The kinetics and mode of nucleation and growth of fibers by 5alpha-cholestan-3beta-yl N-(2-naphthyl)carbamate (CNC), a low-molecular-mass organogelator (LMOG), in n-octane and n-dodecane have been investigated as their sols were transformed isothermally to organogels. The kinetics has been followed in detail by circular dichroism, fluorescence, small-angle neutron scattering, and rheological methods. When treated according to Avrami theory, kinetic data from the four methods are self-consistent and describe a gelation process involving one-dimensional growth and "instantaneous nucleation". As expected from this growth model, polarized optical micrographs of the self-assembled fibrillar networks (SAFINs) show fibrous aggregates. However, their size and appearance change abruptly from spherulitic to rodlike as temperature is increased. 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. This study also provides a methodology for investigating aggregation phenomena of some other self-assembling systems, including those of biological and physiological importance.
The kinetics of the isothermal transformation of sols, comprised of a low molecular-mass organogelator (LMOG) and an organic liquid, to their organogel phases have been followed by circular dichroism (CD), fluorescence, small angle neutron scattering (SANS), and rheological methods. The thixotropic properties (in the sense that severe shearing followed by rest lead to reestablishment of viscoelasticity) of the gels have been examined as well by rheological measurements. The compositions of the samples were either 5alpha-cholestan-3beta-yl N-(2-naphthyl) carbamate (CNC) in an n-alkane (n-octane or n-dodecane) or 3beta-cholesteryl N-(2-naphthyl) carbamate (CeNC) in ethyl acetate. Values of Df, the mass fractal dimension of the microcrystalline self-assembled fibrillar networks (SAFINs) in the gels, have been 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 with the gel network structures observed by optical microscopy. In addition, comparison of the temperature dependence of both n (the Avrami component) and K (the Avrami "rate constant") for CeNC/ethyl acetate gelation with those reported previously for gelation of CNC/n-alkane sols demonstrate that the very small change of a single bond in CNC to a double bond in CeNC causes significant differences in their gelation abilities and gel properties. The rheological measurements on CNC/n-alkane gels with spherulitic SAFIN units, formed by incubation of their sols at < or =28 degrees C, indicate that they are thixotropic. Gels with the same chemical composition but formed by incubation of their sols at > or =30 degrees C, leading to fiberlike SAFIN units, remain liquidlike after shearing regardless of the periods they are at rest. The time-dependent viscoelastic properties of the gel networks 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 the extreme sensitivity of the SAFINs and viscoelastic properties of such organogels to slight modifications in LMOG structure or sample history.
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