Crystallization and phase behavior of fatty acid esters of 1,3-propanediol

Chemistry and Physics of Lipids 149 (2007) 14–27

Crystallization and phase behavior of fatty acid esters of 1,3-propanediol I: Pure systems Madjid Abes, Suresh S. Narine ∗ Alberta Lipid Utilization Program, Department of Agricultural Food and Nutritional Science, 4-10 Agriculture/Forestry Centre, University of Alberta, Edmonton, Alberta, Canada T6G 2P5 Received 9 February 2007; received in revised form 6 May 2007; accepted 7 May 2007 Available online 16 May 2007

Abstract Six pure fatty acid esters of 1,3-propanediol (PADE) molecules were investigated. A careful analysis of XRD, DSC as well as SFC results has allowed the determination of their structure and phase behavior. Two ␤ polymorphs were observed for C10–C18 and three ␤ polymorphs for C8. The same first polymorph (␤1 ) was observed for all the samples. The second polymorph (␤2 ) observed for C12–C18 was different from the second ␤-form observed for C8 and C10. For all properties, the short chain length C8 and C10 samples were distinguished from the C12 to C18 samples and this explained much of the observed trends in behavior. Their lamellar packing was similar and has been explained by a simple addition of multiples of the length of a carbon bond to a primitive structure. The estimated long-range order highlighted a geometric effect that enabled the small chain molecules to better order than the longest molecules. The XRD results have been confirmed by DSC. The difference in property between the short and long chain molecules has also been clearly verified by the evolution of the energy of activation for nucleation as well as the enthalpy of melting and confirmed by microscopy measurements. For all the samples, the hardness which increased with increasing chain length is correlated with final %SFC. Avrami analysis of SFC versus time indicated heterogeneous nucleation and spherulitic crystal development from sporadic nuclei, and suggested that the rate of nucleation was higher for longer chain molecules. © 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: DSC; Polymorphism; Melts; Nucleation; Crystal growth

1. Introduction 1,3-Propanediol esters (PADE) consist of a propanediol moiety with each hydroxyl group esterified to a fatty acid at positions sn-1 and sn-3 (Fig. 1a). PADEs have not been the subject of as much extensive studies as their structurally similar counterparts, the 1,3-diacylglycerols (DAG) but are expected to have the similar potential for various high-value uses. The polymorphism and

∗ Corresponding author. Tel.: +1 780 492 9081; fax: +1 780 492 8855. E-mail address: [email protected] (S.S. Narine).

phase behavior as well as the crystallization kinetics of PADEs have never been reported in the literature, although this information is of significant importance to the effective utilization of these materials. Physical and chemical investigations of PADEs have been limited by the relative difficulty, and hence high cost, of their synthesis (Jacobson et al., 1987). The preparation and isolation of PADEs although not straightforward (Snyder, 1991; Adams and Bhatnagar, 1977) is easier than that of glycerol esters because the propanediol moiety has the advantage over glycerol of being symmetrical around the central carbon atom. While DAG conjugates that have been utilized to carry drugs such as adenosine analogues (Jacobson et al., 1987), AZT

0009-3084/$ – see front matter © 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.chemphyslip.2007.05.001

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ciable concentrations in glycerolipids, other than in a restricted range of commercial fats and oils. The evolution of melting and crystallization, polymorphism, solid fat content, microstructure, and hardness of the PADE molecules was investigated and reported as a function of chain length expressed as a number of carbon atoms in one fatty acid. 2. Experimental 2.1. Materials

Fig. 1. (a) 1,3-Propanediol ester structure. (b) General Tris conjugate structure.

(Steim et al., 1990), phenytoin (Scriba et al., 1995) and acyclouir (Welch et al., 1985) have enhanced biological properties, the chemistry to prepare them is complicated and is often not suitable for commercial scaleup. PADEs have been considered as drug carrying molecules which enhance the therapeutic index by increasing the lipophilicity of drugs (D’Alonzo et al., 1982; Wells et al., 1999; Davey et al., 2002; Adhikari et al., 2002). The methyl group of 1,3-propanediol can provide an ideal attachment site for drugs to provide efficient drug carrying conjugates (Fig. 1b). For example, 2-amino-2-(hydro-xymethyl)-1,3-propanediol, a chemically modified PADE known as Tris, has been used with or without linkers, as a carrier of active compounds (D’Alonzo et al., 1982). Initially, the technology was applied for the delivery of vaccines (Reilly et al., 1991) but it has been used to link various drugs to fatty acids (Whittaker et al., 1993). Similar to DAGs which have been used in cosmetic preparations to impart or improve properties such as adhesion and humidity absorption (Keiko and Masahiko, 1997), PADEs are also of interest for the cosmetic industry. The straight propanediol diester chains with high melting temperature have been already investigated and tried as agents for imparting iridescent luster to cosmetics (Wells et al., 1999). In this study, we report on the properties and phase behavior of six pure PADEs which have the same fatty acid molecule in both sn-1 and sn-3 positions (R1 = R3) covering fatty acid’s chain lengths from the C8 to C18 homologues. Note that, all the even-numbered saturated fatty acids from C2 to C30 have been found in nature, but only the C8–C18 are likely to be encountered in appre-

The six different pure PADE samples investigated in this study (1,3-propanediol distearate C39 H76 04 (SS or C18), 1,3-propanediol dipalmitate C35 H68 04 (PP or C16), 1,3-propanediol dimyristate C31 H60 04 (MM or C14), 1,3-propanediol dilaurate C27 H52 04 (LaLa or C12), 1,3-propanediol dicaprate C23 H44 04 (CiCi or C10) and 1,3-propanediol dicaprylate C19 H36 04 (CaCa or C8), were provided by Bunge Oils (Bradley, IL, USA) with a purity of 98.8, 95.3, 99.5, 98, 99.8 and 100%, respectively. The purity of the PADEs has been obtained using gas chromatography with flame ionization detection (GC-FID). In the following text, the PADEs will be simply referred to as C8, C10, etc. 2.2. Methods 2.2.1. Crystallization procedure The samples were melted at 90 ◦ C and held for 5 min to remove crystal memory and to ensure the absence of germ nuclei, then cooled down to a set holding temperature, Th . For DSC, XRD and microscopy, the holding temperature Th was chosen to allow for the crystallization events to complete in order to reveal all possible transitions and polymorphic types of the samples. Th was set to 3 ◦ C for C10–C18 and −20 ◦ C for C8. For the hardness and SFC measurements, Th was set to 20 ◦ C to mimic industrial processing protocols and practical uses. Apart from DSC measurements where the samples were crystallized using five different cooling rates (0.1, 3, 5, 10.0 and 20.0 ◦ C/min), crystallization was carried out at 3 ◦ C/min cooling rate in a “Linkam LTS 350” temperature-controlled stage (Linkam Scientific Instruments, Tadworth, Surrey, UK). 2.2.2. XRD measurements “Bruker AXS X-ray diffractometer” equipped with a filtered Cu K␣ radiation (λ = 0.154059 nm) was used for XRD analysis. The procedure was automated and commanded by Bruker AXS’s “General Area Detector Diffraction System” (GADDs V 4.1.08) software. The

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M. Abes, S.S. Narine / Chemistry and Physics of Lipids 149 (2007) 14–27

XRD samples were prepared by filling quartz capillary tubes with the molten sample. The tube was fitted with the XRD sample holder and processed in the Linkam as described above. After reaching Th , the sample was held isothermally for 5 min then quickly transferred to the XRD stage where the temperature was already set up and maintained at Th ± 0.5 ◦ C by an air jet cooling system (Kinetics-Thermal Systems, USA) for analysis. The XRD frames, obtained after 450 s exposure, were processed using GADDS software. The spectra were analyzed using Bruker AXS’s “Topas V 2.1” software. 2.2.3. DSC measurements DSCQ100 model (TA Instruments New Castle, DA) was used to monitor the melting and crystallization behavior of the samples. The calibration of the equipment for temperature and for enthalpy was performed using indium (onset temperature for melting = 429.8 K, H for melting = 28.45 J/g). Samples of approximately 5–10 mg were hermetically sealed in an aluminum pan and the experiments were performed under a nitrogen flow of 50 mL/min. An empty aluminum pan was used as a reference. The crystallization curve was obtained by processing the sample following the crystallization procedure detailed above and a subsequent first melting curve was obtained by heating up the sample to 90 ◦ C at 5 ◦ C/min. The sample was held at this temperature for 5 min and crystallized again by cooling it down to Th at the same constant rate as the first crystallization. All curves were normalized to a uniform sample mass of 15 mg. The data was analyzed using the “TA Universal Analysis” software coupled with a method developed by our group (Bouzidi et al., 2005). 2.2.4. SFC measurements A Bruker Minispec mq 20 pulse nuclear magnetic resonance spectrometer with a retro-fitted temperaturecontrolled chamber was used to monitor SFC evolution. Thermal processing in the NMR chamber was controlled and maintained by water baths connected to it. For a detailed description of the system, the reader is referred to Narine and Humphrey (2004). The cooling rate was developed using canola oil in an NMR tube filled to the same level as the sample. The sample was heated to 90 ◦ C, held at this temperature for 5 min and then cooled down to 67 ◦ C outside of the machine. At this temperature, the sample was quickly transferred to the NMR chamber where it was cooled down to 20 ◦ C with a constant cooling rate of 3 ◦ C/min. %SFC was recorded every 10 s. As a consequence of the imposed cooling rate, a calibration curve of apparent SFC versus real SFC was plotted and used to correct the data. The plateau

region and final %SFC were unambiguously determined using the first and second derivatives of %SFC curve with respect to time. 2.2.5. Hardness measurements The methodology and parameters used were chosen according to an exhaustive study of hardness measurements of lipid samples and optimization of the measurement parameters carried out in our laboratory (Boodhoo et al., 2007). Each sample was melted at 90 ◦ C and stirred for 2 min using a motorized mechanical stirrer before pipetting approximately 300 mg of the molten sample into aluminum DSC pans. The sample was cooled down from 90 to 20 ◦ C at 3 ◦ C/min rate in the Linkam then quickly transferred to a temperaturecontrolled chamber fitted to a “TA XT.plus Structure Analyzer” (Stable Microsystems, Surrey, UK) for hardness measurements. The texture analyzer was fitted with a 1.0 kg load cell. The probe was an 8◦ angle truncated cone with tip diameter of 0.14–0.16 mm (ASTM D 1321-65)). The hardness measurements were carried out immediately after processing, using a penetration depth of 1.5 mm and speed of 0.5 mm/s. Sample penetration and data acquisition were controlled by the Texture Exponent 32 software. Force versus displacement was recorded and the maximum force determined and plotted as a function of chain length. 2.2.6. Microscopy measurements The microstructure was investigated using a Leica DMRX polarized light microscope (Leica Microsystems, Wetzlar, Germany) fitted with a Hamamatsu digital camera (C4742-95) and a “Linkam LTS 350” (Linkam Scientific Instruments) temperature-controlled stage. The microscope/camera assembly was controlled by Impovision’s “Openlab 4.02” software (Improvision, Coventry, UK). The sample was heated to 90 ◦ C, held there for 5 min, then cooled at a rate of 3 ◦ C/min down to Th where they were held for 60 min. Images were then obtained at a magnification of 50×. For hardness measurements, the reported values are averages of 10 replicates and for all other measurements, the values are averages of 3 replicates. The reported errors are the subsequent standard deviations. 3. Results and discussion 3.1. XRD results 3.1.1. Long spacings Fig. 2 shows the XRD spectra of the pure PADE samples. The bottom line represents the C18 XRD line

M. Abes, S.S. Narine / Chemistry and Physics of Lipids 149 (2007) 14–27

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Fig. 2. XRD spectra of pure PADE samples.

and each subsequent curve represents PADEs with progressively decreasing chain length, with the C8 being the uppermost curve. The 2θ range is divided into two regions: the small angle region where 2θ ≤ 18◦ (also called long spacing region) and the wide angles region with 18◦ ≤ 2θ ≤ 30◦ (also called short spacing region). For C8 in the long spacing region, there appear a series (series P) of three distinct diffraction peaks (labeled P1 , P2 , and P3 ). For C10, C12, C14, C16 and C18 another series (series P ) of three distinct diffraction peaks (labeled P1 , P2 and P3 ) showed up along with series P. P1 appeared at a position which corresponds to a d1 -spacing double that of the d2 -spacing of its harmonic P2 and triple that of the d3 -spacing of its harmonic P3 giving ratios of d-spacing of d1 :d2 :d3 = 1:1/2:1/3. Similarly d1 , d2 and d3 d-spacing, corresponding to the diffraction peaks P1 , P2 and P3 , respectively, had ratios of d1 : d2 : d3 = 1 : 1/2 : 1/3. The series P and the series P obviously represent two different plane families, characteristic of two lamellar periodicities. The long d-spacing varied linearly with chain length (Fig. 3a). One can note that as expected, the slope of d2 is double that of d3 and the slope of d2 is double that of d3 . The linear increase of the d-spacing indicated

Fig. 3. (a) Variation of d2 , d2 , d3 and d3 long spacing as a function of carbon chain length. The solid lines passing through experimental data are linear fits. (b) Stacking mode of PADE samples. d1 and d1 represent the parallel and perpendicular lamellar periodicities and θ is the angle of tilt. (c) Variation of the order parameter as a function of carbon chain length.

that the packing of all the PADEs was similar. It can be explained by a simple addition of multiples of the length of a carbon bond to a primitive structure. The lamellar packing can be represented by parallel and perpendicular

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M. Abes, S.S. Narine / Chemistry and Physics of Lipids 149 (2007) 14–27

Table 1 Periodicities and angles of tilt for the pure PADE samples PADE

˚ d1 (A)

C8 C10 C12 C14 C16 C18

23.7 28.3 31.3 35.5 39.5 41.9

± ± ± ± ± ±

˚ ) d1 (A 1.2 0.9 1.2 1.3 1.4 0.9

23.7 24.8 27.5 30.4 32.6 34.9

± ± ± ± ± ±

obviously a geometric effect which enabled the small chain molecules to have sufficient room (and time) to better order than the longest molecules.

θ (◦ C) 1.2 0.8 0.9 0.7 0.9 0.7

45 48.8 48.7 49.3 50.4 50.3

± ± ± ± ± ±

0.0 2.7 1.7 1.6 1.4 2.1

periodicities d1 and d1 , and an angle of tilt θ as shown in Fig. 3b. The periodicities and the angle of tilt for all the samples are listed in Table 1. The distance ac–c between two carbon atoms in the fatty acid chains can be deduced from the difference u between d parameters of two adjacent PADE samples. Using d2 for example, u was found equal to ˚ When a carbon atom is added to each 1.23 ± 0.04 A. fatty acid chain, the parallel length of the structure is ˚ increased by u = 2ac–c cos θ giving ac–c = 1.43 ± 0.04 A, a value very close to the distance between two carbon ˚ for a saturated bond (Harrison and Walter, atoms (1.42 A) 1980).

3.1.3. Short spacings In the short spacing range, C18, C16, C14 and C12 XRD spectra have similar shapes with six wellresolved peaks, each originating from the same family of plane. The angular first structure (18–22◦ ) presented two resolved peaks and the second (22–25◦ ) presented four (C12–C18) or more resolved peaks (C8 and C10). The corresponding short d-spacing (shown in Fig. 4a) revealed the presence of two subcells both in the ␤form. The crystal forms have more or less the same triclinic subcells and differ primarily in the angle of tilt of the molecules. The polymorph (labeled ␤1 ) is

3.1.2. Long-range order of the PADE samples The diffracted intensity was estimated in order to have access to the long-range-order state. The longrange-order parameter η, defined as the concentration difference between two (0 0 1) or (1 0 0) successive planes (Warren, 1990), has been calculated using the relative intensity of P2 (I2 ) and the relative intensity of P3 (I3 ). The relative intensities of the peaks have been obtained from the integrated 2θ-curves corrected by the usual polarization and Lorentz effects. A normalized order parameter, η/ηmax , is simply expressed by the following relation (Warren, 1990):  I2 η = (1a) ηmax I3 where ηmax is the long-range-order parameter for the C8 sample; the most ordered pure PADE. Notice that the relative intensity I2 of P2 and the relative intensity I3 of P3 can also be used:  I2 η = (1b) ηmax I3 The results are shown in Fig. 3c for both periodicities. As can be seen, the order parameter decreases sharply with increasing chain length, underlining the increasing difficulty of the molecule to order. This is

Fig. 4. (a) Variation of short spacing as a function of carbon chain length. (b) Lamellar periodicities of the ␤1 and ␤2 subforms. (c) Triclinic subcell.

M. Abes, S.S. Narine / Chemistry and Physics of Lipids 149 (2007) 14–27

characterized by a strong lattice spacing line at near ˚ (line (1) d␤ in Fig. 4a) and by strong lattice spacing 4.7 A 1 ˚ ((2) d␤ and (3) d␤ ) is present lines between 3.6 and 4.0 A 1 1 for all the samples. The second polymorph (labeled ␤2 ) is characterized by a strong lattice spacing line at near ˚ (line (1) d␤ in Fig. 4a) and by strong lattice spac4.5 A 2 ˚ ((2) d␤ and (3) d␤ ), and ing lines between 3.6 and 4.0 A 2 2 is present for C12–C18. C10 presented a second short d-spacing off the linear trend hinting to a ␤-form which is different from ␤2 . C8 presented three ␤-subcells. The polymorphs observed for C12–C18 were different compared with C8–C10. The same first polymorph (␤1 ) was observed for all the samples. The second polymorph (␤2 ) observed for C12–C18 was different from the second ␤form observed for C8 and C10. This is also illustrated in the variation of different ␤-short d-spacing (Fig. 4a). The linear trend is obviously disturbed for the second polymorph in the C10 and C8 samples. The presence of different ␤-forms is common in TAG solutions. Precht and Frede (1983) pointed out that the ␤-modification of saturated mono-acid TAGs C12–C1 8 can occur in two different chain packings (denoted ␤III and ␤IV -forms). These forms are determined by the architecture of the methyl terrace which is reflected in the angle of tilt. De Jong and Van Soest (1978) found that there are two or three solutions for the crystal structure of each ␤ phase TAG. They showed that each modification (their A and E forms correspond to the ␤III and ␤IV -forms of Precht and Frede) is associated with three series of TAG and each TAG may pack in at least two different ways. The A and E forms have an angle of tilt of θ = 52◦ and 60◦ , respectively. Similar results have been reported by Jensen and Mabis (1966) for TAG C10 and by Lutton and Stewart (1970) for TAG C8. For the pure PADE samples, only one angle of tilt was revealed by XRD measurements. This is not surprising since only one architecture of the methyl terrace exists for them. For each form, two monolayers are situated on the same plane with their propanediol groups facing each other at a distance which is the same for all pure PADE samples. The difference between ␤1 and ␤2 polymorphs results from the shifts of a double molecule in crystal network as shown in Fig. 4b. Fig. 4b shows two possible configurations of the PADE crystal network. In fact, the crystal network is a set of pure PADE bilayers generated by two set of discrete translation operations similar to the two possible configurations found by Precht for the ␤IV -form (Precht and Frede, 1983). Each set of discrete operations of translation give a crystal subnetwork which explained the difference found in the values of lattice parameters of the ␤1 and ␤2 -subcells.

19

The thickness (K) of the crystal layers in the pure PADE samples for both submodifications (␤1 and ␤2 ) varied linearly with chain length, N (K = 1.937N + 8.532). Compared to the thickness of the submodifications of ␤IV in pure TAG systems studied by Precht and Frede (1983) (K = 2.047N + 3.567), there ˚ between the distances of adjacent is a difference of 5 A molecule layers of TAG systems and those of PADE systems which can be simply explained by the addition of two H–C covalent bond and the distance which separate two H atoms linked by van der Vaals forces. The three most pronounced X-ray diffraction lines in the short-spacing originated from the triclinic subcell reflections 1 0 0, 0 1 0 and 1¯ 1 0 (Precht and Frede, 1983). The three shortest spacings (see Fig. 4c) are linked by the following relation: d1¯2 1 0 = d12 0 0 + d02 1 0 + 2d1 0 0 d0 1 0 cos(γs )

(2)

where γs = (as , bs ) is the angle between the vectors of the triclinic subcell as and bs . In our case, (1) d␤1 = d0 1 0 , (2) d␤1 = d1¯ 1 0 and (3) d␤1 = d1 0 0 for the ␤1 submodification, and (1) d␤2 = d0 1 0 , (2) d␤2 = d1¯ 1 0 , (3) d␤1 and (3) d␤2 = d1 0 0 for the ␤2 submodification. Eq. (2) yielded values of γs␤1 = (125.6 ± 0.1)◦ and γs␤2 = (126.6 ± 0.1)◦ for the ␤1 and ␤2 -subcells, respectively. Note that the difference between the angle of the ␤1 subcell and that of the ␤2 -subcell is very small (1◦ ). It is also interesting to note that these angles are exactly the same for each PADE regardless of the chain length. These values are relatively close to the values found in the literature for the saturated mono-acid TAG (Precht and Frede, 1983; De Jong and Van Soest, 1978). For example, Precht has found an angle γ s = 122.5◦ for the C18 TAG which is very close to γs␤1 and γs␤2 values found in this study. 3.2. DSC results 3.2.1. Crystallization A single crystallization peak is observed for all the samples as shown in Fig. 5a representing stacked crystallization curves of the samples cooled at 3 ◦ C/min. It is not surprising to observe a single crystallization peak as the polymorphs demonstrated by the sample were very similar. As can be seen, very intense heat (heat flow of about 3 W/g was recorded during crystallization) was released during crystallization resulting in skewed crystallization peaks. This impeded accurate determination of the crystallization parameters, particularly the full width at half maximum (FHWM), the height and the enthalpy of crystallization. Start temperature (TC-start )

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perature at which the recorded heat flow was the highest could be considered as a reasonably good approximation for the crystallization temperature TC . However, TC is not used as it is unreliable. The onset temperature of crystallization TOC (defined by the intersection of the steepest tangent from DSC peak maximum to turning point and the baseline), as estimated using the TA software, was rather very close to TC . TOC (see Fig. 5b for the cooling rate of 3 ◦ C/min) increased with increasing chain length, spanning from approximately −3 ◦ C for C8 to +58 ◦ C for C18. Note that C10 and C8 samples were still liquid at room temperature and all other samples were solid at room temperature. For all the cooling rates, the variation of TOC with chain length fits very well exponentials of the form:   n T = T0 + Tf exp − (3) n0

Fig. 5. (a) Crystallization curves obtained with a cooling rate of 3 ◦ C/min. (b) Variation of onset of crystallization temperature (TOC ) with chain length. (c) Variation of start (TC-start ), end (TC-end ) temperatures and TC = TC-start − TC-end vs. carbon chain length. The dashed lines passing through experimental data are fits to exponentials of the form: y = y0 + A e−N/N0 . The solid lines passing through experimental data are linear fits.

and end temperature (TC-end ) (TC-start and TC-end correspond to the points where the signal starts to depart from the baseline) were however well defined and permitted a good estimation of the span of the thermal event. Considering the narrow span of the crystallization and the fact that the peak did not stretch out outside the span, the tem-

where Tf is the temperature that an infinitely long chain would have (final temperature of fit) and n0 is a characteristic number of carbon atoms. The fitted values for TOC are T0 = 78.0 ± 3.8; Tf = 240.5 ± 13.9; n0 = 7.3 ± 0.6. Fig. 5c shows TC-start , TC-end and the span of crystallization (TC = TC-start − TC-end ) versus chain length for the cooling rate of 3 ◦ C/min. For all cooling rates, TC-start and TC-end increased with increasing chain length n, and fit very well exponential functions of the same form as in Eq. (3). TC increased linearly with increasing chain length. This supports the assertion made earlier on the basis of XRD data, that longer chain length PADEs were demonstrably more difficult to pack into regular crystalline lattices, resulting in correspondingly longer crystallization events. Crystallization, onset, start and end temperatures for each sample decreased exponentially with increasing cooling rate (φ) as illustrated in Fig. 6a for C18. The span of crystallization (TC ) for each pure PADE sample increased linearly with increasing cooling rate (Fig. 6b). The rate at which this increase occurred (i.e. the slope of TC (φ)) well fitted with a sigmoidal function (Fig. 6c), showed two plateau regions (C8–C10) and (C14–C18). The decrease of TC (mirrored by TOC and TC-start ) compared to the decrease in TC-end is relatively small (Fig. 6a). This is of course mirrored in the linear increase of TC as a function of cooling rate (Fig. 6b). This suggests that while an increase in cooling rate and its concomitant increase in supercooling would result in modest speeding up of the beginning of crystallization, that the resulting increases in viscosity would limit significantly the mass transfer of the sample, so as to delay the crystallization event due to mass transfer limitations to ordering of the molecules. As ascertained earlier, the inertial mass

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exposure parameter β, by: J Jmax

= k e−k

√ β

(4)

This function is often used to model diffusional processes which take place with constant probability per unit time. J/Jmax can also be related to the cooling rate (φ) by: J Jmax

Fig. 6. Variation of (a) onset, maximum, end and start temperatures with cooling rate for C18. (b) Span of crystallization TC and for each pure sample. (c) Slope of linear fits (from Fig. 5c) with chain length.

of the sample is related to its ease of ordering, so that the smaller chain lengths C8 and C10 samples are clearly less affected by the mass transfer limitations as on the C14–C18 chain lengths, leading to the stepwide relationship shown in Fig. 6c. It is clear that the difference in polymorphic state between C8 and C10 compared with C12–C18 form explains much of the observed trends in behavior. 3.2.2. Determination of energy of activation for nucleation The energy of activation for nucleation was estimated using an approach developed by Marangoni et al. (2006a,b). In this model, the normalized nucleation rate (J/Jmax ) of a fat system is related to a supercooling-time

√ φ

= k e−X/

(5)

where k is a constant and β = 1/2(TCM )2 /φ with TCM = TM − TC-start being the difference between the melting (TM ) and start (TC-start ) temperatures. X is a factor defined as Qm = ZX J/g where Qm√is the energy of activation for nucleation and Z = Cp 2/k with Cp the specific heat. This model was justified using statistical considerations for the nucleation process (Marangoni et al., 2006a). Using microscopy measurements results, Marangoni recently showed that the nucleation events take place with a constant probability per unit supercooling-time exposure (β) (Marangoni et al., 2006b). The parameter β is directly linked to the supercooling and to the induction time under nonisothermal conditions. Cp was determined for each PADE sample from heat capacity measurements using modulated DSC. The nucleation rate was estimated from the inverse of the induction time of nucleation (ts ) (Marangoni et al., 2006a). Table 2 lists the following parameters (TM , TC-start and ts ). The nucleation rate defined by J = φ/TCM was determined from DSC measurements. Variation and √ fit of the normalized nucleation rate J/Jmax versus β and its variation and fit as a func√ tion of 1/ φ for each sample, are shown in Fig. 7a and b, respectively. The resulting series of exponential fits to the experimental data in Fig. 7a and b were excellent. Table 3 lists the results (k, Z, X and Qm ) of the fits. The values of the energy of activation for the nucleation process in pure PADE samples are close to those reported by Marangoni for palm oil and milkfat. It is interesting to note that the variation of the energy of activation for nucleation vary linearly with increasing chain length up to n = 14 then plateaus for longer chains (Fig. 7c). 3.2.3. Melting The cooling rate had no effect on the subsequent melting curves. Stacked melting curves for the different samples are represented in Fig. 8a with a cooling rate of 3 ◦ C/min. A single melting peak is observed for

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M. Abes, S.S. Narine / Chemistry and Physics of Lipids 149 (2007) 14–27 Table 2 Melting (TM ) and crystallization (TC-start ) temperatures, and induction times (tc ) of the pure PADE samples Cooling rate (◦ C/min)

TM (◦ )

C8

0.1 3 5 10 20

4.0 6.3 7.5 9.3 10.1

± ± ± ± ±

0.2 0.3 0.4 0.5 0.6

1.8 −2.0 −2.8 −3.8 −4.8

± ± ± ± ±

0.2 0.8 1.1 1.0 1.4

1320 166 124 79 45

± ± ± ± ±

188 22 18 9 6

C10

0.1 3 5 10 20

23.1 25.3 25.7 26.6 27.1

± ± ± ± ±

0.1 0.3 0.4 0.3 0.3

20.0 18.6 18.2 17.7 14.0

± ± ± ± ±

0.2 0.4 0.5 0.5 0.6

1860 135 89 53 30

± ± ± ± ±

159 14 11 5 2

C12

0.1 3 5 10 20

36.7 38.9 39.0 40.8 40.8

± ± ± ± ±

0.1 0.2 0.3 0.4 0.3

33.0 31.5 30.6 31.0 28.8

± ± ± ± ±

0.2 0.9 0.6 0.6 0.4

2220 148 101 59 36

± ± ± ± ±

137 22 9 6 2

C14

0.1 3 5 10 20

44.3 44.0 43.0 42.0 41.6

± ± ± ± ±

0.3 0.3 0.3 0.3 0.3

47 50.2 50.7 52.5 53.2

± ± ± ± ±

0.1 0.4 0.3 0.5 0.4

1620 125 92 63 35

± ± ± ± ±

198 14 7 5 2

C16

0.1 3 5 10 20

55.2 56.3 56.8 57.0 57.8

± ± ± ± ±

0.2 0.3 0.3 0.2 0.3

52.5 52.4 51.8 51.0 40.8

± ± ± ± ±

0.1 0.2 0.4 0.4 0.7

1308 77 60 36 21

± ± ± ± ±

156 10 8 4 3

C18

0.1 3 5 10 20

61.5 62.4 63.1 65.4 65.8

± ± ± ± ±

0.2 0.3 0.4 0.5 0.4

58.2 58.0 57.5 57.2 57.1

± ± ± ± ±

0.1 0.3 0.3 0.2 0.3

1980 89 63 49 26

± ± ± ± ±

180 12 8 4 2

PADE

TC-start (◦ )

TC (s)

all the samples and as evidenced by XRD was attributed to the melting of the ␤ polymorphs. Fig. 8b shows start (TM-start ) and end (TM-end ) temperatures and span of melt (TM = TM-start − TM-end ) and Fig. 8c shows the variation of maximum (TM ) and onset (TOM ) temperatures of melt versus chain length. As can be seen, the span of melt is practically the same for all

Fig. 7. (a) Variation of the normalized nucleation rate J/Jmax vs. the supercooling-time exposure (β). (b) Variation of the normalized nucleation rate J/Jmax vs. the inverse square root of cooling rate. (c) Variation of the activation energy for nucleation with carbon chain length.

Table 3 Exponential constants (k, Z and X) and energy of activation for nucleation (Qm ) of the pure PADE samples PADE

k (K−1/2 s−1/2 )

C8 C10 C12 C14 C16 C18

0.14 0.19 0.18 0.20 0.23 0.30

± ± ± ± ± ±

0.03 0.02 0.01 0.02 0.03 0.04

Z (J g−1 K−1/2 s1/2 ) 16.7 16.1 17.3 17.2 16.3 15.7

± ± ± ± ± ±

1.5 1.1 0.7 0.9 1.0 1.3

X 5.0 5.2 4.7 4.7 4.6 4.3

Qm (kJ/mol) ± ± ± ± ± ±

0.8 0.7 0.4 0.9 0.7 0.7

27 32 36 40 41 41

± ± ± ± ± ±

7 6 5 6 5 3

M. Abes, S.S. Narine / Chemistry and Physics of Lipids 149 (2007) 14–27

23

Fig. 8. (a) DSC melting curves of the pure PADE samples. (b) Variation of start (TM-start ), end (TM-end ) temperatures, and span of melt (TM = TM-start − TM-end ) vs. chain length. (c) Variation of maximum (TM ) and onset (TOM ) temperatures vs. chain length. (d) Variation of the enthalpy of melting vs. chain length.

samples and is equal to 20 ◦ C. The melting, onset, start and end temperatures fit very well exponential functions of the form reported in Eq. (3). Fig. 7d shows the enthalpy of melting versus chain length. The melting enthalpy increased with increasing chain length. Note that the curve of melting enthalpy is given by two linear segments of different slopes intersecting at the chain length of n = 12 as shown in Fig. 7d. The melting enthalpy depends on the sum of two contributions given by the following equation (Himawan et al., 2006; Wesdorp, 1990). Hm (n) = pn + po

(6)

where n is the chain length and p and po are the constants. The first contribution is linked to the hydrocarbon chains and varies linearly with the chain length. This contribution depends only on the way hydrocarbon chains are packed, and therefore on the polymorphic form. The second is a contribution of the end and head groups (independent of chain length) and is specific to each class of lipid. Thus, a linear increase of melting enthalpy is expected for chain length of n = 12–18 as the polymorphic form is the same for this chain length range as shown in Fig. 7d. The values of p = 7.10 ± 0.5 and po = −5.4 ± 5.1 were found for the second segment (the C12–C18 range). The first segment was also well fitted by a line having a slope p = 15.1 ± 1.2 and a constant

po = 104 ± 12 (Fig. 7d). The difference between the two sets is understandable since the C8 and C10 samples have different polymorphs from the other samples. 3.3. SFC results The evolution of solid fat content (%SFC) has been measured for C12, C14, C16 and C18 samples and is shown in Fig. 9. C8 and C10 samples have not been measured because they presented a liquid phase at room temperature. The final %SFC increased with increasing chain length (see Table 4). The shapes of SFC curves are a good source of valuable information on the crystallization mechanism of a fat component (Sharples, 1966). The experimentally determined SFC versus time curves did not demonstrate a monotonic increase. Clearly there are evident disruptions and significant changes occurring in growth kinetics suggesting changes in the growth mode of the crystals, in the thermodynamic driving forces, or limitations to molecular transport. There are three obvious crystallization segments for each sample. Because of that, a fit by conventional Avrami sigmoids (not shown) was notably poor for all data sets. A modified form of the Avrami model (Narine et al., 2006) that takes into consideration the variances within the growth curve has been used to fit the SFC versus time

24

M. Abes, S.S. Narine / Chemistry and Physics of Lipids 149 (2007) 14–27

Fig. 9. %SFC vs. time plots and nonlinear fits of experimental data with identified line segments using Eq. (6) (solid lines). (a) C12; (b) C14; (c) C16; (d) C18. Table 4 %SFC and Avrami exponents determined using a modified form of the Avrami model of the pure PADE samples PADE

C12

C14

C16

C18

%SFC m1 A1 × 10−16 (s) r12 m2 A2 × 10−16 (s) r22 m3 A3 × 10−16 (s) r32

94.0 ± 0.1 6.30 ± 0.05 5.7 ± 0.3 0.9972 8.33 ± 0.07 5.3 ± 0.4 0.9983 1.13 ± 0.11 13 ± 3 0.9712

94.3 ± 0.1 6.16 ± 0.04 7.4 ± 0.3 0.9989 9.36 ± 0.20 7.5 ± 0.2 0.9985 1.66 ± 0.22 14 ± 1 0.9512

95.8 ± 0.2 4.95 ± 0.04 9.3 ± 0.4 0.9988 8.02 ± 0.05 8.4 ± 0.4 0.9984 1.77 ± 0.12 15 ± 4 0.9605

96.1 ± 0.2 4.00 ± 0.05 8.8 ± 0.7 0.9976 6.45 ± 0.05 9.9 ± 0.5 0.9990 0.68 ± 0.09 7±1 0.9942

data. In this model, the crystallization of a lipid system is regarded as a succession of p different crystallization events, occurring in steps with different incubation times τ i . Each step i (i = 1, 2, . . ., p) is characterized by a constant growth rate Gi and is described by an Avrami equation: Fi (t) = Fi∞ (1 − exp[−Ai (t − τi )mi ])

(7)

where Fi (t) is the absolute crystallinity at time t, Fi∞ is the crystallinity at some time when either the growth rate or the nucleation conditions change, and Ai and mi are the Avrami constant and exponent applicable to the nucleation, growth, and dimensionality of the crystallizing lipid over that segment of time. The total absolute crystallinity is the sum of the p individual absolute crys-

Fig. 10. Variation of hardness of pure PADE samples with carbon chain length.

M. Abes, S.S. Narine / Chemistry and Physics of Lipids 149 (2007) 14–27

tallinities (Narine et al., 2006): F (t) =

p 

Fi (t)

(8)

i=1

The number of crystallization segments and the values of incubation times have been determined empirically using ln[−ln(1 − F) versus ln(t) plots of experimentally determined SFC versus time data. The details of the procedure can be found in Narine et al. (2006) publication.

25

The resulting series of sigmoids provided exceptional fits to the experimental data as shown in Fig. 9. Results of the fits are listed in Table 4. The m1 and m2 values were above 4 for all PADE samples. The m of 4 or above for the PADE samples suggests heterogeneous nucleation and spherulitic crystal development from sporadic nuclei (Avrami, 1940). These values suggest that addition of carbons to the acid chain did not change the nucleation and growth mechanisms of the samples. However, it has been suggested

Fig. 11. Microstructure of pure PADE samples obtained at 50× magnification (bar = 50 ␮m).

26

M. Abes, S.S. Narine / Chemistry and Physics of Lipids 149 (2007) 14–27

that m larger than 4 indicate that growth rate increases with time (Christian, 1975; Doremus, 1985). The effect of chain length on the growth rate of PADEs is significant. The Avrami exponents decrease as the chain length increases (Table 4). This might be attributed to the time needed for the ordering of the methyl chains the longer the chain, the longer the ordering time. This is corroborated by the XRD results which have shown that the order parameter decreases with increasing chain length (Fig. 2b) and which was explained by the difficulty for the large chain molecules to reach maximum order. A1 and A2 values increased with increasing chain length. In general, as the m values decreased, A values increased. Such increase in Avrami constant indicates an increase in nucleation rate (Sharples, 1966). It is therefore probable that in the case of PADE samples that the rate of nucleation increases with increasing chain length (it would be the lowest for C8 and the highest for C18). 3.4. Hardness results Fig. 10 shows maximum force versus chain length. The C8 and C10 samples have not been measured since these samples presented a liquid phase at room temperature. The hardness generally increases with increasing chain length. This is correlated with the %SFC which also increased with increasing chain length (Table 4). 3.5. Microscopy results The microstructure of the pure PADE samples is shown in Fig. 11 at 5× magnification. C10–C18 micrographs were taken at 3 ◦ C and C8 micrographs were taken at −20 ◦ C. All the samples demonstrated a spherulitic growth. Note that this has been predicted by the modified Avrami model. The microstructure of the C8 and C10 samples were however notably different from the other samples. This related to their differences in the polymorphic forms and could also explain the evolution of melting enthalpy (Fig. 7d) where the two groups (C8, C10) and (C12, C14, C16 and C18) were clearly distinguished. The structures of the C8 and C10 samples consisted of birefringent (crystalline) arms which are separated by plainly visible layers of uncrystallized melt. These interfibrillar layers which appeared dark because of optical extinction remained uncrystallized even after the sample has been cooled for 1 h at −20 ◦ C. The spherulites observed in Fig. 10 for the C12, C14, C16 and C18 samples had finer textures and appeared more compact and more uniform. An observation which we recognize from familiarity with spherulites structures indicated more profuse branching. The radial smooth-

ness of the growth was gradually lost with increasing chain length except for C18 sample where one can observe a smooth radial and flat growth with very clean and straight grains boundaries. 4. Conclusion A careful analysis of XRD, DSC as well as SFC results of six pure fatty acid esters of 1,3-propanediol molecules having chain lengths ranging from C8 to C18 has allowed the determination of their structure and phase behavior. The PADEs presented two or three different but close ␤ polymorphs, the difference resulting only from the shifts of a double molecule in the crystal network. Two ␤ polymorphs were observed for C10–C18 and three ␤ polymorphs for C8. The polymorphs observed for C12–C18 were different compared with C8–C10. The same first polymorph (␤1 ) was observed for all the samples. The second polymorph (␤2 ) observed for C12–C18 was different from the second ␤form observed for C8 and C10. Their lamellar packing was similar and has been explained by a simple addition of multiples of the length of a carbon bond to a ˚ for the distance primitive structure. A value of 1.43 A between two carbon atoms has been calculated based on long spacing data, and agrees well with literature values. The long-range order as estimated using the diffracted intensities highlighted a geometric effect that enabled the small chain molecules to better order than the longest molecules. This has been confirmed by DSC results that indicated that the increases in viscosity imposed significant mass transfer limitations to ordering of the molecules which have led to delay the crystallization event. The difference in polymorphic form of C8 and C10 compared with C12–C18 explains much of the observed trends in behavior. The smaller chain length C8 and C10 samples were clearly less affected by the mass transfer limitations than the C14–C18 chain length samples. The energy of activation for nucleation as determined using DSC data, varied linearly with increasing chain length up to C14 then plateaued for longer chains. Avrami analysis of SFC versus time suggested heterogeneous nucleation and spherulitic crystal development from sporadic nuclei and that the rate of nucleation increases with increasing chain length. The enthalpy of melting showed two different linear segments intersecting at C12 and was related to the way hydrocarbon chains are packed, and therefore to the polymorphic form. This difference has been also clearly observed by microscopy. C8 and C10 showed birefringent (crystalline) arms separated by plainly visible layers of uncrystallized melt and C12, C14, C16 and C18 showed finer textures and appeared more compact.

M. Abes, S.S. Narine / Chemistry and Physics of Lipids 149 (2007) 14–27

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