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Studies involving solids-stabilised foams have been limited and few have focused on the benefits of complex systems such as those involving mixtures of more than one surfactant. Little is known about the effectiveness of using mixed surfactant systems as foam stabilisers at the bulk level. The purpose of this project was to gain further understanding and insight into foam stability, on a bulk scale, in the absence, and in the presence, of solid colloidal particles, and for systems involving single or mixed surfactants. Foams were produced using sodium dodecyl sulphate (SDS), dodecanoic acid (DA), or a mixture of both surfactants at varying molar ratios. The surface tension at a given concentration, the limiting surface tension (at high concentrations), the critical micelle concentration, foamability and foam stability were all significantly affected by the molar ratio of SDS to DA. The initial pH of mixed surfactant solutions played a role in the shift of surface tension and CMC. The higher pH values, the lower surface activity, therefore resulted in higher surface tension. At a given pH, however, the mixed surfactant solutions with higher molar ratio of SDS to DA appeared to have lower CMC. The foam stabilising ability of colloidal dispersions of four hydrous metal oxides, namely hydrous iron oxide (formed by hydrolysis of Fe(III) solutions and referred to as HFO), hydrous zinc oxide (formed by hydrolysis of Zn(II) solutions and referred to as HZO), hydrous chromium oxide (formed by hydrolysis of Cr(III) solutions and referred to as HCO), and hydrous nickel oxide (formed by hydrolysis of Ni(II) solutions and referred to as HNO) were studied at varying concentrations. Generally foam stability increased as the solid concentration increased. Foams stabilised by HNO were found to be the most stable. Foams stabilised by HFO were found to be unstable regardless of the solid concentration. It is believed that the instability of such foams is primarily due to the large aggregated size of HFO flocs. The aggregate size of hydrous metal oxides was influenced by the concentration of NaOH used to hydrolyse the metal ion solution, and by sonication treatment immediately following solid formation. However, the final pH of the colloidal dispersions did not significantly change the aggregate size. Solids stabilised foams are believed to be highly dependent on the state of hydrophobicity of the solids used, and this is in turn controlled by adjustment of the pH dependent surface charge and potential. Electrophoretic mobility is a commonly used tool to probe the potential near the surface and was used in this thesis to determine the affect of surfactant adsorption (particularly from solutions containing mixtures of SDS and DA) on surface properties of the solids. The electrophoretic mobility of all hydrous metal oxide aggregates decreased as the concentration of SDS/DA increased. Specific adsorption was evident in all cases and resulted in charge reversal for most cases. Electrophoretic mobility data for surfactant adsorption, as a function of total surfactant concentration, was consistent with a three-stage model of surfactant adsorption involving (1) electrostatic adsorption, (2) cooperative adsorption and (3) surface saturation. The influence of surfactant adsorption on electrophoretic mobility was found to be consistent with models requiring the stabilising solids to be in a controlled state of flocculation, where the zeta potential (as probed by electrophoretic mobility) must be sufficiently high to prevent total flocculation of the solid and thus collapse of the foam, but sufficiently low that the solids have some degree of hydrophobicity such that they prefer to be only partially wetted (and thus reside at the air/water interface). The total percentage adsorption of SDS/DA surfactant mixtures on both hydrous iron oxide and hydrous nickel oxide was found to be independent of time. However, the proportion of adsorption due to SDS and DA was dependent on time. Moreover, the initial and final ratio of SDS to DA adsorption did not reflect their ratio in solution. SDS adsorbed, initially, to a greater extent that would be predicted from its solution concentration, but this trend was reversed after a period of time.

Recently designed Pepfactants® are an innovative type of nano-technological products, which could potentially replace conventional surfactants in broad-ranging applications. Currently, Pepfactants® technology is still in an initial design period at the laboratory scale. In order to develop the industrial-scale production of Pepfactants®, the design group has proposed simulated strategies for industrial-scale Pepfactants® manufacture and a desire to improve these strategies with regards to sustainability. This project aimed to assist Pepfactants® designers to understand the environmental footprint of simulated Pepfactant® AM1 manufacturing process, using the methodology of Life Cycle Assessment (LCA) – a comprehensive tool to quantify the environmental impacts from products and processes. To find the environmental shortcomings of the proposed manufacturing process for Pepfactant® AM1, the LCA outcomes were compared with published life cycle information of traditional chemical surfactant Lineal Alkylbenzene Sulphonate (LAS) production. Following LCA methodology, a life cycle inventory was compiled based on the simulated AM1 manufacture, which determined the environmental impact assessment for both AM1 and LAS production. In the LCA boundaries disregarding the usage of both surfactants, the quantitative LCA comparison results indicated that raw material and energy requirements of AM1 manufacture were much higher than LAS production, estimated to be 3,186 t/t AM1 against 31.1t/t LAS and 1,564,000MJ/t AM1 against 69,870MJ/t LAS respectively. Additionally, compared with LAS production, enormous water consumption (2,651 t/t AM1) and CO2 emission (522 t/t AM1) were also shown to be severe environmental problems for AM1 manufacture. Furthermore, the AM1 manufacture presents apparent problems with environmental impacts of nutrification, human toxicity, photochemical oxidant formation and acidification in comparison with LAS production. Other than providing the optimisation point in the view of environmental impacts for Pepfactant® AM1 manufacture, the results of experimental work in this project showed that as the surfactant concentration increases a greater foam height of Pepfactant® AM1 was achieved than when (from 7mm to 52mm between 15μM and 100μM) compared with LAS (from 8mm to 53mm between 31.3μM and 2,000μM) in the same aeration duration. This result demonstrated the great potential of AM1 to replace LAS based on the LCA functional unit – 1 tonne of products. The experiments results implied that 1 tonne of AM1 is able to have the same foaming ability as approximate 25 tonnes of LAS. Consequently, the environmental impacts from Pepfactant® AM1 manufacture are reduced by 25 times in the extended LCA boundaries linked to the quantitative usage comparison of these two surfactants. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1317664 / Thesis (M.Eng.Sc.) -- University of Adelaide, School of Chemical Engineering, 2008