Aggregation And Air/Water Interface Adsorption Of Novel Surfactants

This work mainly focused on the relationship between the chemical structures and the physico-chemical properties of gemini surfactants. Three series of gemini surfactants are involved in this thesis. Various techniques, including surface tension, electrical conductivity, steady-state fluorescence, isothermal titration microcalorimetry (ITC), nuclear magnetic resonance (NMR) and nutron reflection are used to investigate the aggregation properties and adsorption behavior of these surfactants. The main conclusions are listed as follows:1. Aromatic groups in the hydrophobic chains of surfactants can facilitate the micellization process. Cationic surfactants N10TAB and N10-6-10N with nitrophenoxy groups at the end of alkyl chains have very strong self-aggregation ability and can form rather small micelles in solution. The micro-environment of different groups changes upon micellization: the hydrophilic headgroups of N10TAB and N10-6-10N are transferred from the aqueous bulk phase to the micelle/water interface; the nitrophenoxy groups partially insert into the micelles, and face the middle methylenes in the hydrophobic chains. In addition, the micelles formed by N10-6-10N in solution can convert into vesicles with the increase of surfactant concentration.2. The thermodynamic behavior is unique for the micellization of the gemini surfactants Nn-6-nN. The absolute value of micellization enthalpyΔH_mic is negative, large and linearly increases with the alkyl chain length. The average difference in micellization enthalpy for Nn-6-nN is about -1.7±0.1 kJ mol^–1 K^–1 per methylene at 298.15 K. Moreover, the exothermic effect induced by each nitrophenoxy group is a constant value for the micellization of Nn-6-nN, which is about -14.0±0.5 kJ/mol per nitrophenoxy group. The micellization heat capacityΔC_p,mic can be obtained from the slope of the variation ofΔH_mic with temperature. The averageΔC_p,mic value decreases about -40±2 J mol^–1 K^–1 per methylene for Ns-6-sN. TheΔC_p,mic originates almost exclusively from the dehydration of hydrophobic alkyl chains, indicating that the nitrophenoxy groups in Nn-6-nN molecule are largely exposed to water even in the aggregates. The aggregation numbers n can be obtained from the simulation of ITC data, which are approximately 13, 12 and 8 for N8-6-8N, N10-6-10N and N12-6-12N, respectively.3. The surface coverage and structure of adsorbed layers of N10TAB and N10-6-10N at the air/water interface are investigated by the combination of neutron reflection and isotopic labeling. N10-6-10N molecules are more loosely packed than N10TAB at the air/water interface. The overall thicknesses of the surfactant layers at CMC are about 19 and 16 ? for N10TAB and N10-6-10N respectively. The chains of both surfactants are tilted away from surface normal, with the tilt increasing in the outer part of the layer. The broad nitrophenoxy distribution indicates significant positional disorder of the nitrophenoxy groups along the surface normal direction and their intermixing with alkyl chains in the adsorbed layer.4. The structures of surfactant headgroup have great impact on the aggregation morphology. The 12-s-12 (OH) surfactants start to form dimers well below their critical micelle concentrations (CMC). Above the CMC, these surfactants form both micelles and vesicles spontaneously with a micelle-to-vesicle transition. The hydrogen bond among the OH groups of headgroups and water molecules as well as the strong hydrophobic interaction among the hydrocarbon side chains are the main origins for these unique aggregation behaviors of these gemini surfactants.5. Many environmental factors can be used to modify the surfactant aggregation process. The aggregation properties of the anionic gemini surfactant SDUC have been characterized at different pH values. At pH 7.0, SDUC can form vesicles; while at pH 12.0, the SDUC molecules can only form micelles. As a small amount of Cu²⁺ ions are added to the vesicle solution of pH 7.0, fission of vesicles into small vesicle clusters occurs. Nevertheless, when Cu²⁺ ions are added to the SDUC solution of pH 12.0, vesicles are formed due to the formation of Cu²⁺- SDUC complexes.