Self-assembly into Soft Materials of Molecules Derived from Naturally-occurring Fatty-acids
Weiss, Richard G
The self-assembly of molecular gelators has provided an attractive route for the construction of nanostructured materials with desired functionalities. A well-defined paradigm for the design of molecular gels is needed, but none has yet been established. One of the important challenges to defining this paradigm is the creation of structure-property correlations for gelators at different distance scales. This dissertation centers on gaining additional insights in the relationship between small changes in gelator structures derived from long-chain, naturally-occurring fatty acids and the properties of the corresponding gels. This approach offers a reasonable method to probe the rational design of molecular gelators.Thus, results from the investigation of molecular gels with a wide range of liquids and several structurally related molecular gelators are reported. The structures of the initial, naturally occurring fatty acids, each with a cis-double bond, were modified systematically to include the trans-isomer of one and its ammonium carboxylate salts, dihydroxyl derivatives, and an α-diketo compound and its metal carboxylate salts. A variety of physical, thermal, structural, molecular packing and mechanical properties of the gels have been studied in detail and compared for each type of gelator. Some important correlations have been made between the changes in gelator structure, the nature of the liquids gelated or not gelated, and the properties of the gels. One of the most important insights gained from these structure-property correlations is that differences in molecular packing arrangements and the magnitudes of the driving forces for nucleation and one-dimensional growth of the molecular gelators are key factors that modulate the efficiency of gelation.In addition, the kinetics of gel formation, and the stimulus-responsiveness of the corresponding gels have been examined in some cases. Some organogels of the α-diketo derivative and its metal salts respond to both thermal and mechanical stimuli in ways that include significant changes in their luminescent properties. Mechanisms for both the formation and the change in the physical properties of these gels have been proposed.Although a general paradigm for prediction of what constitutes an efficient gelator has not been successfully created here, and the possibility for finding one even in the next decade is extremely low, the systematic structure-property correlations presented here constitute an important advance in that quest. They are useful to understand better the design of molecular gelators and their gels, including more complicated systems.
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