Ion mediation and surface charge density in phase transition of micelle templated silica

Abstract

Micelle templated silicas prepared according to the {S+,I−} pathway exhibit phase transitions from lamellar, to hexagonal through the cubic form depending on the lipophilic nature of the anions (Cl−, NO−3, SO2−4), their concentration, and on the presence of cations (Na+, and TMA+) in the synthesis gel. This ion effect is evidenced by the as-synthesized forms of silica mesophases obtained from two series of syntheses, both using cetyltrimethylammonium (CTMA) as surfactant. In the first series, different amounts of various mineral acids HX (X−=Cl−, NO−3 or 1/2SO2−4) were added to the synthesis gel while, in the second one, the concentration of sodium nitrate was varied. The solids were characterized using small angle XRD, transmission electron microscopy, elemental analysis and titration of silanolate groups, CTMA+, and anions of the as-synthesized form. According to chemical analysis, the interface can be described by the following formula {(1−p)S+, pC+, mH2O, nX−, (1−n)I−} where “p” cations C+ and “n” anions X− are inserted in the Helmholtz planes of the electrical interface, respectively. On one hand, these guest ions are competing with cationic surfactants and negative silanolate groups during the synthesis and affect both the long range order and stability of the silica mesophases. On the other hand, they play a mediation role in the electrical balance and in the slight charge density mismatch that control both the interface curvature and the mesophase characteristics.