Nanostructures of mixed surfactant aggregates: Adsorption, solubilization and viscoelasticity

In this work, solution and adsorption behavior of surfactant systems including two key compounds, n-dodecyl-beta-D-maltoside and a viscoelastic surfactant, APA-T, has been systematically investigated using traditional techniques including surface tensiometry, ultrafiltration, depletion, FTIR, viscometer and rheometer as well as a range of new techniques including analytical ultracentrifuge (AUC), fluorescence spectroscopy, Cryogenic-temperature Transmission Electron Microscopy (Cryo-TEM) and 2D Nuclear Magnetic Resonance (NMR) to obtain information on the nanostructures of the aggregates in solution and at solid/liquid interface. This comprehensive study helps to elucidate the mechanisms of micellization and adsorption behavior of the surfactant mixtures and understand the roles of nanostructures of surfactant aggregates in determining their properties.; Although nonionic, the adsorption of n-dodecyl-beta-D-maltoside was found to be significantly pH-dependent at solid/liquid interfaces. For instance, the adsorption density on alumina at pH 7 was observed to be 50 times higher than that at pH 4. The adsorption was found to be proportional to the concentration of surface -AlOH group which is in equilibrium with other surface species governed by pH through surface ionization reactions. Evidently, the surface -AlOH group determines the formation of hydrogen bonds between the surfactant molecules and the solid surface and thus the adsorption. The discovery accounts for the adsorption behavior of n-dodecyl-beta-D-maltoside on a number of other solids, including hematite and silica. Based on the findings, a general model was developed to predict the adsorption of this sugar based surfactant on solids.; In addition, the mixture of n-dodecyl-beta-D-maltoside (DM) and sodium dodecyl sulfonate (SDS) was found to show tunable synergistic or antagonistic interaction depending on pH. The composition of the adsorbed layer was found to be correlated to their monomer concentrations that are governed by the micellization behavior in solution. Synergistic interaction was identified in this nonionic/anionic mixture and the micellar size and shape was found to be dominated by the presence of SDS. Interestingly, the coexistence of a second micellar species was discovered using AUC at high DM ratios.; For removal of pollutant molecule, the micellization behavior of n-dodecyl-beta-D-maltoside in the presence of aromatic phenol was investigated. The intermolecular interaction in turn has a profound impact on the removal of aromatic pollutants from aqueous using micellar enhanced ultrafiltration technique. Direct evidence was obtained for the first time that phenol molecules residue in the palisade layer of the DM micelles with the benzene group interacting with the hydrocarbon chain, particularly the first methylene group. To further understand the mechanism, Cryo-TEM and AUC were employed and the results have shown the micellar shape transition from spherical to worm-like and the coexistence of two types of micelles. Based on the results obtained, a multi-step micellar growth model is proposed to describe the micellar evolution in the system.; When surfactant molecules assemble into giant micelles that undergo entanglement, the fluid systems exhibit significant viscoelasticity analogous to polymer solutions. The steady shear and dynamic rheology of APA-T solutions were investigated systematically as well as in the presence of different additives. The worm-like micelles were revealed using Cryo-TEM, while surface tensiometry and fluorescence spectroscopy supplied supplementary information on the molecular packing in this system. A theoretical model is proposed based on the structural transition of the micelles corresponding to temperature to predict the thermosensitive behavior of worm-like micelle systems and the model prediction is in good agreement with the experimental results. (Abstract shortened by UMI.)...