Latent trialkylphosphine and trialkylphosphine oxide organogelators activated by Bronsted and Lewis acids

The weak bases tri-n-alkylphosphines (R3P, where R = C14H29, C18H37) and phosphine oxides (R3P=O) have been reacted with the strong Lewis acid BF3 to form zwitterions (R3P+-OBF3- or R3P+-BF3-) and with Bronsted acids (PKa < 5) to form salts (R3P+-OH X-, where X- is several anion types, including chloride, p-dodecylbenzenesulfonate, and p-toluenesulfonate). The abilities of the zwitterions and some of the hydroxyphosphonium salts to gel a variety of organic liquids have been investigated and compared with those of the phosphine oxides (R3P=O, R = C10H21, C14H29, C18H37). The gelating abilities of the zwitterions are similar to those of the phosphine oxides in that only liquids capable of donating protons or promoting ionic interactions could be gelled, and both are less efficient than the corresponding hydroxyphosphonium salts. Alcohols with short alkyl chains (<5 carbon atoms) were gelled preferentially by (C18H37)(3)P+-OH X- (X- = chloride or p-toluenesulfonate), while those with longer alkyl chains were gelled better by the less polar p-dodecylbenzenesulfonate salt or (C18H37)(3)P=O. These observations suggest the participation of the solvent at the moment of network formation. Powder X-ray diffraction and temperature-dependent NMR measurements reveal that gel formation from the (C18H37)(3)P+-OH X- salts involves a competition among several concurrent processes, some of which are a consequence of salt dissociation during heating of sols. The one(s) that dominate determine the nature of the networks responsible for immobilization of the liquid components of the gels. Thus, some gel networks were comprised of (C18H37)(3)P=O despite the fact that (C18H37)(3)P+-OH Cl- was the added gelator and the same liquid was not gelled by the phosphine oxide alone! A hypothesis for the role of the acid in these gelation processes and models for molecular arrangements of the gelators in the neat solids and gel networks are presented.