Bitumen Refractive Index and ATR-FTIR baseline correction (dataset)

ATR-FTIR, SARA fractionation, IR Ellipsometry and Brightfield Microscopy of Bituminous Materials

Overview

This repository contains the data used in the study for developing an ATR-FTIR baseline correction script:

• ATR-FTIR measurements
• Transmittance FTIR measurements
• Brightfield Microscopy
• SARA fractionation 
• IR Ellipsometry 

File Format

All files are stored inside zipped folders for each measurement technique. They were converted from original device environment file formats (Opus, WVASE, etc) to .tiff, .dat and .csv to increase accessibility of the data.

Materials

Table 1: The samples used in this study, indicating their grade, modification, ageing protocols and presence of mineral filler

Experimental Equipment

Diamond ATR-FTIR

A Bruker FTIR Vertex 80v spectrometer with Pike GladiATR unit, is operated under vacuum conditions to eliminate atmospheric absorptions. The Attenuated Total Reflectance (ATR) unit is placed in an evacuated side chamber and equipped with a diamond crystal exposed to the environment, allowing direct analysis of solid and liquid samples without extensive preparation. All spectra are recorded with a liquid nitrogen cooled MCT (mercury-cadmium-telluride) detector in a spectral range from 4000 cm^-1 to 600 cm^-1, at a spectral resolution of 2 cm^-1. Four replicate samples were prepared per binder, and each sample was measured four times resulting in a total of 16 measurements per binder. When a spectrum was deemed insufficient, a new measurement was performed. Pressure was only applied for strongly aged samples, when otherwise no proper contact with the ATR crystal could have been achieved. The angular spread of the GladiATR accessory could not be determined from the available instrument documentation and is therefore not reported as a numerical value. However, the angular distribution of the incident radiation is an important parameter for the interpretation and reproducibility of ATR measurements and should be considered when comparing ATR configurations.

Germanium ATR-FTIR

A Bruker Tensor 37 FTIR with a Bruker Platinum ATR was used to measure Germanium ATR crystal mid IR spectra. A spectral range from 4000 cm-1 to 400 cm-1 with a resolution of 2 cm-1 was recorded. Three replicate samples, four repetitions and 48 scans were taken of several bituminous samples. Moderate pressure was only applied for strongly aged samples, when otherwise no proper contact with the ATR crystal could have been achieved. No atmospheric compensation was applied. The Ge-ATR measurements were performed using the manufacturer's 45° nominal incidence angle.

Transmittance FTIR

Transmittance FTIR spectra were recorded with a Bruker FTIR Vertex 80v; a similar set-up as described above, but with a two 2-mm BaF₂ windows in the transmission cell and dissolving the bitumen in dichloromethane (DCM) at a concentration of 10mg/mL. A comparable spectral range was applied; however, the lower limit was restricted to ~800 cm^-1 due to the transmission cutoff of the BaF₂ windows. The resolution was set to 2 cm^-1, and each sample was measured using four repetitions of 24 scans. The cell spacing was determined from interference fringes, yielding an approximate pathlength of 0.14 mm.

IR Ellipsometer    

An Infrared Variable Angle Spectroscopic Ellipsometer (IR-VASE) by J.A. Woollam infrared Ellipsometer was used to characterize the optical properties of bitumen thin films in the mid-infrared spectral region. The instrument measures changes in polarization state – specifically the amplitude ratio (Ψ) and phase difference (Δ) – of IR radiation reflected off the sample surface. Measurements were performed over a wavenumber range of 4000 cm⁻¹ to 400 cm⁻¹. The collected Ψ and Δ data were converted using Fresnel equations and ellipsometry theories to extract the complex refractive index. Samples were ensured to have smooth surfaces; by spin coating a drop of bitumen-n-toluene solution (25 mg/mL). The measurements were taken at different angles of incidence (45, 55 and 65^o), at a resolution of 32 cm^-1 over a spectral range of 8000-500 cm^-1; collecting 15 spectra created by 20 scans each; to reproduce the conditions within the ATR crystal.

Microscope

A Nikon Eclipse Ci optical microscope was employed to examine the surface morphology and microstructural features of bitumen samples. Bitumen samples, roughly 5-10 g, were heated and homogenised at 150° C for not more than 2 minutes on a metal spoon, poured on pre-heated (at 150°C) microscopy slides with an indent to promote a flat surface. These were then cooled to room temperature for at least 15 minutes and then observed under brightfield illumination at a magnification of 100x. Images were captured with a digital camera integrated into the microscope system to document morphological features corresponding to spectroscopic measurements.

Spin Coater

A POLOS SPIN150i spin coater was used to prepare uniform thin films of both bitumen and its SARA fractions for infrared ellipsometry measurements. The sample procedure was based on work from Hung and Fini [1]. Bitumen was first dissolved in toluene to obtain a homogeneous solution (25 mg/mL), after which a small volume was dispensed onto clean glass substrates (40 µL). The samples were then spin-coated at controlled rotation speeds, an acceleration of 500 rpm/s and a speed of 1000 rpm for 30 seconds, to ensure uniform film thickness and smooth surface morphology. After spin coating, the films were allowed to dry to ensure complete solvent evaporation prior to analysis (~24 hours). This preparation method enabled reproducible thin-film formation suitable for optical characterization.

Fast-SARA  

SARA fractionation was performed after the recommendations made by the simplified technique by Sakib & Bhasin in 2018 [2]. For each SARA separation, 400 ± 40 mg of bitumen was placed in a 250 mL jar with 40 mL of HPLC-grade n-heptane. The mixture was magnetically stirred for 24 ± 2 hours at 200 rpm. Asphaltenes (n-heptane insoluble) were separated using 0.2 µm PTFE syringe filters (25 mm, Thermo Scientific™ Titan3™). The resulting filtrate (maltenes) was divided into four vials; two were dried at 120 °C under a nitrogen stream and the remaining mass was used to determine the maltene to asphaltene ratio. The remaining maltenes were diluted to 3.33 mg/mL for solid phase extraction (SPE) with Thermo Scientific™ HyperSep™ Silica Cartridges (25 mL, 40–60 µm). After pre-washing the cartridges with 10 mL each, they received 15 mL of the maltene solution, followed by 10 mL n-heptane. Next, 25 mL of toluene/n-heptane (80:20 v/v) and 40 mL of DCM/methanol (90:10 v/v) were added sequentially at separate manifold positions. The resulting SAR fractions were dried under nitrogen on a heating plate until constant mass. Asphaltene fractions were dissolved in toluene, transferred to glass vials, and evaporated similarly.

[1] Hung, A. and E.H. Fini, (2020), Surface morphology and chemical mapping of UV-aged thin films of bitumen. ACS Sustainable Chemistry & Engineering. 8(31): p. 11764–11771. DOI: https://doi.org/10.1021/acssuschemeng.0c03877.

[2] Sakib, N. and A. Bhasin, (2019), Measuring polarity-based distributions (SARA) of bitumen using simplified chromatographic techniques. International Journal of Pavement Engineering. 20(12): p. 1371–1384. DOI: https://doi.org/10.1080/10298436.2018.1428972.