Pre-formulation of biofuels: Kinematic viscosities, low-temperature phase behaviour and nanostructuring of ethanol/“ethanolotrope”/rapeseed oil mixtures

Abstract

As ethanol and rapeseed oil are not miscible, tributyl citrate, 2,5-dimethylfuran (DMF), 2-methylfuran (2-MF), 2-methyltetrahydrofuran (2-MTHF), methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), rapeseed oil FAME-biodiesel (FAME), 1-heptanol (HepOH) and 2-ethylhexyl nitrate (EHN) are used as green mixing agents or “ethanolotropes”. The area of existence of clear and monophasic mixtures as well as biphasic liquid/liquid solutions, determined in a previous work, are reported at 25°C using ternary phase diagrams. In the present work, some of these ternary diagrams are completed by investigating their critical points (CPs). Further, the kinematic viscosity (kV) of different binary and ternary, clear and monophasic mixtures is measured at 25 and 40°C and their phase behaviour is studied at 0, −15 and −20°C. Dynamic light scattering (DLS) experiments are performed on various samples at 25°C and the results are discussed with respect to previous measurements including DLS, static light scattering (SLS), small and wide angle X-ray scattering (SWAXS) and conductivity. Close to the CP of the investigated systems, the highest scattering intensities (SIs) can be observed as well as a distinct increase of the slope of the kV versus the rapeseed oil concentration. Evaluating these results, the high viscosities can be associated with the formation of a rapeseed oil continuum. Further, the monophasic domain close to the CP can be considered as bicontinuous-like. For every used “ethanolotrope”, the mixtures meet the required ASTM D6751 standard for the kV of biodiesel. Thus, rules for possible adjustments to reach this standard by varying the nature and the quantities of the “ethanolotropes” are proposed. Ternary and binary melts with high amounts of rapeseed oil can meet the kV standard and are clear and monophasic below 0°C or even below −15°C. Binary mixtures of DMF or 2-MF with high amounts of rapeseed oil (70wt%) show no phase transitions during several hours at −20°C and form reversible, monophasic and clear gels. As the kV of these two binary mixtures is slightly above 6mm2/s at 40°C, they nearly meet the kV standard. Replacing ethanol by “ethanolotropes” like 2-MTHF or HepOH can lead to an increase of the monophasic region of these ternary mixtures at low temperatures. Moreover, this temperature dependence could be associated with the nanostructuring, since mixtures located in the bicontinuous-like regions seem to be more sensitive regarding their phase behaviour.