Design and synthesis of benign, N- and O-containing, organic ligands for surface engineering
Renz, Robert Phillip
This thesis is concerned with the development of new organic ligands as corrosion inhibitors. The introductory chapter presents a review of the chemistry involved in the corrosion of metal surfaces. Chapter 2 considers the requirements necessary to develop new ligands. The ligand-surface complex must have a high thermodynamic and kinetic stability. The inclusion of multifunctionality allows the potential for intermolecular ligand-ligand or ligand-surface secondary bonding which can generate a multisite attachment mode of binding. For example, Irgacor 419©, 3-(4-methylbenzoyl)-propionic acid, is used commercially in surface coatings as a corrosion inhibitor. Its efficacy is rationalised by the strong binding of the carboxylic acid group to iron(III) atoms and the secondary bonding of the keto group to a surface hydroxyl. Its bromo-analogue, 3-(4-bromobenzoyl)-propionic acid, was studied for use in Quartz Crystal Microbalance studies. Use of a QCM and the adsorption isotherm of a ligand from a solution phase onto high surface area solid substrate, goethite, were considered as analytical techniques to investigate ligand-surface interactions. Chapter 3 considers the synergistic corrosion inhibition effect obtained by combining two knonwn corrosion inhibitors for mild steel, 2-hydroxyl-5-nonyl-benzaldehyde oxime (P50 oxime) and polyisobutylene succinic anhydride (PIBSA) which has been patented by ICI. Reactions between salicyaldehyde or benzaldehyde oxime and succinic or glutaric anhydride are shown to result in ring opening and generation of a series of ligands, including salicyaldehyde O-(4-carboxypropanoyl) oxime and benzaldehyde O-(5-carboxybutanoyl) oxime. A search of the CSD shows that salicylaldoximes and similar structures have a propensity for formation of polynuclear complexes. Measurement of adsorption isotherms on goethite indicate that the inclusion of a phenolic hydroxyl to these oxime ester molecules allows them to generate multilayers when attaching to iron(III) oxide surfaces which is useful in generating hydrophobic thick-films. The absence of a phenol to surface interaction in surface binding is supported by the binding of 2-hydroxy-benzaldehyde-O-methyl-oxime being weaker than that of 2-hydroxy-benzaldehyde oxime. In addition, these are both weaker than P50 oxime suggesting that the corrosion resistance synergism observed when combining P50 oxime with PIBSA is partly due to the high number of alkyl chain groups held in close proximity to the surface which assists in forming films with good hydrophobicity. In the fourth chapter, the synthesis of several benzoic acid derivatives was used to determine the effect of the systematic introduction of cooperative intermolecular bonding groups to a ligand. In addition, the inclusion of a sulfur atom was used to allow adsorption isotherm determination to be carried out using ICP-OES. The binding strength of 4-[3-(4-Methylsulfanyl-phenyl)-ureido]-benzoic acid, as determined by adsorption isotherms, is considerably higher than that of 4-ethylsulfanyl benzoic acid. A series of methionine derivates generated 2-[4-(1-Carboxy-3-methylsulfanyl-propylcarbamoyl)-benzoylamino]-4-methylsulfanyl-butyric acid, which has a large number of surface ligating and hydrogen bonding groups. This ligand forms a very stable ligand-surface assembly and has a tendency to create second or further layers. Carboxylic acids with a single carbon atom link to a carbonyl group are considered in chapter 5. Adsorption isotherm determinations demonstrate that 2-(4-chloro-phenyl) malonic acid binds more strongly to goethite that the commercial reagent, Irgacor 419©, and much more strongly than (4-chloro-phenyl) acetic acid. Molecular modelling identifies surface binding modes which could account for the higher stability of the malonate surface complexes. The most plausible models involve multisite attachment with a binucleating mode which leaves on carboxylic acid group free to form hydrogen bonds to adjacent ligands or, a trinucleating arrangement with malonate in its dianionic form. The development of commercial corrosion inhibitors is considered. A propensity for decarboxylation, observed when attempting to prepare polynuclear complexes as model compounds and when attempting to prepare new 2-substituted malonic acids may limit their efficacy in surface protection and surface engineering.