N-alkyl perfluoroalkanamides as low molecular-mass organogelators.

Abstract

A new class of low molecular-mass organogelators (LMOGs), N-alkyl perfluoroalkanamides, F(CF(2))(n)CONH(CH(2))(m)H, is described. The molecules are designed to exploit the incompatibilities of their three molecular parts, and the results demonstrate that this strategy can be used to tune molecular aggregation and gel stability. The gelating properties of these LMOGs have been examined in a wide variety of organic liquids (including alkanes, alcohols, toluene, n-perfluorooctane, CCl(4), and DMSO) as a function of the N-alkyl and perfluoroalkyl chain lengths by X-ray diffraction, polarizing optical microscopy, infrared spectroscopy, differential scanning calorimetry, and small-angle neutron scattering (SANS). The gels are thermally reversible and require generally very low concentrations (<2 wt %) of LMOG. Several of the gels are stable for very long periods at room temperature. The incompatibility of the fluorocarbon and hydrocarbon segments causes the LMOGs to aggregate, probably into lamellae within the fibrils that constitute the basic unit of the gel networks. The SANS studies show that the cross-sections of fibers in the gel networks of LMOGs with shorter perfluoroalkyl chains are much larger than those with longer ones. Comparisons with the gelating properties of some analogous esters (F(CF(2))(n)CO(2)(CH(2))(m)H) and diblock perfluoroalkylalkanes (F(CF(2))(n)(CH(2))(m)H) indicate that additional ordering within the aggregate units is enforced by the intermolecular H bonding among amide groups that is evidenced by IR spectroscopy. Analyses of these results and structure/solvent correlations are provided.