Solid-state properties of pink clay from Jequitinhonha Valley in Brazil for pre-formulation study

Abstract

Clay minerals are still widely used in pharmaceutical products for human health and cosmetic purposes. Pre-formulation studies were conducted to identify solid-state properties of pink clay, a sample from Diamantina, Brazil. Among the solid properties to be analyzed, we have selected type identification, iron phases, crystallinity, powder flow characteristics, thermal behavior, and non-isothermal phase transition kinetics. The pink clay is composed of (1:1) clay type and kaolinite as the main component. The Mössbauer spectrum of pink clay shows Fe³⁺(α-Fe₂O₃)

hematite, Fe²⁺, and Fe³⁺ with large Δ/2ξq of about 2.80 and 2.69 mm.s^-1 respectively, related to iron silicates, most likely pyroxene, and a superparamagnetic Fe³⁺. Pink clay exhibits poor flow properties. The thermal behavior indicates a phase-transition between 400 - 600 ºC associated with the dehydroxylation of the pink clay system requiring ~300 kJ mol^-1, being constant until the process reaches a conversion of ~50% when the energy is enhanced to ~530 kJ mol^-1, concluding the whole dehydroxylation process (α=80%). Solid-state properties and characteristics found for the pink clay must be considered for the proper design of formulations. This type of clay shows unique pharmaceutical properties that can be favorably exploited by the cosmetic industry.

Keywords:
Pink clay mineral; Thermal behavior; Iron phases; Powder technology; Solid-state stability; Kinetic study