Formation of aggregates or clusters

It should be noted that the formation of such aggregates in the gas phase causes a diminution of entropy. To be possible, the formation of such aggregates must be exothermic. Furthermore, if the partners have similar basicities or affinities for the cation, the cluster is more stable. Otherwise it dissociates, one of the partners taking the proton, or the cation, according to the relative stability. This is why associations of two or more identical molecules are observed more often or at higher abundances. Indeed, they have the same affinities of course. 

The formation of oligomers has as a consequence the diminution of the number of molecules in the gas phase, and thus occurs with a diminution of the entropy. As the reaction must be exergonic to occur, it must be sufficiently exothermic, at least to compensate for the entropy loss. Once formed, the internal energy of the oligomer should be reduced, since it contains a sufficient amount of energy to dissociate. This needs a collision with a third partner, and this requires a sufficient pressure. 

As a general rule, the abundance is reduced when the number of associated molecules increases. However, some specific aggregates, resulting from particularly important interactions, are present at particularly high abundance. This occurs often with organometallic compounds, as the metal tries to complete its electronic shell. 

Aggregates are rarely observed in the negative ion mode, because the presence of the negative charge causes an expansion of the electronic shell, thus reducing the electric field around the negatively charged ion. This reduces the interactions between the partners. 

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By analogy with the works which dealt with cellulose micro crystal-reinforced nanocomposite materials, microcrystals of starch [95] or chitin [96, 97] were used as a reinforcing phase in a polymer matrix. Poly(styrene-co-butyl acrylate) [95,96], poly(e-caprolactone) [96], and natural rubber [97] were reinforced, and again the formation of aggregates or clustering of the fillers within the matrices was considered to account for the improvement in the mechanical properties and thermal stability of the respective composites processed from suspensions in water or suitable organic solvents. 

Hortal et al. [52,53] and Martinez-Haya et al. [54] showed that the LD-MS spectrum of an asphaltene depended on the concentration of species in the sample and on the laser fluence apphed.The thickness of sample applied to the target plate was a factor in the formation of aggregates or clusters of molecules in the reactive gas plume formed by the laser shot thin layers were less likely to form cluster ions than thick layers. High laser fluences were more hkely to lead to cluster ion formation than fluences slightly above ionization threshold. However, these works did not examine fractions of asphaltenes where low- and high-mass molecules were separated from each other. 

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