Papers by Author: Radosław Zaleski

Abstract: The dynamics of n-heptane adsorption and desorption on two silicas (Si-A and SBA-15) with different porosity was studied using positron annihilation lifetime spectroscopy (PALS). Simplified method of spectra analysis consisting in the summation of counts in the long-lived part of PALS spectra was used. It allowed to observe changes occurring in the minute time scale. The dependence of the number of counts on time could be fairly well approximated by exponential functions. It was found that the time required for reaching equilibrium in long cylindrical pores is up to ten times longer than in bell-mouthed interconnected ones. The longest time constants (ca. 5 min in Si-A and ca. 60 min in SBA-15) were observed for desorption in both samples.

Abstract: The porous structure of polymer-silica composites, based on three polymer templates, which differ in a porosity and hydrophobicity, was examined using positron annihilation lifetime spectroscopy. Additionally, the investigation of silicas obtained after removal of polymers during calcination of composite materials, was performed. In composites based on hydrophobic polymers, silica condensates only in larger free volumes, while SiO₂ deeply penetrates spaces between polymer chains, when the template is polar. Moreover, the structure of the silica gel, obtained after polymer removal, depends on chemical character of the template, rather than its porosity.

Abstract: Five samples of porous polymers were investigated using positron annihilation lifetime spectroscopy and liquid nitrogen sorption techniques. Temperature dependencies of o-Ps lifetime were used to estimate the ETE model parameters (R and Δ) for porous polymers. The value of the Δ parameter was found to be 0.13-0.15 nm. A significant difference between pore radii obtained by both techniques was found.

Abstract: Positron annihilation lifetime spectroscopy study of n-heptane adsorption and desorption on silica gel Si-60 gives insight into a free volume evolution during those processes. Analysis of the lifetime distribution allows to ascribe particular ortho-positronium components to silica walls, mesopores and n-heptane. Hysteresis loop of the intensities of the mesopore and n-heptane related components similar to classic adsorption/desorption isotherms is observed. Transition from an adsorbate multilayer structure to liquid when pores become completely filled, manifests itself as an intensity change. Kinetics of desorption and adsorption studies allows to determine the time of reaching the equilibrium by the Si 60–n-heptane system at various pressures.

Abstract: The modification of the extended Tao-Eldrup model accounting ortho-positronium quenching in air is presented. Taking into account quenching by oxygen molecules adsorbed on the surface of porous material gives reasonable agreement between results of the model and the experimental positron annihilation lifetime spectroscopy data. Pore size distributions calculated using this model from the spectra for MCM 41 mesoporous sieve obtained in air, oxygen or vacuum are compared and discussed taking into account effect of ortho-positronium migration from small open pores to the larger ones. The rates of ortho-positronium quenching by air (47.2 µs-1 MPa 1), oxygen (220 µs-1 MPa-1) or nitrogen (1.7 µs-1 MPa-1) obtained from pressure dependences of the lifetimes observed in MCM 41 agree reasonably with the experimental results of other authors, if the correction for oxygen adsorbed on the surface is applied.

Abstract: The ordering of n-nonadecane confined in the pores of MCM-41 silica was investigated by Positron Annihilation Lifetime Spectroscopy. No irradiation effects caused by the positron source were observed. The ortho-positronium components found in the lifetime spectra correspond to the free volumes observed in the bulk alkane plus additional space at the interface between n-nonadecane and the silica wall. Phase transitions rigid-rotator and rotator-liquid were shifted towards lower temperatures by 28 and 16 degrees respectively.

Abstract: A new method of analysis of PALS spectra of porous materials is proposed. The model considers both the thermalization process of positronium inside the pores and the pore size distribution. The new model is fitted to spectra of mesoporous silica MCM-41 and MSF. The resulting parameters are compared with parameters obtained from fitting the “conventional” models, i.e. a sum of exponential components with discrete or/and distributed lifetimes.