Abstract
The intended purpose of this work is to form a more complete understanding of how to maintain pristine, foulant-resistant stainless steel surfaces in aqueous proteinaceous or microbial environments such as those found on ship hulls, food processing plants, or in the human body. Biofouling is a worldwide problem requiring thin film engineering at the molecular level for prevention. This requires detailed understanding of adhesive and interfacial aspects of thin film materials that are unique to the underlying substrate material. Surface chemistry and aqueous interfacial phenomena are the central concepts here.
A chromium(III) (hydr)oxide dried film was used as a model substrate for the stainless steel passivation layer. In-house synthesised, X-ray amorphous and crystalline commercial particles were used in technique specific aqueous adsorption studies. The substrate materials were characterised by scanning electron microscopy, electrophoretic mobility, infrared spectroscopy, and thermogravimetric analysis. The dried substrates were comprised of non-spherical particles with mean diameters of 292 nm and 263 nm particles (broad size distribution), respectively. Each substrate had a zetapotential of around 30 mV and isoelectric points of pH 7.1 (commercial) and pH 7.9 (synthetic). The synthetic material was found to be a hydrated gel matrix with approximately 30% water content and the commercial material was a crystalline material with 1.5% water content.
Organic anions such as methylphosphonate, polyphosphonate, phosphonated poly(ethylene oxide), polysulfonate, and polycarboxylate were adsorbed and the interaction with the chromium(III) (hydr)oxide model substrate was established in terms of binding mode, substrate affinity, rinsing stability, surface coverage, and polymer surface conformation. The phosphonates and polycarboxylates demonstrated the highest affinity to the particles, both formed inner sphere complexes with the Cr3+ surface cations, and both formed complete monolayers at minimum. They were followed by polysulfonates in bond affinity, which formed an ion pair-like outer sphere structure with poly(vinylsulfonate) and an inner sphere structure with the (2-acrylamido-2-methyl-1-propanesulfonate) polymer. Fundamental aspects of the adsorption process were investigated with pH and concentration based assessment of the surface structures.
The phosphonates were adsorbed to 316L stainless steel foil and sputtered stainless steel substrates. The films formed with these phosphonate binders were investigated by quartz crystal microbalance, spectroscopic ellipsometry, and atomic force microscopy. Films were formed at concentrations greater than 0.1 mg mL-1 of methyl- and poly- phosphonate and showed resistance to washing. Surface coverages for the phosphonated poly(ethylene oxide) was between 0.5 and 2.8 poly(ethylene oxide) chains per nm-2, which suggests possible antifouling potential on stainless steel. Protein resistance experiments demonstrated 67%, 53%, and 77% antifouling for the 600, 2000, and 5000 molecular weight polymers, respectively, as compared to bare stainless steel.
Synthetic pathways were undertaken to develop phosphonated polymers that could serve as antibacterial, or antifouling, chemical motifs. Phosphonation reactions were attempted on quaternised polymers via free radical polymerisation of vinylphosphonic acid with vinylbenzyl trimethylammonium chloride and 3-[(methacryloylamino)propyl] trimethylammonium chloride, which yielded a phosphonic acid incorporation of 9.5% and 25%, respectively. Poly(allylamine) structure and low molecular weight chitosan were phosphonated by the Kabachnik-Fields reaction, which yielded phosphonic acid incorporation of 11% and 3%, respectively.