engleski

Interactions of organic additives with ionic crystal hydrates: The importance of the hydrated layer

The interactions of two groups of hydrated model crystals, calcium hydrogen phosphate dihydrate (DCPD) vs. octacalcium phosphate (OCP) and calcium oxalate monohydate (COM) vs. calcium oxalate dihydrate (COD) with different organic additives are considered. DCPD precipitates as plate-like crystals with the dominant faces shielded by hydrated layers and charged lateral faces. In the second system COM has charged surfaces, while all faces of COD are covered with layers containing water molecules. The organic molecules tested include negatively charged, flexible and rigid small and macromolecules (glutamic and aspartic acid, citrate, hexaammonium polyphosphate, phytate and polyaspartate) and anionic surfactants (sodium dodecyl sulphate, SDS, sodium diisooctyl sulfosuccinate, AOT, sodium cholate, NaC and disodium oleoamido PEG-2 sufosuccinate PEG). Two types of effects have been demonstrated: (1) Effect on crystal growth morphology. Flexible organic molecules with high charge density and anionic surfactants affected the growth morphology of DCPD and COM by selectively interacting with the charged lateral aces while rigid molecules (phytate, polyaspartate) specifically recognized the dominant (010) face of DCPD due to structural and stereochemical compatibility. (2) Effect on phase composition: Anionic surfactants at concentrations above cmc promoted growth of OCP and COD respectively by selectively adsorbing at, and inhibiting growth of nuclei of DCPD and/or COM, which were dominant in the respective control systems. The effect was especially pronounced in the calcium oxalate precipitation system, where in some cases complete reversal of the phase composition occurred. The important role of the hydrated layer, as part of the structure of the investigated crystak hydrates, in the above crystal additive interactions is discussed.

sodium diisooctyl sulfosuccinate; calcium-oxalate crystals; micellar solutions; crystallization; surfactants; whewellite; induction

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