Liquid–liquid equilibria of mixtures with ethyl lactate and various hydrocarbons
Highlights
► Ethyl lactate is completely soluble with 1-hexene, 1-heptene, 1-octene, cyclohexane, benzene and toluene. ► Critical point of (alkane + ethyl lactate) increased approximately 7 K per one carbon atom. ► Presence of π-bond significantly increases mutual solubility.
Introduction
An important area of green chemistry investigations concerns the selection of an appropriate medium in which to carry out a chemical reaction or a separation process. As noted by Holbrey and Seddon [1], there are so far five main strategies for the elimination of volatile organic compounds (VOCs) in which research is being developed: the solvent-free synthesis, and the use of water, fluorous phases, supercritical fluids, and ionic liquids, as solvents. Another option is ethyl lactate which has emerged in recent years. Ethyl lactate is an agrochemical “green” solvent with a potential to substitute toxic industrial solvents due to many attractive features: low volatility, biodegradability, noncorrosive and non-carcinogenic behavior, ozone-friendly performance, broad liquid temperature range and low viscosity. Due to its low toxicity, ethyl lactate is approved by the U.S. Food and Drug Administration (FDA) as a pharmaceutical and food additive [2]. Moreover, ethyl lactate can be easily obtained using carbohydrate feedstocks since it is produced from ethanol and lactic acid that are acquired by fermentation of biomass, such as corn starch crops. Recent review of Pereira et al. [3] shows that ethyl lactate has good prospective to be applied as a green solvent in several applications, such as organic synthesis, pharmaceutical preparations, fragrances, in ink and coating industry, and as food additive.
In 1998, researchers from Argonne's Energy Systems Division won the Presidential Challenge Green Chemistry Award for the development of the technology that significantly reduced the market price of ethyl lactate [4]. This new cost-cutting manufacturing process is based on an integrated purification–separation system. The latter uses a membrane to selectively remove reaction products and bring the reaction to nearly 100% completion providing high-purity product (ethyl-lactate). This huge overcoming of the old (higher) price barrier makes ethyl lactate an economically viable alternative solvent.
According to the study reported by Aparicio and Alcalde [5], the molecular structure of ethyl lactate is defined by a specific topology of hydrogen bonds between hydroxyl and carbonyl groups, whereas the alkoxy position is sterically hindered to develop H-bonding. This indicates the formation of intra- and inter-molecular associations in ethyl lactate which acts either as a proton donor or proton acceptor [6]. On the other hand, ethyl groups tend to arrange in apolar domains across the fluid. Thus, these features make ethyl lactate very versatile solvent capable to solubilize both polar and apolar compounds.
Recent works have reported the potential application of ethyl lactate to extract carotenoids from different sources (tomatoes, carrots, and corn) [7] and sclareol from salvia extracts [8]. Hernández et al. [9] studied the possible use of ethyl lactate to recover squalene from olive oil deodorizer distillates. Also, our group reported the utilization of ethyl lactate for selective separation of α-tocopherol from triglycerides [10]. Ethyl lactate showed its ability to solubilize valuable solid compounds such as caffeine, cafeic acid, ferulic acid, vanillic acid and thymol [11].
Although phase equilibria data are essential for design of any separation technology, data on mutual solubility of systems containing ethyl lactate are scarce. In this work we report experimental liquid–liquid equilibria (LLE) of mixtures containing ethyl lactate and various hydrocarbons in the temperature range from (273.2 to 320.9) K, at atmospheric pressure. To the best of our knowledge, this work is the first report of liquid–liquid equilibria data for mixture comprising ethyl lactate and alkane or alkene.
The obtained binary LLE data for ethyl lactate and hexane or heptane or octane or isooctane or nonane or decane or dodecane or 1-hexadecene, were represented using the UNIQUAC model.
Section snippets
Chemicals
Chemical names, structures, abbreviations and stated purity of all chemicals used in this work are presented in Table 1. Hydrocarbons of high purity were purchased either from Sigma Aldrich or from Merck Chemicals and were used without further purification. Vacuum at room temperature was applied to the ethyl lactate for several days in order to reduce its water content. Karl-Fischer coulometric titration (Metrohm 870 KF Titrino Plus coulometer) was employed to determine the water content before
Results and discussion
The binary mixtures of (ethyl lactate + 1-hexene), (ethyl lactate + 1-heptene), (ethyl lactate + 1-octene), (ethyl lactate + cyclohexane), (ethyl lactate + benzene) and (ethyl lactate + toluene) showed complete miscibility in the studied temperature range from 273 K to 360 K. On the other hand, mixtures containing ethyl lactate and hexane or heptane or octane or 2,2,4-trimethylpentane (isooctane) or nonane or decane or dodecane or 1-hexadecene exhibited upper critical solution temperature (UCST) behavior.
Conclusions
Ethyl lactate is completely soluble with 1-hexene, 1-heptene, 1-octene, cyclohexane, benzene and toluene in the temperature range from 273 K to 360 K.
Partial mutual solubility of ethyl lactate with hexane or heptane or octane or 2,2,4-trimethylpentane (isooctane) or nonane or decane or dodecane or 1-hexadecene was reported. All mixtures exhibited upper critical solution temperature (UCST) behavior. Critical point of (alkane + ethyl lactate) increased approximately 7 K per one carbon atom in alkane.
List of symbols
- ri
volume parameter
- qi
area parameter
- z
coordination number
- aij, aji
temperature-independent binary interaction parameters
- uij, uji
binary interaction energy parameters
- UCST
upper critical solution temperature
- Vw
van der Waals volume (cm3 mol−1)
- Aw
van der Waals area (cm2 mol−1)
- xi
mole fractions of hydrocarbon
- T
temperature (K)
- u
standard uncertainties
Acknowledgements
This work has been supported by Fundação para a Ciência e a Tecnologia (Portugal) through grant no. PEst-C/EQB/LA0006/2011. M.S. Manic and M.E. Zakrzewska are thankful to Fundação para a Ciência e Tecnologia – Portugal for doctoral fellowships SFRH/BD/45323/2008 and SFRH/BD/74929/2010, respectively.
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