Characterization of pore filling of mesoporous host systems by means of positronium annihilation lifetime spectroscopy (PALS)

Positronium annihilation lifetime spectroscopy (PALS) is an alternative method for the determination of pore sizes and pore size distributions. It is a measuring methodology which shows no limitations in the lower nanometer region and works, besides, without destruction of the sample material. It can be used for the characterization of open and closed pores. Additionally, this technology offers the possibility to obtain pore filling ratios of gases, liquids and solids precisely. Polymorphous medicaments, like acetaminophen, show different crystallization behavior within a pore system, depending on the pore size. This property can be used to control the crystalline state of the medicament and to optimize therefore the pharmaceutical use of the active substance. In this study, acetaminophen (C8H9NO2), also known as Paracetamol, is incorporated into different porous systems. The filling of the pores is realized by an acetaminophen melt at 453 K. The silica membranes are dipped into the melt, subsequently removed and cooled. Information concerning the filling ratio of the pores with the pharmaceutical was received with the help of PALS. The extended Tao-Eldrup model forms the theoretical base, taking into account the pore size sensitive annihilation properties of the ortho-positronium in matter.