National Physical Laboratory, Teddington TW11 0LW, UK.
Strathclyde Institute of Pharmacy & Biomedical Sciences, University of Strathclyde, Glasgow G1 1XQ, UK.
Sensors (Basel). 2020 May 31;20(11):3120. doi: 10.3390/s20113120.
The porosity of porous materials is a critical quality attribute of many products ranging from catalysis and separation technologies to porous paper and pharmaceutical tablets. The open porosity in particular, which reflects the pore space accessible from the surface, is crucial for applications where a fluid needs to access the pores in order to fulfil the functionality of the product. This study presents a methodology that uses terahertz time-domain spectroscopy (THz-TDS) coupled with an index-matching medium to measure the open porosity and analyze scattering losses of powder compacts. The open porosity can be evaluated without the knowledge of the refractive index of the fully dense material. This method is demonstrated for pellets compressed of pharmaceutical-grade lactose powder. Powder was compressed at four different pressures and measured by THz-TDS before and after they were soaked in an index-matching medium, i.e., paraffin. Determining the change in refractive index of the dry and soaked samples enabled the calculation of the open porosity. The results reveal that the open porosity is consistently lower than the total porosity and it decreases with increasing compression pressure. The scattering losses reduce significantly for the soaked samples and the scattering centers (particle and/or pore sizes) are of the order of or somewhat smaller than the terahertz wavelength. This new method facilitates the development of a better understanding of the links between material properties (particles size), pellet properties (open porosity) and performance-related properties, e.g., disintegration and dissolution performance of pharmaceutical tablets.
多孔材料的孔隙率是许多产品的关键质量属性，从催化和分离技术到多孔纸和药用片剂都有涉及。特别是开放孔隙率，它反映了从表面可进入的孔空间，对于需要流体进入孔以实现产品功能的应用至关重要。本研究提出了一种使用太赫兹时域光谱（THz-TDS）结合折射率匹配介质来测量粉末压块的开放孔隙率和分析散射损耗的方法。无需了解完全致密材料的折射率即可评估开放孔隙率。该方法已通过压缩药用级乳糖粉末的片剂进行了验证。粉末在四个不同的压力下被压缩，并在浸泡在折射率匹配介质（即石蜡）之前和之后通过太赫兹时域光谱进行了测量。通过确定干燥和浸泡样品的折射率变化，可以计算出开放孔隙率。结果表明，开放孔隙率始终低于总孔隙率，并且随着压缩压力的增加而降低。浸泡样品的散射损耗显著降低，散射中心（颗粒和/或孔径）的大小与太赫兹波长相当或略小。这种新方法有助于更好地理解材料特性（颗粒尺寸）、片剂特性（开放孔隙率）和性能相关特性（例如，药用片剂的崩解和溶解性能）之间的联系。
