Determination of optical and dielectric properties of blends of alcohol with diesel and biodiesel fuels from terahertz spectroscopy

The polarity and polarizability of fuel blend components determine some of the blend physical properties, such as viscosity, surface tension, fluidity or miscibility. This affects the development of the injected fuel jet and, therefore, the performance and pollutant formation in diesel engines. Among polar molecules, such as alcohols (ethanol and butanol), attractive forces exist between dipolar ends with opposite charges. The permanent molecular dipoles lead to the formation of induced dipoles in neighbouring molecules, which interact with the permanent dipoles forming intermolecular attraction. Although less appreciable, interaction between non-polar molecules also occurs in diesel fuels. In this study, the dielectric properties were investigated in the terahertz spectral region (located between infrared and microwaves). This region is typical for vibrational and rotational studies of molecules of non-conductive materials. For all blends tested, the time-domain terahertz spectra show that the amplitude of pulses decreases and the peak position shifts in time as alcohol content increases. For diesel and biodiesel fuels, amplitudes are higher and less delayed than for alcohols, indicating that they are less radiation-absorbing. The optical and dielectric properties (absorption coefficient, refraction index and dielectric constant) of alcohol blends with diesel and biodiesel blends were determined. For the alcohols studied, both absorption coefficient and refractive index decrease for increasing alkyl chain length. Real and imaginary terms of the complex dielectric function show increasing dependency with frequency as the molecule polarity increases. From these properties, the relaxation dynamics can be modelled for blends, and nonlinearities in their physical properties can be explained.