Influence of Silica Nanoparticles on the Crystallization Behavior of and Proton Relaxation in Cesium Hydrogen Sulfate

Abstract

The influence of nanoparticulate SiO_2 on the crystallization behavior of CsHSO_4 from aqueous solution has been quantitatively evaluated using powder X-ray diffraction (XRD) and ^1H magic angle spinning nuclear magnetic resonance (NMR) spectroscopy. It is shown that SiO_2 induces amorphization of a portion of CsHSO_4 and crystallization of the otherwise metastable phase II form of CsHSO_4. The fraction of amorphized CsHSO_4 (as determined from an evaluation of the XRD peak intensity) was found to increase from 0% in the absence of SiO_2 to fully amorphized in the presence of 90 mol % (~70 wt %) SiO_2. Within the crystalline portion of the composites, the weight fraction of CsHSO_4 phase III was observed to fall almost monotonically from 100% in the absence of SiO_2 to about 40% in the presence of 70 mol % SiO_2 (from both XRD and NMR analysis). These results suggest a crystallization pathway in which SiO_2 particles incorporate an amorphous coating of CsHSO_(4-)like material and are covered by nanoparticulate CsHSO_(4-II), which coexists with independently nucleated particles of CsHSO_(4-III). In composites with small molar fractions of CsHSO_4, the entirety of the acid salt is consumed in the amorphous region. At high CsHSO_4 content, the extent of amorphization becomes negligible, as does the extent of crystallization in metastable phase II. The phase distribution was found to be stable for over 1 year, indicating the strength of the stabilization effect that SiO_2 has on phase II of CsHSO_4.