Thermal Expansion of Free-Volume in Hexadecane Confined in the Silica Gel Nanopores

Abstract

Temperature dependence of the free-volume of voids in hexadecane (HXD) con ned in the 3 nm silica gel pores (Develosil-30) was measured by positron annihilation lifetime spectroscopy. Di erent amount of HXD was lled into the pores of matrix. The anomalous thermal expansion was observed in the partially lled pores with extremely low lling. The thermal expansion coe cient of free-volume voids in this solidi ed HXD layer has a negative sign at temperatures below 180 K compared to highor fulllled pores of silica gel. At these temperatures, the positron annihilation lifetime spectroscopy measurements demonstrate the appearance of a new type of the free volume within HXD which has been interpreted as the cracks in the HXD layer on the inner walls of matrix cavities. The di erential scanning calorimetry method con rmed that the processes were enacted within pores and not on the outer surfaces of silica gel grains. In the sample with the extremely low lling, only the one HXD solidi cation/melting e ect connected with con ned states is manifested.