Abstract
The research presented in this thesis explores the behaviour of non-ionic surfactants at air/solution and silica/solution interfaces. The aim was to gain insight into this behaviour to improve the efficacy of detergent and rinse aid surfactant behaviour in dishwashing performance. The project was driven by both academic and commercial foci due to the project being funded through a collaboration between Fisher&Paykel, the University of Otago and the Ministry of Business Innovation and Employment (NZ). The project aims to create methodologies employable on-site at Fisher&Paykel’s product development centre derived from academic insight garnered during the work.
Analysis of non-ionic surfactant behaviour at the silica/solution interface was predominantly interrogated by attenuated total internal reflection infrared (ATRIR) spectroscopy. Such a technique is surface sensitive and informs of both molecular details of surfactant adsorption at this interface and the kinetics of such a process. Two different methodologies were employed to examine non-ionic surfactant adsorption at the silica/solution interface using ATR-IR. The first employed silica particle films, and the second a silicon prism. Particle films were employed to determine conformational behaviour of surfactants and silicon prisms were used for kinetic analysis of the adsorption process.
Stable silica films were formed by decreasing the silica suspension pH from 10 to 2.5 eliminating of electrostatic repulsive forces. Such films were easily prepared and experiments were found to be replicable. IR spectral interpretation of surfactant adsorption to silica films was aided by analyzing the spectral response of silica films exposed to differing relative humidities. Such work spectroscopically
illustrated the room temperature interconversion of surficial siloxane groups to hydrogen-bonded silanols groups. The data gathered gave insight into the IR response of the main νas(Si-O-Si)TO mode at ∼1080 - 1040 cm−1, with a bipolar band noted.
Surface forces experienced in foam and wetting films formed in surfactant solution were studied with the newly developed interferometric atomic force microscopy (i-AFM). Such a technique gives the synchronous measurement of force and separation distance of deformable surfaces, not previously possible using colloid probe AFM. Due to the the micrometer size of bubbles used no deformation of bubble surfaces were observed owing to increased Laplace pressure. Such results were supported by analysis of surfactant adsorption behaviour at silica/solution and gas/solution interfaces using ATR-IR, ellipsometry and interfacial tension measurements. These techniques provided information about surfactant packing, adsorbed layer thickness and phase behaviour at both of these interfaces.
Finally this work presents the extension of analytical methodologies employed to examine fundamental non-ionic surfactant interfacial behaviour to the real world application of machine dishwashing. A methodology for the determination of adsorption rate constants utilizing UV-visible spectroscopy was developed. These measurements gave replicable results and will be utilized in testing the effect of altering product design on surfactant adsorption behaviour within the dishwasher. In addition foam column measurements determined foam stability of detergents formulated with different non-ionic surfactants. Such measurements gave good indications about desired surfactant properties to mitigate foaming in addition to providing an analytical tool that Fisher&Paykel can use on-site.