Exploring the thermophysical properties of waste cooking biodiesel + hexadecane system at atmospheric pressure

Fuel diversification is fundamental for both energy security and environmental protection. In this context, biodiesel is a renewable component of diesel fuels and is legally used in blends with fossil diesel in many countries. Despite its widespread application, a detailed understanding of the thermophysical behavior of biodiesel + diesel blends remains essential, as these properties directly affect fuel handling, flow, and injection performance. Consequently, the study of model systems has been employed to gain deeper insight into the molecular interactions governing the thermophysical properties of such mixtures. In this study, the thermophysical properties of a pseudo-binary mixture of waste cooking oil biodiesel (MM = 291.68 g·mol^-1) and hexadecane (MM = 226.44 g·mol^-1), used as a representative diesel compound, were investigated at  p = 0.1 MPa using a vibrating-tube viscodensimeter (SVM 3001, Anton Paar). Mixtures were synthetized gravimetrically by using an analytical scale with precision of 0.0001 g. Both density ρ and viscosity η were measured in a temperature domain ranging from T = (293.15 to 333.15) K by varying the hexadecane mole fractions (x_C16). Results demonstrate that mixture properties presented a behavior typical of regular liquids, where both ρ and η decreased with T. From these results, the excess properties such as excess molar volume V^E and viscosity deviation ∆η were calculated. The V^E showed positive values, indicating a predominance of repulsive intermolecular interactions, presenting a maximum around x_C16 = 0.5 mol·mol^-1. This behavior can be assigned to the multicomponent characteristic of the biodiesel mixed with only one hydrocarbon, showing a predominance from dispersive forces. Moreover, negative deviation from ideal behavior was observed for ∆η against x_C16, suggesting a disruption of molecular packing, presenting a maximum at composition near to x_C16 = 0.5 mol·mol^-1.