Molecular simulations of surfactants and silanes: Self-assembly in solutions and on surfaces, and friction between monolayers

Abstract

Recent experimental efforts have focused on the development of water-based chemistries to deposit hydrophobic alkylsilane films on the silica surfaces to address the problem of stiction induced failure in MEMS. A detailed molecular level examination of the structure of these films is limited with the available experimental methods. Therefore, this work undertakes an investigation of the structure and properties of self-assembled alkylsilane monolayer films via Monte Carlo (MC), and molecular dynamics (MD) simulations. The existing literature on the surfactant aggregation is used as a guide for modeling the alkylsilanes molecules. However, the current literature is ambiguous whether to describe interactions between the surfactant head groups as short-range or long-range. This work resolves this discrepancy successfully by performing a series of simulations of various structural and interaction models of surfactants. Simulations show that a realistic surfactant aggregation requires an excluded volume of the head groups, necessitating different interaction models in different structural models of surfactants. Next, MC simulations have been used to investigate the impact of the charged group in cationic alkylsilane on the structure of the deposited film. The structure of the films is characterized with the spatial pair correlations at each molecular layer of the deposited films. The long-range correlations are seen for the films of cationic alkylsilanes. Also, the frictional behavior of the alkylsilane films deposited on silica substrates is examined via molecular dynamics simulations. The friction coefficients of the films are obtained as a function of separation between the films, temperature, and velocity of the substrates. The results of the MD simulations support a thermal activation model of friction.