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Volatile corrosion inhibitors (VCIs), specifically formulations based on thiols and amines, can be used to mitigate top-of-the-line corrosion (TLC) that arises during the transportation of wet gas through transmission pipelines. Nevertheless, the VCI inhibition efficiency (IE) can be compromised by the presence of condensable hydrocarbon phases. In this research, the IE of two thiol compounds (decanethiol and hexanethiol) and three combinations of VCIs for TLC scenarios, both in the presence and absence of n-heptane, representing a condensing hydrocarbon phase were studied. The results proved the IE of thiols in a water-only condensing environment, with effectiveness increasing with the alkyl tail length. Conversely, in a water/n-heptane co-condensing environment, a reversed trend was observed, where hexanethiol exhibited higher corrosion IE compared to decanethiol. Molecular simulation results indicated a synergistic adsorption behavior when the alkane was of a similar length as the alkyl tails of the inhibitors, leading to the incorporation of alkane molecules with the inhibitor molecules. A mixture of thiols (decanethiol and hexanethiol) and two mixtures of thiol and amines (decanethiol and diethylamine/t-butylamine) were also considered in both water-only and water/n-heptane co-condensing environments. In the presence of n-heptane, only the thiol mixture, featuring molecules with different tail lengths, demonstrated high IE. This behavior was attributed to the superior IE provided by thiol-based molecules with a shorter alkyl tail (hexanethiol) in the presence of n-heptane. Additionally, the results revealed that the mixtures of decanethiol and amines did not enhance corrosion inhibition in the presence of n-heptane within the system. 

Intermittent oil–water wetting can have a significant effect on the internal corrosion of steel pipelines. This paper presents a combined experimental and molecular modeling study of several influential factors on the surface properties and corrosion behavior of mild steel in CO2 environments. The influence of different model oils (LVT-200 and Aromatic-200) and select surface-active compounds (myristic acid, cyclohexane butyric acid, and oleic acid) on the corrosion behavior of carbon steel during intermittent oil–water wetting was determined by measuring the corrosion rate after intermittent wetting cycles. The interfacial tension measurements were performed to study the incorporation of the oil phase along with surface-active molecules in the protective layer formed on the specimen surface. Results showed that the interfacial tension for an aromatic oil–water interface is lower than that for an aliphatic oil–water interface. To understand this result, molecular dynamics simulations of oil–water interfaces were performed in the presence of surface-active molecules and different oils to analyze the structure of the layer formed at the interface. The simulations supported the hypothesis that aromatic molecules are less structured at the interface, which results in the incorporation of more water molecules into the protective layer formed at the steel surface, causing a higher corrosion rate. On the other hand, the simulations revealed that myristic acid in an aliphatic oil forms a well-aligned structure at the interface, devoid of any water molecules. This is in agreement with the hypothesis that the linear molecular structure of myristic acid favors the alignment of molecules at an aliphatic oil–water interface, resulting in a lower interfacial tension and more effective corrosion mitigation as compared to the other two nonlinear compounds tested. It is concluded that an important factor controlling the corrosion behavior is the molecular structure of the oil–water interface, which is adopted by the steel surface layer through the Langmuir–Blodgett process. 

The geometry, arrangement, and orientation of a quaternary ammonium surfactant flanked by two methyl groups, a benzyl head, and an octyl tail, were assessed at the air-water and air-deuterium oxide (D2O) interfaces using sum frequency generation vibrational spectroscopy (SFG). Remarkably, symmetric and asymmetric N-CH3 stretches (at ~2979 and ~3045 cm-1, respectively, in the SSP polarization combination) were visible in water but were negligible in deuterium oxide. We concluded that D2O addition triggers the average reorientation of the dimethyl amino units parallel to the interface, and possibly changes the overall conformation of the surfactant. A reduced number of gauche defects in the surfactant octyl chain is also observed in D2O. Tilt angles for the octyl chain (1.0 – 10.8º) are consistent with an ordered monolayer at the air-liquid interface.