29a Reunião Anual
Sociedade Brasileira de Química
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:: Richard G. Weiss ::

Structural and Dynamic Aspects of Molecular Organogels

Richard G. Weiss
Department of Chemistry, Georgetown University, Washington, DC 20057-1227, USA

Molecular organogels consist usually of < 2 wt% of a low molecular-mass organic gelator (LMOG) molecule, nominally <2000 Daltons, that forms reversibly a self-assembled
fibrillar network (SAFIN) through physical intermolecular interactions as solutions or sols of an organic liquid are cooled below a characteristic gelation temperature. In this super-saturated concentration regime, phase separation is microscopic, rather than macroscopic.  It leads to a network of objects with very high aspect ratios and, in many cases, mono-disperse, submicron-to-nanometer range cross-sections. Interactions of these objects give the 3-dimensional porous lattice (i.e., SAFIN) that permeates the volume of the sample and immobilizes the liquid component by capillary forces and suface tension.  The strands of SAFINs can take the shapes of fibers, tapes, ribbons, and tubules, among others, and their aggregates can take on distinct forms as well. 
Examples, principally from the speaker’s laboratory, will be used to demonstrate how specific properties of a gel depend upon its history and method of formation, the temperature at which it is kept, its age (because many molecular gels are not thermodynamically stable and undergo phase separation or SAFIN changes with time), LMOG structure and concentration, and liquid type. Both dynamic aspects (i.e., rheological) and structural properties (from the macroscopic to the molecular distance regimes) of gel systems will be discussed.  Also, recent studies to determine the steps involved in the evolution of the (0-dimensional) LMOG molecules into (3-dimensional) SAFINs will be presented. Finally, an application of organogels from the speaker’s laboratory, their delicate application to the surfaces of oil paintings in art restoration, will be shown.
If prompted, the speaker will masticate some Jell-O at forces above a yield stress that leads to its destruction.   However, he will not recreate the thixotropic red fluid purported to be used as a substitute for the blood of St. Januarius in Naples!!
The US National Science Foundation is thanked for its support of this research.