Preparation and Properties of Nano Calcium Fluoride

A spray-drying method, previously used for preparing nano-sized hydroxyapatite, was modified to prepare nano calcium fluoride (CaF₂). Objective: To study the properties of the nano CaF₂ and evaluate effectiveness of a fluoride (F) rinse with nano CaF₂ as the F source. Methods: Dilute calcium hydroxide and ammonium fluoride solutions were fed to a two-liquid spray nozzle and atomized into a dry atmosphere. Nano CaF₂ particles, formed after evaporation of ammonium hydroxide and water, were collected using an electrostatic precipitator. Solubility product (Ksp) of the nano CaF₂ was determined by equilibrating the nano sample in solutions presaturated with respect to crystalline CaF₂. F deposition by a 12 mmol/L NaF rinse or a liquid having the same total F content but with suspended nano CaF₂ particles as the F source was determined using a previously published in vitro filter disc model. Results: BET measurements showed that the nano CaF₂ had a surface area of 35.5 m²/g, corresponding to a particle size of 53 nm. TEM examinations indicated that the nano CaF₂ contained clusters comprising particles of 10 to 15 nm in size. XRD analysis showed pattern of low crystalline CaF₂. Ksp of the nano CaF₂ was (2.3±0.2)x10^-10, which is about 6 times greater than that (3.9x10^-11) for crystalline CaF₂. F deposition by the nano CaF₂ rinse was 2.3±0.3 μg/cm² of surface area (n=5), which was significantly (p<0.01) greater than that (0.31±0.06 μg/cm²) produced by the NaF solution. This result suggests that the nano CaF₂ particle could readily be lodged in the filter disc pores. Conclusions: The nano CaF₂ was significantly more soluble than crystalline CaF₂. The rinse with nano CaF₂ as the F source produced a significantly greater F deposition than a NaF rinse with the same F content.  Supported by NIH grant DE16416, ADAF, and NIST.