pattern was identical toβ ₁ of POP and SOS. This is consistent with crystal habit:β single crystals of POS showed the same shape as those of β₁ of POP and SOS. However, the melting point ofβ (POS), 35.9°C, was lower than those ofβ ₁ of POP, 36.7°C, and of SOS, 43.0°C. Correspondingly, solubility ofβ of POS was lower than that of β₁ of POP below about 13°C, but higher above 13°C. POS did not possessβ ₂ , which is the second stable form in POP and SOS. Two forms of6 and pseudoβ′ occurred, the latter being more stable. The structural properties ofδ showed thatδ is not identical toγ previously observed in POP and SOS. Transformation behavior from the metastable to stable polymorphs of POS showed some differences from those of POP and SOS. Presented at the AOCS annual meeting in Cincinnati, Ohio, in May 1989.     

Molecular motion and transitions in solid tripalmitin measured by deuterium nuclear magnetic resonance

Deuterium nuclear magnetic resonance (NMR) quadrupole echo spectra of deuterated acyl chains and the glyceryl moiety of tripalmitin were found to depend on the crystal form. At 20°C, which is below melting points, line shapes indicate that molecular motions in theβ form (triclinic subcell) are more restricted than inβ′ (orthorhombic) orα (hexagonal). Motional rates in excess of about 20 kHz are responsible for the line shapes. Spin-lattice relaxation, sensitive to motional frequencies in the tens of megahertz range, is much faster for methyl (CD₂) groups inα orβ′ than inβ, indicating that fast motions are also governed by crystal form. General theoretical considerations suggest that motion of methylene groups is dynamically heterogeneous and that motion of methyl (CD₃) groups may be averaged by motions other than rotation about the terminal C-C bond. The isothermal solid-state transition fromα toβ, induced by increasing the temperature to 35°C, was accompanied by NMR lineshape changes consistent with immobilization. The reversible transition ofα to “sub-α” upon cooling, accompanied by orthorhombic-like Bragg spacings and other changes in the X-ray pattern and by corresponding changes in the infrared spectrum, also produced a marked restriction in NMR-detected mobility of the kind seen inβ′ relative toα. The advantages of²H NMR for studies of motions and transitions in solid glycerides are discussed. Presented at the 80th Annual Meeting of AOCS in Cincinnati, May 1989, and at the 81st Annual Meeting in Baltimore, May 1990.     

Isothermal crystallization of tripalmitin in sesame oil

Crystallization of tripalmitin (TP) in sesame oil was investigated under isothermal conditions at a cooling rate similar to the one achieved in industrial crystallizers (1 K/min). The results obtained indicated that, at TP concentrations <0.98%, triacylglycerides of sesame oil developed mixed crystals with TP. However, at concentrations within the interval of 0.98 to 3.44%, tripalmitin crystallized independently from sesame oil. Within this concentration interval, discontinuities were observed in the behavior of the induction time of TP crystallization (T _i) in sesame oil as evidenced by differential scanning calorimetry, polarized microscopy studies, and determination of the Avrami index (n). In general, the discontinuities in T _i were associated with different polymorph states developed by TP in sesame oil as a function of its concentration and crystallization temperature. Thus, TP crystals obtained at temperatures above 296 K with 1.80 and 2.62% TP solutions had n values close to 3 and developed lamellar-shaped crystals that are characteristic of β tripalmitin. In contrast, the crystals obtained at temperatures of 296 K and below with 1.80% and 2.62% TP solutions provided n values close to 3. Axialite-shaped β′ TP crystals were obtained under these conditions. For the 0.98% TP solution, simultaneous production of α and β′ crystals occurred below 291 K. However, at temperatures above 291 K, a crystallization process with n=3 was obtained, and it developed a different polymorph state, i.e., β, with lamellar-shaped TP crystals.     

Computer modeling ofα- toβ′- form phase transitions using theoretical triglyceride structures

Intermolecular energy calculations were performed on theoretical triarachidinα-form structures as selected bond rotations converted them intoβ′-forms with a chain-tilt change in the glycerol region. Interactions across the methyl gaps amounted to only 2-3% of the total energy in initialα- and finalβ′-forms, but computer generated energy profiles duringα- toβ′-phase transitions revealed highly repulsive regions due to the close approach of methyl groups. This methyl gap interaction, plus additional repulsive interactions in the lateral packing of molecules during rigid chain rotations, necessitated modification of certain chain movements during phase transition to reduce excessive repulsive energy. These results suggest that phase transitions proceed in a particular sequence of events that either distribute energy to promote further phase excitation or that lead to collapse into the stable polymorphic form. Phase transition energy curves also reveal that secondaryα- andβ′-forms are possible and are dependent on the startingα-forms, the direction of chain rotation and the subcell arrangement.     

Structural analogy between β′ triacylglycerols and n-alkanes. Toward the crystal structure of β′-2 p.p+2.p triacylglycerols

The structural elements of the β′-2 polymorph of 1,3-dilauroyl-2-myristoylglycerol as found by Birker et al. (J. Am. Oil Chem. Soc. 68:895–906, 1991) were also observed in the crystal structures of other long-chain compounds. This analogy led to the assembly of a β′-2 structure at the atomic level from known crystallographic data. The structure was optimized by molecular mechanics and was consistent with experimental data, including satisfactory reproduction of the X-ray powder pattern. To the best of our knowledge, this is the first β′ structure with a 1,2 configuration and an intramolecular orthorhombic subcell which is fully optimized by molecular mechanics to date. It shows all structural elements found earlier by Birker et al.     

The effect of supercooling on crystallization of cocoa butter-vegetable oil blends

The solid fat content (SFC), Avrami index (n), crystallization rate (z), fractal dimension (D), and the pre-exponential term [log(γ)] were determined in blends of cocoa butter (CB) with canola oil or soybean oil crystallized at temperatures (T _Cr) between 9.5 and 13.5°C. The relationship of these parameters with the elasticity (G′) and yield stress (σ^*) values of the crystallized blends was investigated, considering the equilibrium melting temperature (T _M ^o) and the supercooling (i.e., T _Cr ^o−T _M ^o) present in the blends. In general, supercooling was higher in the CB/soybean oil blend [T _M ^o=65.8°C (±3.0°C)] than in the CB/canola oil blend [T _M ^o=33.7°C (±4.9°C)]. Therefore, under similar T _Cr values, higher SFC and z values (P<0.05) were obtained with the CB/soybean oil blend. However, independent of T _Cr TAG followed a spherulitic crystal growth mechanism in both blends. Supercooling calculated with melting temperatures from DSC thermograms explained the SFC and z behavior just within each blend. However, supercooling calculated with T _M ^o explained both the SFC and z behavior within each blend and between the blends. Thus, independent of the blend used, SFC described the behavior of G′_eq and σ^* and pointed out the presence of two supercooling regions. In the lower supercooling region, G′_eq and σ^* decreased as SFC increased between 20 and 23%. In this region, the crystal network structures were formed by a mixture of small β′ crystals and large β crystals. In contrast, in the higher supercooling region (24 to 27% SFC), G′_eq and σ^* had a direct relationship with SFC, and the crystal network structure was formed mainly by small β′ crystals. However, we could not find a particular relationship that described the overall behavior of G′_eq and σ^* as a function of D and independent of the system investigated.     

13C cross-polarization and magic-angle spinning nuclear magnetic resonance of polymorphic forms of three triacylglycerols

The cross-polarization and magic-angle spinning nuclear magnetic resonance (CP/MAS-NMR) technique has been used to analyze the polymorphic forms of three triacylglycerols, 1,3-dipalmitoyl-2-oleoyl glycerol (POP), 1, 3-racpalmitoyl-stearoyl-2-oleoyl glycerol (POS), and 1,3-distearoyl-2-oleoyl glycerol (SOS). Specific attention has been paid to glycerol, carbonyl, olefinic, and methyl end carbon resonances. Many distinct differences were observed in each polymorphic form of SOS. In the α form, the saturated and unsaturated acyl chains exhibit liquid state-like conformations. However, olefinic conformations of the γ and β′ forms were asymmetric with respect to thecis double bond. Spectral difference between β₂ and β₁ was observed only for the methylene carbon, and not in the other regions. Spectra of corresponding polymorphic forms of POP and POS were almost identical to those of SOS. However, some spectral differences were observed in the glycerol and methyl regions of γ and β′. From the chemical shifts of the methylene carbons, the crystal structures of the polymorphic form have been discussed, particularly in terms of the subcell structures.