Bonded alkyl chains density

The bonded alkyl chains density, pb, is the density of the bonded alkyl chain layer. It is often assumed to be the density of liquid octadecane in the case of Cis columns. Strictly speaking, however, the bonded layer is not a liquid since all the octadecyl chains are bonded, so their average distance is higher than in a liquid, and their entropy lower. This density (and the volume occupied by the layer) depends also on the mobile phase composition, depending on whether these chains are poorly soluble in the mobile phase and collapse on the surface or whether they are soluble and are swollen by the mobile phase or by the organic modifier in the case of the aqueous solutions most often used in RPLC. 

In 1993, Linford firstly reported a quite useful method to prepare monolayers of alkyl chains by thermal hydros-ilylation of hydrogen-terminated silicon surfaces [25]. Alkyl chains are covalently bound to Si surface by Si-C bonds. This thermal hydrosilylation could be attributed to a free-radical process with 1-alkene. First, a diacyl peroxide initiator was used to produce free radicals. However, at higher temperature, only hydrogen-terminated silicon and a neat solution of 1-alkene or 1-alkyne can form Si-C linkages [26]. Furthermore, lately it is found that such Si-C covalent links can be observed even in dilute solutions of 1-alkenes [27]. In that case, the density of monolayer packing strongly depends on the reaction temperature. 

FIGURE 5.22 (See color insert following page 280.) Perspective four-cell view snapshots of various stationary-phase models, which illustrate the contrasting surface topography with bonding density and alkyl chain length. (Adapted from Lippa, K.A., et al., Anal. Chem., 11, 7852, 2005.)... 

Oligomers of perfluorohexyl-ethene fulfilled these expectations in all preclinical studies, in vitro tests as well in animal tests. A radical polymerisation, followed by ultra-purification steps, created a crystal clear gel-like substance. The behaviours of the mixture of dimeric, trimeric and tetrameric star-shaped species with an inner core of hydrocarbon bonds and an outer layer of perfluoro-alkyl chains could be adjusted by the ratio of the dimeric, trimeric and tetrameric species, using a thin layer distillation. In dependence on this ratio, the viscosity could be adjusted in the range between 90 mPas and 1700 mPas, the specific density between 1.60 g/ml and 1.66 g/ml and the interfacial tension against water between... 

The partial charges of the alkyl chain s carbon atoms are quite different (294) the C in the a-position to oxygen possesses the largest density of positive charge. As a result, the scission of Ca-Cp bonds was easier than other C-C bonds. 

Bonded alkyl ligands are flexible, although their flexibility is significantly restricted by immobilization on the surface also, the higher the ligand bonding density, the less flexibility these chains have. 

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