Fatty acids methyl esters: Complementary measurements and comprehensive analysis of vaporization thermodynamics

https://doi.org/10.1016/j.jct.2019.01.007Get rights and content

Highlights

  • Vapor pressures of FAMEs measured by static and transpiration methods.

  • Vaporization heat capacity difference evaluated with 4 methods and analyzed.

  • Enthalpies of vaporization at 298 K derived and compared with the literature.

  • Dispersion interactions assessed from the vaporization enthalpies.

Abstract

Vapour pressures of methyl decanoate, methyl undecanoate, methyl tetradecanoate, methyl hexadecanoate and methyl octadecanoate were measured by using the static technique. Vapour pressures of methyl dodecanoate, methyl tridecanoate, methyl tetradecanoate, methyl heptadecanoate, and methyl octadecanoate were measured by using the transpiration method. In the present work, vapour pressures of methyl esters of the saturated fatty acids from methyl propanoate to methyl eicosanoate were collected from the literature. These data together with own complementary results were used for validation of methods for assessment of the difference ΔlgCp,mo required for the correct adjustment of vaporization enthalpies to the reference temperature 298.15 K. The evaluated vaporization enthalpies for esters from methyl propanoate to methyl eicosanoate demonstrate impeccable linear chain-length dependence. The CH2 group contributions for different homologous series were derived and compared. Quantitative analysis of dispersion interactions in the long-chained esters have been performed.

Introduction

The expected decline of oil supply in the near future makes the development and optimization of the manufacturing of alternative biofuels a hot topic for current science and engeneering. The prevailing share of the modern biofuel market includes bioethanol and biodiesel. Biodiesel is composed of alkyl esters of fatty acids and it usually synthesized by chemical or enzymatic catalysis mainly from renewable resources [1]. Thermodynamic data on esters are indispensable for optimization of the synthesis conditions, heat management, as well as for separation processes. Experimental thermodynamic studies of aliphatic esters has been a long-standing goal in our laboratory [2], [3], [4].

In this work, we focused on the Fatty Acids Methyl Esters (FAME) and their structure–property relationships. In a previous study of a series of n-alkyl acetates (from methyl acetate to tetradecyl acetate) [2], we observed a remarkable linear chain-length dependence for their vaporization enthalpy (referenced to T = 298.15 K). The dependence of vaporization enthalpy on the total number of C-atoms in alkyl chains (for nC ≥ 3), was expressed by the following equation:ΔlgHm°298.15K/kJ·mol-1=4.56±0.06nC+16.0±0.7withR2=0.999

This result can be rationalized in a simple way by the observation that monotonically growing chain length is characterized by a constant contribution of 4.56 kJ·mol−1 for each [CH2] group. From a theoretical as well a practical point of view, we could expect that this [CH2]-contribution is also valid for (FAME) where the monotonically growing chain length is extended from the side of carbonyl group (see Fig. 1).

In order to confirm our expectations, we have thoroughly collected the available literature on vapour pressures and vaporization enthalpies for FAME but we failed to draw any reasonable conclusion regarding the vaporization enthalpy ΔlgHm°(298.15 K) chain-length dependence due to a significant spread of the available literature data. Despite of that, the thermodynamic studies of FAME have been a popular endeavor in the past, since 1926 [5]. Many vapour pressure data of different quality and accuracy have been reported in the literature and some may be affected by systematic errors. Moreover, vapour pressures published in the literature were measured by different techniques and over significantly different temperature ranges. As a consequence, ΔlgHm°(Tav)-values, derived from these measurements, were referenced to different average temperatures Tav. For comparison, ΔlgHm°(Tav)-values must be adjusted to any common temperature, e.g. to the reference temperature T = 298.15 K. Obviously if the temperature Tav is close to T = 298.15 K, the contribution to vaporization enthalpy due to the temperature adjustment is rather small and it hardly would exceed the boundaries of experimental uncertainties (within the range of 1–2 kJ·mol−1). Otherwise, vaporization enthalpy derived e.g. from the high-temperature ebulliometry (e.g. [6]) are significantly affected by a contribution of (5–6) kJ·mol−1 due to large adjustment to T = 298.15 K. The temperature adjustment of ΔlgHm°(Tav)-values is usually performed according to the Kirchhoff's rule:ΔlgHmo298.15K=ΔlgHmoTav+ΔlgCp,mo298.15K-Tavwhere ΔlgHmo is the standard molar vaporization enthalpy and the difference between the isobaric heat capacities of the gaseous Cp,mo(g) and liquid phases, Cp,mo(l), is described by Eq. (3):ΔlgCp,mo=Cp,mog-Cp,ml

Recently [7], we successfully tested four methods to calculate the difference ΔcrgCp,mo between the isobaric heat capacities of the gaseous and crystal phases required for adjustment of sublimation enthalpies to the reference temperature 298.15 K. Tests were performed for ferrocene, for which reliable experimental data are available in the literature. In this work, we focus on methods to evaluate the difference ΔlgCp,mo between heat capacities of the gaseous and liquid phases required for adjustment of vaporization enthalpies to the reference temperature 298.15 K. Numerous experimental vapour pressures available for FAME together with our new complementary measurements have been used to evaluate results of ΔlgCp,mo estimated using different methods. The reconciled ΔlgHm°(298.15 K)-values for FAME with the chain-length NC = 3–19 were analyzed in order to derive the chain length dependence for methyl esters and to compare the [CH2]-contributions for different homologous series. Moreover, the ΔlgHm°(298.15 K)-values of FAME have been used for interpretation of the gas phase dispersion interaction of the alkyl chain and the ester group.

Section snippets

Materials

Samples of FAME were of commercial origin with the mass fraction purities better than 0.99 according to certificate (see Table S1). The purity of the samples used for vapour pressure measurements was determined by gas chromatography (GC) using the Hewlett-Packard 4890 equipped with FID and HP5 capillary column (30 m × 0.32 mm), film thickness 0.25 µm. The final mass purity of FAME as determined by GC was better than 0.998 (see Table S1). Samples were additionally purified during the

General methods for estimation of ΔlgCp,mo

The isobaric heat capacity differences ΔlgCp,mo are required for vaporization enthalpy temperature adjustment according to the Kirchhoff's rule. There are at least four well-established methods I to IV, which can be applied for assessment ΔlgCp,mo-values.

Conclusions

New experimental vaporization enthalpies of the FAMEs were determmined using the static and transpiration methods. Four methods for assessment the heat capacity differences were discussed and applied for adjustment of vaporization enthalpies to the reference temperature 298.15 K. A set of vaporization enthalpies of FAMEs was evaluated and recommended for thermochemical calculations. Dispersion forces in methyl esters have been quantified and discussed. These results will facilitate evaluation

Acknowledgments

This contribution is a part of the special issue dedicated to the 80th birthday of Prof. Gennady Y. Kabo, the founder of thermochemical science school in Belarusian State University. His former students are working all around the world in the field of physical chemistry and thermodynamics. Authors gratefully acknowledge financial support from the Government of Russian Federation (decree №220 of 9 April 2010), agreement №14.Z50.31.0038. SPV acknowledges financial support from DFG, grant VE

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