Lubricating and waxy esters, I. Synthesis, crystallization, and melt behavior of linear monoesters

Chemistry and Physics of Lipids 165 (2012) 38–50

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Lubricating and waxy esters, I. Synthesis, crystallization, and melt behavior of linear monoesters Laziz Bouzidia , Shaojun Lia , Steve Di Biasec , Syed Q. Rizvic , Suresh S. Narinea,b,∗ a b c

Trent Centre for Biomaterials Research, Department of Physics & Astronomy, Trent University, 1600 West Bank Drive, Peterborough, Ontario K9J 7B8, Canada Trent Centre for Biomaterials Research, Department of Chemistry, Trent University, 1600 West Bank Drive, Peterborough, Ontario K9J 7B8, Canada Elevance Renewable Sciences, 175 E Crossroads Parkway, Suite F, Bolingbrook, IL 60440, USA

a r t i c l e

i n f o

Article history: Received 6 August 2011 Received in revised form 8 November 2011 Accepted 9 November 2011 Available online 16 November 2011 Keywords: Jojoba wax esters Dimers Powder X-ray diffraction Differential scanning calorimetry Crystallization Melting Polarized light microscopy Crystal structure Polymorphism Microstructure Solid fat content

a b s t r a c t Four pure jojoba wax-like esters (JLEs), having carbon chain length of 36, 40 (two isomers) and 44, were prepared by Steglish esterification of fatty acids (or acid chlorides) with fatty alcohols at room temperature. Calorimetric and diffraction data was used to elucidate the phase behavior of the esters. The primary thermal parameters (crystallization and melting temperatures) obtained from the DSC of the symmetrical molecules correspond well with the carbon numbers of the JLEs. However, the data also suggests that carbon number is not the only factor since the symmetry of the molecule also plays a significant role in the phase behavior. Overall, the JLEs show very little polymorphic activity at the experimental conditions used, suggesting that they are likely to transform the same way during melting as well as crystallization, a characteristic which may be useful in designing new waxes and lubricants. The XRD data clearly show that the solid phase in all samples consists of a mixture of a ˇ-phase and a ˇ -phase; fully distinguishable by their characteristic diffraction peaks. Subtle differences between the subcell patterns and phase development of the samples were observed. Different layering of the samples was also observed, understandably because of the chain length differences between the compounds. The long spacings were perfectly linearly proportional to the number of carbon atoms. The length of the ester layers with n carbon atoms can be calculated by a formula similar to that used for the layers in linear alkane molecules. © 2011 Elsevier Ireland Ltd. All rights reserved.

1. Introduction Naturally occurring wax esters are chemically diverse and serve a variety of biological functions. They are readily found in plants, animals, and microorganisms (Hamilton, 1995; Samuels et al., 2008). Wax esters are for example major constituents of beeswax and plants, such as jojoba, store large quantities (Blomquist et al., 1980; Busson-Breysse et al., 1994). The molecules of this family of compounds consist of fatty acid/fatty alcohol esters and possess unique properties suitable for a variety of high-end applications, such as cosmetic and medical formulations, lubricants, and foods (Patel et al., 2001). The oil from the jojoba plant has a unique chemical composition which makes it different from all other known seed oils. It is composed mainly of linear wax esters (95%) of chain lengths C38 to C44 . In particular, C40 and C42 ester molecules having a combination of

∗ Corresponding author at: Trent Centre for Biomaterials Research, Department of Physics & Astronomy, Trent University, 1600 West Bank Drive, Peterborough, Ontario K9J 7B8, Canada. Tel.: +1 705 748 1011x6105; fax: +1 705 748 1652. E-mail address: [email protected] (S.S. Narine). 0009-3084/$ – see front matter © 2011 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.chemphyslip.2011.11.003

monounsaturated C20:1 fatty acids and C20:1 and C22:1 fatty alcohols make up to 80% of the oil (Miwa, 1971; Busson-Breysse et al., 1994; Binman et al., 1996; Tobares et al., 2004). It is believed that the narrow-range composition contributes greatly to jojoba oil’s unique characteristics (Wisniak, 1994; Le Dréau et al., 2009). The generalized structure of these compounds is shown in Scheme 1. The jojoba plant has been the major natural source of the liquid wax esters for commercial applications because of the global ban on whale hunting and their numerous potential uses (Wisniak, 1994). Jojoba oil is extensively used in the pharmaceutical and cosmetics industries, foods, foam control agents, and plasticizers (Bhatia et al., 1990; Fuks et al., 1992; Saguy et al., 1996; Selim et al., 2003). They have good lubricity and are therefore suitable as components in high-grade lubricating oil formulations (Bhatia et al., 1990; Bisht et al., 1993). Jojoba oil has been also considered as a potential non-food feedstock alternative to biodiesel production (Chinwuba Victor Ossia, 2010; Shah et al., 2010). However, the high price of jojoba oil has limited the use of these esters. The wax ester replacements, which are produced chemically or by biotechnological processes employing lipases, at present depend on chemically-synthesized fatty alcohols as substrates (Wahlen et al., 2009), making their cost a challenging issue.

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Scheme 1. Composition of jojoba-oil. m = 7, 9, 11, 13; n = 8, 10, 12, 14. Table 1 Nomenclature used for the dimer esters prepared in this work. The column headed “Structure” refers to the generalized structures shown in Scheme 2. Compounds

IUPAC Name

Structure

JLE-36S

Octadec-9-enyl octadec-9-enoate

JLE-40S

Octadec-9-enyl docos-13-enoate

JLE-44S

Docos-13-enyl docos-13-enoate

JLE-40A

Docos-13-enyl octadec-9-enoate

n1 = n2 = 8 m1 = m2 = 5 n1 = n2 = 8 m1 = 9; m2 = 5 n1 = n2 = 8 m1 = m2 = 9 n1 = n2 = 8 m1 = 5; m2 = 9

These factors have led to increased efforts towards the domestication and genetic improvements of the jojoba plant in terms of yield and oil content (Reddy and Chikara, 2010), and have motivated research for the production of jojoba wax-like esters from inexpensive renewable resources, such as fatty acids (Kalscheuer et al., 2006). The present work is a contribution towards such efforts. A study of the chemical and physical properties of the individual constituent wax esters and their mixtures would be an effective method to gain insight into the thermal, structural, and rheological properties of the natural and modified wax esters. We have therefore targeted the closest range of carbon atom numbers occurring in jojoba wax naturally and prepared four pure jojoba wax-like esters (JLEs), having a carbon chain length of 36, 40, and 44. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), polarized light microscopy (PLM), and wide-line pulsed nuclear magnetic resonance (pNMR) were used to investigate the phase transition behavior, crystal structure, microstructure, and solid fat content (SFC), respectively, as a function of symmetry and the carbon chain length. The C36 and C44 compounds have been prepared in the symmetrical conformation (JLE-36S and JLE-44S, respectively) from C18:1 and C22:1 acids and C18:1 and C22:1 alcohols, respectively. The C40 compound has been prepared in both the symmetrical (JLE40S) and asymmetrical (JLE-40A) conformation from C18:1/C22:1 and C22:1/C18:1 alcohols/acids, respectively. Note that strictly speaking, both JLE-40S and JLE-40A are asymmetrical molecules. We have simply arbitrarily assigned JLE to be referred to as symmetric for the ease it allows in discussion, and because its thermodynamic behavior falls within a linear trend with the other, symmetric esters studied. The starting materials were sourced from oleic acid and erucic acid. These model JLE molecules contain two double bonds separated by an ester bond and three functional groups which can be used as the source of useful intermediates or final products. Table 1 summarizes the dimer esters (JLEs) prepared in this work. The generalized structures of the compounds are shown in Scheme 2. 2. Materials and methods 2.1. Synthesis of the dimer esters The jojoba wax-like esters were prepared by Steglish esterification of fatty acids (or their chlorides) and fatty alcohols at room

Scheme 2. Structures of dimer esters.

Scheme 3. (a) Synthesis of erucyl alcohol. (b) Synthesis of esters, Procedure 1. (c) Synthesis of esters, Procedure 2.

temperature in the presence of dicyclohexylcarbodiimide (DCC) and dimethylaminopyridine (DMAP) or pyridine in high yields (Scheme 3b and c). Fatty materials used to make JLEs were erucic acid (90% purity), oleoyl chloride (80%), and oleyl alcohol (85%). Other reagents used were 4-dimethylaminopyridine (DMAP), dicyclohexylcarbodiimide (DCC), lithium aluminum hydride (LiAlH4 ), NaHCO3 , and Na2 SO4 , purchased from Sigma–Aldrich. Chloroform, ethyl acetate, hexane, hydrochloric acid (HCl), and tetrahydrofuran were purchased from ACP Chemical Int. (Montreal, Quebec, Canada) and used without further purification. Erucyl alcohol was prepared from erucic acid using LiAlH4 following a literature procedure (Carroll, 1957). 2.1.1. Synthesis of erucyl alcohol (Scheme 3a) Erucic acid (23 g, 70 mmol) dissolved in 20 mL THF was slowly added to a solution of LiAlH4 (1.6 g, 42 mmol) in 160 mL THF at 0 ◦ C. After addition, the reaction mixture was stirred at room temperature and the progress of reaction followed by the use of TLC. When all of the erucic acid had reacted, the reaction was quenched by adding 5 mL water, followed by 5 mL of 5% aq. NaOH and 5 mL water again to assure neutralization of the excess LiAlH4 . The resulting white solid was removed by filtering through filter aid and the filtrate was poured into 200 mL water and extracted with 2× 100 mL portions of ethyl acetate. Ethyl acetate phase was washed with 100 mL brine and dried using anhydrous Na2 SO4 . 14 g erucyl alcohol was obtained after removal of ethyl acetate through evaporation using a rotary evaporator and used without further purification. 1 H NMR in CDCl ı (ppm): 5.3 (2H, m), 3.6 (2H, t), 2.0 (5H, m), 3 1.6 (2H, m), 1.4–1.2 (30H, m), 0.9 (3H, t).

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The esters were prepared by the following two procedures: 2.1.2. Procedure 1 (Scheme 3b) To a solution of fatty alcohol (10 mmol) in 20 mL chloroform, fatty acid (10.1 mmol) and 4-dimethylaminopyridine (2 mmol) were added. The reaction mixture was cooled in an ice bath, DCC (11 mmol) in chloroform was slowly added, and the mixture stirred at room temperature overnight. The precipitated dicyclohexylurea was removed by filtration and the filtrate diluted with 60 mL CHCl3 . The combined solution was sequentially washed with water (3× 50 mL), 5% aq. HCl (1× 50 mL), 4% aq. NaHCO3 (2× 50 mL), and brine (2× 50 mL); and the organic phase dried using anhydrous Na2 SO4 . Chloroform was removed from the dried phase by using a rotary evaporator and the residue purified via column chromatography using a mixture of ethyl acetate and hexane as an eluent to yield the product as a colorless oil.

2.3. Thermal processing A Linkam LS 350 temperature chamber (Linkam Scientific Instruments, Tadworth, Surrey, United Kingdom) was used to process the samples for XRD and microscopy measurements. The samples were equilibrated for more than 5 min at 50 ◦ C, a temperature and a time over which crystal memory is erased, then cooled at a constant rate of 3.0 ◦ C/min, to a finish temperature Tf of −90 ◦ C, where they were held isothermally for a time th of 10 min. For DSC analysis, the samples were processed in the cell of the instrument using the same cooling protocol described above to allow for comparison with the other techniques used. The sample was subsequently reheated at a constant rate of 3.0 ◦ C/min to 70 ◦ C to obtain the melting profile. All measurements were run in triplicate. The measurement temperatures are reported to a certainty of better than ±0.5 ◦ C. 2.4. Physical characterization

2.1.3. Procedure 2 (Scheme 3c) To a solution of fatty alcohol (10 mmol) in 30 mL of chloroform and acyl chloride (10 mmol), pyridine (12 mmol) was added dropwise. The reaction mixture was stirred at room temperature overnight, diluted with 60 mL Chloroform, and the organic layer washed with water (3× 50 mL), 5% aq. HCl (2× 50 mL), water (2× 50 mL), 4% aq. NaHCO3 (2× 50 mL) and brine (3× 50 mL). After drying the organic phase with anhydrous Na2 SO, chloroform was removed and the residue purified via column chromatography using a mixture of ethyl acetate–hexane as an eluent to give the product as a colorless oil. 2.2. Chemical characterization All compounds were isolated and or analyzed by HPLC and characterized by 1 H NMR spectroscopy. 2.2.1. High performance liquid chromatography (HPLC) Waters Alliance (Milford, MA) e2695 HPLC system fitted with a Waters ELSD 2424 evaporative light scattering detector was used for HPLC analysis. The HPLC system includes an inline degasser, a pump, and an auto-sampler. Waters Empower Version 2 software was used for data collection and data analysis. A 250 × 4 mm Betasil Diol-100 (5 ␮m particle size) column purchased from Thermo Hypersil-Keytone Inc., Bellefonte, PA, was used in normal-phase and isocratic mode. The temperature of the column was maintained at 30 ◦ C by a Waters Alliance column oven. The ELSD nitrogen flow was set at 25 psi with nebulization and drifting tube maintained at 12 ◦ C and 60 ◦ C, respectively. Gain was set at 500. The mobile phase was heptane:isopropyl alcohol (90:10)v run for 30 min at a flow rate of 0.5 mL/min. 1 mg/mL (w/v) solution of sample in heptane:isopropyl alcohol (50:50)wt% filtered through a cotton filter was prepared and a 2 ␮L of sample was injected. All solvents were HPLC grade and obtained from VWR International, Mississauga, ON. Purity of eluted samples was determined using the relative peak area. The reported purity is the average value obtained from at least three separate runs. 2.2.2. Nuclear magnetic resonance spectroscopy (1 H NMR) 1D 1 H NMR spectra were acquired in CDCl3 using Varian VNMR spectrometer [␯(1 H) = 399.75 MHz; Varian Inc., Walnut Creek, USA] equipped with a 5 mm PFG auto switchable 1 H{X} probe. Spectra were obtained at 25 ◦ C over a 30,000 Hz spectral window with a 1 s recycle delay, 32K complex points, and 16 transients.

2.4.1. Differential scanning calorimetry The cooling and heating profiles of all compounds were carried out using a Q200 model DSC (TA Instruments, New Castle, DE, USA) equipped with a refrigerated cooling system (RCS 90, TA Instrument). Approximately 5.0–10.0 (±0.1) mg of fully melted and homogenously mixed sample was placed in an aluminum DSC pan which was then hermetically sealed. An empty aluminum pan was used as a reference and the measurements were performed under a nitrogen flow of 50 mL/min. The samples were processed, as described in Section 2.3. The “TA Universal Analysis” software coupled with a method developed in our group (Bouzidi et al., 2005) was used to analyze the data and extract the main characteristics of the peaks (temperature at maximum heat flow, Tm ; onset temperature, TOn ; offset temperature, TOff ; enthalpy, H; and full width at half maximum, FWHM). The temperature window over which a thermal event occurs is defined as the absolute value of the difference between TOff and TOn of that event. It is labeled TC for crystallization and TM for melting. The characteristics of non-resolved shoulder peaks were estimated using the first and second derivatives of the differential heat flow. The uncertainties associated with the calculated standard deviations are based on at least three runs. 2.4.2. X-ray diffraction A Panalytical Empyrean X-ray diffractometer (PANalytical B.V., The Netherlands) equipped with a filtered Cu-K˛ radiation source ( = 0.1542 nm) and a PIXcel3D detector was used in line-scanning mode (255 lines over 3.347 degree wide detector). The samples were processed as described in section 2.2 in the XRD chamber using a 700 Series Cryostream Plus cooling system (Oxford Cryosystems, Oxford, UK). The temperature was controlled to better than ±0.5 ◦ C. The XRD patterns were generally recorded between 0.1 and 60◦ (2) in 0.013◦ steps, at 45 kV and at 40 mA. The data were processed and analyzed using the Panalytical’s X’Pert HighScore V3.0 software. We refer to the range 2 = [0.05–15◦ ] and [15◦ –up] as the small- and wide-angle scattering regions, respectively. 2.4.3. Solid fat content determination The SFC measurements were performed on a Bruker Minispec mq 20 (Milton, Ontario, Canada) pNMR spectrometer equipped with a combined high and low temperature probe, and a temperature controller (Bruker BVT3000). Approximately 0.57 ± 0.05 mL of fully melted sample was quickly pipetted into the bottom 1.0cm portion of the NMR tube. The samples were subjected to the same thermal protocol as described above in the DSC experimental section.

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Scheme 4. Synthesis of octadec-9-enyl octadec-9-enoate (JLE-36S).

Scheme 5. Synthesis of octadec-9-enyl docos-13-enoate (JLE-40S).

The system was calibrated with highly unsaturated canola oil (with heat capacity similar to that of the saturated oils, which do not crystallize until −20 ◦ C) using an external data logger (Oakton, Eutech Instruments, Singapore) and a type K probe (TRP-K, Omega, Stamford, Connecticut) immersed in the oil. The accuracy of the measurement was determined to be greater than ±0.1 ◦ C. In the low temperature region, the probe is supplied with N2 gas, obtained by evaporating liquid nitrogen. The temperature was controlled to better than ±0.1 ◦ C. Bruker’s Minispec V2.58 Rev. 12 and Minispec plus V1.1 Rev. 05 software were used to collect SFC data as a function of time and as a function of time and temperature, respectively. The SFC values are reported as the ratio of the intensity of the NMR signal of the solid part to the total detected NMR signal in percent [labeled as SFC (%)]. The uncertainties associated are the calculated standard deviations based on at least three runs.

3. Results and discussion 3.1. Synthesis and chemical characterization The HPLC chromatograms and the 1 H NMR spectra of the JLEs were consistent with the structures proposed. The JLE purities exceeded 95%, as estimated by HPLC. The synthesis of the individual dimer esters is provided below:

Yield: 98.5% 1 H NMR in CDCl ı (ppm): 5.4 (4, m), 4.1 (2, t), 2.3 (2, t), 2.1–2.0 3 (8, m), 1.7–1.56 (4, m), 1.44–1.20 (42, m), 0.86–0.76 (6, t) Purity: >95% 3.1.2. Octadec-9-enyl docos-13-enoate (JLE-40S) Compound JLE-40S was prepared from erucic acid and oleyl alcohol in the presence of DCC and DMAP following the general procedure described before and shown in Scheme 5. Pure compound JLE-40S was a colorless oil obtained by column chromatography using ethyl acetate:hexane (1:40)v mixture as an eluent. Yield: 91.8% 1 H NMR in CDCl 3

ı (ppm), 5.4 (4, m), 4.1 (2, t), 2.3 (2, t), 2.1–2.0 (8, m), 1.7–1.56 (4, m), 1.44–1.20 (50, m), 0.86–0.76 (6, t) Purity: >95%

3.1.3. Docos-13-enyl docos-13-enoate (JLE-44S) Compound JLE-44S was prepared from erucic acid and erucyl alcohol (cis-13-docosenol) in the presence of DCC and DMAP following the general procedure described before and shown in Scheme 6. Pure JLE-44S was a colorless oil obtained by column chromatography using ethyl acetate:hexane (1:40)v mixture as an eluent. Yield: 95% 1 H NMR in CDCl 3

3.1.1. Octadec-9-enyl octadec-9-enoate (JLE-36S) Compound JLE-36S was prepared from oleoyl chloride and oleyl alcohol in the presence of pyridine following the general procedure already described and shown in Scheme 4. Purified JLE-36S was a colorless oil obtained by column chromatography using ethyl acetate:hexane (1:30)v mixture as an eluent.

ı (ppm), 5.4 (4, m), 4.1 (2, t), 2.3 (2, t), 2.1–2.0 (8, m), 1.7–1.56 (4, m), 1.44–1.20 (58, m), 0.86–0.76 (6, t) Purity: >95%

3.1.4. Docos-13-enyl octadec-9-enoate (JLE-40A) Compound JLE-40A was prepared from oleoyl chloride and erucic acid following the general procedure described before and

Scheme 6. Synthesis of docos-13-enyl docos-13-enoate (JLE-44S).

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Scheme 7. Synthesis of docos-13-enyl octadec-9-enoate (JLE-40A).

shown in Scheme 7. Pure JLE-40A was a colorless oil obtained by column chromatography using ethyl acetate: hexane (1:40)v mixture as an eluent. Yield: 94.5% 1 H NMR in CDCl ı (ppm), 5.4 (4, m), 4.1 (2, t), 2.3 (2, t), 2.1–2.0 (8, 3 m), 1.7–1.56 (4, m), 1.44–1.20 (50, m), 0.86–0.76 (6, t) Purity: >95% 3.2. Crystallization behavior of the jojoba wax-like esters Typical cooling thermograms of purified JLE-36S, JLE-40S, JLE44S and JLE-40A are shown in Fig. 1a and the corresponding crystallization values are shown in Fig. 1b. As can be seen in Fig. 1a, the cooling thermograms of the JLE samples obtained at a cooling rate of 3.0 ◦ C/min have broad leading exotherms followed by a main crystallization event (P1 and P2 in Fig. 1a). P2 extends asymmetrically to lower temperatures and is followed by a more or less resolved low temperature third thermal event in the case of JLE-36S, JLE-40A and JLE-44S (P3 in Fig. 1a). The onsets and peak maximums of crystallization in these samples occurred at fairly high temperatures (Fig. 1b), ranging from −25 ◦ C to about 5 ◦ C for onsets, indicating clearly that these base esters have the ability to behave like lubricants as well as like waxes, depending on the numbers of carbon atoms present. The presence of different thermal events with notable net temperature differences (larger than 10 ◦ C) for such pure substances indicates a polymorphic behavior characteristic of the manner in which these esters crystallize. This is not unexpected, given that these are relatively long chain esters, which can “crystallize” into metastable forms with a measurable amount of exotherm long before the most stable solid arrangement is realized. However, one is not able to assess the nature of the transformation that occurs using solely the calorimetric data. However, simultaneous consideration of the calorimetric and diffraction data clarifies the phase transformation/development in the JLEs and further explains the nature of the primary peaks. The main crystallization values (onset, offset, and peak maximum temperature of crystallization) of the symmetrical molecules coincide well with the carbon numbers and vary linearly with them (dashed lines in Fig. 1b, R2 > 0.950). However, it is clear that the carbon number is not the only factor since JLE-40S and JLE-40A, with the same number of carbon atoms, have different crystallization values. Interestingly, JLE-40S and JLE-40A have similar onset temperatures of crystallization but the asymmetrical JLE-40A displays lower peak maximums and in contrast with the symmetrical JLE-40S exhibits a third exotherm at low temperature (−18.6 ◦ C); evidently the two samples are not identical in their lowtemperature behavior. As will be discussed later, further evidence of the effect of symmetry on phase development can be inferred from the crystallization enthalpy associated with different individual exotherms, shown in Table 2. Clearly, the symmetry of the molecule plays a significant role in the phase development of the linear esters.

Fig. 1. (a) Cooling thermograms (3.0 ◦ C/min) of the jojoba wax-like linear esters (JLEs). (b) Corresponding main characteristics of the crystallization DSC signal as a function of number of carbon atoms in the ester. The symbols are as follows: center maxima (TC ), 䊉; onset (TOn ), ; and offset (TOff ) temperature of crystallization, . The values of the asymmetric ester JLE-40A are represented by unfilled symbols. The dashed lines are linear fits.

Table 2 Enthalpy of crystallization of the JLEs. Enthalpy of crystallization (J/g) JLE-36S H1 (P1) H2 (P2) H3 (P3) HC

12.3 104.9 19.7 117.2

± ± ± ±

1.3 11.2 2.3 12.5

JLE-40S

JLE-40A

19.0 ± 3.2 104.7 ± 3.3 – 123.7 ± 6.3

9.6 129.1 4.9 143.6

± ± ± ±

JLE-44S 2.8 6.0 0.4 3.8

12.7 115.9 20.9 149.6

± ± ± ±

1.2 8.2 1.7 11.0

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Comparison of thermodynamic data of JLE-40S and JLE-40A indicates that (a) the leading exotherm in JLE-40A has the lowest enthalpy; a sign that in this case less material is involved in the first liquid–solid transformation, (b) the main peak maximum in JLE-40A is lower, indicating that the main (second) phase formation in JLE-40A is less stable than that in JLE-40S and (c) the presence of a third exotherm indicates that a further transformation, either in the remaining melt or in the solid state, occurs in JLE-40A and not in JLE-40S. This can be explained by simple geometrical considerations – the asymmetrical molecules are less susceptible to pack in the most stable form than the symmetrical ones at the same conditions. For this class of compounds, this is very important fundamental knowledge, as it gives a predictive manner of changing thermal characteristics which may be useful to utility of these esters. The symmetry of the molecule in this case refers to the carbon lengths and position of the double bond on either side of the ester bond – in the case of JLE-40S, the C22 moiety is derived from a fatty acid, while in the case of JLE-40A, the C22 moiety is derived from the fatty alcohol. This means the double bonds are at carbon lengths of 13/9 and 9/13 on either side of the ester bond for JLE-40S and JLE40-A, respectively. The hydrogen-bonding introduced at the ester bonds, coupled with the positioning of the gauche introduced by the double bonds, results in significant differences to the ease of packing of these two isomers into regular crystal lattices. Clearly, these considerations results in more of a barrier to crystallization for the asymmetric molecule compared to the symmetric, resulting in lower maximum peak temperatures for crystallization exotherms, and to a much lower crystallization event being recorded for JLE-40A but not for JLE-40S. Interestingly, however, despite the requirement of higher levels of undercooling for crystallization of JLE-40A, resulting in fractions of JLE-40A crystallizing at lower temperatures compared to JLE-40S, the overall enthalpy of crystallization of JLE-40A is higher than that of JLE40S, suggesting that the compounds formed by the crystallization of JLE-40A are thermodynamically more stable than those formed by JLE-40S. Therefore, the suggestion is that the asymmetric isomer’s molecules which pack into the crystal lattices, although requiring a higher thermodynamic driving force (undercooling) for crystallization, have stronger forces of bonding between them than does the symmetric molecules. 3.3. X-ray data analysis and polymorphism of JLEs The common crystal structure models of waxes and related aliphatic compounds were established in numerous studies. The molecules take different orientations relative to their neighbors by rotation around their long axis, forming different crystal structure symmetries of the methylene subcells (Larsson, 1994). Three basic symmetries, commonly denoted as ˛, ˇ and ˇ, are described in the literature (Larsson, 1986). The chains of the ˛-polymorph either pack in a hexagonal symmetry with nonspecific chain-chain interactions or they can rotate around the long axis (so-called rotator phase). The ˛-polymorph is characterized by one strong wide-angle ˚ originatline in the XRD pattern at a lattice spacing of ∼4.2 A, ing from the (1 0 0)˛ basal plane reflection. The common subcell packing of the ˇ -polymorph is orthorhombic, with the alternate acyl chains packing in planes almost perpendicular to each other (O⊥ ), and is characterized by two strong wide-angle lines at lattice spacings of 4.2–4.3 A˚ originating from the (1 1 0)ˇ reflection and 3.7–3.9 A˚ originating from the (2 0 0)ˇ reflection. The hydrocarbon chains of the ˇ-polymorph are commonly packed parallel to each other in a triclinic (or monoclinic, if the angles ˛ and  are 90 ◦ C) parallel subcell (T// ). The ˇ-form in the wide-angle region is characterized by a lattice spacing of ∼4.6 A˚ originating from the (0 1 0)ˇ ˚ The reflection and a number of other strong lines around 3.6–3.9 A.

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Table 3 Characteristic peaks and related Miller indices of the ˇ-form (triclinic) and ˇ -form (orthorhombic). ˇ -form

ˇ-form ˚ d (A)

hkl

˚ d (A)

hkl

4.60 4.40 3.81 3.65 3.48

010 011 1¯ 0 0 100 110

4.18 4.10 4.02 3.71

110 1¯ 1 0 020 200

ˇ-polymorph is the most stable crystal form, with the highest melting temperature, and the ˛-polymorph is the least stable crystal form, with the lowest melting temperature (Ghotra et al., 2002; Timms, 2003). The “long spacing” peaks provide information about the layer order. The d-value of the (0 0 1)-reflection represents the thickness of the molecular layers (Fahey et al., 1985). A series of higher order (0 0 l)-reflections indicates a regular, periodic structure and represents the periodical sequence of electronic density differences in multiple layers. In the case of hydrocarbons, such as alkanes, the series of (0 0 l)-peaks originates from the region of lower scattering density in the gap between the molecular layers (Dorset, 1995, 2002). The X-ray patterns of the purified JLEs obtained at low temperature (−75 ◦ C) are shown in Fig. 2a and b, for the long and short spacing regions, respectively. The d-spacings corresponding to the main peaks and the related indices are reported in Table 3. From data in Table 4, one can notice subtle differences between the subcell patterns amongst samples and the similarity of the crystal structure of JLE-40S and JLE-40A compounds. However, the layering between compounds is quiet different, as evidenced by the obvious differences in the position of the small angle diffraction angles, understandably because of the chain length differences (Table 4). A definitive probe on the assignment of the different spacings was achieved by monitoring the diffraction pattern of each sample with temperature. Fig. 3a–d shows selected diffraction patterns of compounds JLE-36S, JLE-40S, JLE-44S and JLE-40A, respectively, obtained at different temperatures during cooling of the melt. The temperatures shown are close to the different DSC cooling peak temperatures, provided in Fig. 1 as peak crystallization temperatures. The XRD patterns obtained in the melt and at −75 ◦ C are also shown. The XRD data clearly show that the solid phase in all samples consists of a mixture of two phases, the triclinic ˇ-phase and the orthorhombic ˇ -phase, fully distinguishable by their characteristic diffraction peaks. No hexagonal phase was observed as there was no evident indication that the intensity ratio of the (1 1 0) and (2 0 0)-peaks of the orthorhombic structure is altered by an extra contribution. The XRD pattern of JLE-40S and JLE-44S obtained at the first peak temperature shows a relatively strong (1 1 0) line at 4.16 A˚ and a ˚ both characteristic of the ˇ -form. One weak (2 0 0) line at 3.74 A, can also observe the relatively large width of the two peaks, usually associated with poor homogeneity and short-range order. JLE-36S and JLE-40A show extra very weak lines, some of which originate Table 4 (0 0 1) d-spacing values obtained for the layering of the ˇ-form (triclinic) and the ˇ -form (orthorhombic) for the different jojoba wax-like linear esters. Sample

Number of carbons

˚ d(0 0 1)ˇ (A)

JLE-36S JLE-40S JLE-40A JLE-44S

36 40 40 44

40.46 45.54 44.77 50.54

± ± ± ±

0.19 0.37 0.09 0.12

˚ d(0 0 1)ˇ (A) 45.13 50.78 52.57 58.51

± ± ± ±

0.77 1.08 1.34 1.20

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L. Bouzidi et al. / Chemistry and Physics of Lipids 165 (2012) 38–50

Fig. 2. XRD patterns of the purified jojoba wax-like linear esters (JLEs) obtained at −75 ◦ C. (a) Long and (b) short spacing regions. Selected characteristic Miller indices are reported on top of the corresponding peaks.

from a ˇ-phase co-growing with ˇ -phase. Starting from the end of the first exotherm, the intensity of the (1 1 0)ˇ peak did not change significantly with temperature, indicating that the growth of the ˇ phase was completed at this early stage. The (0 1 0) reflection of a ˇ ˚ was detected at form, located at higher basal spacing of about 4.6 A, the end of the first exotherm for all samples. Its intensity increased as the sample was cooled (see Fig. 4a–d) indicating the increased growth from the melt of the ˇ-phase. However, subtle differences in phase development between the samples were observed and are described in light of the evolution of characteristic lines of the ˇ and ˇ-phases. For all samples and up to the temperature at the end of the first exotherm, most of the XRD spectra constituted a wide halo characteristic of the liquid phase. The crystallinity [X(%)], as estimated by the relative intensity of the crystalline peaks to the total intensity in the wide-angle region, was less than 5% at the offset of the first exotherm, a value which roughly compares with that estimated from the enthalpy of the first exotherm to the total enthalpy of crystallization. At −75 ◦ C, no liquid phase was detected. The crystallinity at the different temperatures (Fig. 5) was calculated using the area of the peaks originating from crystal phase, normalized to the area of the peaks measured at −75 ◦ C. As can be seen, a sigmoidal shape was obtained for all samples with induction temperatures consistent with the respective onset of crystallization. However, one can notice obvious differences in the slopes, an indication of the stark differences in growth rate. This is particularly evident in the case of the asymmetrical molecule, which displays an abrupt change in crystallinity, right after the end of the first exotherm (up triangles in Fig. 5). For the symmetrical molecules, the effect of increased carbon number can be appreciated with the increase of the slope of crystallinity versus temperature curves. Both ˇ - and ˇ-phases were detected at the peak temperature of the first exotherm in JLE-36S and JLE-40A. In these cases and as indicated by the variation of the intensity of (1 1 0)ˇ with temperature, the ˇ -phase content does not measurably change from the end of the first exotherm. However, a clear decrease in the d-spacing values of the peaks characteristic of ˇ phase as well the appearance

of extra, albeit weak reflections, was observed. As exemplified in Fig. 6 by JLE-36S, the d-spacings associated with the (1 1 0), (0 2 0), and (2 0 0) reflections of ˇ decreases linearly (R2 ≥ 0.9650), indicating an ordering of this crystal form. This may be in fact a transition from a disordered to an ordered ˇ -form, which is analogous to the phase transition occurring in n-alkanes from the so-called rotator form, having the face centered orthorhombic symmetry (space group Fmm), to the ordered orthorhombic form (space group Pcam) (Metivaud et al., 1998). The rotator form is characterized by a rotational disorder of the molecule along its c long axis. The same changes in the ˇ -phase were also observed in JLE-40S and JLE-44S. This analysis is reasonable as the rotator form is usually formed from the melt first and is very much affected by temperature. Striking differences are observed in changes affecting the ˇphase between samples. The intensity of its characteristic (0 1 0)ˇ peak changes continuously and steadily for JLE-36S, JLE-40S, and JLE-44S; but in JLE-40A increases suddenly at the end of the first exotherm and then remains almost constant (Fig. 4). This indicates a very fast growth of this phase in JLE-40A and a slow growth in others. There are practically no changes in the position of the peaks of the ˇ-form detected in JLE-36S and JLE-40S, suggesting that it has achieved its final order at the early stages of the growth process. In the case of JLE-40A, the ˇ-phase is characterized by different XRD traces at each peak temperature, a sign of the occurrence of different subforms of the ˇ-phase. These changes can be associated with different exotherms appearing in the cooling thermogram of this sample. This can be explained by the asymmetrical nature of the molecule, which through successive transformation adopts slightly different conformations as it is cooled. Notice that the cooling thermogram of JLE-40A has a very resolved exotherm at ∼18 ◦ C. Interestingly in the case of JLE-44S, the position of peak (0 1 0)ˇ shifted to lower angles (higher d-spacing), whereas, the other peak did not, indicating a continuous rearrangement of the ˇ lattice at the 0 1 0 basal plane level explainable by an increase of the repulsion forces at the detriment of the attractive forces. This may be occurring primarily at the ester group level. Note also that at the lowest temperature (−75 ◦ C), the reflection doublets at 3.86 A˚ and

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45

Fig. 3. Selected diffraction patterns of jojoba wax-like linear esters (JLEs) (a) JLE-36S, (b) JLE-40S, (c) JLE-44S and (d) JLE-40A, respectively, obtained during cooling of the melt. The temperatures shown are close to the different DSC peak temperatures recorded during the cooling (3.0 ◦ C/min) cycle.

3.80 A˚ as well as at 3.71 A˚ and 3.61 A˚ of the ˇ -form in JLE-36S are overlapped by the strong reflections at 3.80 A˚ and 3.61 A˚ of the ˇ-form. This is probably due to the predominant ˇ-phase which may have also developed to another subform. If one considers that the (0 1 0) peak can be assigned to the average distance between chains of molecules linked by hydrogen bonds (Le Poulennec et al., 2000), the transformations occurring in the ˇ-form maybe due to adjacent layers interacting via van der Waals interactions between methylene-containing part of the hydrocarbon chains.

As can be seen in Fig. 2a, the (0 0 1) peaks are intense but the intensity of these higher-order peaks decreases rapidly. Reduced intensity or extinction of certain (0 0 l) reflections of the long spacing peaks may be due to several reasons (Ensikat et al., 2006). Because reduced intensity caused by oxygen atoms present at a specific position in long-chain molecules is characteristic and occurs commonly in waxes (Ensikat et al., 2006), one would surmise that the reduction of intensity of the higher order (0 0 l) reflections in the JLEs is probably due to electrons of the oxygen atoms present in

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Fig. 4. Relative area (%) of (0 1 0), ; and (1 1 0), , ˇ-phase content (Cˇ , %) by relative area, 䊉; and ˇ-phase content (Cˇ , %) calculated using peak heights, . The sample name is specified in the title of each panel.

the ester functionality. The presence of the additional oxygen atoms compensates the lower electronic density in the gap region as they form a plane of higher scattering density between the low-density planes, causing a destructive interference of certain (0 0 l) peaks, thereby dramatically lowering their intensity and that of the others. This is aptly illustrated by the extinction of the (0 0 2) reflection for all samples except JLE-36S measured at −1.5 ◦ C in which a very weak line was detected (Fig. 7). In all samples but JLE-40A, the 0 0 4 reflection was weaker than 0 0 5, which in turn was weaker than 0 0 3, probably due to the closeness of the ester group to the periodicity site. For any given order, the reflections detected in JLE-36S (36 carbons) were higher than those in JLE-40S and JLE-40A (40 carbons), which in turn were higher than those in JLE-44S (44 carbons). The intensity of the 0 0 4 reflection was higher than that of the 0 0 5

Fig. 5. Crystallinity (%) of the jojoba wax-like linear esters (JLEs). Symbols are: JLE36S, 䊉; JLE-40A, ; JLE-40S, ; JLE-44S, .

reflection in JLE-40A, a feature which can be attributed to differences in electronic distribution close to the ester group amongst the asymmetrical and symmetrical molecules. One would assume also, based on the data published on the molecular architecture of the three-dimensional waxes (Koch et al., 2010), that lateral oxygen atoms at the side of the molecules require additional space, causing local disorder between the molecules and hindering the formation of the orthorhombic structure. As can be seen in Fig. 7 that represents the small angle region of the XRD pattern of JLE-36S, a first series [0 0 1, 0 0 2 and 0 0 3] with a long spacing of 45.13 ± 0.77 A˚ appears at −1.5 and −5 ◦ C and another series [0 0 1, 0 0 3 and 0 0 4 and 0 0 5] with a long spacing

Fig. 6. Short d-spacing measured in the linear ester JLE-36S. The corresponding Miller indices of the ˇ reflections (up triangles) and ˇ reflections (filled circles) are labeled accordingly. Dashed lines are linear fits (R2 ≥ 0.9650), and dotted lines are the average values of the data point through which they pass.

L. Bouzidi et al. / Chemistry and Physics of Lipids 165 (2012) 38–50

47

Fig. 8. (0 0 1) long spacings of the ˇ-form (䊉) and ˇ -form () versus number of carbon atoms scale perfectly linearly (R2 > 0.999). Unfilled symbols are values from the asymmetric ester JLE-40A.

Fig. 7. Small angle region of the XRD pattern of sample JLE-36S. The series of long d-spacing reflections corresponding to the ˇ-form (triclinic) and ˇ -form (orthorhombic) are labeled accordingly and indicated with arrows.

˚ much more intense, appears for the temperatures of 40.46 ± 0.19 A, higher than −5 ◦ C. The two series coexist and are not altered upon further cooling. The respective peaks for the reflection with order higher than 1 were well resolved in this case, a further indication of the coexistence of the two phases. The first and second series are probably linked to the layering of the chains in the ˇ - and the ˇ-form, respectively. Note that no long spacing was detected in the liquid state. Such first order spacings are surprisingly close to those observed in tilted orthorhombic and triclinic subcell 18carbon bilayers, such as those of the oleic acid (Kodali et al., 1985; Larsson, 1994; Rousseau et al., 1998). The (0 0 l) long spacing as well as several higher order reflections ˚ associated with the ˇ-form have been detected at 45.54 ± 0.37 A, ˚ and 50.54 ± 0.12 A˚ for JLE-40S, JLE-40A, and JLE-44S, 44.77 ± 0.09 A, respectively. Only the first order spacing associated with the ˇ crystals was observed distinctly in JLE-40S, JLE-44S and JLE-40A at all temperatures. The higher order reflections, appearing as shoulders, could not be accurately fully resolved as they overlapped with the left asymmetry of the (0 0 l) reflections associated with the ˇcrystals. As can be seen in Fig. 8, the (0 0 1) long spacings were perfectly linearly proportional to the number of carbon atoms (R2 > 0.999). The length of the ester layers (molecules) with n carbon atoms can be calculated by the formula (1.260n − 4.887) A˚ in the ˇ-form and (1.673n − 15.135) A˚ in the ˇ -form. The effect of extra carbon atoms on the length of triclinic linear esters can be compared to that of alkane molecules in which length can be calculated by the formula (1.273n + 1.875) A˚ (Dorset, 1995).

melting values of the main endotherm as a function of the number of carbon atoms of the ester. As can be seen from the heating profiles of the esters (Fig. 9a), a very small endotherm is leading the transformations in JLE-40S, JLE-40A and JLE-44S (arrows in Fig. 9a) but not in JLE-36S. The melting point of the symmetrical esters, similarly to crystallization, is clearly a function of the total number of carbon atoms (Fig. 9b), a not too surprising a result. Onset of melting and temperature at maximum heat flow of the main peak increased linearly with the increasing number of carbon atoms (dashed lines in Fig. 9b, R2 > 0.995). JLE-40S and JLE-40A have similar onsets of melting, again not too surprising a result, given that both have the same total amount of carbon atoms. However, there is a clear anomaly between JLE-40S and JLE-40A isomers (unfilled symbols in Fig. 9b). Note that the difference is finite and reproducible. Table 5 provides enthalpy of melting data for JLEs. The differences in enthalpy values between JLE-40S and JLE-40A are probably related to the fact that the former has C22 moiety derived from the alcohol and the latter has C22 moiety derived from the acid, despite the fact that the total carbon chain length in both is C40 . In particular, the differences in melt enthalpy of JLE-40S and JLE-40A are similar to the differences in their crystallization enthalpies, supporting the discussion above of the impact on the thermodynamic stability of the crystalline lattices formed by each respective isomer of the symmetry of the molecule. The very low enthalpy measured for the leading endotherm indicates that it is the expression of the melting of a small low stability phase formed during the early stages of crystallization. This first endotherm can be safely associated with the leading peak observed during crystallization and represents the event of melting of the orthorhombic phase, which was formed at the early stages of crystallization and which coexisted with the ˇ-phase, a deduction clearly evidenced by temperature resolved XRD. Note again that the asymmetrical ester JLE-40A showed the largest leading endotherm,

Table 5 Enthalpy of melting of the JLEs. Enthalpy of melting (J/g) JLE-36S

3.4. Melting behavior of the jojoba wax-like esters Fig. 9a presents heating thermograms of the linear esters and Fig. 9b provides the corresponding main characteristics of the

H1 (P1) H2 (P2) H3 (P3) HM

0.2 107.7 11.2 119.1

± ± ± ±

JLE-40S 0.1 10.3 1.2 11.6

1.2 114.9 8.7 125.9

± ± ± ±

JLE-40A 0.2 7.1 1.2 8.3

8.9 125.8 10.4 145.1

± ± ± ±

JLE-44S 2.3 8.1 4.4 2.4

2.1 140.5 7.6 150.2

± ± ± ±

0.2 10.1 0.4 10.5

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L. Bouzidi et al. / Chemistry and Physics of Lipids 165 (2012) 38–50

the previous researchers (Patel et al., 2001), who found that melting temperature (Tm ) values are higher for compounds having alcohol and acid chains of the same length on either side of the ester moiety. When the ester link is moved to less central locations along the molecule the Tm decreases. Overall, the esters show very little polymorphic activity, at the processing conditions applied. Note that the enthalpy of melting for each ester is similar to the enthalpy of crystallization, suggesting that the esters tend to transform the same way on melting as during crystallization, a characteristic which may be useful for developing new wax-based products. 3.5. Solid fat content of the JLEs

Fig. 9. (a) Heating thermograms (3.0 ◦ C/min) of the jojoba wax-like linear esters (JLEs). (b) Corresponding main characteristics of the melting values of the main endotherm as a function of number of carbon atoms of the ester. The symbols are as follows: center maximum (TM ), 䊉; onset (TOn ), ; and offset (TOff ) temperature of melting, . The values of the asymmetric ester JLE-40A are represented by unfilled symbols. The dashed lines are linear fits.

a sign that its solid phase contains the highest amount of the low stability ˇ -phase. The main endotherm is obviously the occurrence of the melting of the ˇ-phase (also clearly evidenced as the predominant phase by XRD at the completion of the crystallization process in all esters). There was no observation of a small leading peak for JLE-36S. This is in agreement with the XRD findings which indicate a ˇ-phase formation concomitantly with the ˇ -phase, the latter probably overwhelms the former. The very weak XRD signal characteristic of a ˇ -phase detected in JLE-36S probably originated from such a small amount of crystal that its heat of melting may have been either buried in the broad asymmetrical left shoulder of the main endotherm or was not detected at all by DSC. As commonly known, structural variations (acid and alcohol chain length, ester position, unsaturation) significantly affect the properties of the wax esters. Our DSC results agree with those of

The SFC (%) versus temperature curves obtained for the different purified ester compounds are shown in Fig. 10a and b, for the cooling cycle and the heating cycle (3.0 ◦ C/min), respectively. The induction time is the time at which the SFC deviates from the baseline and the final SFC is the SFC achieved by the mixtures at the completion of crystallization; both are presented in Fig. 10c and d, respectively. As can be seen, the curves display non-sigmoidal shapes that are particularly evident in the cooling cycle. The crystallization mechanism and consequently the kinetics, as evidenced by SFC versus temperature, are not constant during the crystallization process. The application of the modified Avrami model, which takes into consideration the variances within the growth curve (Narine et al., 2006), yield three distinct temperature regions. In the modified Avrami model, each step is characterized by a constant growth rate and is described by an Avrami equation with an Avrami constant and Avrami exponent, applicable to nucleation, growth, and dimensionality of the crystallizing molecules over that step. SFC data indicates that the overall crystal growth in the JLEs occurs in three main steps. The first sigmoid-shaped region can be associated with the leading event observed in the DSC crystallization curve. The second and third regions yield two different but intricately related segments, suggesting that more than one process takes place in these regions. The competition between growth and activated secondary nucleation, as has been observed in the growth of crystals of organic molecules (Patel et al., 2001), could explain the appearance of these segments. As crystallization progresses, the relative contribution of the different processes to the overall rate constant may change, leading to a change in the apparent activation energy (Mullin, 2001; Bouzidi and Narine, 2010). The SFC data strongly indicate that overall, these compounds crystallize in a similar fashion but with subtle differences, corroborating findings from the DSC crystallization experiment. The SFC induction time, ti , for the symmetrical compounds [Fig. 10c], and hence induction temperature, coincide very well with carbon number [Fig. 10c]. A linear regression showed that the addition of 1 carbon atom facilitated crystallization by approximately 1.15 ± 0.06 min. Note that a small but still significant difference was observed for the asymmetrical compound outlining the effect of symmetry on the crystallization behavior of the JLEs – i.e., the asymmetrical JLE needs a longer induction time for crystallization to occur. This is of course supported by the arguments above explaining the same effect in the occurrence of the crystallization exotherm peak maximums. The final SFC (%) for symmetrical JLEs also matches relatively well with the number of carbon atoms [Fig. 10e], with the asymmetrical JLE demonstrating a higher SFC value than its symmetrical isomer, suggesting that the asymmetry in the molecule introduces more favorable packing modes. This is consistent with the DSC results – suggesting that while the ease of packing of the asymmetrical molecules is less than its symmetrical counterpart, and therefore the induction time of crystallization is delayed and higher levels of undercooling are required for crystallization, that the

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Fig. 10. SFC data obtained at 3.0 ◦ C/min cooling and heating rate of the purified jojoba wax-like esters (JLEs). SFC (%) versus temperature curves: (a) cooling cycle and (b) heating cycle. (c) Induction time (min). (d) Final SFC (%) achieved by the JLEs. Unfilled squares represent the values of the asymmetrical compound JLE40A. Dashed lines in (c) and (d) are fits of the data obtained for the symmetrical compounds to a line.

asymmetric molecules crystallize into lattices where the molecular bonding is stronger. This behavior is also seen in TAGs where asymmetric isomers of TAGs generally crystallize with higher final SFC values, but take longer to begin crystallizing, presumably due to the increased free energy required for an asymmetric molecule to diffuse over to a growing nucleus in a conformation which allows it to participate in the growing surface (Yang and Nagle, 1988; Sato and Garti, 2001). Note that the same final SFC (%) value was observed during heating [Fig. 10d]. The JLEs all crystallized to final SFC values close to 100%, indicating that, notwithstanding the variations mentioned before, these compounds form well-organized, plastic fats with predictable solidification behavior. However, with the exception of JLE44S, none of the other samples nearly fully crystallized above 0 ◦ C, and at room temperature all of these materials are viscous liquids. The relationships of solid fat content, temperature, and carbon number for symmetric JLEs elucidated here provide the formulator with specific information on the required chain lengths for particular applications related to waxes, lubricants, and cosmetic chemicals.

4. Conclusions Crystallization onsets, onsets of melt, peak maximums and enthalpies of crystallization and melt scale in a predictable manner with carbon number for the linear monoesters, although distinct thermodynamic differences are recorded in asymmetric isomers. Furthermore, the monoesters demonstrated very little polymorphic hysteresis on melt/cool cycling, suggesting that they are materials which can be exploited well as lubricants and waxes in applications such as greases, cosmetics and pharmaceuticals. This work suggests that single-molecule esters, much easier to produce

from cheap fatty acid feedstock, can perform as well as and more predictably than the feedstock derived from scarce and expensive feedstock such as jojoba oil. The monoesters all demonstrated a mixture of ˇ and ˇ polymorphs, with the long spacings being exactly predictable from theory. Although there were distinct, if subtle, differences in phase development and subcell patterns, the polymorphism of these samples is uncomplicated and unpredictable. Importantly, measurable differences in induction time and final solid content were detected in the asymmetric isomers, leading to informed choices on what alcohols and acids should be used in the synthesis of similar monoesters for specific applications.

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