14560 Phys. Chem. Chem. Phys., 2010, 12, 14560–14566 This journal is c the Owner Societies 2010 Stability and transformation mechanism of weddellite nanocrystals studied by X-ray diffraction and infrared spectroscopy Claudia Conti,* a Luigi Brambilla, b Chiara Colombo, a David Dellasega, c G. Diego Gatta, d Marco Realini a and Giuseppe Zerbi b Received 19th May 2010, Accepted 13th September 2010 DOI: 10.1039/c0cp00624f This study is focused on the stability of weddellite, the dihydrate phase of calcium oxalate [CaC 2 O 4 Á(2 + x)H 2 O], mainly detected in kidney stones and in oxalate films found on the surfaces of several ancient monuments. Its occurrence is a critical issue since, at environmental conditions, weddellite is unstable and quickly changes into whewellite, the monohydrate phase of calcium oxalate (CaC 2 O 4 ÁH 2 O). New single crystal X-ray diffraction experiments have been carried out, which confirm the structural model of weddellite previously published. Synthesised nanocrystals of weddellite have been kept under different hygrometric conditions in order to study, by X-ray powder diffraction, the influence of humidity on their stability. Moreover, the mechanism of transformation of weddellite nanocrystals has been investigated by infrared spectroscopy using D 2 O as a structural probe. Introduction Calcium oxalate crystallizes in nature in three different forms: (1) calcium oxalate monohydrate, or whewellite (CaC 2 O 4 ÁH 2 O), which represents the most common form; (2) calcium oxalate dihydrate, or weddellite [CaC 2 O 4 Á(2 + x)H 2 O] which is significantly less frequent than whewellite; (3) calcium oxalate trihydrate, caoxite, rarely observed. 1 Whewellite and weddellite are found naturally in plant tissues, in sediments and also in kidney stones. Calcium oxalate, calcium phosphate, uric acid, ammonium hydrogen urate and magnesium ammonium phosphate are the main components of kidney stones, with different distribution depending on the population groups examined. Nowadays, calcium oxalate is the main component, in the form of whewellite and/or weddellite, of more than 70% of all kidney stones in Western countries. 2,3 Calcium oxalates are extensively studied also in the field of Cultural Heritage, because oxalate films have been found on the surfaces of several ancient monuments. 4 They consist of whewellite, weddellite and other minor components as gypsum, calcite, silicates and some accessory minerals. 4 Their colours range from pale pink to yellow, to ochre or brown and they have been detected both on natural stone materials as marble, calcarenite, sandstone, granite and on artificial materials as plaster or terracotta. 5 The undiscussed peculiarity of these films is their chemical stability, proven by their persistence over the centuries on surfaces exposed to atmospheric conditions, in spite of the strong increase of atmospheric acidity in all urban sites in the last century. The stability of calcium oxalate is easily explained by its water solubility; for instance, whewellite has a very low solubility (0.05 mmole l À1 at pH between 5 and 11) still at acid pH (0.37 mmole l À1 at pH 2.5). 6 In the field of conservation science calcium oxalate films are a hot issue and some problems are still unsolved. Indeed, whewellite is stable at environmental conditions, while weddellite, at the same conditions, quickly changes into whewellite. Some questions arise: why have whewellite and weddellite been often detected within the same film? Which conditions can govern the formation of whewellite and weddellite, together or separately? Moreover, one of the most controversial issues is to explain the transformation of one species into the other. 7 This work aims at contributing to the solution of some of these problems. In particular the research is devoted to explore (i) the stability of suitably synthesised nanocrystals of weddellite in different experimental conditions and (ii) the transformation mechanism of weddellite into whewellite. Experiments have been carried out using powder X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). Moreover, a crystallographic study by single-crystal X-ray diffractometry has been carried out. In this work we have considered the influence of humidity on the stability of calcium oxalates. The effect of temperature on the stability of calcium oxalates has been already exten- sively studied by many authors, 8–16 thus in this work it is not treated; however, all data suggest that, at environmental temperature, the only calcium oxalate actually stable is whewellite. Regarding the assignment of the absorption bands of the two phases we refer to the wide bibliography. 8,9,17–19 Previous studies Experiments have previously been carried out in order to investigate the stability of weddellite kept under different a Istituto per la Conservazione e la Valorizzazione dei Beni Culturali, CNR, via Cozzi 53, 20125 Milano, Italy. E-mail: c.conti@icvbc.cnr.it b Dip. Chimica, Materiali e Ingegneria Chimica ‘‘Giulio Natta’’, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy c Dip. Energia, Politecnico di Milano, via Ponzio 34/3, 20133 Milano, Italy d Dip. Scienze della Terra, Universita ` degli Studi di Milano, via Botticelli 23, 20133 Milano, Italy PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics Downloaded by Politecnico di Milano on 24 May 2011 Published on 07 October 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00624F View Online