Abstract
The ability of surfactant-like compounds to inhibit the corrosion of aluminum in NaCl solution was systematically investigated. The basic idea of this study was to scrutinize the effect of type of backbone chain (alkyl and perfluoroalkyl), length of backbone chain (number of carbon atoms 7, 10, and 17), various anchor groups (carboxylic, thiol, and imidazole) and presence of alkylene and benzene spacers between perfluoroalkyl chain and anchor group. To tackle these effects, three model studies were designed for alkaline etched, superhydrophilic aluminum surface and then approached experimentally and by density functional theory modeling. This enabled us to decouple the adsorption affinity of selected anchor groups on the hydroxylated aluminum surface from the lateral intermolecular cohesive interactions between hydrophobic backbone chains. Fourteen compounds were used to study the changes in the surface composition, wettability and the electrochemical barrier properties. For the carboxylic anchor group, the length and type of chain are important for barrier properties and also for tuning the wettability of the surface. The addition of alkylene spacer to perfluoroalkyl chain significantly affects the properties of the modified surface. Thiol and imidazole anchor groups, however, are not efficient inhibitors regardless the type and length of backbone chains.
Highlights
Fourteen organic compounds were purposely prepared to study the effect of the type and length (x = 7, 8, 10, and 17) of backbone chain and the type of anchor group (COOH, SH, and ImiH) on adsorption on a superhydrophylic, hydroxylated aluminum surface
The methodological approach consisted of four levels: (i) synthesis or purchase of appropriate organic compounds, (ii) fabrication of adsorbed organic layers on Al etched in alkaline NaOH solution; (iii) their experimental characterization using electrochemical and surface-analytical (SEM/energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and ToF-SIMS) techniques; and (iv) density functional theory (DFT) modeling of adhesion and self-assembly of organic layers on oxidized Al substrates
By this integrative experimental-modeling approach applied on the three types of targeted systems— (1) Al-surface/anchor-group/alkyl-backbone, (2) Al-surface/anchor-group/perfluoroalkyl-backbone, and (3) Al-surface/anchor-group/spacer/perfluoroalkyl-backbone—we gained valuable information concerning the mechanism of interaction of various backbone chains and anchor groups with Al
Summary
Presentation and synthesis of organic compounds.—Ten perfluoro compounds were investigated (Fig. 1), among them seven were synthesized in-house (Scheme 1–6). Model study 2.—The effect of alkylene (−(CH2)2– or C2) and benzene (Bn) spacers was studied at a selected length of perfluoroalkyl and alkyl chains (x = 8) and constant type of anchor group (COOH) (Fig. 4b). Even the standalone shortest-chain C1–COO* molecule ( shown in Fig. 17) displays a similar value This confirms that neither the type nor the length of the organic backbone significantly affects the binding of the anchor group to the surface. Thiol function cannot provide strong chemisorption with such an etched Al surface.[2] For the Cf8-ImiH (528 g.mol−1), only a fragment of the molecule, corresponding to the C3N2H3− imidazole anchor group, is detected (m/z = 67.2) (Fig. 20b) This may suggest the possibility that only imidazole anchor group bonds to the surface, without the alkyl chain. In addition to XPS, it was shown that the variations in durable long-term protectivity can be followed by electrochemical measurements.[50,58,59]
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