Heterogeneous Deposition of Calcium Phosphates at the Silicon (Hydrous) Oxide — Water Interface

Abstract

Calcium phosphates (CP) are found in significant quantities in almost all kidney stones. With a relatively high surface free energy, but only a moderate supersaturation in human urine, it is likely that nucleation of CPs requires a pre-existing surface. As a consequence, there have been a number of studies that have focused on the heterogeneous nucleation of CPs on a variety of materials, from calcium oxalate monohydrate to uric acid and the urate salts ^1-3. However, there have also been studies that have implicated silicon and its oxide compounds, hydrous oxides, or soluble silicate ions, in the nucleation and growth of CPs and hydroxyapatite, the thermodynamically stable form, in particular. These past studies tended to focus on organic silicon or soluble silicates ^{4, 5}. However, silicon has a concentration in human urine in the range of 6 to 17 mg per 24 hours ⁶. With a solubility of about 100 ppm in water, solid forms of silicon such as silicon dioxide or the more thermodynamically stable solid silicon (hydrous) oxide are likely to be present in urine as particles.