Phase transitions in some n-alkanes and petroleum waxes - investigation byphotoacoustic and exoelectron emission techniques

The use of the photoacoustic (PA) effect and photostimulated exoelectron em ission (EEE) technique in the investigation of first- and second-order phas e transitions in pure n-alkanes such as n-C-22, n-C-28 and n-C-32 and in th ree petroleum waxes having different carbon number distributions, have been examined. Changes in the amplitude and phase of the PA signal with tempera ture and the temperature dependence of the intensity of photostimulated EEE across the phase transitions have been measured. These measurements have s hown that both the polymorphic phase transitions and melting of these hydro carbons 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-li quid phase transitions. These results are further confirmed by a comparison with those obtained by differential thermal analysis (DTA) control measure ments performed at the same heating and/or cooling rate. The PA and EEE res ults thus obtained are discussed in the light of phase transition temperatu res obtained from other techniques. (C) 2000 Elsevier Science B.V. All righ ts reserved.