Molecular Simulation Study of Nanoscale Friction between Alkyl Monolayers on Si(111) Immersed in Solvents

Non-equilibrium molecular dynamics simulations were performed to study nanoscale friction between two Si(111) surfaces covered with alkyl monolayers immersed in liquid solvents. Three pairs of interfaces, ranging from hydrophobic CH3/CH3 to hydrophilic OH/OH, were studied. Three solvents, including water, methanol, and n-decane were used to represent different solvent polarities. It was showed that friction was dependent not only on surface hydrophobicity, but also on solvent polarity. In polar solvents (e.g., water), friction is much larger for hydrophilic than hydrophobic interfaces, while in non-polar solvents, (e.g., n-decane), friction has no significant difference for both hydrophobic and hydrophilic monolayers. A fundamental understanding of friction at different interfaces in various solvents is very important to micro- or nano-electromechanical systems (MEMS/NEMS), particularly, bio-MEMS/NEMS.

Simulation Method and Results:

The simulation box used in this work is shown in Figure 1. This simulation box was constructed from the one we used previously by placing a bulk solvent on each side along the y direction. Solvent molecules are confined between the two solid walls covered by alkyl monolayers. Top and bottom walls have opposite sliding directions at a rate of U* and the sliding directions are along the x direction. The solid walls are infinite in the x direction, but finite in the y direction. The confined solvent is in contact with its bulk on both sides.

Figure 1. A schematic representation of the simulation box. Two Si(111) surfaces coated with C9 alkyl monolayers with terminal groups of -OH (hydrophilic) and/or -CH3 (hydrophobic) are sheared against each other.

The Si(111) were coated with C9 alkyl monolayers with terminal groups of -CH3 (hydrophobic) and/or -OH (hydrophilic). Variation of the composition of mixed -OH and -CH3 monolayers will results in different surface hydrophobicities. Three compositions were used, i.e., X=0 (terminated with –CH3 only), X=50% (terminated with a mixture of 50% –CH3 and 50% –OH), and X=100% (terminated with –OH only). In this work, three pairs of surface interactions were studied, i.e., both surfaces were covered with alkyl monolayers having the terminal groups of -CH3 (denoted as CH3/CH3 interface), of 50% -CH3 and 50% -OH (denoted as CH3+OH/CH3+OH interface), and of -OH (denoted as OH/OH interface). The solvents used are in the decreasing polarity: water (polar) > methanol >> n-decane (non-polar).


Figure 2. Snapshots from MD simulations for different interfaces immersed in water at 300K: (a) CH3/CH3, (b) CH3+OH/CH3+OH, and (c) OH/OH.

Figure 3. Snapshots from MD simulations for the OH/OH interface immersed in (a) water, (b) methanol, and (c) in n-decane.

Figure 4. Frictional coefficients for the CH3+OH/CH3+OH interface in water as a function of the length of the solid walls in the y direction.