No description available.

No description available.

Gemini surfactants are superior to their corresponding monomeric surfactants in features such as much lower CMC values and greater efficiency. In spite of a lot of research on them there are, however, still many basic questions are unanswered. Even the determination of such a simple quantity as the coverage is uncertain at a level which would be unacceptable with other surfactants. The motivation of the work in this thesis was to try and make a systematic study of a range of surface properties associated with (a) the architecture of the gemini surfactant ion and (b) the counterion.

Polymer/surfactant systems are increasingly being used in a wide range of applications. Weakly-interacting systems comprise ionic surfactants and neutral polymers, while strongly-interacting systems contain ionic surfactants and oppositely charged ionic polymers. The complex nature of interactions in the mixtures leads to interesting and surprising surface tension profiles as the concentrations of polymer and surfactant are varied.

In order to improve understanding of the relationship between molecular structure and interfacial structure, surfactant monolayers at the air-water (A-W) and oil-water (O-W) interfaces, and counterions bound to surfactant monolayers, have been studied using sum-frequency spectroscopy (SFS) and complementary techniques, including neutron reflection (NR) and ellipsometry. The non-linear optical technique of SFS is applied first to the study of surfactants at the A-W interface. Various theoretical models used to explain peak intensities in the resulting vibrational spectra are critically assessed. Values of mean tilt and degree of conformational disorder of adsorbed surfactant molecules are inferred. It is observed that the molecular conformation of monolayers of different surfactants cannot be predicted solely from the area per hydrocarbon chain, depending also on the nature of the head-group. At constant area per molecule, monolayer structure is found to be unaffected by chain length. The behaviour of binary mixtures of a non-ionic and an anionic or zwitterionic surfactant is found to be indicative of ideal mixing thermodynamically in the monolayer, whereas strong synergistic interactions are observed for an anionic-zwitterionic mixture. Two counterions known to induce viscoelasticity in surfactant solutions are then investigated. The orientations of benzoate and p-tosylate ions bound to surfactant monolayers at the A-W interface are quantified. Both are found to be located deep within the hydrophobic region of the monolayer. The presence of p-tosylate causes structural changes in the monolayer, and p-tosylate is found to be acting as both ion and cosurfactant. Specifically, through the joint application of SFS and NR, the structure of a monolayer of hexadecyltrimethylammonium p-tosylate is established, revealing that the hydrocarbon chains of the surfactant rise steeply away from the water before turning over to lie flatter in the region above the p-tosylate ions. These findings are related to micelles in solution to provide an insight into the sphere-to-rod transitions thought to be responsible for dramatic changes in the physical properties of surfactant solutions in the presence of specific counterions. Finally, an entirely new experimental arrangement is developed to address the significant technical problems standing in the way of obtaining SF spectra of surfactants adsorbed at the O-W interface. The same basic design was later used to acquire the first SF spectra of surfactant monolayers at this interface.

This work is concerned with the interfacial behaviour of a range of non-ionic and anionic surfactants mainly at the sapphire/water interface. At the air/liquid interface, the adsorption of a series of a series of specially synthesized pentaethylene oxide surfactants was characterized at the molecular level by using specular neutron reflection in conjunction with isotopic labeling. Ihe information obtained about composition and the fragment distributions in the adsorbed layers helps to explain the variations in their macroscopic properties. The results indicate that the architecture of the surfactant hydrophobe, such as the branching and the position of aromatic group in the alkyl chain, has a strong influence upon the surface absorbed amount and limiting surface tension.

With regard to CO2-based processing and handling technologies, research into optimising COrphilic surfactant design is especially important. Here, surfactant structure design is focused on anionic surfactant analogues of Aerosol-OT (sodium bis (2-ethylhexyl) sulfosuccinate). The surfactants were systematically synthesised with modification centred on four strategies including (1) alkyl chain type (2) introduction of a third chain (3) counterion type and (4) surfactant template. The work presented in this thesis has aimed to investigate the relationship between the architecture and the properties (aggregate formation and interfacial activity) of AOT analogue in water and water-in-C02 (wlc) microemulsion. The surfactants have been investigated by a range of techniques including high-pressure phase behaviour, small-angle neutron scattering (SANS) and air-water (alw) surface tension measurements. The results show fluorination is a crucial factor influencing AOT surfactant analogue compatibility with CO2• The presence of fluorine in the surfactant chemical structure gives rise to an optimum interfacial packing density needed to promote favourable interactions with CO2• On the other hand, CO2- philicity of hydrocarbon surfactants can be enhanced by introducing a third chain as well as terminal methyl groups. However, despite low cost and being environmentally friendly, a major limitation of these hydrocarbon surfactants is inability to disperse water in the CO2 continuous phase. The results also highlight the challenges in controlling the CO2-philicity of hydrocarbon surfactants by changing bulky tetrapropylammonium (TPA) counterion and hydroxyaluminium disoap headgroup. It has now been established that both modifications are detrimental to CO2 compatibility. Here, a novel hybrid surfactant, with one F-chain and another H-chain, was synthesised where attention has been paid to minimizing the fluorine content while retaining' an acceptable level of COrphilicity. The work has advanced the understanding of how CO2-philicity of AOT analogues can be enhanced in CO2. The results obtained are beneficial for expanding CO2 industrial applications and realising its potential using the most economic and efficient CO2- philic surfactants.

A widely accepted model in obtaining the mixed micelle composition is Rubingh's thermodynamic model, in which CMC data from surface tension and conductivity measurements of mixed surfactant solutions are used to calculate compositions of mixed micelles. The validity of this model has not been previously challenged. In this study, the behaviour of binary mixtures of an anionic; sodium dodecylsulfate (SDS) and a cationic; dodecyltrimethyl ammonium bromide (DTAB) with non-ionic surfactants; N -dodecyl-N ,N -di mcthyl-3-ammoni o-l-prop<mesulfonatc (SB), N-dodecyl maltoside (DM), Octaethyleneglycol monododccylcthcr (C12Es) and 7,7-bis([( 1,2,3,4,S pentahydroxyhexanamido) methyl]-n-tridecane (di-GLU) in aqueous solutions has been studied using surface tensiometry, conductometry and small-angle neutron scattering (SANS) techniques. Firstly we investigated the critical micelle concentrations (CMCs) of aqueous solutions of single and binary mixtures of the surfactants and found large discrepancies in the CMCs obtained from surface tension and conductivity measurements even when those of the individual components are fairly constant. CMCs obtained from conductivity measurements are found to be larger, in fact, in some mixtures, about twenty times higher than those obtained from surface tension measurements. SANS measurements suggest that the size of mixed micelles vary with surfactant concentration and composition above CMC. Therefore, it is possible that surface tension measurements detect CMC at the point of maximum surface coverage above which mixed micelle size and composition continue to change. This is in agreement with the conductivity measurements, which suggest that only the presence of micelles with an appreciable size that is able to influence the conductivity of the mixed solution is detected. We conclude that the CMC of surfactant mixtures is not a well defined thermodynamic parameter as previously thought. Secondly, mixed micelle compositions calculated from the CMCs of the single and mixed surfactants using Rubingh's thermodynamic model and a new conductivity model developed in this work arc compared with those obtained from SANS model. The results from SANS model and the conductivity model show that the compositions of mixed micelles are close to ideal compositions in contrasts to those calculated from Rubingh's thermodynamic model which deviate widely from ideality. Mixed micelle compositions obtained from conductivity measurements using the new model show very good agreement with SANS results while those from surface tension and conductivity data applied to Rubingh's model differ appreciably from both. Thirdly. the values of Rubingh' s interaction parameter between the mixing surfactants calculated using CMCs determined from conductivity (CMC.J and surface tension (CMCy) measurements also show discrepancies. Taken together, our results in all the eight different binary mixed surfactant systems studied suggest that Rubingh's model cannot correctly predict the compositions of surfactant mixtures and thus its thermodynamic foundation needs further re-examination.

This study presents part of a multifaceted study of microbial biosurfactants and their industrial potential. The first step was the assembly of a microbial culture collection of a variety of biosurfactant-producing organisms with the choice of biosurfactant types informed by the industrial partner. The choice was based on two main criteria, the ease of product recovery and its final yield. Two groups of glycolipid biosurfactants were selected for further study: rhamnolipids produced by Pseudomonas aeruginosa and sophorolipids by Candida spp. Efforts to manipulate the biosurfactant chemical profiles by changing the cultivation media (carbon source in particular) and conditions in shake-flasks, demonstrated that there is only a limited possibility for changing the biosurfactant composition. This raised the question of the extent to which biosurfactant production is constrained by genetic determinants? To overcome the limitations of the flask-scale production, selected P. aeruginosa and Candida strains were used in bioreactors, and rhamnolipids and sophorolipids were synthesised in quantities sufficient for extraction and purification. The purified biosurfactants were used by the project partners for further characterisation and formulation in trial industrial products. Rhamnolipid yields were approximately 10 g/L whilst sophorolipid production exceeded 100 g/L. Detailed examination of the orcinol assay, which is widely used for the determination of rhamnolipid yields, showed that the method is flawed and provides an overestimate of yield when compared to quantification following extraction and purification. The culture approach had demonstrated the restricted possibilities for manipulating rhamnolipid production profile in P. aeruginosa and therefore a wider range of strains from different environmental niches were selected for genetic analysis. The aim of this part of the investigation was to establish the extent of natural gene variation which could be exploited for customised biosurfactant production. The comparative analysis of the rhl genes, coding the factors involved in rhamnolipid biosynthesis, was carried out on different P. aeruginosa strains isolated from water, soil and including pathogenic strains infecting cystic fibrosis patients. The extent of the gene sequence diversity resulted < 5%, which indicated that the rhl genes are conserved and are part of the core genome of P. aeruginosa. The single polymorphisms that occurred on the gene sequences, which gave rise to several variants, revealed no clear effect on the phenotype but appeared rather random. These variants showed also no specific correlation with different habitats. The analysis of the codon usage further supported the confidence in the highly conservative nature of the rhl genes. Most amino acids were encoded by highly preferred codons, some of which were selected upon the optimal translational efficiency, some others were instead determined by the high GC content of P. aeruginosa genome. On the whole, our data correlated well in showing that rhamnolipid biosurfactants produced by P. aeruginosa are conserved in their structure and profile as well in the genetic makeup. As involved in many important biological activities under a variety of environmental conditions, rhamnolipids developed as highly fit molecules. In the light of this, it appears that the likelihood that isolates of P. aeruginosa displaying unusual rhamnolipid profiles and features occurring naturally would be limited. Alternative routes for the development of industrially customised rhamnolipid biosurfactants are suggested as either the genetic manipulation of P. aeruginosa or the screening of rhamnolipid-producing organisms closely related.

No description available.