Chemical indexes and in-vitro solubility

Abstract

Different chemical indexes have been proposed in the literature, in order to predict the influence of the chemical composition of mineral fibers on their properties. One of these properties is the acellular in-vitro dissoludon 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 expensive 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 could easily be predicted through conventional chemical indexes already reported 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 (ΔG°) of hydration as evaluated from tables in Paul [7]; - "Kanzerogenitätsindex" (KI) as defined in [8]. The relationship between the acellular in-vitro solubility expressed as KDISSiO2 or KDISCaO and these four different indexes has been analyzed. Nene of these indexes could be reliably used to prediet the in-vitro solubility at pH 7.4 for a broad range of composidons. KI is an acceptable indicator of 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 prediet the results of in-vitro dissolution tests; and thus the need for a reliable standardized in-vitro test method is emphasized.