Thermal stability and antimicrobial properties of pure and modified pyrophyllite (PYRO/Ag) clay

Abstract

Clays, inorganic materials with layered crystal structure, are widely used as a pharmaceutical excipients and active substances. The pyrophyllite, Al2Si4O10(OH)2, is an aluminosilicate mineral, that together with talc belongs to a specific sub-clay group. In this study, we examined some aspects of the use of pyrrophylite as a talc substitute. In this study, two pyrophyllite modifications have been investigated: the original pure pyrophyllite (labeled as PYRO), and that modified by incorporation of nanodispersed silver (labeled as PYRO/Ag). For pyrophyllite application in medicine, its thermal stability is of great importance. As an accelerated study of thermal stability, we employed simultaneous thermogravimetry and differential thermal analysis (TG/DTA) methods in dynamic (non-isothermal) regime in inert (N2) atmosphere. Pyrophyllite clay decomposition has been proved to proceed in two steps: moisture removal (I stage) and dehydroxilation (II stage). It must be stressed that these two degradation stages are not strictly separated. The kinetic parameters of decomposition reactions were examined by both isoconversional and model-fitting analytical procedures. The isoconversional Friedman model amounted activation energy (Ea) value on 65 kJ mol−1 and 214 kJ mol−1 for vaporization and dehydroxilation process, respectively. Fitting procedure revealed that vaporization process is of the n-th order (with activation energy, frequency factor and reaction order Ea = 66.6 kJ mol−1, A = 3.05 × 108 s−1, n = 3, respectively), while the dehydroxilation process, is best described with distributed activation energy models (Discrete: Ea = 232.2 kJ mol−1 (50.1%), with initial minimum frequency factor 3.66 × 109 s−1 and initial maximum frequency factor 3.66 × 1011 s−1). In addition to thermal properties, antimicrobial activity of pure (PYRO) and modified pyrophyllite (PYRO/Ag) were tested. Modified PYRO/Ag showed excellent antimicrobial properties, providing sterile conditions against Staphylococcus aureus ATCC 25923 and Escherichia coli ATCC 25922. The minimum bactericidal concentration (MBC) of Ag was found to be 25 mg L−1.