Structure and Dynamics of Self-Assembling Aluminum Didodecyl Phosphate Organogels

Abstract

A self-assembled organogel of aluminum isopropoxide and didodecyl phosphate surfactant in decane is studied as a function of the ratio of aluminum:surfactant. The rheology and structure is shown to depend sensitively on this ratio. Small angle neutron scattering shows the aggregates to be locally cylindrical and molecularly thin at all compositions studied. In the presence of excess surfactant, the system displays Maxwellian rheology and behavior reminiscent of transient, entangled networks of wormlike micelles. A large transition in the relaxation time of the system occurs over a narrow composition range near stoichiometric equivalence (aluminum:surfactant = 1:3), indicating the formation of a connected network with physical gel behavior. At a large excess of aluminum this results in a phase separation into gel and nonviscoelastic liquid. The formation of the gel network and subsequent phase separation are rationalized in terms of localized branch points formed in the aggregates by the excess of aluminum isopropoxide.