FTIR-Based Quantification of a Corrosion Inhibitor in Solution: Experimental Investigation and Quantum-Chemical Modeling

Abstract Residual corrosion inhibitor (RCI) monitoring is essential for ensuring the effectiveness of corrosion mitigation programs in oil and gas production systems. Among the available analytical techniques, Fourier transform infrared spectroscopy (FTIR) offers advantages such as rapid analysis, minimal sample preparation, and the absence of chemical reagents. In this study, an FTIR-based method was developed for the quantification of 2-phenyl-2-imidazoline (IMDZ), a corrosion inhibitor, and the influence of typical produced-water constituents on its spectroscopic response was investigated. Calibration curves were constructed using IMDZ concentrations ranging from 100 to 1000 ppm in Milli-Q water. Two characteristic spectral regions were evaluated, centered at approximately 1500 and 784 cm–1. The band at 1500 cm–1 exhibited a linear response with concentration (R2 = 0.9922), while the band at 784 cm–1 showed a logarithmic response. The estimated limit of detection (LOD) and limit of quantification (LOQ) for the 1500 cm–1 band based on within-pellet repeatability were approximately 32 and 96 ppm, respectively. The effects of acetic acid, NaCl, CaCl2, FeCl2, NaHCO3, and dissolved CO2 were evaluated individually at a fixed IMDZ concentration of 500 ppm. Significant matrix effects were observed, particularly in the low-frequency spectral region. Acetic acid and FeCl2 produced the largest spectral perturbations, whereas NaCl caused substantial reductions in peak area. Overall, the 1500 cm–1 band exhibited greater resistance to interference than the 784 cm–1 band. Quantum-chemical calculations revealed strong hydrogen-bond formation between IMDZ and acetic acid, as well as cation−π interactions involving dissolved metal cations. The combined experimental and computational results demonstrate that the spectral region centered at approximately 1500 cm–1 is the most suitable analytical marker for FTIR-based quantification of IMDZ in brine environments.