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 CHARACTERIZATION OF INTERACTION BETWEEN WHEY PROTEIN ISOLATE AND XANTHAN/CURDLAN HYDROGEL TO IMPROVE FREEZE-THAW STABILITY(2014) ghorban shiroodi, Setareh; Lo, Y. Martin; Food Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Syneresis, the liquid separation from a food product, is one of the major challenges in the frozen food industry. It is often accelerated by unintentional temperature fluctuations, i.e. repeated freezing and thawing during distribution, transportation, storage, and consumption. It has been demonstrated that the combination of xanthan and curdlan is capable of reducing syneresis up to five freeze-thaw cycles (FTCs) with relatively stable rheological and textural properties. The present study aimed at developing an effective mixture of whey protein isolate (WPI) and xanthan-curdlan hydrogel complex (XCHC) to minimize moisture migration over multiple FTCs. The addition of XCHC to WPI solution significantly reduced the syneresis of heat-induced gels, increased the storage modulus (G') of the gels, reduced the minimum concentration of whey protein isolate required to form a gel, and minimized the discrepancies of G' in frequency sweep tests over 5 FTCs. By comparing the microstructure of mixed WPI-XCHC and pure WPI gels, it was found that XCHC served as a pore-forming agent, namely increasing the porosity, reducing the pore size, and consequently improving the gel's water retention over multiple FTCs. Results from dynamic rheological measurements showed that both G' and the gelation temperature of mixed WPI-XCHC complex were strongly pH-dependent. Moreover, the interactions between WPI and XCHC in aqueous solution were characterized. An edible coating solution containing the mixture of WPI, xanthan, and curdlan was tested on mushroom and green bell pepper. Fresh mushroom and green bell pepper samples treated with WPI-XCHC significantly prevented moisture migration after 10 days of frozen storage. The coating also decreased the changes of whiteness and greenness in mushroom and bell pepper, respectively, while significantly improved the firmness of bell peppers. Such information could provide useful guidelines when designing novel food products utilizing the unique properties provided by WPI-XCHC.