I. Structures and kinetics of formation of molecular organogels of steroid-based gelators. II. Syntheses of porous silica using self-assembled fibrillar networks of molecular organogels as templates

One of the research projects presented in this thesis is focused on a kinetic study of two low molecular-mass organogelators (LMOGs), 5alpha-cholestan-3beta-yl N-(2-naphthyl) carbamate (CNC) in n-octane and n-dodecane and 3beta-cholesteryl N-(2-naphthyl) carbamate ( CeNC) in ethyl acetate. The gelation processes have been investigated in detail as their sols were transformed isothermally to organogels by circular dichroism (CD), fluorescence, small angle neutron scattering (SANS), optical microscopy, and rheological methods. The kinetics and mode of nucleation and growth of CNC or CeNC fibers are revealed by using a statistical model based upon Avrami theory. Kinetic data from all methods are self-consistent and describe a gelation process involving one-dimensional growth. In addition, values of Df, the mass fractal dimension of the self-assembled fibrillar networks (SAFINs) in the gels, have been also extracted from the kinetic data using a model developed by Dickinson (J. Chem. Soc., Faraday Trans., 1997, 93, 111). The Df values, 1.1-1.3 for the CeNC gels and 1.3-1.4 or 1.6-1.8 (depending on the temperature of incubation of the sol phase) for CNC gels, are consistent the results of Avrami model and the fibrous gel network structures observed by polarized optical micrographs.;In CNC gels, a phase transition from spherulitic to rod-like as temperature is increased is observed. This morphological change is attended by corresponding excursions in static and kinetic CD, fluorescence and rheological data. Furthermore, the rheological measurements reveal an unusual linear increase in viscoelastic moduli in the initial stages of self-assembly. Each of the methods employed becomes sensitive to changes of the system at different stages of the transformation from single molecules of the LMOG to their eventual SAFINs. However, no phase transition is observed in CeNC/ethyl acetate gel formation, but a change in nucleation mechanism is revealed as temperature is increased. Our results demonstrate that the very small change of a single bond in CNC to a double bond in CeNC causes significant differences in their gelation ability and gel properties.;Rheological measurements also reveal that the CNC/ n-alkane gels with spherulitic SAFIN units, formed by incubation of their sols at ≤28 °C, are thixotropic. Gels with the same chemical composition but formed by incubation of their sols at ≥30 °C, leading to fiber-like SAFIN units, do not reform their gel state after being sheared beyond the destruction limit. The recovery of thixotropic gel networks after shearing has been followed by time-dependent dynamic rheological measurements and the data are treated according to a stretched exponential model. The observations from these studies provide detailed insights into the mechanisms of formation of molecular organogel phases and demonstrate how the properties of such organogels can be modulated.;Another research project is to make porous silica by templating the SAFINs of organogels via sol-gel processes. Ionic and uncharged LMOGs have been successfully applied as templates to make porous, spherulitic, and tubular silica in the size range of several microns to tens of nanometers. Their specific shapes and sizes depend on the specific conditions of the hydrolytic sol-gel process, including the nature of the catalyst. The electrostatic interaction or H-bonding between silicate intermediates and gelator strands is the driving force for templating. The template effect is strongly influenced by factors which include: (1) the competition between silicate/solvent and silicate/template interactions; (2) the period of the sol-gel process; (3) the hardness of the anion of the gelator salt; (4) the properties of the solvent, such as its surface tension; (5) and the sequence of drying and template removal processes. The specific nature of the objects and the conditions under which they can be formed are discussed. Templated titanium oxide is also made by the same method.