Chemical and Biomolecular Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2751
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Item Self-Assembled Photoresponsive and Thermoresponsive Fluids with Tunable Rheology(2009) Kumar, Rakesh; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluids whose rheological properties can be tuned by light or heat (termed as photorheological (PR) or thermorheological (TR) fluids, respectively) have attracted a lot of attention as they can be useful in numerous applications such as drug delivery, coatings, sensors, and valves for microfluidic devices. However, current formulations of these fluids suffer from several limitations: in particular, they often require synthesis of complex organic molecules by elaborate procedures, and this limits the widespread use of these fluids. In this dissertation, we seek to develop and investigate new classes of PR and TR fluids based on organic molecules that are readily available and quite inexpensive. Since no new synthesis is required, these systems could prove to be more attractive for a variety of applications. In the first part of this study, we describe a new aqueous photorheological (PR) fluid based on the zwitterionic surfactant, erucyl dimethyl amidopropyl betaine (EDAB) and the photosensitive molecule, ortho-methoxy cinnamic acid (OMCA). EDAB/OMCA fluids exhibit photogelling, i.e., a large (~ 10,000 fold) increase in viscosity upon exposure to UV radiation. We show that this photogelling is caused by the growth of long wormlike micelles in the sample. This structural change, in turn, is induced by the UV-induced isomerization of OMCA molecules from their trans to cis form. Evidence from zeta-potential studies, small-angle neutron scattering (SANS), and rheology are used to systematically reveal the molecular and microstructural mechanism for our results. In the second part of this study, we turn our attention to non-aqueous solvents and demonstrate a new class of PR fluids using such solvents. The PR effect here relies on transformations of "reverse" micellar structures formed by a well-known lipid (lecithin) in conjunction with para-coumaric acid (PCA). Lecithin/PCA fluids exhibit a substantial decrease in viscosity upon exposure to UV light (i.e., photothinning). Initially, the molecules self-assemble into long wormlike micelles, leading to highly viscoelastic fluids. Upon UV irradiation, PCA is photo-isomerized from trans to cis. This change in geometry induces a transition from long to short micelles. In turn, the solution viscosity is decreased by more than three orders of magnitude. Small-angle neutron scattering (SANS) is used to confirm the dramatic reduction in micellar length. In the last study, we report a class of aqueous fluids whose viscosity increases upon heating (i.e., thermo-thickening). These fluids are mixtures of telechelic associating polymers (HEURs) and a type of supramolecules called cyclodextrins (CDs) in water. Interestingly, we observe this behavior only with a particular type of CDs, called alpha-CDs, and not with the other common CD types, i.e., beta- and gamma-CDs. These results are explained in terms of a competition between the hydrophobic end-caps and the hydrophilic backbone of the polymer for complexation with alpha-CD molecules. We have also investigated the effect of amphiphiles (single-tailed surfactants and double-tailed lipids) on the thermo-thickening. The addition of lipids substantially enhances the thermo-thickening behavior, which is explained to be due to an enhancement of the connectivity of hydrophobic junctions by lipid vesicles.Item Micelles of Polybutadiene-b-Poly(Ethylene Oxide) in a Binary Solvent System(2008-05-01) Ploetz, Christopher D; Greer, Sandra C; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We studied the assembly behavior of a polybutadiene-b-poly(ethylene oxide) diblock copolymer in methanol, cyclohexane, and the corresponding partially miscible binary solvent system. Dynamic light scattering indicates that the copolymer forms coexisting spherical and cylindrical micelles in both of the pure solvents. In the binary solvent system, spherical micelles form in the methanol-rich phase for a wide range of temperatures. Conversely, micelles are present in the cyclohexane-rich phase only near the critical temperature. At the critical solvent composition, micelles form in the single phase region above the critical temperature. Size exclusion chromatography results for the binary solvent system show that the copolymer generally prefers the methanol-rich phase. The preference becomes more pronounced as temperature decreases.Item SELF-ASSEMBLY OF AMPHIPHILIC MOLECULES IN ORGANIC LIQUIDS(2007-08-06) Tung, Shih-Huang; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Amphiphilic molecules are well-known for their ability to self-assemble in water to form structures such as micelles and vesicles. In comparison, much less is known about amphiphilic self-assembly in nonpolar organic liquids. Such "reverse" self assembly can produce many of the counterparts to structures found in water. In this dissertation, we focus on the formation and dynamics of such reverse structures. We seek to obtain fundamental insight into the driving forces for reverse self-assembly processes. Three specific types of reverse structures are studied: (a) reverse wormlike micelles, i.e., long, flexible micellar chains; (b) reverse vesicles, i.e., hollow containers enclosed by reverse bilayers; and (c) organogel networks. While our focus is on the fundamentals, we note that reverse structures can be useful in a variety of applications ranging from drug delivery, controlled release, hosts for enzymatic reactions, and templates for nanomaterials synthesis. In the first part of this study, we describe a new route for forming reverse wormlike micelles in nonpolar organic liquids. This route involves the addition of trace amounts of a bile salt to solutions of the phospholipid, lecithin. We show that bile salts, due to their unique "facially amphiphilic" structure, can promote the aggregation of lecithin molecules into these reverse micellar chains. The resulting samples are viscoelastic and show interesting rheological properties. Unusual trends are seen in the temperature dependence of their rheology, which indicates the importance of hydrogen-bonding interactions in the formation of these micelles. Another remarkable feature of their rheology is the presence of strain-stiffening, where the material becomes stiffer at high deformations. Strain-stiffening has been seen before for elastic gels of biopolymers; here, we demonstrate the same properties for viscoelastic micellar solutions. The second reverse aggregate we deal with is the reverse vesicle. We present a new route for forming stable unilamellar reverse vesicles, and this involves mixing short- and long-chain lipids (lecithins) with a trace of sodium chloride. The ratio of the short to long-chain lipid controls the type and size of self-assembled structure formed, and as this ratio is increased, a transition from reverse micelles to vesicles occurs. The structural changes can be explained in terms of molecular geometry, with the sodium chloride acting as a "glue" in binding lipid headgroups together through electrostatic interactions. The final part of this dissertation focuses on organogels. The two-tailed anionic surfactant, AOT, is well-known to form spherical reverse micelles in organic solvents. We have found that trace amounts (e.g., less than 1 mM) of the dihydroxy bile salt, sodium deoxycholate (SDC) can transform these dilute micellar solutions into self-supporting, transparent organogels. The structure and rheology of these organogels is reminiscent of the self-assembled networks formed by proteins such as actin in water. The organogels are based on networks of long, rigid, cylindrical filaments, with SDC molecules stacked together in the filament core.Item Functionalization of Nanoparticles for Biological Applications(2005-12-09) Koh, Isaac; Ehrman, Sheryl H.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Functionalization of metal oxide nanoparticles enables their use in biological applications via hybridization of biological molecules and modification of surface properties. This Ph. D research is aimed at increasing knowledge of the process of metal oxide nanoparticle functionalization for biological applications. The achievements presented in this dissertation can be divided into three categories: i) a fluorescence-based quantitative evaluation of surface coverage and bio-activity of antibodies immobilized on magnetic nanoparticles (MNPs), ii) differential functionalization of SiO2/TiO2 mixed nanoparticles via preferential binding of phosphonic acids to TiO2 and subsequent trimethyl silyl group binding to the remaining surface, and iii) X-ray scattering (XRS)-mediated detection of peak shifts of a biological substrate, Escherichia coli (E. coli), as a function of applied magnetic field strength and magnetic nanoparticle concentration in a cell growth medium. In a study of MNP surface modification, quantitative evaluation of anti-mouse IgG binding on MNPs and bioactivity on MNPs was conducted via fluorescence assays. Nanosize -Fe2O3 particles were hybridized with anti-mouse IgG via silane chemistry with 3-aminopropyltriethoxy silane and glutaraldehyde activation. A chemisorption isotherm via fluorescence assays demonstrated that immobilization of anti-mouse IgG can be stoichiometrically controlled with the surface coverage at saturation corresponding to 36% of the theoretical limit. The immobilized anti-IgG retains ~50% of its bioactivity at saturation. Differential functionalization of SiO2/TiO2 mixed nanoparticles was demonstrated via aqueous-phase preferential binding of phosphonic acids to TiO2 and subsequent binding of trimethyl silyl group to the remaining surface. SiO2/TiO2 mixed nanoparticles with three different mole ratios of Si/Ti together with pure SiO2 and TiO2 nanoparticles were used in comparative XPS study of differential functionalization. Differential functionalization of metal oxide-metal oxide mixed nanoparticles demonstrated herein adds a route to multifunctional nanoparticles. An In situ XRS study of E. coli in applied magnetic fields up to 423 mT was performed. Two peaks, a sharp peak at q = 0.528 Å-1 (1.189 nm) and a diffuse peak at q = 0.612 Å-1 (1.027 nm), were detected in XRS of MNP-absent E. coli culture. The presence of SiO2/-Fe2O3 MNPs at 40 mg/L in E. coli growth medium changes the sharp peak to the lower side of q as a function of applied magnetic field strength, while the position of the diffuse peak is invariable. 362 mT was found to be a critical magnetic field strength, at which the sharp peak disappears. This study demonstrates magnetic field-assisted interactions between E. coli cell membranes and MNPs.Item Coil-to-Helix Transition of Poly(ethylene oxide) in Solution(2004-07-30) Alessi, Michael Louis; Greer, Sandra C.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Poly(ethylene oxide) (PEO) is a simple polymer with repeating units [- C - C - O -] soluble in organic and aqueous solvents. The carbon atoms are hydrophobic; the oxygen atoms are hydrophilic and participate in hydrogen bonding. In all solvents in which PEO has previously been studied, PEO forms a coil in solution. Neutron scattering studies of PEO in isobutyric acid show that PEO undergoes a coil-to-rod transition in a solution of isobutyric acid (IBA). The stiffening is seen to progress smoothly with the addition of IBA, from a coil in D2O to a rod in pure deuterated-IBA. In addition to a solvent driven transition, a reversible rod-to-coil transition was seen to occur as a function of temperature, between 55 and 60 oC. Polarimetry experiments show that the rod formed by the PEO in solution is actually a helix, the conformation that PEO has in the solid state. It is also shown that, through the use of chiral impurities and temperature, the direction of the helix can be affected, allowing polymer folding to be influenced on a molecular level.