Effects of Aluminate and Silicate on the Structure of Quaternary Ammonium Surfactant Aggregates

The effect of ionic aluminate and silicate species on the structure of surfactant aggregates used in the preparation of mesoporous aluminosilicates was investigated using dynamic light scattering and rheological techniques. Aqueous cetyltrimethylammonium chloride (CTAC) solutions saturated with sodium aluminate, sodium silicate, or tetramethylammonium silicate were studied over a wide range of CTAC concentrations (1.0−25.0 wt %), which included both spherical and rod-shaped micellar regimes. Sodium aluminate was observed to have no effect on the structure of CTAC micelles and so plays no role in aggregate formation in the prereaction stage of the production of aluminosilicates such as MCM-41. Silicate anions from sodium silicate and tetramethylammonium silicate strongly affected CTAC micelle formation, promoting the formation of flexible wormlike micelles at dilute concentrations in which elongated wormlike micelles are not entangled. The condensation of silicate ion with the tetramethylammonium cation in CTAC/tetramethylammonium silicate solution inhibited the condensation of silicate ion at the surface of the CTAC micelles. Results strongly suggest that, for the two silicate sources studied, flexible CTAC wormlike micelles surrounded by silicate oligomers are formed first in the preparation of aluminosilicate materials; the subsequent intermicellar condensation of silicate oligomers then leads to the gradual arrangement of the micelles into hexagonal liquid crystalline arrays. The effect of a chloride ion (at a constant molar Cl-/CTA+ ratio of 3.98) was also investigated using sodium chloride. Unlike the aluminate and the silicate ions, a transition between the rodlike and flexible wormlike micellar regions was observed. The semidilute regime, defined as the concentration range over which the collective diffusion coefficient is described by the power law model for single-chain polymer in good solvent, was observed at 11.0−21.6 wt %. The transition into the liquid crystalline regime was observed to begin at 21.6−25.0 wt %.