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.

摘要 黏土矿物目前仍广泛应用于人类健康相关的医药产品及美妆领域。本研究针对巴西迪亚曼蒂纳地区产出的粉色黏土样品开展了制剂前研究，以表征其固态属性。本次分析的固态属性涵盖物相鉴定、铁物相、结晶度、粉体流动特性、热行为以及非等温相变动力学。该粉色黏土以1:1型黏土矿物及高岭石为主要组成成分。本样品的穆斯堡尔谱（Mössbauer spectrum）显示存在Fe³+(α-Fe₂O₃)赤铁矿、Fe²+以及两种大Δ/2ξq值的Fe³+，其数值分别约为2.80 mm·s⁻¹和2.69 mm·s⁻¹，前者对应铁硅酸盐（大概率为辉石），后者则为超顺磁性Fe³+。该粉色黏土的粉体流动性能较差。热行为分析结果显示，该粉色黏土体系在400~600 ℃区间发生相变，对应脱羟基过程，该阶段的活化能约为300 kJ·mol⁻¹，且该值保持恒定直至反应转化率达到约50%；随后活化能提升至约530 kJ·mol⁻¹，直至整个脱羟基过程完成（α=80%）。在开展合理的制剂设计时，必须充分考量该粉色黏土的各项固态属性与特征。该类黏土具备独特的医药相关特性，可被美妆行业加以高效利用。