Organogelation Properties of a Series of Oligoamides Rolf Schmidt, Marc Schmutz, Marc Michel, Gero Decher, and Philippe J. Me ´sini* Institut Charles Sadron, Chemistry of Associating Systems, 6, rue Boussingault, 67000 Strasbourg, France Received October 12, 2001. In Final Form: May 6, 2002 The monodisperse oligoamides 1, 2, 3, and 4 have been tested as organic gelators. It was found that gelation occurs only above a critical degree of oligomerization of 2, that is, for the trimer 3 and the tetramer 4, and in aromatic solvents. Thermotropic and rheological properties of the trimer and the tetramer gels are reported. The tetramer gels have an elastic shear modulus 1 order of magnitude lower than that of the trimer gels. A structural study of the gels has been conducted by transmission electron microscopy. Introduction Organogelators are a growing class of materials that display the ability to form gels at low concentrations with organic solvents. 1 Typically, the microscopic structure of those gels consists of supramolecular aggregates which form long fibrous structures, which in turn assemble in an entangled network. Those structures entrap the solvent molecules and are responsible for the rheological proper- ties of the binary system, including a large viscosity increase and the ability to form soft solid systems. These materials have applications in various fields such as lubricating greases, 2 biomedical applications, 3 and oil spill recovery. 4 An important group of gelators uses H-bonds as the driving force to self-assemble. These molecules can be depicted as two-part systems, where one part secures interactions between molecules by H-bonds and the other part promotes the solubility of the molecule and the unidirectionality of the crystallization. This rough view applies to structures such as amides, 5 ureas, 6 peptide derivatives, 7 pepsipeptide 8 or peptidomimetic structures, 9 sugars, 10 and alcohols. 11 Many other gelators without H-bond donors assemble through weaker interactions such as dipole interactions, van der Waals forces, or solvophobic forces. For example, steroids, 12 aromatic derivatives, 13,14 and very simple molecules such as alkanes 15 or semiflu- orinated alkanes 16 fall into this category. Due to the wide structural variety of potential gelators, their properties cannot be easily predicted. Very subtle variations or derivatizations from a given gelator structure can result in a drastic change of the gel stability or suppress the gelation properties completely. Amides constitute an important class of organogelators, since all peptide gelators can be included in this category. In the course of our studies, we synthesized a series of oligoamides shown in Chart 1. In this paper, we report the gelation properties versus the degree of oligomerization of the species, especially the critical degree of oligomerization above which the gelation can be observed. The thermotropic and rheological prop- erties of those gels have been investigated. A preliminary structural analysis of the gels has been carried out by electron microscopy. Experimental Section Materials. The synthesis of the monomer 1, dimer 2, and tetramer 4 has been described elsewhere. 17 Oligmers obtained by the classical polymerization/separation method are usually * To whom correspondence should be addressed. E-mail: mesini@cerbere.u-strasbg.fr. (1) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133-3159. Van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263-2266. (2) Vold, M. J.; Vold, R. D. J. Colloid Sci. 1950, 62,1-19. Hotten, B. W.; Birdsall, D. H. J. Colloid Sci. 1952, 7, 284-294. (3) Murdan, S.; Gregoriadis, G.; Florence, A. T. Eur. J. Pharm. 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