UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
2 results
Search Results
Item Self-assembly in aqueous solutions of a non-ionic hydrotrope(2012) Subramanian, Deepa; Anisimov, Mikhail A; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hydrotropes are amphiphilic molecules, too small to cause spontaneous self-assembly towards equilibrium mesoscale structures in aqueous solutions, but they form dynamic, noncovalent assemblies, which may create microscopic regions of lowered polarity. This enhances the solubilization of hydrophobic compounds, also known as solubilizates, in aqueous solutions and may cause further aggregation to larger structures. In this work, unusual mesoscopic properties of aqueous solutions of a non-ionic hydrotrope, namely tertiary butyl alcohol (TBA) have been investigated by light scattering, microscopy, and chromatography. Aqueous TBA solutions show anomalous thermodynamic and structural properties in the range of concentrations 3-8 mol % TBA and temperatures 0 - 25 °C. These anomalies appear to be associated with short-lived, short-ranged micelle-like structural fluctuations, distinctly different from usual concentration fluctuations in non-ideal solutions. Molecular dynamics simulations and neutron-scattering experiments show clustering of TBA molecules on a nanometer scale, interacting through hydrogen bonds with a shell of water molecules. In this concentration range, TBA aqueous solutions, although macroscopically homogeneous, occasionally show the presence of "mysterious" inhomogeneities on a 100 nm scale. We have found that the emergence of such inhomogeneities strongly correlates with impurities present in commercial TBA samples. Experiments with controlled addition of a third component, such as propylene oxide, isobutyl alcohol, or cyclohexane, reveal the mechanism of formation of these inhomogeneities through stabilization of micelle-like fluctuations by a solubilizate. These structures are long-lived, i.e., stable from a few days up to many months. We have confirmed that mesoscale structures in aqueous solutions can be generated from self-assembly of small molecules, without involvement of surfactants or polymers. This kind of self-assembly may potentially result in the development of novel nanomaterials.Item PHASE BEHAVIOR AND INTERFACIAL PHENOMENA IN TERNARY SYSTEMS(2009) Subramanian, Deepa; Anisimov, Mikhail A; Adomaitis, Raymond A; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Phase behavior in multi-component systems has a wide variety of applications in the chemical process industry. In this work, the interfaces in two-phase, three-component systems were modeled and studied. Direct calculations of the asymmetric concentration profiles near the critical points of fluid phase separation are very difficult since they are affected by mesoscopic fluctuations. In this study a "complete scaling" approach was used to model interfacial profiles for a highly asymmetric, dilute ternary mixture near the critical point of liquid-liquid separation. The symmetric order parameter profile, the density profile of the lattice gas model, was used to further calculate the asymmetric interfacial concentration profiles at the mesoscale. Fluid asymmetry has been introduced through mixing of the physical field variables into the symmetric scaling theoretical fields. The system-dependent mixing coefficients were calculated from experimental data and a mean-field equation of state, namely, the Margules model. The resultant interfacial profiles for the concentration of water across the methanol-rich and cyclohexane-rich phases show the asymmetry associated with the contribution of the entropy into the symmetric order parameter profile.