Organic–inorganic composite micro-granules by evaporation induced assembly: role of trapped water in structural evolution

Abstract

Nano-structured composite micro-granules are promising candidates for many technological and medical applications. Understanding the evolution of the mesoscopic structure of these granules, under external thermo-dynamic perturbation, is crucial to assure their use in desired applications. Silica-surfactant micro-granules, primarily of doughnut shape, have been synthesized using one step evaporation-induced assembly through spray drying of mixed dilute colloidal dispersions of silica and sodium dodecyl sulfate (SDS). Composite hollow micro-granules, consisting of densely correlated SDS molecules and silica nano-particles, are obtained because of rapid assembly during spray drying. The trapped water, retained in the spray dried granules, ensures the structural correlation among the SDS molecules in the spray dried granules. While the free water left the granules around 100 °C as expected, the trapped-water in the nano-pores remained even up to 175 °C. Expulsion of the trapped water results in immediate collapse of the correlated surfactant molecules and in turn leads to the rapid restructuring of the silica nano-particles within the granules. In situ X-ray scattering, differential scanning calorimetry and thermo-gravimetric analysis have been utilized to probe the real time thermal evolution of the mesoscopic structure of the synthesized composite granules.