Understanding Phase Stability in Silica/Surfactant Composites through the Study of a Curvature Driven Rectangular Intermediate

• Published in: Langmuir Vol. 17; no. 11; pp. 3496 - 3504
• Main Authors: Gross, Adam F, Le, Van H, Kirsch, Bradley L, Tolbert, Sarah H
• Format: Journal Article
• Language: English
• Published: American Chemical Society 29.05.2001
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la0018080

Hydrothermal chemistry is used to alter phase stability in ordered silica/surfactant composites. These materials, which are studied using real time X-ray diffraction, display a direct hexagonal-to-lamellar transformation when heated in water. When treated in a pH 11 buffer, however, low angle X-ray diffraction (XRD) reveals an intermediate centered rectangular phase during the phase transformation. By examining the kinetics of this transformation under a range of temperature conditions, we can understand how silica chemistry, interfacial charge density, and surfactant packing interplay to control phase stability. For example, if a lower transformation temperature is used, a hexagonal-to-rectangular-to-lamellar phase progression is observed. Higher transformation temperatures display more complex phase behavior, showing both a direct hexagonal-to-lamellar phase transition and the hexagonal-to-rectangular-to-lamellar transformation occurring at the same time. The observed ordering of phases is consistent with activation energies calculated using the Ozawa method for nonisothermal experiments and the Avrami equation for Arrhenius-based isothermal kinetics. The hexagonal-to-rectangular transformation has an activation energy of 104 ± 7 kJ/mol (Ozawa) while the rectangular-to-lamellar phase transition has an activation energy of 147 ± 7 kJ/mol (Ozawa) or 140 ± 20 kJ/mol (Arrhenius). The unit cell area of the material can also be tracked over the heating ramp to learn more about the transformation. As the material transforms to the rectangular structure, the unit cell area drastically decreases, suggesting that curvature, rather than surfactant volume, drives the rearrangement. The results in this work provide a basis for better understanding the factors that affect phase stability and the relationship between atomic scale silica chemistry and nanoscale order in periodic surfactant templated silicas.