Terahertz Time-Domain Spectroscopy (THz-TDS) is used to investigate water and soot contaminations in oils, exhibiting different dilution modes. For synthetic polyglycol oils, the water is dissolved due to the polar behavior of the oil, whereas in non polar mineral oils the water-oil compound forms an emulsion. This behavior is modeled with an effective medium approximation (EMA) formalism. Small soot agglomerates are remaining in suspension when mineral oils are polluted with soot particles. In this case, the absorption spectrum is dominated by scattering effects. Due to the small particle size of the soot agglomerates compared to the THz wavelength, coherent scattering is the dominant process.
The water content in polyglycol oils is investigated by Terahertz Time-Domain Spectroscopy (THz-TDS). These oils are able to dissolve a certain percentage of water. Changes in the absorption coefficient and refractive index are observed related to the amount of water added to the pure oils. Comparison of the experimental data with predictions based on Beer-Lambert and Lorentz-Lorenz-theory, respectively, exhibits an excellent agreement. Analyses with Fourier Transform Infrared (FTIR) Spectroscopy reveal sensitivity similar to the THz-TDS experiment. THz-TDS may offer powerful tools to quantitatively determine the water concentration in petroleum products.