Keng Chang Chou
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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - May 2021)
This dissertation uses molecular dynamics (MD) simulations to mainly focus on the study of the interaction between neutral surfactants-water and anionic surfactant-anionic polyelectrolyte on the water surface. Besides, this study devotes to finding the possible routes of improving the design of a drug candidate, polyethylene-glycol-linked cationic binding groups (PEG)n-HBG, to inhibit polyphosphate (polyP) thrombotic activities. It is found that the behavior of the nonionic polyoxyethylene glycol alkyl ether on the water surface is more anionic-like, even though the surfactant is overall neutral. The non-ionic surfactant increases the depth of the surface anisotropic layer and the average number of hydrogen bonds per water molecule. MD simulation showed that the negatively-charged O atoms have the most impact on the orientation of water as most water molecules arrange with their H atoms pointing toward the surface. In contrast, the behavior of the zwitterionic surfactant, N-dodecyl-N, N-dimethyl-3-ammonio-1-propanesulfonate, on the water surface is more cationic-like as the positively charged group is more capable of orienting interfacial water. The zwitterionic surfactant orients water molecules with their OHs mostly pointing toward the liquid water. While the complex formation between highly-charged surfactants and polyelectrolytes of the same charge is generally expected to be prohibited by the electrostatic repulsion, my study shows it is possible to form thermodynamically stable complexes in the presence of excess ions. With excess Na⁺ ions, the charge screening effect allows anionic polyelectrolyte to weakly interact with anionic surfactant via hydrogen bonds. In the presence of divalent Ca²⁺ ions, the surfactant and the polymer is strongly coupled by forming Ca²⁺ ion bridges and hydrogen bonds.The mechanism of complex formation between (PEG)n-HBG and polyP are studied using metadynamics simulations with the all-atom and coarse-grained force fields. It is shown that the PEG length does not have any impact on the interaction between the (PEG)n-HBG and polyP. However, it mostly improves the drug’s hemocompatibility by preventing the cationic drug from binding to other negatively- charged biomolecules. Increasing the number of the positive charges on the headgroup strengthens drug binding to polyP. It is found that the binding of (PEG)n-HBG remains intact against various lengths of polyP.
This dissertation studies the silica/water interface using sum frequency generation spectroscopy. The effects of alkali chloride ions and temperature on the hydrogen bonding network at the interface are examined. We observed that the structure of water in the Stern layer depends on the identity of the cation. The ability of a cation to displace the hydration water on silica surface is in the order of Mg²⁺ > Ca²⁺ > Li⁺ > Na⁺, consistent with the trend of the acid dissociation constant of the salt. We conclude that ions with a high pKa, such as Mg²⁺ and Ca²⁺, have a local electrostatic field strong enough to polarize water molecules in the hydration shells of the ions. These partially hydrolyzed water molecules form linkages with the negative charges on the silica, forming solvent shared ion pairs. During freezing of pure water, we observed a transient phase of ice at water/mineral interfaces, which had enhanced IR-visible sum frequency generation intensity for several minutes. Most forms of ice are centrosymmetric but a possible explanation of for the transient phase is the formation of stacking-disordered ice during the freezing process. Stacking-disordered ice, which has only been observed in the bulk ice at temperatures lower than -20 °C, is a random mixture of layers of hexagonal ice and cubic ice. The transient phase at the ice/mineral interface was observed at temperatures as high as -1 °C. This observation suggests that the mineral surface may play a role in promoting the formation of the stacking-disordered ice at the interface. The effect of ions during freezing at the silica/water interface was investigated. Ice is the first phase to form. NaCl·2H₂O forms below the eutectic temperature, indicating that the formation and growth of ice does not push the ions out of the interfacial region. We compared the surface freezing diagram with the bulk equilibrium phase diagram of aqueous sodium chloride solutions. Although the concentration of ions is higher at a charged surface, we observe that freezing point depression at the surface is analogous to freezing point depression for homogeneous freezing and bulk equilibrium phase diagram.
Single-molecule localization microscopy has greatly improved our understanding of biology by providing super-resolution images of biological processes and structures. However, it is still very challenging to apply this technique to thick tissues. A 3D imaging system based on single-molecule localization microscopy is presented to allow high-accuracy drift-free (
This dissertation studies the surface chemistry of water and charged molecules using phase-sensitive sum frequency generation (SFG) vibrational spectroscopy. The studied molecules include surfactants, polyelectrolytes, bitumen, and ionic liquid, which are related to technological processes, such as surface modification, catalysis, and bitumen production. Interactions of the polyelectrolyte partially hydrolyzed polyacrylamide with water and cations at air/liquid interfaces were studied. The polyelectrolyte caused water molecules to re-orient with the hydrogen pointing toward the air. The addition of Na⁺ counteracted the negative charges of the polyelectrolyte. Divalent cation Ca²⁺ formed a polymer-ion complex with the polymer and completely destroyed the ordered water structure. The addition of polyelectrolytes to a surfactant solution caused a complex behavior of the surface tension. SFG studies showed that the complex surface tension behavior was the result of a surface charge reversal. A better ordered interfacial molecules produced low surface entropy, which counteracted the surface enthalpy decrease and kept the surface tension nearly unchanged at a low surfactant concentration. The ordering of water was found to play a role in surface tension. Four types of surfactants were studied: nonionic, zwitterionic, anionic, and cationic surfactants. Particularly, ionic surfactants decreased the surface entropy to near zero or even negative, which was associated with a surfactant-induced ordering of surface water molecules and an increase in hydrogen bond formation. Both effects lead to the reduction of water’s surface entropy. Studies of bitumen/water interfaces using phase-sensitive SFG showed that the bitumen surface carried negative charges, which induced a well-ordered water structure at the bitumen/water interface. The presence of salt neutralized the surface charge and nearly destroyed the ordered water structure. Both anionic and cationic surfactants interacted with the bitumen surface. Finally, the water structure at the air/1-butyl-3-methylimidazolium tetrafluoroborate aqueous solution interface was studied. The orientation of water molecules indicated that a charge reversal occurred at the interface when the concentration of the ionic liquid (IL) changed. The imidazolium cations resided at the water surface at a low IL mole fraction. However, with an increased IL mole fraction, the surface number of anions increased making the surface negatively charged.
Surface properties of polymers and competition adsorption of toluene and heptaneon silica were studied using IR-visible sum frequency generation (SFG) vibrationalspectroscopy. SFG is intrinsically surface sensitive because the second-order opticalprocess is forbidden in media with inversion symmetry, such as bulk polymers andliquids. This nonlinear optical technique provides surface vibrational spectra underambient conditions without the need of an ultra-high vacuum environment. Polymersurface properties, including surface relaxation temperature of poly(methyl methacrylate)(PMMA) and surface electronic states of poly[2-methoxy, 5-ethyl (2’-hexyloxy) paraphenylenevinylene] (MEH-PPV), were investigated. It was found that there aresignificant differences between the surface and bulk properties for these polymers. ForPMMA, a new surface structure relaxation was identified at 67°C, which does not matchany known structure relaxation temperatures for bulk PMMA and is 40°C below the bulkglass transition temperature. For MEH-PPV, SFG electronic spectra, which wereobtained by scanning the frequencies of incident visible and JR beams, indicated that theelectronic states at the polymer/solid and air/polymer interfaces are red-shifted withrespect to that of the bulk. Finally, SFG was employed to study the competitionadsorption of toluene and heptane on silica surfaces. Experimental data showed thatheptane adsorbed favorably compared to toluene. Using a Langmuir adsorption isotherm,the changes of Gibbs free energy for the adsorption processes were calculated to be —12.1± 1.8 (kJ/mol) for toluene and —16.5 ± 2.3 (kJ/mol) for heptane.