Simulated monolayers orientational order

For all the temperatures studied, the simulations lead to crystalline monolayers with strong orientational ordering of the chains in a distorted hexagonal structure even in the absence of an applied pressure. The chains are tilted at an angle of 30° to the surface normal. Most of the gauche defects are found near the ends of the chains, as observed in crystals of n-alkanes, but even at 400 K the average number of gauche conformations per chain is only about one. 

Figure 21. Orientational order-disorder (squares) and translational melting (circles) transition temperatures for N2 monolayers on graphite as a function of the island size N from Monte Carlo simulations with free boundary conditions. The arrows with the corresponding symbols at the end mark the infinite system size estimates for these quantities within the same model. (Adapted from Fig. 1 of Ref. 185.)...

Near strongly attractive surfaces, liquid structure differs noticeably from the bulk one. This is caused by the packing effect due to the localization of molecules in a plane(s) parallel to the wall and by specific fluid-wall interactions, such as H-bonds. Density oscillations of liquids near solid substrates were observed in experiments [143, 144, 417-419] and in numerous computer simulations of confined fluids. Besides, fluids with strongly anisotropic interactions (such as water) unavoidably undergo orientational ordering near the wall. It is important to know the character of this ordering and its intrusion into the bulk liquid. In the present section, we consider structural properties of adsorbed water layers in the liquid, bilayer, and monolayer phases. 

Tarek et al. [388] studied a system with some similarities to the work of Bocker et al. described earlier—a monolayer of n-tetradecyltrimethylammonium bromide. They also used explicit representations of the water molecules in a slab orientation, with the mono-layer on either side, in a molecular dynamics simulation. Their goal was to model more disordered, liquid states, so they chose two larger molecular areas, 0.45 and 0.67 nm molecule Density profiles normal to the interface were calculated and compared to neutron reflectivity data, with good agreement reported. The hydrocarbon chains were seen as highly disordered, and the diffusion was seen at both areas, with a factor of about 2.5 increase from the smaller molecular area to the larger area. They report no evidence of a tendency for the chains to aggregate into ordered islands, so perhaps this work can be seen as a realistic computer simulation depiction of a monolayer in an LE state. 

The variation of the friction with wall geometry, interaction potentials, temperature, velocity, and other parameters has been determined with simulations using simple spherical or short-chain molecules to model the monolayer [24,25,30,61,194]. In all cases, the shear stress shows a linear dependence on pressure that is consistent with Eq. (2) up to the gigapascal pressures expected in real contacts. Moreover, the friction is relatively insensitive to parameters that are not controlled in typical experiments, such as the orientation of crystalline walls, the direction of sliding, the density of the layer, the length of the hydrocarbon chains, and so on. Variations are of order 20%, which is comparable to variations in results from different laboratories [44,217]. Larger variations are only seen for commensurate walls, which may also exhibit a nonlinear pressure dependence [218]. 

Examination of the structure of the packing of 3-centre carbon dioxide at different pore sizes revealed a clear pattern of change with respect to pore size, associated with layer formation. Both the density profiles and the profiles of molecular orientation showed a monolayer of carbon dioxide oriented parallel to the pore walls at small pore size below about H = 0.71 nm. However, with increase in pore width above 0.68 nm there is a tendency for the flat molecules to rotate, in order to permit additional molecules to adsorb. Above about 0.71 nm an additional layer is formed, with molecules near the wall tending to tie flat and those at the centre tending to rotate relative to the axis. This pattern of behaviour is repeated as the pore size is increased. Fig. 3. depicts snapshots of the structure at different pore sizes. They reveal the formation of a central relatively flat layer, followed by rotation and subsequent separation of two distinct rotated layers. This pattern is consistently followed as the pore size increases, and in this way additional layers are created. This was confirmed fix)m the simulation results, by examination of profiles of density and the molecular orientation. Simulations with the 3-centre fluid without charges at the sites were also conducted, and yielded similar trends as given by the fluid with charges, with only a small reduction in capacity. Thus, the difference in liehavior compared to the U fluid is clearly related to the different molecular shape represented by the 3-center fluid, and not to electrostatic effects. 

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