Phase Transitions of Benzene Confined in Nanoporous Silica

We studied the effects of confinement in the freezing and melting of benzene confined in nano pores (7-100 nm) using a Differential Scanning Calorimeter.  The DSC is able to measure and record (thermograms) the power required to heat or cool a material.  During a phase transition, the thermograms display a peak. From this we were able to determine the melting and freezing temperatures and compare it to the expected results in accordance with the Gibbs-Thomson equation.  In agreement with the equation, the temperature difference between the onset of the confined and bulk material decreases linearly with the inverse of the pore radii.  However, the heat of fusion is assumed to remain constant across all pore sizes. This behavior is not observed in experimentation as the change in the heat of fusion begins to decrease at smaller pore sizes. Although, this does not agree with the Gibbs-Thomson equation, other researchers have observed similar behavior for benzene and other materials as well. One possible reason why the apparent heat of fusion is smaller than its bulk counterpart is due to the chance that materials at the interface may not readily freeze and do not contribute to the energy of transition. In an attempt to correct this, we annealed the sample but found no significant difference in the heat flow of the non-annealed and annealed samples of the same pore size however there was evidence of crystallization memory with the confined samples.