Crystallization of silver stearate from sodium stearate dispersions

Silver carboxylates, the common silver source used for photothermographic imaging materials, are normally obtained from the reaction between sodium soap (e.g., sodium stearate) and silver nitrate. They form platelet-like crystals with a lamellar structure in water at room temperature. Light microscopy investigations reveal that the formation of silver stearate (AgSt) crystals follows a diffusion-controlled mechanism. The reaction between the sodium soap and silver nitrate preferentially occurs in solution rather than on the soap fiber solid interface. Cryogenic transmission electron microscopy, together with an on-the-grid reaction technique, provides a useful tool to directly image silver stearate microstructures at the initial stages of AgSt precipitation. The AgSt reaction product first forms particles about 5 nm in size, which is similar to the d-spacing of final AgSt crystals. Those particles aggregate to produce larger and loosely packed embryonic crystals, the precursors to the ultimate silver stearate crystals.