Direct molecular dynamics simulations of the liquid-liquid interface of water–n-alkane and water-methanol–n-alkane systems have been performed in order to study the interfacial properties of these systems. The simulations were carried out using the NERD revised force field of Nath et al. for the n-alkanes, the simple point charge extended model for water, and the optimized potential for liquid simulations model for methanol. In order to validate the model employed in this work for the n-alkanes we calculated the coexisting densities, surface tension, and thickness of the interface for pure n-pentane. For all the systems studied the interfacial tension and thickness were calculated at 298.15 K. Our results show that, by adjusting the number of molecules to reproduce the liquid densities in the direct simulation method of the liquid-liquid interface in multicomponent systems, we are able to reproduce available experimental data for interfacial tension. The interfacial thickness is underpredicted and a constant negative deviation of $\sim 2.5\mathrm{\AA }$ from the experimental data is usually observed. We find that methanol acts like surfactant when it is added to the water–n-alkane mixtures, reducing the interfacial tension of the liquid-liquid ternary system. The interfacial tension results agree quantitatively well for the range of concentrations of methanol studied.

The mechanical properties of sliding carbon nanotubes have been investigated by classical molecular dynamics simulations in the canonical ensemble. In particular we have studied damped oscillations in the separation between the centres of mass of the inner and outer tubes of double-walled carbon nanotubes (DWCN). Incommensurate DWCNs forming (7, 0)@(9, 9) structures were simulated for systems at 298.15 K with axial lengths from 12.21 to 98.24 nm. The oscillations exhibited frequencies in the range of gigahertz with the frequency decreasing as the length of the system increases. The time until oscillations become negligible exhibited a nearly linear dependence on the length of the system. Two macroscopic models were developed in order to understand the forces involved in terms of macroscopic properties like friction and shear. The first model considered constant restoring forces during the whole event, while in the second the value of these constant restoring forces depended on the initial conditions of each oscillation. Both models reproduced the oscillations quite well, while the second model allows us to predict the dynamic shear strength in terms of the axial length of the system for tubes with the same diameters. The calculated dynamic shear strength exhibited monotonic behaviour with an inverse dependence on the length of the system. For systems with unequal axial lengths, the restoring force, which drives the oscillation, is reduced compared to the system with equal lengths, regardless of whether the outer nanotube is longer or shorter.