Organogels with Complexes of Ions and Phosphorus-Containing Amphiphiles as Gelators. Spontaneous Gelation by in Situ Complexation

Abstract

The properties of thermally reversible organogels that are formed spontaneously upon mixing a phosphonic acid monoester, monophosphonic acid, or bisphosphonate ester, each containing a long alkyl chain substituent, with one of several compounds of aluminum(III) and boron(III) in an organic liquid were studied by IR, NMR, optical microscopy, X-ray diffraction, and rheological techniques. Attempts to form gels with zirconium(IV) were unsuccessful. Gelation occurred at room temperature upon complexation, leading to the formation of entangled networks of elongated objects similar to giant, worm-like micelles. On the basis of the diversity of the liquids gelated, the minimum concentration of gelator required to make a gel at room temperature (typically <5 wt %), and the temporal and thermal stabilities of the gels, Al complexes of phosphonic acid monoesters were found to be better gelators than bisphosphonate complexes. Several of the gels formed from the monophosphonate-Al complexes were stable for very long periods when they were kept in sealed tubes at room temperature. When heated, they reverted to sols over wide temperature ranges. The nature of the gels and the complexes from which they were formed were correlated, especially for those with the phosphonic acid monoester. The results describe an interesting class of two-component gelators that can be made from freely flowing solutions by mixing the components at room temperature, without the need for a catalyst, radiation, or sonication. The properties of the gels can be modulated by careful choice of the structural variables in the phosphorus-containing latent gelators.