In the last decade, the development, formulation and use of renewable fuels have been focused on biofuels. At the early stage, the biofuels were obtained from agricultural biomass but due to the ethical uses of the land, the... [ view full abstract ]
In the last decade, the development, formulation and use of renewable fuels have been focused on biofuels. At the early stage, the biofuels were obtained from agricultural biomass but due to the ethical uses of the land, the actual production of biofuels is focused on unconventional sources such as water-based microalgae. Considering the experience in fossil fuels, where diesel is more efficient than gasoline, the type of biofuel that is being studied with great interest by the scientific community is biodiesel, a mixture of alkyl esters of long chain fatty acids derived from lipid raw material. From a chemical point of view; esters possess the consistency of conventional hydrocarbon oils. In fact, the chemical structures of these esters resemble triglycerides that constitute the major portion of vegetable oils, animal fats and/or from water-based microalgae. Both fatty acid methyl esters (FAMEs) and fatty ethyl esters (FAEEs) are good substitutes for petroleum-based fuels due to they can be used in their neat form or blended with fossil diesel inside of compression ignition engines. In addition to the use as a fuel, FAMEs and FAEEs are more eco-friendly and highly biodegradable than traditional fuels.
Based on their natural source, the thermodynamic characterization of ester + water mixtures are fundamental to optimize the application of esters as fuel and to evaluate their impact in the environment. In this work, we are focused on the thermodynamic determination and theoretical modeling of liquid - liquid equilibria, bulk densities and interfacial tension of a select FAMEs (from methyl formate to methyl heptanoate) + water systems.
Mass density determinations are carried out in a vibrating tube densimeter, whereas a spinning drop tensiometer is used for performing interfacial tension (IFT) measurements. Additionally, the IFT data have been correlated by using a simplified version of the Square Gradient Theory (SSGT) as applied to the Non-Random Two Liquids (NRTL) activity coefficient model.
According to the experimental results, the mass densities of both aqueous and organic (FAMEs) bulk phase decreases with temperature, whereas the IFT for FAMEs + water mixtures increases with the temperature and reaches a maximum value, which is related to the maximum tie line in the liquid – liquid equilibrium. For higher temperatures, IFT starts to decrease with temperature. The SSGT + NRTL approach was found to be suitable for correlating the IFT of the mixtures and to provide a route to explore other interfacial properties, such as interfacial profiles, which are difficult to measure. Based on the calculated interfacial profiles along the interfacial region, the components (FAMEs and water) neither exhibit surface activity nor adsorption activity and the interfacial width decreases with temperature as it is expected.
