(a) Imperial College
Figure 5: Snapshots of the C10C18 bilayer at a normal pressure
of 210±10 MPa
and 298.15K after shearing at 1ms-1 for 500000 time-steps. The white spheres
are the nitrogens, the blue lines represent the C10 chain and the last 8 atoms
in the C18 chain are red. The top snapshot shows the high density system
(50Å2/32Å) and the bottom is the low density system (77Å2/21.05Å).
The Comfort 96 code1 is a simulation package for modelling dialkyl- dimethy-ammonium cationic bilayers2 sandwiched between two cotton surfaces. This code is standard Fortran 77 and has been successfully ported to a number of serial and parallel systems including the Cray T3D. The PVM and MPI message passing libraries are used.
The Molecular Dynamics simulation of bilayers of R2N+Me2Cl- adsorbed onto cotton surfaces as a function of shearing velocity, headgroup area, normal pressure and chain length has been performed. Bilayers with surface areas of 40Å2 and 77Å2 at shearing velocities from 1 to 100 ms-1 were studied. The effect of chain length was made by simulating C18C18, C10C18 and C10C10 bilayers.
The friction coefficient can be calculated from the tangential and normal pressures m=pxz/pzz. The friction coefficient decreases with increasing normal pressure, increases with decreasing amphilphile density, increases with increasing shearing velocity and increases with shorter chain lengths.
The friction coefficient of the low density system is higher in the case of the C18C18 and C10C10. However, for the C10C18 bilayers this is reversed. In the 50A2 system there is no steric interaction between C10 chains and this leads to greater inter-digitation of the C18 chains at the interface. This can be seen in the stronger overlap area between the two layers in figure 5. In the 77A2 system the interactions between C10 and C18 chains leads to a sharper interface between the two monolayers. There is a positive correlation between the overlap area and the friction coefficient.