New Models for Calcium Phosphate Scale Formation and Dissolution
Scale inhibitors are usually evaluated by performing empirical precipitation tests and by assessing the resulting change in the mass of scale formed. Little attention is usually paid to the molecular events that result in the observed inhibition with a reduction in the crystal nucleation, growth, or dissolution of these sparingly soluble minerals. Recently, there has been considerable activity in studies aimed at elucidating the mechanisms of these reactions. Using a thermodynamic approach together with atomic force microscopy (AFM), new light has been shed on the ways in which ions and water molecules can assemble to form complex crystals. Scale inhibitors may in uence the growth and dissolution of different crystal faces, resulting in marked changes in morphology that can in uence crystal adherence to scaling surfaces. In the dispersal of scale crystals by dissolution, a newly developed model reveals that when crystals are suf ciently small, the surfaces may be unable to support etch pits of suf cient size, thus inhibiting the dissolution process. Crystals then resist dissolution even when the aqueous phases are undersaturated. These new models and their applicability to scale formation are discussed in this chapter.