Abstract
An experimental study of minimum autoignition temperatures (AIT) reveals unexpected behavior of pure, higher-carbon-number n-alkanes (C16 and larger) not predicted by current AIT prediction methods nor current phenomenological understanding of autoignition processes. The well-documented AIT trend for normal alkanes shows high AIT values for smaller members of the family with decreasing AIT, as carbon chain length increases, to a roughly constant value for C7 and larger. Measurements in this work show that values start to increase again at C16 and show a dramatic and previously unexplained step increase between C25 and C26.The impact of measurement methods on AIT values is discussed. Experimental modifications to compensate for AIT measurements at altitude are presented. Measured values for several n-alkanes using the ASTM E-659 method are reported and compared to data from other sources. Temperature-versus-time plots, external autoignition experiments, lag times, and decomposition measurements using differential scanning calorimetry are also reported and examined for several n-alkanes to illucidate the unexpected trends. A mechanistic explanation of these trends is proposed. Finally, the implications of this behavior for trends in other chemical families are discussed.
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