New insights into the mesophase transformation of ethane-bridged PMOs by the influence of different counterions under basic conditions

Abstract

The counterions are of crucial importance in determining the mesostructure and morphology of ethane-bridged PMO materials synthesized under basic conditions. By using CTABr as the surfactant, the final PMO materials show a 2-D hexagonal (p6mm) mesophase, while PMO materials with cubic (Pmn) mesostructure are obtained when CTACl or CTA(SO₄)_1/2 are used. With gradually replacing CTABr by CTACl or CTA(SO₄)_1/2 while keeping the total surfactant concentration constant, a clear p6mm to Pmn mesophase evolution process is observed. For a given gel composition, the mesophase of ethane-bridged PMO materials can also be adjusted by the addition of different sodium salts. In short, the effect of the counterions on the mesophase can be attributed to the binding strength of the ions on the surfactant micelles, which follows the Hofmeister series (SO₄²⁻ < Cl⁻ < Br⁻ < NO₃⁻ < SCN⁻). Furthermore, it is found that the hydrolysis and condensation rate of the organosilica precursor also plays an important role in the formation of the final mesostructure