Anomalous thermal expansion of thin cetane layer solidified at the inner surface of confining nanoporous silica gel

Silica gel with extremely small pores of 3 nm diameter was filled with liquid cetane. Samples with various coefficients of filling k of cetane were prepared. After the solidification of confined cetane, its free volume has been investigated. The positron annihilation lifetime spectroscopy at 24–300 K and differential scanning calorimetry (DSC) at 233–303 K measurements have been performed. Decreasing the temperature, only small changes in dimensions of silica gel matrix and pronounced contraction of confined cetane have been indicated. In the case of k < 0.2, however, via cooling below 180 K, the temperature dependence changes its sign and the free volume in confined cetane increases. Decreasing the cetane content, the negative apparent expansion coefficient dramatically grows. This anomalous temperature dependence is interpreted by the cracking of thin layer of solid cetane which is in contact with the walls of silica gel pores.