Structural rearrangements in confined n-hexane at elevated temperature. Isobars in pore characterization

Abstract

A novel approach to thermoporometry is proposed. Thermal transformations in a confined liquid were investigated using positron annihilation lifetime spectroscopy (PALS). The SBA-3 silicas with a regular pores with sizes of about 2 nm or 3 nm wetted with n-hexane were used in experiments as model solid/liquid systems. The information obtained from in situ monitoring by PALS allows to infer about the evolution of the amount of n-hexane remaining in the pores as well as empty pore space. Moreover, the changes in the coverage of silica by alkane and the degree of pore opening are simultaneously detected. The investigations provided a deeper insight into the mechanism of thermal evacuation of the condensate from the pores. The most important finding is that during thermodesorption, liquid n-hexane filling the pores splits into plugs separated by empty volumes rather than being removed layer-by-layer from the surface. The pore size distributions derived from the PALS results obtained at the saturated vapor pressure of n-hexane and the results of thermogravimetry at atmospheric pressure are consistent with each other. However, comparing them with the results of low-temperature nitrogen adsorption suggests that using the Kelvin equation is not suitable for n-hexane in such small pores.