ABSTRACT
Two desirable characteristics of corrosion inhibitors is their high aqueous solubility and their low tendency to micellize. In this work, we propose that addition of a polar moiety to the terminal end of the alkyl tail of the quaternary ammonium-based corrosion inhibitors improves both these characteristics. Our molecular simulation results show that corrosion inhibitors with a hydroxyl group as the terminal group of alkyl tail results in three orders of magnitude increase in the aqueous solubility and also significantly decreases the micellization tendency. These results are significant for the purpose of designing more potent corrosion inhibitor molecules.
Oil and gas pipelines are generally made of low alloy carbon steel. These pipelines are prone to internal corrosion due to the presence of water in the oil and gas stream. Mitigation of internal corrosion by means of injecting corrosion inhibitor molecules in the oil stream is an effective and low cost solution.1–3 In the bulk phase, corrosion inhibitors aggregate as micelles above their critical micelle concentration (CMC). The CMC of corrosion inhibitors decreases with the length of the alkyl tail. In addition, inhibitors prefer to accumulate at the oil-water and gas-water interfaces because of their amphiphilic nature. The aqueous solubility of corrosion inhibitors is inversely proportional to the length of their alkyl tail. Thus, while long alkyl tails result in undesirable properties of higher micellization tendency and lower aqueous solubility, the corrosion inhibition efficiency is understood to increase with the length of the alkyl tail. Thus, an important problem is how one can decrease the micellization tendency and increase the aqueous solubility of long alkyl tails. In this work, we address this problem by modifying the chemical make-up of quaternary ammonium-based inhibitors (henceforth referred to as quat), which are widely used inhibitors in the oil and gas industry.4,5 We have attached a hydroxyl (−OH) group at the terminal end of the alkyl tail of quat inhibitors. As a result of this modification, we find that the aqueous solubility of the inhibitor increases by three orders of magnitude. In our previous works, we showed that the cationic inhibitors have a strong tendency to form micelles (free energy of ∼ 68 kBT) and that these micelles experience a long-range free energy barrier to adsorption on the metal surfaces.6,7 The molecule with the polar −OH group on the alkyl tail shows a weaker tendency to form a micelle.
