Packing alkane chain

Fig. 1. (a) Short, flat aromatic planes (seen end-on), when bonded to 5 A-wide alkane chains, pack poorly if not tilted (b) the same aromatic planes, if tilted and side-slipped, will achieve better packing, at the expense of good overlap, (c) Longer aromatic planes, e.g. perylene, can tilt and also overlap better, with best packing. 

The strength of the London forces between alkane molecules increases as the molar mass of the molecules increases hydrocarbons with unbranched chains pack together more closely than their branched isomers. Alkanes are not very reactive. but they do undergo oxidation (combustion) and substitution reactions. 

Figure7.11 Orientation and packing for 4,4 -biphenylsubstituted alkaneselenolate molecules on Au and Ag showing the difference between an odd and even number of CH2 groups in the alkane chain. Reproduced with permission from Shaporenko eto/. [196], Copyright (2007) American Chemical Society.

X-ray diffraction studies have revealed that alkane chains with an even number of carbon atoms pack more closely in the crystalline state => attractive forces between individual chains are greater and melting points are higher. 

Water, being a polar solvent, can interact with the anion [NO3] but the long alkane chains (Cjo) of the cation are hydrophobic hence, there is little interaction of water with the cahon, and thus the anion entity is favored by the (IL + water) mixtures. When one alkyl chain (Cjg) was changed by the benzyl group and the other one by the longer alkane chain (C12-C14) as the substituents at the cation, the decrease of solubility was reported. These new substituents had strong influence on packing effects in the liquid phase. 

Fig. 1 An ideal self-assembled monolayer of alkanethiolates supported on a gold surface. The terminal groups are amenable to synthesis, providing a means to immobilize and present ligands from the surface the alkyl chain spacer promotes tight, regular packing, and the gold-sulfur interface stabilizes surface atoms and alkane chains during packing...

Once the film capacitance is known, the dielectric constant of the layer can be determined by using Equation (4.10), provided that the thickness of the assembly is known. For self-assembled monolayers of long-chain alkane thiols, the experimentally determined dielectric constants fall in the range 23-2.6 and are consistent with a close-packed layer of alkane chains with essentially no penetration by electrolyte solvent or ions. 

Figure 8.12. The d-l phase diagram for Pd nanocrystals thiolized with different alkane-thiols. The mean diameter, d, was obtained from the TEM measurements on as-prepared sols. The length of the thiol, /, is estimated by assuming an all-trans conformation of the alkane chain. The thiol is indicated by the number of carbon atoms, Cn. The bright area in the middle encompasses systems that form close-packed organizations of nanocrystals. The surrounding darker area includes disordered or low-order arrangements of nanocrystals. The area enclosed by the dashed line is derived from calculations from the soft sphere model. (Reproduced with permission from...

It has also been argued that the polymer chains are stacked with a distance between thiophene rings of 3.8 A, based on the weak large-angle peak [79]. A kind of two-dimensional lamellar ordering of the PT chains would then occur [80]. The distance between successive rings along the PT chain, and also the distance between close-packed alkane chains measured... 

The second step of adsorption is much slower and is zeroth order with respect to the thiol concentration20. Saturation is usually reached after 16-24 h of exposure. During this step, the monolayer reorganizes, producing well-packed structure and freeing some more gold surface which is immediately occupied by additional thiol molecules. The alkane chains straighten and the order and orientation of the monolayer... 

In crystalline materials, with the alkane chains tightly packed and in an all-//cum conformation, the asymmetric stretch of the methylene groups va(CH2) shows at 2918 cm-1, whereas in liquid state, with disordered orientation of alkane chains and high concentration of gauche interactions, the position of this peak is shifted towards higher frequencies (2921-2924 cm-1). 

---