Phase transitions in some n-alkanes and petroleum waxes — investigation by photoacoustic and exoelectron emission techniques

Abstract

The use of the photoacoustic (PA) effect and photostimulated exoelectron emission (EEE) technique in the investigation of first- and second-order phase transitions in pure n-alkanes such as n-C 22, n-C 28 and n-C 32 and in three petroleum waxes having different carbon number distributions, have been examined. Changes in the amplitude and phase of the PA signal with temperature and the temperature dependence of the intensity of photostimulated EEE across the phase transitions have been measured. These measurements have shown that both the polymorphic phase transitions and melting of these hydrocarbons are accompanied by jumps or peaks of the PA amplitude and phase and EEE intensity, thus enabling the detection of the solid–solid and solid–liquid phase transitions. These results are further confirmed by a comparison with those obtained by differential thermal analysis (DTA) control measurements performed at the same heating and/or cooling rate. The PA and EEE results thus obtained are discussed in the light of phase transition temperatures obtained from other techniques.