In Situ Calorimetric Study of the Hexagonal-to-Lamellar Phase Transformation in a Nanostructured Silica/Surfactant Composite

Abstract

Restructuring of hexagonal silica/surfactant composites under hydrothermal conditions was studied using in situ scanning microcalorimetry to understand the energetic changes associated with these rearrangements. Thermal processes can be associated with either changes in packing of the organic template or with chemistry of the cross-linked inorganic framework. To sort these out, calorimetric data were collected as composites were heated in water, where a hexagonal-to-lamellar phase transformation occurs, and as composites were heated in an acidic boric acid buffer, where no phase change is observed. The scanning calorimetric data were correlated with in situ low angle XRD to explore the relationship between rearrangements of the nanoscale architecture and the various energetic processes occurring in these materials. 29 Si NMR, which tracks changes in framework bonding, TGA and 1 H NMR, which measure surfactant loss from the composite, and 13 C NMR, which tells us about surfactant rearrangement and degradation, were also correlated with the calorimetric data. Both samples showed an endotherm at 70-71 °C that was assigned to an order-disorder transformation of the organic surfactant of the composite. In this same moderate temperature range, broad exotherms observed in both samples were associated with condensation of the silica framework. Two endotherms were observed in calorimetric scans of the water-treated composites that were not present in data collected on composites treated in boric acid. These endotherms were thus associated with the hexagonal-to-lamellar phase transformation, which has an enthalpy change of +0.5 ( 0.1 kJ/(mol SiO2 )o r+2.4 ( 0.3 kJ/(mol surfactant) and entropy changes of + 1JK -1 (mol SiO2) -1 or + 6JK -1 (mol surfactant) -1 . The results quantify differences in thermodynamic stability in silica/surfactant composites and identify the physical, molecular, and nanoscale changes that influence stability in these materials.