Thermal Expansion of Free-Volume in Hexadecane Confined in the Silica Gel Nanopores
O. Šaušaa, V. Majerníka, E. Illekováa, E. Macováb and D. Berekb
aInstitute of Physics, Slovak Academy of Sciences, Dúbravská cesta 9, SK-845 11 Bratislava, Slovakia
bPolymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, SK-845 41 Bratislava, Slovakia
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Temperature dependence of the free-volume of voids in hexadecane (HXD) confined in the 3 nm silica gel pores (Develosil-30) was measured by positron annihilation lifetime spectroscopy. Different amount of HXD was filled into the pores of matrix. The anomalous thermal expansion was observed in the partially filled pores with extremely low filling. The thermal expansion coefficient of free-volume voids in this solidified HXD layer has a negative sign at temperatures below 180 K compared to high- or full-filled 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 differential scanning calorimetry method confirmed that the processes were enacted within pores and not on the outer surfaces of silica gel grains. In the sample with the extremely low filling, only the one HXD solidification/melting effect connected with confined states is manifested.

DOI: 10.12693/APhysPolA.125.798
PACS numbers: 68.60.-p, 78.67.Rb, 78.70.Bj