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Nanoaggregation of Polyaromatic Compounds Probed by Electrospray Ionization Mass Spectrometry

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journal contribution
posted on 05.04.2016 by Lan Liu, Johan Sjöblom, Zhenghe Xu
This paper reports the results of the first detailed experimental study on probing nanoaggregation of a polyaromatic compound. Electrospray ionization mass spectrometry (ESI–MS) was used to monitor the self-association of a well-defined polyaromatic compound, N-(1-hexylhepyl)-N′-(5-carboxylicpentyl)-perylene-3,4,9,10-tetracarboxylicbisimide (C5Pe), under various solution conditions. Gaseous ions corresponding to nanoaggregates of C5Pe molecules were directly observed on ESI mass spectra. The dominant aggregation number (n) was found to be less than 10, although larger nanoaggregates with an aggregation number larger than 10 were also observed. The aggregation number of C5Pe decreased by replacing toluene with xylene, while it increased with the C5Pe concentration or upon the addition of heptane to toluene as the solvent. The consecutive aggregation number was found only for small C5Pe nanoaggregates (2 ≤ n ≤ 11), which suggests a stepwise self-association at n ≤ 11. The larger nanoaggregates (n > 11) were formed by interactions between small nanoaggregates. The presence of naphthenic acids (NAs) was observed to hinder C5Pe self-association. The dispersive effect of NAs was found to be in the order of 1-methyl-1-cyclohexanecarboxylic acid ∼ cyclohexanebutyric acid < stearic acid < 5β-cholanic acid < 1-naphthalene pentanoic acid. The nanoaggregation behavior of C5Pe was compared to that of two other polyaromatic compounds.