Cryogenic transmission electron microscopy as a probe of microstructural transitions in complex fluids.

摘要

This dissertation explores the relationships between the self-assembled microstructure, phase behavior, and rheology of aqueous surfactant solutions. The explicit intent is to selectively tune surfactant solution properties by altering macroscopic parameters, such as temperature, surfactant ratio, pH, or addition of a salt or hydrotrope. Self-assembled surfactant structures have been extensively characterized using cryogenic transmission electron microscopy (cryoTEM). This technique was used to characterize self-assembled morphology and reconcile visually observed phase behavior with thermodynamics. Branched micelles associated with peaks in viscosity at zero shear rate were identified using cryoTEM and their structure studied under flow using simultaneous steady shear rheology and small angle neutron scattering experiments (flow-SANS).;Structural transitions were explored in several related surfactant systems. Sugar based nonionic surfactant like n-dodecyl-beta-d-glucoside (C 12betaG1) are made from renewable resources and are non-toxic and environmentally benign. Their use, however, is often restricted by limited solubility in water. Introducing a small amount of the anionic surfactant sodium dodecyl sulfate (SDS) reduces the miscibility gap for C12betaG 1, and the concentration of SDS (or any added salt) is an excellent control parameter for tuning a wide variety of available microstructure as well as macroscopic physical properties. Alkyl polyoxyethylene sulfate surfactants (AOES) are anionic surfactants, commonly used in the personal care industry, that contain a hydrophilic head group with distinct ionic and nonionic moieties. Addition of the cationic hydrotrope p-toluidine hydrochloride (PTHC) induces phase separation at high concentrations, but a lower concentration produces a wide range of micelle morphologies with AOES. Temperature and PTHC concentration are both excellent control parameters for tuning the micelle structure. Finally, use of pH or UV light as a control parameter was explored using aqueous solutions of the anionic surfactant sodium dodecylbenzene sulfate, the amino acid arginine, nitric acid, and the photo acid generator diphenyliodonium nitrate. A spontaneous micelle-to-vesicle transition was induced by the addition of acid or exposure to UV light.;Peaks in zero shear rate viscosity were observed in solutions containing the anionic surfactant sodium 2-(dodecyloxy)ethyl sulfate (C12E 1S) as a function of PTHC concentration and in mixtures of C12 betaG1 and SDS as functions of both surfactant ratio and added salt. CryoTEM confirms the presence of slightly branched micelles before the peak, slightly branched micelles at the peak, and highly branched micelle networks beyond the peak in zero shear viscosity. The viscosity peak does not correspond to the onset of micelle branching but rather to a critical degree of branching. All of the micellar solutions examined shear thin. Flow-SANS experiments on mixtures of C12E1S and PTHC correlate this shear thinning to flow-induced alignment of the linear and slightly branched micelles. The highly branched micelle network exhibits a similar degree of shear thinning but with substantially less micelle alignment. This suggests the branched micelle network dissipates stress under flow without disentangling.