研究目的
To apply terahertz time-domain spectroscopy as a non-destructive technique for monitoring pure and degraded oils and hydrocarbon chemicals, and to investigate the optical properties and degradation processes in these materials.
研究成果
THz-TDS is a viable non-destructive technique for identifying and monitoring oils, showing discriminative features in refractive index and absorption coefficient. RI correlates with molar mass in hydrocarbons and mono-functional esters but not in multi-functional esters. Degradation processes, such as hydrolysis and micelle formation, lead to non-uniform changes in optical properties, with critical micelle concentration playing a key role. The technique is effective for heavily deteriorated oils but requires improvements for higher accuracy and broader application.
研究不足
The accuracy of measurements is affected by sample thickness and multiple reflections in the cuvette, requiring thicker samples for optimal results in low-absorption oils. The technique has limited sensitivity to low concentrations of degradation products, with unambiguous detection only above certain thresholds (e.g., 16.7% for OA). Water contamination up to 1000 ppm shows minimal changes in optical properties, making detection challenging. The method may not fully capture oxidative degradation processes, which involve different chemical mechanisms.
1：Experimental Design and Method Selection:
THz time-domain spectroscopy (TDS) was used in transmission mode to measure optical properties (refractive index and absorption coefficient) of various oils and chemicals. The method involves analyzing transient waveforms of THz radiation.
2：Sample Selection and Data Sources:
Samples included edible oils (e.g., Sunflower, Olive, Coconut, Hemp, Flax, Camelina, Fish, Rapeseed), technical oils (e.g., 10W-40 motor oil), hydrocarbons (Hexane, Cetane), and esters (Acetate, Palmitate, Adipate). Degradation was simulated by adding Oleic Acid (OA) and water to refined Rapeseed oil.
3：List of Experimental Equipment and Materials:
THz-TDS spectrometer (T-spec from Ekspla-Teravil), HDPE cuvette for sample holding, femtosecond laser, photoconductive antennas (LT-GaAs emitter and detector), parabolic mirrors, delay line, and various oils and chemicals as listed.
4：Experimental Procedures and Operational Workflow:
Samples were placed in a 4 mm thick HDPE cuvette. THz pulses were generated and detected, with waveforms recorded as a function of time delay. Measurements were conducted at room temperature and 12% relative humidity. Data analysis involved calculating RI and absorption coefficient from waveforms.
5：Data Analysis Methods:
Equations from supplementary information were used to compute RI and absorption coefficient. Integrated areas under curves were analyzed to study dependencies on concentration changes.
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