Molecular clouds with PAHs

Carbonaceous material (Fig. 12.8b) is intimately mixed with silicates and is very abundant (carbon abundance averages 13% and varies up to 50%) in CP IDPs. Some carbon is elemental (graphite), but C-H stretching resonances in infrared spectra show that aliphatic hydrocarbons are also present. Polycyclic aromatic hydrocarbons (PAHs) also occur. Nanodiamonds have been identified in cluster IDPs, but not in smaller CP IDPs. Enormous D/H and 15N/14N anomalies have been measured in bulk IDPs, and the hydrogen isotopic anomalies are correlated with organic-rich domains. Ratios of D/H as high as 25 times the solar ratio suggest the presence of molecular cloud materials. 

It is now generally accepted that many of the neutral molecules observed in interstellar clouds are formed in the gas phase by positive ion-molecule reactions which produce a wide variety of molecular positive ions which then dissociatively recombine with electrons to form the observed neutral molecules. The final neutralization step in the conversion of a positive molecular ion to a neutral molecule may also be the transfer of a proton from the ion to another molecule or the transfer of an electron to the molecular ion from a species of low ionization energy (e.g. a metal atom). Very recently, it has been proposed that negatively-charged macromolecules, PAH" (i.e. negatively- charged polycyclic aromatic hydrocarbons, PAH s), may be present in interstellar... 

One disadvantage of using some nonionic surfactants in the preconcentration step of PAHs is their tendency to undergo clouding and liquid-liquid separation at elevated temperatures [226], Sometimes, the high temperatures used in the extraction provoked losses of PAHs with low molecular weight [227], It has been observed that the observed cloud-point temperature of a mixtoe of two nonionic surfactants is intermediate between the individual cloud-point temperatures of the nonionic surfactants involved [230], Therefore, it is often very convenient to use mixtures of surfactants in order to modify and control the cloud-point temperature for a given use [185]. 

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