J Am Oil Chem Soc (2009) 86:843–856
`DOI 10.1007/s11746-009-1423-2
`
`O R I G I N A L P A P E R
`
`A Comprehensive Evaluation of the Melting Points of Fatty Acids
`and Esters Determined by Differential Scanning Calorimetry
`
`Gerhard Knothe Æ Robert O. Dunn
`
`Received: 16 January 2009 / Revised: 26 May 2009 / Accepted: 5 June 2009 / Published online: 26 June 2009
`Ó AOCS 2009
`
`Abstract The melting point is one of the most important
`physical properties of a chemical compound and it plays a
`significant role in determining possible applications. For
`fatty acid esters the melting point is essential for a variety
`of food and non-food applications, the latter including
`biodiesel and its cold-flow properties. In this work, the
`melting points of fatty acids and esters (methyl, ethyl,
`propyl, butyl) in the C8–C24 range were determined by
`differential scanning calorimetry (DSC), many of which for
`the first time. Data for triacylglycerols as well as ricinoleic
`acid and its methyl and ethyl esters were also acquired. For
`some compounds whose melting points have been previ-
`ously reported, data discrepancies exist and a comprehen-
`sive determination by DSC has not been available.
`Variations in the present data up to several °C compared to
`data in prior literature were observed. The melting points
`of some methyl-branched iso- and anteiso-acids and esters
`were also determined. Previously unreported systematic
`effects of compound structure on melting point are pre-
`including those for x-9 monounsaturated fatty
`sented,
`acids and esters as well as for methyl-branched iso and
`anteiso fatty acids and esters. The melting point of a pure
`fatty acid or ester as determined by DSC can vary up to
`
`Product names are necessary to report factually on available data;
`however, the USDA neither guarantees nor warrants the standard of
`the product, and the use of the name by USDA implies no approval of
`the product to the exclusion of others that may also be suitable.
`G. Knothe (&) R. O. Dunn
`US Department of Agriculture,
`Agricultural Research Service,
`National Center for Agricultural Utilization Research,
`1815 N. University St, Peoria, IL 61604, USA
`e-mail: gerhard.knothe@ars.usda.gov
`
`approximately 1 °C. Other thermal data, including heat
`flow and melting onset temperatures are briefly discussed.
`Keywords Fatty acids Differential scanning
`calorimetry Butyl esters Ethyl esters Melting point
`Methyl esters Propyl esters
`
`Introduction
`
`The melting point (MP) of a substance is one of its most
`important physical properties. It plays a major role in
`determining the suitability and applicability of a substance
`in countless food and non-food applications. Numerous
`structural factors influence the MP of an organic com-
`pound, including the molecular weight, the number and
`configuration of double bonds, triple bonds, branching,
`stereochemistry, and the presence of one or more polar
`groups
`such as OH, with interrelationships existing
`between these features for specific compounds or classes of
`compounds. These effects are also observed in long-chain
`fatty compounds.
`Several reasons prompted the present investigation of
`the MP of fatty compounds. First, while MP data for many
`common fatty acids and esters are given in reference works
`[1–5] or primary literature [6–10], such data for many other
`similar compounds are not readily available. Especially
`data for compounds with MP at lower temperatures (below
`ambient) are often less readily available. Therefore, in this
`work additional MP data are provided that are not or not
`readily available in the existing literature including several
`new systematic structure-property effects. Furthermore,
`most MP data that are routinely used were determined with
`varying equipment before the advent or more widespread
`use of differential scanning calorimetry (DSC). Thus a
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`844
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`J Am Oil Chem Soc (2009) 86:843–856
`
`second goal of this work is to provide compile and evaluate
`such information using DSC. It may be noted that photo-
`pyroelectric measurements of melting phenomena provide
`information complementary to DSC [11]. A third reason is
`that MP data compiled from the literature (Tables 1, 2)
`display discrepancies for many compounds, providing an
`incentive to determine more accurate values.
`Previous work has shown that the melting and crystal-
`lization behavior of fatty acids and esters depend strongly
`on structural features such as chain length, position and
`configuration of double or
`triple bonds or
`functional
`groups, with some systematic effects of the structure on
`MP having been established. It was recognized early [12]
`that the MP of a saturated fatty acid with an odd number of
`carbon atoms is slightly lower than that of the even-num-
`bered fatty acid with one less carbon atom, an observation
`confirmed by later studies cited above. Besides the afore-
`mentioned studies on common fatty acids and esters, other
`literature reports MP data with systematic influences for
`fatty acids and esters with a variety of structures, including
`octadecynoic [7, 13], pentadecynoic acids [13], methyl
`epoxyoctadecanoates [14], methyl-substituted octadeca-
`noic acids [15], vinyl fatty acids [16], and wax esters [17].
`Recently, melting data of some methyl and iso-propyl
`esters of branched (iso and anteiso) acids [18] and various
`oleates [19] were reported.
`An increasingly common industrial application of fatty
`esters is biodiesel [20, 21], defined as the mono-alkyl esters
`of vegetable oils or animal fats. While biodiesel is gener-
`ally technically competitive with conventional diesel fuel
`derived from petroleum (petrodiesel), it features advanta-
`ges in terms of domestic production, renewability, reduc-
`tion of most regulated exhaust emissions except nitrogen
`oxides, biodegradability, inherent lubricity, and safer han-
`dling due to higher flash point. Technical problems with
`biodiesel include nitrogen oxides exhaust emissions, oxi-
`dative stability and cold flow. The latter problem is
`exemplified by the relatively high cloud and pour points of
`biodiesel [22]. A major factor influencing this cold flow
`behavior is the high MP of saturated fatty esters found in
`biodiesel.
`Data analyzed from DSC melting and cooling (freezing)
`curves were correlated with measured cold flow properties
`of biodiesel [23]. Both cloud point (CP) and low-temper-
`ature flow test (LTFT) data indicated good probabilities of
`correlating to melting curve peak maximum temperatures.
`Completion of melt temperatures from DSC melting curves
`were used to predict crystallization temperatures of FAME
`from conventional and low-palmitic acid soybean oil [24]
`and mono-alkyl esters from tallow and waste grease [25,
`26].
`Thus cold flow is a major factor when considering which
`fatty esters to enrich in biodiesel when ‘‘designing’’ an
`
`123
`
`optimized biodiesel fuel composition. In recent work, it
`was discussed that esters of palmitoleic acid and decanoic
`acid may be candidate compounds for enrichment in bio-
`diesel to help improve cold flow while not comprising
`other important fuel properties [27]. Biodiesel produced by
`transesterification of oils or fats with medium—(C2 or C4)
`or branched—chain alcohols, instead of the more com-
`monly used methanol, generally has better cold flow
`properties [24–26, 28–30]. Thus, for determining optimum
`mono-alkyl fatty ester compositions and for modeling the
`cold flow behavior of mixtures of esters such as biodiesel,
`accurate and consistent MP data are critical, thus providing
`another reason for this study.
`In light of the issues mentioned above, in this work the
`MP of fatty acids as well as a variety of methyl, ethyl and
`other alkyl esters,
`including fatty acids with methyl
`branching (iso and anteiso fatty acid chains) in the C8–C24
`range, which covers the most common fatty acids, were
`determined by DSC. The MP of numerous compounds
`previously not investigated for this property were deter-
`mined. Systematic influences for x-9 fatty acids and esters
`as well as methyl-branched iso acids and methyl esters are
`presented for the first time. The results are compared to
`literature data.
`
`Experimental
`
`Straight-chain fatty acids and esters were obtained from Nu
`Chek Prep (Elysian, MN). Branched fatty acids and esters
`were obtained from Sigma-Aldrich (Milwaukee, WI) or
`Matreya LLC (Pleasant Gap, PA). To ensure purity and
`nature of
`the samples, some samples were randomly
`checked by GC-MS and NMR (solvent CDCl3; 500 MHz
`for 1H-NMR, 125 MHz for 13C-NMR). All samples were
`found to be of advertised purities or higher ([98–99%).
`NMR also served structure verification purposes, including
`the double bond position for unsaturated fatty acids and
`esters. The position and separation of the olefinic carbon
`signals in 13C-NMR of monounsaturated fatty acid chains
`depends on the proximity of the double bond to the ter-
`minal ester or methyl group [31]. Thus, for a double bond
`‘‘migrating’’ from the 5- to the 15-position in the mono-
`unsaturated samples investigated here, a decrease in the
`difference of the signals of the olefinic carbons from
`2.9 ppm (C20:1 D5) to 0.03 ppm (C22:1 D13) to 0 ppm
`(C23:1 D14 and C24:1 D15) was observed.
`Differential scanning calorimetry analysis is based on
`determining heat flow through a sample by simultaneous
`measurements through a sample pan and an empty refer-
`ence pan. Hermetically sealed aluminum pans were used
`for
`the present analyses. A computer-controlled DSC
`model Q2000 (TA Instruments, Wilmington, DE) was used
`
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`J Am Oil Chem Soc (2009) 86:843–856
`
`845
`
`Table 1 Literature data for melting points (°C) of saturated straight-chain fatty acids and esters investigated here
`
`Chain Acid
`
`Methyl ester
`
`Ethyl ester
`
`Propyl ester
`
`Butyl ester
`
`Triacylglycerol
`
`8:0
`
`16.5 [1, 3]; 16.3 [2]
`
`16, 16.5 [4]
`
`16 [5]
`
`-40 [1–3]
`
`-37.27 [5]
`
`9:0
`
`12.2 [1], 12.3 [2],
`
`-35 [4]
`
`12.4 [3]
`
`12.5, 15 [4]
`
`12.52 [5]
`
`10:0
`
`31.5 fr [1]; 31.9 [2],
`
`31.4 [3]
`
`31.6, 31.5 [4]
`
`31.39 [5], 31.0 [5]
`
`-18 [1–3]
`
`-13.34 [5]
`
`-43.1 [1–3]
`
`-43.1 [5]
`
`-36.7 [1–3]
`
`-44.5 [4]
`
`-36.7 [5]
`
`-20 [1–3]
`
`-19.9 [5]
`
`11:0
`
`28.6 [1–3]
`
`-16 [1], -15 [2, 3]
`
`29.3, 28.2–28.6 [4]
`
`28.20 [5]
`
`-15 [5]
`
`-46.2 [1–3]
`
`-42.9 [1–3]
`
`-45 [3]
`
`-43 [3]
`
`-38 [2, 3]
`
`-38 [4]
`
`32 [3]
`
`32 [4]
`
`31.2 [4]
`
`12:0
`
`44 [1], 43.2 [2], 43.8 [3]
`
`5.2 FP [1], 5.2 [2, 3] -1.8 FP [1], -10 [2, 3]
`
`-7 [4]
`
`46.4 [1]
`
`44.8, 44 [4]
`
`44.1 [5], 44.2 [5]
`
`13:0
`
`41.5 [2, 3]
`
`41.8, 44.5–45.5 [4]
`
`41.76 FP [5]
`
`14:0
`
`58 [1], 53.9 [2],
`
`54.2 [3]
`
`54.4, 54 [4]
`
`5 [4]
`
`4.80 [5]
`
`6.5 [2, 3]
`
`6.5 [4]
`
`5.5 [5]
`
`19 [1–3]
`
`18.5 [4]
`
`17.86 [5]
`
`18.8–19.1 [9]
`
`-15.5 [5]
`
`-4.8 [5]
`
`44.5 [4]
`
`12–13 [1], 12.3
`
`9.4–10.0 [9]
`
`6.2–7.0 [9]
`
`56.5 [2], 58.5 [3]
`
`[2, 3]
`
`12.3 [4]
`
`11 [5]
`
`58.5 [4]
`
`57.4 [5], 54.25 [5]
`
`15:0
`
`53–54 [1], 52.3 [2, 3]
`
`18.5 [1–3]
`
`52.5, 53 [4]
`
`52.40 [5]
`
`16:0
`
`63 [1], 63.1 [2],
`
`62.5 [3]
`
`62.9, 63–64 [4]
`
`62.8–63.0 [5]
`
`18.5 [4]
`
`18.5 [5]
`
`19.2–19.4 [9]
`
`30 [1–3]
`
`30.5 [4]
`
`29.5 [5]
`
`30.0–30.3 [9]
`
`17:0
`
`62–63 [1], 61.3 [2, 3]
`
`30 [1–3]
`
`61.3, 62–63 [4]
`
`60.85 [5]
`
`18:0
`
`71.2 [1], 68.8 [2],
`
`69.3 [3]
`
`70.1, 69.7 [4]
`
`68.9 [5], 69.5 [5]
`
`19:0
`
`69.4 [1–3]
`
`69.4, 68.7 [4]
`
`68.6 [5]
`
`30 [4]
`
`29 [5]
`
`29.7–30.2 [9]
`
`39.1 [1–3]
`
`39.1 [4]
`
`37.85 [5]
`
`41.3 [n]
`
`38.9 [4]
`
`39.5–40.5 [5]
`
`38.7–39.0 [9]
`
`12.6–13.0 [9]
`
`14 [4]
`
`11.5 [5]
`
`12.7–13.2 [9]
`
`11.9–12.1 [9]
`
`6.8–7.1 [9]
`
`a: 24 b: 19.3 [1],
`
`20.4 [1, 2]
`
`16.9 [1–3]
`
`66.4 [1], 66.5 [2.3]
`
`24 [2, 3]
`
`19.3 [4]
`
`23.2 [5]
`
`24.3–24.8 [9]
`
`28 [1, 2]
`
`25.2 [5]
`
`25.0–25.4 [9]
`
`20.9–21.9 [9]
`
`16.8–17.1 [9]
`
`66.4 [4]
`
`24.2–24.7 [7, 9]
`
`20.4–21.0 [9]
`
`64 [4]
`
`31–33 [1], 33
`
`28.9 [1, 2]
`
`27.5 [1, 2],
`
`73 [1]
`
`[2]
`
`33.4 [4]
`
`33.5 [5]
`
`37–38 [5]
`
`35.7–36.2 [9]
`
`28.6 [4]
`
`30.8–31.4
`
`[7]
`
`73.5 [4]
`
`27[3]
`
`27.5 [4]
`
`26.6–27.2 [7]
`
`33.0–33.9 [7]
`
`29.2–31.4 [7]
`
`71 [4]
`
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`

`846
`
`Table 1 continued
`
`J Am Oil Chem Soc (2009) 86:843–856
`
`Chain Acid
`
`Methyl ester
`
`Ethyl ester
`
`Propyl ester
`
`Butyl ester
`
`Triacylglycerol
`
`20:0
`
`77 [1], 76.5 [2, 3]
`
`54.5 [1–3]
`
`76.1, 77 [4]
`
`73.35 [5]
`
`45.8–46.3 [4]
`
`45.8 [5]
`
`50 [1, 2]
`
`41.4–42 [4]
`
`40.54 [5]
`
`45.8–46.3 [9]
`
`41.4–42.0 [9]
`
`21:0
`
`75.2, 73–74 [3]
`
`74.3 [4]
`
`48–50 [3]
`
`48–49 [4]
`
`46.7–47.0 [8]
`
`22:0
`
`80 [1], 81 [2], 81.5 [3]
`
`54 [1–3]
`
`80, 81–82 [4]
`
`79.95 [5]
`
`54 [4]
`
`53.2 [5]
`
`45 [4]
`
`44.3–44.5 [8]
`
`50 [1, 2]
`
`50 [4]
`
`48.25 [5]
`
`52.2–52.8 [9]
`
`47.8–48.5 [9]
`
`23:0
`
`79.6, 79.1 [4]
`
`79.1 [5]
`
`24:0
`
`87.5 [2, 3]
`
`55.6 [1]
`
`53.4 [4]
`
`55–56 [5]
`
`53.3–53.5 [9]
`
`59.5–60 [4]
`
`84.2, 87.5–88 [4]
`
`57.8 [5]
`
`52–53 [5]
`
`50.8–51.2 [9]
`
`56–57 [4]
`
`54.35 [5]
`
`84.15 [5]
`
`57.8–58.3 [9]
`
`54.2–54.5 [9]
`
`FP freezing point
`
`38.3–39.0 [7]
`
`36.6–36.8 [7]
`
`78 [4]
`
`42.4–43.1 [6]
`
`40.1–40.4 [6]
`
`75.9 [4]
`
`45.7–46.2 [7]
`
`43.8–44.1 [7]
`
`82.5 [3]
`
`48.4–49.2 [7]
`
`47.3–47.5 [9]
`
`51.8–52.3 [7]
`
`50.6–50.8 [9]
`
`86 [4]
`
`for determining the melting phase transitions of experi-
`mental samples. The system consisted of a measurement
`cell fitted with a refrigerated cooling system and a model
`5000 PC-based controller for conducting experiments and
`analyzing the resulting scans. Reference and sample pans
`were placed in precise positions within the cell by an
`autosampler. The sample purge gas was dry nitrogen at
`50.00 mL/min.
`Melting phase transitions were analyzed by the proce-
`dure given in American Oil Chemists’ Society (AOCS)
`method Cj 1-94 [32]. The three-step temperature program
`consisted of: (1) heating the sample to the starting tem-
`perature and holding it isothermally for 10 min to ensure
`complete transition into the liquid phase; (2) cooling
`rapidly to form a solid phase and holding for 30 min; and
`(3) heating at 5 °C/min back across the melting region to
`the final temperature.
`While DSC instruments are commonly calibrated with
`indium, the performance of the instrument at temperatures
`below the MP of indium (156.79 °C; enthalpy of melt-
`ing = 28.66 J/g) was checked with additional materials due
`to the lower MP of fatty esters. Thus benzil, a common MP
`standard, and water were selected as compounds for com-
`parison. The peak melting temperature values for benzil and
`water (distilled) were 94.37 (average of three determina-
`tions; standard deviation 0.08) and -0.04 °C (three deter-
`minations,
`standard deviation 0.34),
`respectively. A
`literature value for the MP of benzil is 94.87 °C [3].
`
`For each DSC peak, an onset, peak and completion
`temperature (Ton, Tpeak, Tcom, respectively) can be deter-
`mined directly from the scan by use of the system software.
`As discussed below, Tpeak is given in the tables as MP.
`Typical scans are shown in Figs. 1 and 2.
`
`Results and Discussion
`
`In this work, the MP of saturated and unsaturated fatty
`acids and esters with chain lengths C8–C24 were deter-
`mined by DSC using AOCS method Cj 1-94. The exam-
`ined chain length range covers most fatty acids as they
`occur naturally in vegetable oils or animal fats. Several
`methyl-branched saturated acids of the iso (branching at
`Cn-1 in the chain) and anteiso (branching at Cn-2) types
`were also studied. Typical DSC heating scans are shown in
`Figs. 1 and 2. Tables 1, 2 and 3 contain literature data on
`the MP of compounds investigated here while Tables 4, 5,
`6, and 7 present data determined during the course of this
`work. Table 4 presents the MP of saturated fatty acids,
`methyl esters, ethyl esters as well as some propyl and butyl
`esters for this range of fatty acid chain lengths. Table 5
`lists the same information for a variety of olefinic fatty
`acids and esters with chain lengths C11–C24. Table 6 gives
`this information for some methyl-branched (mainly iso
`and anteiso) acids and esters with a total of C11–C20
`including the methyl branch. The MP of triacylglycerols
`
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`J Am Oil Chem Soc (2009) 86:843–856
`
`847
`
`Table 2 Literature data for melting points (°C) of unsaturated fatty acids and esters investigated here
`
`Chain
`
`Acid
`
`Methyl ester
`
`Ethyl ester
`
`Propyl ester Butyl ester
`
`Triacylglycerol
`
`-27.5 [2, 3]
`
`-27.5 FP [4]
`
`-27.5 [5]
`
`-38 [2, 3]
`
`-37.5 [5]
`
`\15 [5]
`
`\15 [5]
`
`C11:1 D10
`
`C14:1 D9c
`
`C16:1 D9c
`
`C16:1 D9t
`C18:1 D6c
`
`24.5 [2, 3]
`
`24.5 [4]
`
`24–24.5 [5]
`
`-4 [2]
`
`-4.5 [4]
`
`-4.5 to -4 [5]
`
`-0.1 [2], 0.5 [3]
`
`0.5 [4]
`
`-1.0 [5]
`
`32, 32–33 [4]
`
`29.8 [3], 29, 31, 33 [4]
`
`31 [10]
`
`28–29 [6], 29 [8]
`
`28.6 [5]
`
`C18:1 D6t
`
`52.7–53.4, 54–59 [4]
`
`53 [10]
`
`53-54 [6], 54 [8]
`
`53.6 [5]
`
`28 [4]
`
`52 [4]
`
`C18:1 D9c
`
`16.3 [1], 13.4 [2, 3]
`
`-19.9 [1–3]
`
`\-15 [5]
`
`-26.4 [1–3] -5.5 [1], -32 [2], -4 [3]
`
`12 (lab), 16 (stab), 16.2 [4] -19.6 to -19.9 [5]
`
`-26.4 [4]
`
`5 [4]
`
`13.2 [5]
`
`13 [10]
`
`10–11 [6], 11 [8]
`
`C18:1 D9t
`
`45 [1–3]
`
`C18:1 D11c
`
`45.5, 45–45.5 [4]
`
`43.68 [5]
`
`44 [10]
`
`44.5–45.5 [6], 45 [8]
`
`15.5, 14.5–15.5 [4]
`
`13–14 [5]
`
`15 [10]
`
`12.5–13.5 [6], 13 [8]
`
`C18:1 D11t
`
`44 [2, 3]
`
`C18:1 D9c,12c
`
`44.1, 43.5–44.1 [4]
`
`43.5–44.5 [5]
`
`44 [10]
`
`43.5–44.5 [6], 44 [8]
`
`-5 [1]
`
`-5 [2]
`
`-5 [10]
`
`-5.2 to -5.0 [5]
`C18:1 D9c, 12c, 15c -11.3 [1]
`
`-11 [2]
`
`-11 [10]
`
`-11.3 to -11 [5]
`
`27, 26–27 [4]
`
`52.5–54 [4]
`
`23–23.5 [4]
`
`C20:1 D5c
`C20:1 5t
`C20:1 D9c
`
`13.5 [3]
`
`13–13.5 [4]
`\15 [5]
`
`5.8 [3]
`
`5.1 [4]
`
`-12.5 [4]
`
`10.6 [4]
`
`\15 [5]
`
`-11 [4]
`
`-35 [1, 3]
`\15 [5]
`
`-45.5 [3]
`\15 [5]
`
`123
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`
`

`

`848
`
`Table 2 continued
`
`Chain
`
`C20:1 D9t
`C20:1 D11c
`
`C20:1 D11t
`
`Acid
`
`54 [4]
`
`24 [2, 3]
`
`25, 24–25 [4]
`
`22 [5]
`
`49–51 [4]
`
`53–54 [5]
`
`J Am Oil Chem Soc (2009) 86:843–856
`
`Methyl ester
`
`Ethyl ester
`
`Propyl ester Butyl ester
`
`Triacylglycerol
`
`-45 [4]
`
`-15 [5]
`
`C22:1 D13c
`
`33–34 [1], 33.5 [2],
`
`2–3 94 [5]
`
`32 [4]
`
`34–35 [4]
`
`34–35 [5]
`
`30.5 [4]
`
`30–30.5 [5]
`
`58 [4]
`
`-14.10°C
`
`146.2J/g
`
`-13.06°C
`
`34.7 [3]
`
`33.5 (-51, -7, 2, 14) [4]
`
`33.5 [5]
`
`C22:1 D13t
`
`61.5 [1], 61.9 [2, 3]
`
`C24:1 D15c
`
`61.5 [4]
`
`60 [5]
`
`39–39.5 [5]
`
`45–41 [4]
`
`1
`
`0
`
`-1
`
`-2
`
`-3
`
`Heat Flow (W/g)
`
`Fig. 1 DSC heating scan of
`methyl decanoate (data included
`in calculation in Table 5)
`
`-4
`-100
`
`-80
`
`-60
`
`-20
`-40
`Temperature (°C)
`
`0
`
`20
`
`40
`
`(triglycerides) of fatty acids with chain lengths C8–C24 are
`contained in Table 7. Figs. 3, 4 and 5 are visualizations of
`the data in Tables 4, 5, 6 and 7.
`
`Procedure
`
`For sake of brevity and reasons given below, only the values
`of Tpeak are given in Tables 4, 5, 6 and 7. Although gen-
`erally an increase in heat flow was observable for increased
`chain lengths, heat flow varied considerably and was
`probably affected by how the sample itself was distributed
`within the pan. Therefore, no heat flow data are given. The
`variability of heat flow was confirmed by analyzing some
`samples sequentially five times. The heat flow decreased
`considerably within such a sequence, being only a fraction
`of the initial value when the fifth run was conducted.
`
`Melting onset and completion can also be determined by
`DSC and these parameters are usually reported when
`AOCS method Cj 1-94 is applied to analyze oils and fats.
`However,
`these temperatures,
`rather
`the differences
`between melting peak maxima and onset and completion
`temperatures, also depend on the amount of sample, so that
`the melting onset and completion temperatures are not
`given here. The values for Tpeak appear to be less influ-
`enced by this aspect. Generally, the difference between
`melting onset temperature and peak temperature (given as
`MP) was greater (range of 1.5–3 °C) than that between
`peak temperature and melting completion temperature
`(range 0.5–1.5 °C). The peak melting temperature is given
`as MP in Tables 4, 5, 6 and 7. It may be noted that, for
`example, the peak melting temperature given for water and
`benzil (see ‘‘Experimental’’ section) is the closest of all
`
`123
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`
`

`

`J Am Oil Chem Soc (2009) 86:843–856
`
`849
`
`-7.59°C
`
`11.34°C
`
`28.59J/g
`
`-5.90°C
`
`150.6J/g
`
`1
`
`0
`
`-1
`
`-2
`
`Heat Flow (W/g)
`
`Fig. 2 DSC heating scan of
`oleic acid (data included in
`calculation in Table 5)
`
`-3
`-100
`
`-80
`
`-60
`
`-40
`
`0
`-20
`Temperature (°C)
`
`12.64°C
`
`20
`
`40
`
`60
`
`Table 3 Literature data for melting points (°C) of branched fatty
`acids and esters investigated here
`
`Branched (iso)a
`
`Acid
`
`Methyl ester
`
`materials exhibiting such behavior is beyond the scope of
`this work. In order to record any such observations, peaks
`indicating polymorphism are indicated in Tables 4 and 5
`by peak maxima given in italics. The MP given correspond
`to the stable modification of the compounds investigated
`here. Note that in case of discrepancies with literature
`values, the MP determined here are usually either slightly
`lower than the previous literature values or tend towards
`the lower end of reported values.
`
`Comparison to Prior Data
`
`The data in Tables 1, 2 and 3 show that in many cases the
`MP data in the literature agree well with data observed
`here. However, in some cases significant differences can be
`observed. Furthermore, there are numerous cases in which
`prior literature data vary, so that the present data can be
`used to verify or confirm more accurate data.
`Results for several compounds deviated considerably
`from reported literature data. The MP was -2 °C in the
`present work for ethyl dodecanoate compared with lower
`literature values of -10 and -15 °C (Table 1). Likewise,
`the MP of propyl heptadecanoate was 22.3 °C compared
`with 24.2-24.7 °C in the literature. Similar observations
`were made for comparison of results with literature values
`in Table 2 with corresponding data for alkyl oleates. Propyl
`oleate has MP = -30.5 versus -27.2 °C from the litera-
`ture (Table 2); butyl oleate has MP = -34.8 versus -26.4
`to -31.7 °C (Table 2); and butyl elaidate has MP = 0.2
`versus 10.6 °C. The MP of methyl linoleate was -43.1 °C
`in the present work though some scans showed a minor
`peak near -37.8 °C, compared with a value of -35 °C
`from the literature. Methyl cis-11-eicosenoate had an
`MP = -7.8 °C for the present work, a value that was
`than -45 °C as reported in the
`considerably higher
`literature.
`
`123
`
`10-Me C11
`
`12-Me C14
`
`13-Me C14
`
`14-Me C15
`
`15-Me C16
`
`16-Me C17
`
`17-Me C18
`
`41.2 [4]
`
`41.2 [5]
`
`53 [4]
`
`50.5–51 [5]
`
`52.5–53 [4]
`
`61.8–62.4 [4]
`
`61.8–62.4 [5]
`
`60.3–60.5 [4]
`
`68.8–69.7 [5]
`
`67.3–67.8 [4]
`
`67.3–67.8 [5]
`
`66.5–67 [15]
`
`18-Me C19
`
`75.3 [5]
`
`Branched (anteiso)
`
`10–Me C12
`
`12-Me C14
`
`13-Me C15
`
`14-Me C16
`
`2.7 [18]
`
`16.5 [18]
`
`18.3–18.9 [4]
`
`25.7 [18]
`
`26/27.5 [4]
`
`26–28 [5]
`
`-31.8 [18]
`
`-10.9 [18]
`
`65–66 [5]
`
`(S): 39.5–40, 39–40 [4]
`
`3.8 [18]
`
`a For example, the acid termed 10-Me C11 is 10-methyl undecanoic
`acid with 11 carbons in the chain but possessing a total of 12 carbon
`atoms when counting the branching at C11
`
`three temperatures (onset, peak, melting) to its known MP,
`providing a justification for using only this value.
`Variations in the DSC-based MP determinations of up to
`approximately ±1 °C were observed. The MP points of all
`samples were thus determined at least in triplicate. The
`resulting standard deviation (SD) for a sample is given in
`parentheses in Tables 4, 5, 6 and 7.
`The DSC scans of some compounds investigated here
`are more complex than only exhibiting one peak corre-
`sponding to the MP. Investigating the polymorphism of all
`
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`
`

`

`850
`
`J Am Oil Chem Soc (2009) 86:843–856
`
`52.32(0.44),51.17(1.22)
`
`45.29(0.17)
`
`55.92(0.16)
`
`51.22(0.73)
`
`48.64(0.25)
`
`43.66(0.37),36.30(0.28)
`
`35.14(0.07),20.27(0.02),23.60(0.23)
`
`37.15(0.07),31.99(0.09)
`
`41.33(0.66)
`
`30.71(0.03),15.84(0.06),18.87(0.10)
`
`32.94(0.24),25.36(0.15)
`
`35.28(0.41),27.30(0.53)
`
`38.03(0.31),31.89(0.13)
`
`25.63(0.18),10.51(0.11),14.09(0.14)
`
`28.10(0.30),24.93(0.81)
`
`32.98(0.33),30.43(0.33)
`
`37.66(0.25)
`
`19.68(0.20),9.18(0.23)
`
`16.07(0.26)
`
`6.26(0.06),3.82(0.31)
`
`5.57(0.12)
`
`-8.48(0.25)
`
`-6.53(0.19),-7.42(0.09)
`
`-23.69(0.14)
`
`-22.96(0.21)
`
`-43.10(0.10)
`
`-43.33(0.06)
`
`Butylester
`
`22.32(17.81)
`
`20.27(0.23)
`
`24.70(0.32),19.92(0.41)
`23.23(0.42)
`
`9.24(0.37)
`
`-4.35(0.19)
`
`-19.70(0.08)
`
`-21.84(0.19)
`
`-41.81(0.13)
`
`-45.68(0.32)
`
`Propylester
`
`11.81(0.29)
`
`12.52(0.60)
`
`-2.07(0.22)
`
`-1.78(0.24)
`
`-19.43(0.41)
`
`-20.44(0.42)
`
`-43.56(0.22)
`
`-44.74(0.14)
`
`Ethylester
`
`5.17(0.35)
`
`4.30(0.54)
`
`Standarddeviationsinparentheses.Secondarypeakslikelyindicatingadditionalphasetransitionsareitalicized
`
`58.61(0.71)
`
`53.38(0.45)
`
`53.22(0.26)
`
`47.58(0.34)
`
`46.43(0.38)
`
`28.58(0.06),25.85(0.37)
`28.48(0.44)
`
`18.47(0.50)
`
`18.09(0.42)
`
`83.82(0.42)
`
`78.74(0.26)
`
`79.54(0.34)
`
`73.69(0.34)
`
`74.76(0.27)
`
`67.76(0.27),66.53(0.23)
`
`69.29(0.19)
`
`60.85(0.34),57.27(0.15)
`62.20(0.19)
`
`52.15(0.17),46.00(0.35)
`
`53.47(0.31),50.52(0.16)
`
`41.37(0.20),34.64(0.46)
`
`43.29(0.34),42.96(0.03)
`
`-12.17(0.33)
`
`27.32(0.05),17.42(0.26)
`
`-13.48(0.52)
`
`30.80(0.31)
`
`-34.99(0.53)
`
`11.28(0.50),-9.31(0.19)
`
`-37.43(0.26)
`
`15.41(0.18)
`
`24:0
`
`23:0
`
`22:0
`
`21:0
`
`20:0
`
`19:0
`
`18:0
`
`17:0
`
`16:0
`
`15:0
`
`14:0
`
`13:0
`
`12:0
`
`11:0
`
`10:0
`
`9:0
`
`8:0
`
`Table4Meltingpoints(°C)ofsaturatedfattyacidsandestersdeterminedinthepresentwork
`
`Methylester
`
`Acid
`
`Chain
`
`123
`
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`
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`

`J Am Oil Chem Soc (2009) 86:843–856
`
`851
`
`aSingledetermination
`Secondarypeakslikelyindicatingadditionalphasetransitionsareitalicized
`
`123
`
`-22.69
`
`-58.61a
`-51.50(0.63)
`
`-57.63a
`
`-0.23(0.03),-0.55
`
`-0.03(0.60)
`
`-30.50(0.34)-34.76(0.43)
`
`-11.35(0.14)
`
`-24.35(0.12)
`
`-12.83(0.16)
`
`-52.61(0.36),-54.10(0.14)
`
`-66.19(0.07)
`
`1.21(0.42)
`
`13.27(0.15)
`
`24.50(0.34)
`
`-10.54(0.22)
`
`5.63(0.32),3.00(0.06)
`
`14.11(0.43)
`
`-8.80(0.26)
`
`3.14(0.75)
`
`9.49(0.28)
`
`18.22(0.13)
`
`29.36(0.14)
`
`8.47(0.15)
`
`20.76(0.41)
`
`-7.79(0.34)
`
`9.11(0.43)
`
`-8.57(0.45),-46.95(0.51),-18.92(0.44)
`-7.51(0.36)
`-61.71a
`
`2.39(0.20),-45.52(0.21)
`-2.33(0.49)
`
`-43.09(0.71),-37.77(0.02)-56.72(0.57),-58.80(1.23)
`
`4.10(0.32)
`
`-36.49(0.68)
`
`4.17(0.36)
`
`-20.32(0.36)
`
`9.45(0.30)
`
`-7.74(0.49)
`
`-20.02(0.32),-21.94(0.41)
`
`-10.99(0.29)
`
`9.94(0.24)
`
`-24.29(0.72)
`
`9.94(0.24)
`
`-20.21(0.51)
`
`19.16(0.20)
`
`-0.97(0.26)
`
`-16.02(0.26)
`
`-2.99(0.39)
`
`-34.10(0.26),-43.53(0.14)-36.65(0.29)
`
`-65.35(0.38)
`
`-32.24(0.31)
`
`-52.26(0.82)
`
`-24.63(0.23)
`
`32.96(0.14)
`
`51.94(0.31)
`
`23.37(0.14)(-4.68),
`
`35.13(0.26)(34.09)
`
`26.61(0.57)(4.92)
`
`22.47(0.29)
`
`-7.15(0.69)
`
`43.37(0.65)
`
`15.40(0.29)
`
`43.35(0.24)
`
`12.82(0.15),-5.59(0.30)
`
`52.38(0.22)
`
`29.11(0.14)
`
`15.05(0.19),-0.62
`
`32.22(0.33)
`
`1.22(0.46),-23.77(2.69)
`
`-3.91(1.17)
`
`23.91(0.14)
`
`24:1D15c
`23:1D14c
`22:1D13t
`22:1D13c
`21:1D12
`20:1D11t
`20:1D11c
`20:1D8c
`20:1D5c
`19:1D10c
`18:3D9c,D12c,D15c-11.58a
`18:2D9c,D12c
`18:1D11t
`18:1D11c
`18:1D9t
`18:1D9c
`18:1D6t
`18:1D6c
`17:1D10c
`16:1D9t
`16:1D9c
`14:1D9c
`11:1D10
`
`42.87(0.27)
`
`43.23(0.11),39.28(0.23)
`
`59.16(0.39)
`
`32.18(0.37),7.69(0.19),29.16(0.62)-3.05(0.43)
`
`Butylester
`
`Propylester
`
`Ethylester
`
`Methylester
`
`Acid
`
`Chain
`
`Table5Meltingpoints(°C)ofunsaturatedfattyacidsandestersdeterminedinthepresentwork
`
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`
`

`

`852
`
`J Am Oil Chem Soc (2009) 86:843–856
`
`Table 6 Melting points (°C) of some branched fatty acids and methyl
`esters determined in the present work
`
`Chain
`
`Iso acids
`
`10-Methyl C11
`
`11-Methyl C12
`
`12-Methyl C13
`
`13-Methyl C14
`
`14-Methyl C15 (isopalmitic)
`
`15-Methyl C16
`
`16-Methyl C17 (isostearic)
`
`17-Methyl C18
`
`18-Methyl C19
`
`19-Methyl C20
`
`Anteiso acids
`
`12-Methyl C14
`
`13-Methyl C15
`
`14-Methyl C16
`
`Acid
`
`Methyl ester
`
`40.28
`
`40.57
`
`53.13
`
`51.35
`
`61.94
`
`59.78
`
`69.23
`
`66.34
`
`74.68
`
`72.51
`
`24.05
`
`37.10
`
`-13.02
`
`-6.87
`
`3.48
`
`6.37
`
`16.80
`
`17.55
`
`26.82
`
`27.98
`
`36.43
`
`-5.29
`
`-13.34
`
`7.62
`
`The present data can also be employed to verify more
`accurate data reported in the literature. For example, ethyl
`nonanoate had a MP = -43.6 °C (Table 4) compared to
`values of -36.7 and -44.4 °C in Table 1. Other examples
`included methyl decanoate (MP = -13.5 herein vs. -18 to
`-13 °C in the literature), ethyl heptadecanoate (24.7 vs.
`25–28 °C), methyl eicosanoate (MP = 46.4 vs. 46–
`54.5 °C), ethyl eicosanoate (MP = 41.3 vs. 41–50 °C), and
`tetracosanoic acid (83.8 vs. 84–88 °C). The common
`methyl palmitate had MP = 28.5 °C in comparison with
`values of 24 and 29.5–30.5 °C reported in the literature.
`In the present work, the MP of oleic acid was determined
`to be around 12.8 °C. Literature values for the MP of oleic
`acid given in the literature varies in the range of 10–16 °C,
`probably due to different crystalline modifications, although
`one paper [33] reports 5 °C as the MP for oleic acid as
`determined by DSC, a result not in agreement with the
`present work or with other investigations reported in the
`literature. For the common ester methyl stearate, a MP of
`37.7 °C (Table 4) was found, a value that generally agreed
`well with MP data given in the literature (37–39.1 °C).
`Another work [18] provides MP for various oleate
`esters. In that reference, the peak melting temperature of
`methyl oleate is reported as -17.3 °C, which is several °C
`higher than the value reported here. The lowest melting
`reported was for isopropyl oleate around -33 °C. In con-
`trast, analysis of DSC cooling curves for methyl oleate
`yielded crystallization onset
`(freezing point)
`tempera-
`tures = -17.5 °C [34] and -19.8 °C [35], values compa-
`rable to those in Tables 2 and 5.
`Another study [36] reported MP data for several pure
`FAME measured by an automated ‘‘mini-cloud point’’ test
`
`123
`
`Table 7 Melting points of some triacylglycerols determined in the
`present work (quadruplicate determinations; standard deviations in
`parentheses)
`
`Chain
`
`Saturates
`
`Melting point (°C)
`
`8:0
`
`9:0
`
`10:0
`
`11:0
`
`12:0
`
`13:0
`
`14:0
`
`15:0
`
`16:0
`
`17:0
`
`18:0
`
`19:0
`
`20:0
`
`21:0
`
`22:0
`
`23:0
`
`9.44 (0.14)
`
`9.46 (0.09)
`
`30.37 (0.24)
`
`27.98 (0.84)
`
`46.29 (0.04)
`
`44.60 (0.25)
`
`57.35 (0.19)
`
`55.46 (0.05)
`
`65.45 (0.27)
`
`64.11 (0.06)
`
`72.67 (0.32)
`
`71.31 (0.29)
`
`77.67 (0.12)
`
`76.36 (0.43)
`
`82.50 (0.08)
`
`81.85 (0.22)
`
`Unsaturates
`16:1 D9c
`
`18:1 D6c
`18:1 D9c
`18:1 D11c
`18:2 D9c, D12c
`19:1 D10c
`20:1 D11c
`21:1 D12c
`22:1 D13c
`24:1 D15c
`
`-22.75; 25.68, -21.81; -27.75, -21.88 (two
`peaks)
`
`26.24 (0.06)
`
`3.98 (0.59)
`
`1.04 (0.08)
`
`-12.70 (0.35)
`
`26.12 (0.05)
`
`10.11 (0.10), 17.80 (0.14)
`
`37.97 (0.08)
`
`29.78 (0.23)
`
`41.43 (0.09)
`
`apparatus. Although this experimental procedure is better
`characterized as analysis of crystallization onset tempera-
`tures, data reported as MP for methyl myristate (17 °C) and
`stearate (37 °C) were within 1–2 °C of results reported in
`the present work. An earlier work [37] analyzed and
`reported Tonset and Tpeak from DSC heating scans per-
`formed on methyl palmitate, stearate and oleate. Onset
`temperatures were 1.8–2.1 °C lower than measured peak
`maxima for these esters. Taking the MP at peak maximum
`temperature resulted in values for methyl palmitate, stea-
`rate and oleate (29.9, 39.0, -18.9 °C) that agreed well with
`results shown in Table 4.
`
`Saturated Unbranched Fatty Acid Chains
`
`The data in Table 1 for saturated fatty acids confirm
`previous observations [9, 12] regarding alternating MP,
`
`Purdue 2014
`Collegium v. Purdue, PGR2018-00048
`
`

`

`J Am Oil Chem Soc (2009) 86:843–856
`
`853
`
` Acid
` Methyl ester
`
`75
`70
`65
`60
`55
`50
`45
`40
`35
`30
`25
`20
`15
`10
`
`05
`
`-5
`-10
`-15
`
`Melting Point (oC)
`
`22
`21
`20
`19
`18
`17
`16
`15
`14
`13
`12
`Number of carbon atoms in the fatty acid chain (incl. branching)
`
`Fig. 5 Melting points (Tpeak) of iso acids and their methyl esters. The
`number of carbon atoms includes those in the methyl branches
`
`decreasing with the number of carbons in the ester moiety.
`In this case, the esters of odd-numbered fatty acids show
`only minor differences, often less than 1 °C, to those of the
`preceding even-numbered fatty esters. Furthermore,
`the
`MP differences between acids their corresponding esters
`decrease with increasing chain length.
`
`Unsaturated Fatty Acid Chains
`
`The introduction of one cis double bond in a fatty acid
`chain reduces the MP considerably compared to the satu-
`rated chain with the same number of carbon atoms. A trans
`double bond has comparatively little effect on the MP
`compared to the saturated chain with the same number of
`carbon atoms. This is a result of a cis double bond intro-
`ducing a bend in the hydrocarbon chain while in the case of
`a trans double bond, the chain is propagated in a fashion
`close to that of the corresponding saturated species. This
`structural similarity of chains with trans double bonds to
`fully saturated chains is also expressed in other physical
`properties, such as viscosity [38].
`The cis x-9 fatty acids in the range of C18–C24 show MP
`alternation reversed in comparison to the saturated fatty
`acid chains. For these fatty acids, the even-numbered cis
`x-9 fatty acid possesses a slightly lower MP (or only slightly
`higher, see C20 vs. C19) than the preceding odd-numbered
`fatty acid or the increase is very small (Table 5; Fig. 4).
`Another reversal is the strong alternation observed for the
`cis x-9 methyl and ethyl esters in comparison to the acids.
`This effect is also shown by the difference in MP between
`the acids and the corresponding methyl esters. For the cis
`x-9 chains with an even number of carbons, the difference
`in MP between the acid and the corresponding me