Article pubs.acs.org/jced
The Use of Antioxidants to Improve Vapor Pressure Measurements on Compounds with Oxidative Instability: Methyl Oleate with tertButylhydroquinone Jason A. Widegren,* Casey E. Beall, Audrey E. Tolbert, Tara M. Lovestead, and Thomas J. Bruno Applied Chemicals and Materials Division, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305-3337, United States S Supporting Information *
ABSTRACT: The vapor pressure (psat) of methyl oleate was measured with and without the addition of 0.2 mass % of the antioxidant stabilizer tert-butylhydroquinone (TBHQ). The measurements were made by the gas saturation method with a temperature range of 303.15−343.15 K. In the absence of TBHQ, oxidative decomposition severely compromised the measurements, as evidenced by dramatic decreases in the measured psat for repeat measurements at 323.15 K. When combined with a room-temperature N2 flush of the apparatus, the addition of 0.2 mass % TBHQ limited the decomposition to insignificant levels and resulted in repeatable measurements of psat. Simultaneous measurements on the control sample neicosane (C20H42) yielded values of psat that were in excellent agreement with reference correlations.
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INTRODUCTION The goal of this work was to develop procedures for vapor pressure (psat) measurements with the gas saturation method on compounds with oxidative instability. The gas saturation method, which is also known as the transpiration method, is a commonly used measurement technique for the determination of psat < 1 kPa.1−10 The basis of this method is the saturation of an inert carrier gas with vapor from a condensed phase. From measurements of the amount of vapor solute and the amount of carrier gas, one can calculate psat. An important advantage of this method is that the effect of impurities is relatively small and predictable, which means that samples of limited purity can be measured. A drawback of the gas saturation method is that, because of the need for a carrier gas, such devices are not usually designed for rigorous degassing of samples in situ. This creates a problem for oxygen- and moisture-sensitive samples because the apparatus is typically full of air at the beginning of a psat measurement. When working with samples of limited stability, procedures to detect sample decomposition must be in place. An excellent, underutilized11,12 way to detect sample decomposition is to do replicate psat measurements on the same sample. Sample decomposition will usually change the measured value of psat over time; therefore, the last measurement in any series should replicate the first measurement in the series. Under some circumstances, more frequent replicate measurements can be useful. For example, for a measurement series of increasing temperatures, regular replicates of the initial measurement allow one to determine the point in the measurement series where significant sample decomposition has occurred. Another This article not subject to U.S. Copyright. Published 2016 by the American Chemical Society
useful means of detecting significant sample decomposition is to analyze the sample that remains in each saturator at the end of the measurement series. This approach is not definitive because relatively volatile decomposition products are continually swept away with the carrier gas and relatively heavy decomposition products may not be detected (by gas chromatography, for example). Nevertheless, reanalysis of the sample at the end of the measurements can distinguish sample decomposition from other measurement problems. Of course, if the initial and final purities differ, it is advisible to report them both. Another means of detecting sample decomposition is by the appearance of new peaks in the chromatographic or spectroscopic analysis of the trapped vapor. The relative size of such impurity peaks can be deceptive due to potentially large differences in volatility between the sample compound and its decomposition products, but this is often the first warning sign of sample decomposition. When unacceptably high levels of sample decomposition do occur, measurement conditions must be altered. It may be sufficient to lower the measurement temperature or to use a rapid measurement technique,13−15 but such approaches are not always possible or desirable. In the case of oxidative instability, there are some additional measures that can be taken. We employed two primary strategies to decrease the extent of oxidative decomposition. First, the gas saturation apparatus was flushed with nitrogen carrier gas at room Received: September 21, 2016 Accepted: December 6, 2016 Published: December 19, 2016 539
DOI: 10.1021/acs.jced.6b00821 J. Chem. Eng. Data 2017, 62, 539−546
Journal of Chemical & Engineering Data
Article
Table 1. Purity Determinations, with Combined Expanded (k = 2) Uncertainties, for Compounds at the Beginning of the Vapor Pressure Measurements compound methyl oleate
a
CASRN
GC-FIDb,c
H NMRb,d
112-62-9
0.995 ± 0.003
1
C NMRb,e
13
0.999 ± 0.005 0.995 ± 0.002 n-eicosane n-octadecane
112-95-8 593-45-3
0.998 ± 0.003 0.992 ± 0.003
a Methyl cis-9-octadecenoate. bCalculated as the fraction of raw peak areas. cGas−liquid chromatography with flame ionization detection. d1H nuclear magnetic resonance spectroscopy. e13C nuclear magnetic resonance spectroscopy.
temperature before the thermostat was set to the higher measurement temperature. The goal of this process is to remove atmospheric oxygen from the system at a relatively low temperature where decomposition is less problematic. If the flush volume is small compared to the measurement volume, or the flush temperature is low compared to the measurement temperature, this flush does not introduce a significant measurement error. Second, a small amount of antioxidant stabilizer was added to the sample compound. Such stabilizers have been widely studied as a means to improve the storage stability of biodiesel fuel.16−21 Effective concentrations depend on storage temperatures and storage periods but are typically ∼0.1 mass % for that application.21 For the long series of psat measurements reported herein, 0.2 mass % of tert-butylhydroquinone (TBHQ) was necessary to avoid significant decomposition of the methyl oleate. The use of a minimal amount of additive causes only a small decrease in sample purity. Furthermore, as noted above, impurities have a relatively small and predictable effect on psat measurements by the gas saturation method; thus, this approach can be employed without introducing a significant measurement error. On the basis of an extensive search of the literature, we believe that this is the first reported use of an antioxidant stabilizer to improve the quality of psat measurements. We report psat measurements on methyl oleate with and without the addition of 0.2 mass % of the antioxidant stabilizer TBHQ. Simultaneous measurements on the control sample neicosane (C20H42) are also reported. The measurements were made with a gas saturation apparatus in the temperature range 303.15−343.15 K. We chose methyl oleate as the sample compound for these experiments for a variety of reasons. To begin with, in the presence of oxygen (and at temperatures below ∼523 K), oxidation is expected to be the primary mode of decomposition.21−23 The availability of other unsaturated fatty acid methyl esters (FAMEs), such as methyl linoleate and methyl linolenate, with similar vapor pressure curves12,15,24 but different reactivities to oxygen21,22 will allow for further testing of this methodology. Finally, the psat curves of FAMEs are important for understanding biodiesel fuel performance and for refining and formulating biodiesel fuel.12,25−27 The control sample, n-eicosane, was chosen because it is chemically stable, available commercially in good purity, and has a well-known vapor pressure curve, which has been measured with a variety of techniques5,9,28−32 and correlated.33,34
measurements were made. These initial purities (shown in Table 1) are consistent with the manufacturers’ purity statements. For these analyses, research-grade nitrogen was used as the carrier and makeup gas. The FID was maintained at 300 °C. The split/splitless injection inlet was used in the splitless mode and maintained at 300 °C. The samples were separated on a 30 m capillary column coated with a 0.25 μm film of 5% phenyl 95% methylpolysiloxane. The temperature program consisted of 1 min at 100 °C, followed by a 100 °C min−1 gradient to 130 °C, followed by a 20 °C min−1 gradient to 300 °C, with a 6.2 min hold at 300 °C. The initial purity of the methyl oleate was also measured by 1 H and 13C nuclear magnetic resonance (NMR) spectroscopy. A commercial 600 MHz NMR spectrometer with a cryoprobe, operated at 150.9 MHz for 13C, was used to obtain 1H and 13C NMR spectra for the methyl oleate. A sample for 1H NMR spectroscopy was prepared by dissolving 17 mg of methyl oleate in 1 g of chloroform-d. A sample for 13C NMR spectroscopy was prepared by dissolving 150 mg of methyl oleate in 1 g of chloroform-d. In both cases, the NMR solvent contained 0.05% tetramethylsilane (TMS), which served as the chemical shift reference. The samples were maintained at 25 °C for the NMR measurements. A quantitative 1H NMR spectrum was obtained with a 30° flip angle and a long interpulse delay (5.45 s acquisition time, 14.55 s relaxation delay). A sweep width of 12019.23 Hz (−4 to 16 ppm) was used. Spectral processing was done without zero filling and with exponential line broadening of 0.3 Hz. After 128 scans, the spectrum had a signal-to-noise ratio of 2.1 × 105 (based on the tallest peak). A quantitative 13C spectrum was obtained by use of inverse-gated WALTZ-16 proton decoupling, a 30° flip angle, and a long interpulse delay (1.82 s acquisition time, 18.2 s relaxation delay). A sweep width of 36057.69 Hz (−20 to 220 ppm) was used. Spectral processing was done without zero-filling and with exponential line broadening of 1.0 Hz. After 4096 scans, the spectrum had a signal-to-noise ratio of 6300 (based on the tallest peak). The purities of the other chemicals used in this work were also determined. Reagent-grade acetone and n-octane, used as solvents, were obtained from commercial sources and used as received. Our analysis by GC-FID indicated purities ≥0.99 (based on raw peak areas), which is consistent with the manufacturers’ purity statements. The antioxidant stabilizer tertbutylhydroquinone (TBHQ) was analyzed by GC-FID and found to have a purity of 0.996 ± 0.003 (based on raw peak areas). The carrier gas for the gas saturation apparatus was ultrahigh purity nitrogen (mole fraction purity = 0.999995 according to the manufacturer), which was obtained from a commercial source. Before use, the nitrogen carrier gas was transferred into
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EXPERIMENTAL SECTION Chemicals. Methyl oleate (methyl cis-9-octadecenoate, CAS Number 112-62-9), n-eicosane, and n-octadecane were obtained from commercial sources and used as received. These compounds were analyzed by gas−liquid chromatography with flame ionization detection (GC-FID) before any psat 540
DOI: 10.1021/acs.jced.6b00821 J. Chem. Eng. Data 2017, 62, 539−546
Journal of Chemical & Engineering Data
Article
#6 (short), #9 (long), #12 (short), #15 (long), and #18 (short) were coated with n-eicosane. The coating procedure consisted of the following steps. First, a 10% (mass/mass) solution of each sample in hexane (for n-eicosane) or acetone (for methyl oleate) was made. The glass beads in each saturator were wetted with ∼1.5 mL of this solution, and the excess was poured out. Then, the solvent was removed by a gentle flow of helium through the adsorber at room temperature for 0.5 h. The coated saturators were then immediately installed in the temperature-controlled chamber of the apparatus. Measurement of the Temperature, Mass of Carrier Gas, and Mass of Sample Vapor. The temperature in the temperature-controlled chamber was measured with an ITS-90 calibrated platinum resistance thermometer (PRT). During the course of the vapor pressure measurements, the calibration of the PRT was checked with both a water triple-point cell (T = 273.16 K) and a gallium fixed-point cell (T = 302.9146 K). The temperatures measured by the PRT deviated from the two fixed points by
