Investigating refractive index effects in bitumen ATR-FTIR spectroscopy

Attenuated Total Reflectance Fourier-Transform Infrared (ATR-FTIR) spectroscopy is widely used to characterise the ageing and modification of bitumen because of the molecular information it provides, its high signal-to-noise ratio, minimal sample preparation, and applicability to solid materials. However, interpretation of ATR-FTIR spectra remains challenging, particularly regarding baseline shifts observed during ageing or modification, which are not well understood yet. To tackle this issue, this study demonstrates that these baseline variations originate from changes in the real part of the refractive index of bitumen, which directly influence the penetration depth of the evanescent wave in ATR-FTIR measurements. By combining ATR-FTIR spectroscopy with ellipsometry, microscopy, and SARA fractionation, a quantitative relationship between refractive index evolution and spectral baseline behaviour was established for different binder sources, ageing states, polymer modifications, and filler additions. Using IR ellipsometry the refractive indices of the saturates, aromatics, resins, and asphaltenes (SARA fractions) were determined, and the changes in fractional composition affecting the bulk refractive index of bitumen was related to aliphatic band vibrations measured with FTIR.Based on these findings, a baseline correction model was developed to compensate for refractive-index-induced distortions in ATR-FTIR spectra. The corrected spectra allow extraction of a predicted bulk refractive index as an alternative ageing index, consistent with conventional markers such as carbonyl and sulfoxide band growth. This approach improves the quantitative reliability, comparability, and inter-laboratory consistency of ATR-FTIR-based ageing assessment.