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

Micelle templated silicas prepared according to the {S+,I-} pathway exhibit phase transitions from lamellar, to hexagonal through the cubic form depen ding on the lipophilic nature of the anions {Cl-, NO3-, SO42-}, their conce ntration, and on the presence of cations (Na+, and TMA(+)) in the synthesis gel. This ion effect is evidenced by the assynthesized forms of silica mes ophases obtained from two series of syntheses, both using cetyltrimethylamm onium (CTMA) as surfactant. In the first series, different amounts of vario us mineral acids HX (X- = Cl-, NO3- or 1/2 SO42-) were added to the synthes is gel while, in the second one, the concentration of sodium nitrate was va ried. The solids were characterized using small angle XRD, transmission ele ctron microscopy, elemental analysis and titration of silanolate groups, CT MA(+), and anions of the as-synthesized form. According to chemical analysi s, the interface can be described by the following formula {(1 - p)S+, pC(), mH(2)O, nX(-), (1 - n)I-} where "p" cations C+ and "n" anions X- are ins erted in the Helmholtz planes of the electrical interface, respectively, On one hand, these guest ions are competing with cationic surfactants and neg ative 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 densit y mismatch that control both the interface curvature and the mesophase char acteristics. (C) 2001 Elsevier Science B.V. All rights reserved.