Chemical indexes and in-vitro solubility

Different chemical indexes have been proposed in the literature, in order t o predict the influence of the chemical composition of mineral fibers on th eir properties. One of these properties is the acellular in-vitro dissoluti on behavior (noted thereafter: solubility) in synthetic lung fluids at pH 7 .4, which has been shown to be a reliable predictor of the results of expen sive chronic long-term in-vivo tests conducted up to now with fibers [1 and 2]. This in-vitro solubility has been reported as being a function of the chemical composition [3 and 4]. The aim of this investigation was to assess if the in-vitro solubility coul d easily be predicted through conventional chemical indexes already reporte d in the literature; such as the following: nonbridging oxygen (NBO) as calculated by Moore and Brown [5]; optical basicity (OB) as defined by Baucke and Duffy [6]; free enthalpy (Delta G(0)) of hydration as evaluated from tables in Paul [7 ]; "Kanzerogenitatsindex" (KI) as defined in [8]. The relationship between the acellular in-vitro solubility expressed as KDI SSiO2 Or KDISCaO and these four different indexes has been analyzed. None o f these indexes could be reliably used to predict the in-vitro solubility a t pH 7.4 for a broad range of compositions. KI is an acceptable indicator o f in-vivo biopersistence [9], in a narrow range (15 to 40). It is the best of all the analyzed indexes giving a general trend for the solubility. The calculated correlation coefficient between KI and KDISSiO2 is then 0.78. This work shows that calculated indexes derived from the literature are not sufficient to predict the results of in-vitro dissolution tests; and thus the need for a reliable standardized in-vitro test method is emphasized.