Influence of the Structure and Composition of Mono- and Dialkyl Phosphate Mixtures on Aluminum Complex Organogels

The rheological properties and structure of organogels formed by the in situ complexation and self-assembly of aluminum isopropoxide and didodecyl phosphate surfactant in decane are investigated as mono-n-dodecyl phosphoric acid and bis(2-ethylhexyl) phosphoric acid complexing agents are added. At low loadings, the bulky bis(2-ethylhexyl) additive disrupts the physical gel structure by changing the packing around the aluminum centers, weakening the transition from viscoelastic fluid to physical network of branched cylinders, and completely suppresses gelation at high loadings. Monododecyl phosphate affects coordination at the Al center. At low substitution, it shifts the composition at which the transition to a physical gel occurs while simultaneously improving long-term stability. Structures deduced from the rheological response are confirmed by small-angle neutron scattering, which shows that the aggregates are locally cylindrical and molecularly thin at all compositions studied, although the cross section of the cylinders depends on the alkyl chain structure and composition of the organic phosphate mixtures.