Theses and Dissertations from UMD
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Item Self-Assembly in Polar Organic Solvents(2019) Agrawal, Niti; Raghavan, Srinivasa R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Self-assembly of amphiphilic molecules occurs extensively in water, and can result in a variety of large, nanoscale aggregates, including long cylindrical chains called wormlike micelles (WLMs), as well as nanoscale containers called vesicles. However, in organic solvents of polarity lower than water, such as formamide, glycerol, and ethylene glycol, self-assembly has been demonstrated only to a limited extent. While there are reports of small micelles in these solvents, there are no reports of large structures such as WLMs and vesicles (with at least one length scale > 100 nm). In this dissertation, we show that both WLMs and vesicles can be formed in these solvents, and thereby our work expands the possibilities for self-assembly to new systems. Applications for the fluids developed here could arise in cosmetics, pharmaceutics, antifreeze agents, and lubricants. In the first part of this study, we demonstrate the formation of WLMs in polar solvents like glycerol and formamide. WLMs in water are induced by combining a cationic surfactant and a salt, but the combinations that work for water mostly do not work in polar solvents. The combination that does work in the latter involves a cationic surfactant with a very long (erucyl, C22) tail and an aromatic salt such as sodium salicylate. These WLMs display viscoelastic and shear-thinning rheology, as expected. By using a low-freezing mixture of glycerol and ethylene glycol, we are able to devise formulations in which WLMs remain intact down to sub-zero temperatures (–20°C). Thereby, we have been able to extend the range for WLM existence to much lower temperatures than in previous studies. Next, in the second part, we focus on the dynamic rheology of WLMs in glycerol, which is shown to be very different from that of WLMs in water. Specifically, WLMs in glycerol exhibit a double-crossover of their elastic (G′) and viscous (G″) moduli within the range of frequencies accessible by a rheometer. We believe that the high viscosity of glycerol influences the rheology at high frequencies. We also hypothesize that the WLMs in glycerol are shorter and weakly entangled compared to WLMs in water. Moreover, in terms of their dynamics, we suggest that WLMs in glycerol are similar to polymers – i.e., the chains will remain intact and not break and re-form frequently. In the last study, we demonstrate the formation of vesicles in polar solvents (glycerol, formamide and ethylene glycol) using the simple phospholipid, lecithin. Lecithin dissolves readily in polar solvents and gives rise to viscous fluids at low concentrations (~ 2 to 4%). At higher concentrations (> 10 wt%), lecithin forms clear gels that are strongly birefringent at rest. Dynamic rheology of the latter reveals an elastic, gel-like response. Images from cryo-scanning electron microscopy (cryo-SEM) indicate that the concentrated samples are ‘vesicle gels’, where multilamellar vesicles (MLVs, also called onions), with sizes between 50 to 600 nm, are close-packed across the sample volume. This structure explains both the rheology and the birefringence.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.