Interaction between epsomite crystals and organic additives

Abstract

A number of phosphonates and carboxylates were tested as potential crystallization inhibitors for epsomite (MgSO4·7H 2O). Epsomite nucleation is strongly inhibited in the presence of amino tri(methylene phosphonic acid) (ATMP), diethylenetriaminepentakis (methylphosphonic acid) (DTPMP), and poly(acrylic acid) sodium salt (PA). These additives also act as habit modifiers promoting the growth of acicular crystals elongated along the [001] direction. Environmental scanning electron microscopy (ESEM), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and molecular modeling of additive adsorption on specific epsomite (hkl) faces are used to identify how these additives inhibit epsomite crystallization. Additives attach preferentially on epsomite {110} faces, at edges of monolayer steps parallel to [001]. Step pinning and the eventual arrest of step propagation along (110) directions account for the observed habit change. Hydrogen bonding between the functional groups of additive molecules and water molecules in epsomite {110} appears to be the principal mechanism of additive-epsomite interaction, as shown by FTIR and molecular modeling. Molecular modeling also shows that DTPMP displays a high stereochemical matching with epsomite {110} surfaces, which can explain why this is the most effective inhibitor tested. The use of such effective crystallization inhibitors may lead to more efficient preventive conservation of ornamental stone affected by epsomite crystallization damage. © 2008 American Chemical Society.