Nanoscale uniformity of pore architecture in diatomaceous silica: A combined small and wide angle X-ray scattering study

Combined small and wide angle IZ-ray scattering (SAXS and WAXS) analysis wa s applied to purified biogenic silica of cultured diatom frustules and of n atural populations sampled on marine tidal flats. The overall WAXS patterns did not reveal crystalline phases (WAXS domain between 0.07 to 0.5 nm) in this biogenic silica, which is in line with previous reports on the amorpho us character of the SiO2 matrix of diatom frustules, One exception was the silica of the pennate species Cylindrotheca fusiformis Reimann et Lewin, wh ich revealed wide peaks in the WAXS spectra, These peaks either indicate th e presence of a yet unknown crystalline phase with a repetitive distance (d -value approximate to 0.06 mn) or are caused by the ordering of the fibrous silica fragments; numerous girdle bands. The SAXS spectra revealed the siz e range of pores (diameter d between 3.0 and 65 nm), the presence of distin ct pores (slope transitions), and structure factors (oscillation of the spe ctra). All slopes varied in the range of -4.0 to -2.5, with two clear commo n regions among species: d < 10 nm (slopes 4, denoted as region I and also called the Pored region), and 10.0 < d < 40.0 nm (slopes -2.9 to -3.8, deno ted as region LT), The existence of these common regions suggests the prese nce of comparable form (region I) and structure (region II) factors, respec tively the shape of the primary building units of the silica and the geomet ry of the pores. Contrast variation experiments using dibromomethane to fil l pores in the SiO2 matrix showed that scattering was caused by pores rathe r than silica particles. Electron microscopic analysis confirmed the presen ce of circular, elliptical, and rectangular pores ranging in size from 3 to 65 nm, determining the structure factor. The fine architecture (length/wid th ratio of pore diameters) and distribution of the pores, however, seemed to be influenced by environmental factors, such as the salinity of and addi tions of AlCl3 to the growth medium, The results indicate that diatoms depo sit silica with pores <50 nm in size and are highly homologous with respect to geometry. Consequently, it is suggested that in diatoms, whether pennat e or centric, the formation of silica at a nanoscale level is a uniform pro cess.