Greener Surface Anchoring Anti-Bacterials

Abstract

This research thesis has been focused on the interfacial chemistry involved in the anchoring on various substrates of commercially significant biocides of the type Dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (C26H58ClNO3Si). These biocides are referred throughout this thesis as Anchoring/Anchored Quaternary Ammonium Salts (AQAS). The strategic objective in this study is to develop "greener" biocide systems with reduced background environmental toxicity and reduced human health impacts. Three diverse substrates with varied potential end-user applications were studied to evaluate the adsorption, coverage and leachability of the anchored biocide systems. The first section of the study (Chapter 2) is concerned with elucidating the basic interfacial chemistry of mono-molecular oriented island mat-films of the AQAS biocides on an atomically flat quartz surface. Chapter 2 is essentially a platform study of the basic science underpinning the entire thesis. Imaging of the molecular layers at nano-scale resolution is possible using Atomic Force Microscopy (AFM). Morphological evidence of bacterial degradation on the biocide layers was observed using high-resolution Atomic Force Microscopy (AFM) and Confocal Scanning Microscopy. X-ray Photoelectron Spectroscopy (XPS) was extensively used for the surface analysis of anchored molecular layers. The sampling depth of XPS being ~ 10 nm gave detailed information regarding the ordering and clustering of the AQAS layers. The XPS N1s binding energy peaks were observed to give noticeable shifts attributed to higher-layer disordering and conformation effects associated with the long C18 alkyl chains of AQAS, which, under ordered tight packing conditions, might adopt an all-trans orientation. Increasing the hydrophilicity of the quartz substrate using oxygen plasma treatment increased the population of anchoring hydroxyls and so also increased the adsorption capacity and multi-layer coverage of the quartz surface (Chapter 3). The latter sections of the thesis are focussed on the anchoring of the AQAS biocides on substrates with potential end-applications in high hygiene environments, surfaces and medical devices (Chapters 4 and 5). Expanded Perlite, a siliceous thermally pre-treated substrate with a honeycomb cellular structure was studied for the sustained release of the biocide under aqueous leaching conditions over a period of 2 days (Chapter 4). This system may find future applications in food handling hygiene and wastewater treatment applications. The formation of oligomeric forms of AQAS in higher layers following curing at 160°C was identified using Solid State Nuclear Magnetic Resonance (SSNMR) spectroscopy. SSNMR technique was able to detect the AQAS oligomers due to the multi-layered nature of the oligomers mimicking a bulk material in this study, in contrast to the few molecular layers detected on a quartz surface in Chapter 2, which are below the detection limits of conventional SSNMR. This study led to a quantitative evaluation of the AQAS sustained release kinetics observed for the biocide loaded expanded perlite honeycomb system under aqueous leaching studies. The leaching was followed using Fourier Transform Infrared Attenuated Total Reflectance Spectroscopy (FTIR-ATR) and Thermal Gravimetric Analysis (TGA). The final experimental section of the thesis (Chapter 5) comprises a study of the chemisorption of AQAS anchored directly onto optically transparent and heat resistant thermoset polyurethane films. This is a simple, low-dosage and cost-effective approach to form antibacterial hygienic films on an important polymer substrate used in medical devices, displays and touch screen devices in the electronics industry. The adsorption capacity of the polyurethane film surfaces was increased by enhancing the hydrophilicity of the surface using plasma treatment. In a parallel set of experiments, siloxane-polyurethane blend surfaces were subjected to oxygen-plasma treatment and these modified surfaces revealed substantially increased adsorption capacity and surface coverage.