Sodium Sulfate Crystallization in the Presence of Phosphonates:  Implications in Ornamental Stone Conservation

Phosphonates were tested as potential crystallization inhibitors for sodium sulfate, one of the most damaging soluble salts in historic building and sculpture. Although mirabilite (Na 2 SO 4 ·10H 2 O) crystallization is promoted in the presence of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), aminotris(methylenephosphonic acid) (ATMP), and diethylenetriaminepentakis(methylphosphonic acid) (DTPMP) at salt solution natural pH (6.4), crystallization is inhibited at moderately alkaline pH (8−8.5). Molecular modeling of additive adsorption on specific mirabilite ( hkl ) faces, in addition to ESEM observations, allowed us to identify how phosphonates inhibit mirabilite crystallization. ATMP and DTPMP display the best stereochemical matching with mirabilite {100} surfaces, which explains why they are the most effective inhibitors. Inhibition of sodium sulfate crystallization in the presence of phosphonates may promote the growth of efflorescence on porous ornamental stone, rather than damaging subefflorescence. The use of phosphonates may lead to more efficient preventive conservation of ornamental stone exposed to sodium sulfate damage.