Monte Carlo simulations of aqueous surfactant solutions have been performed on a two-dimensional lattice in a canonical ensemble. The simulations are based on four different models of surfactants with chainlike geometry and several types of interactions between the surfactant headgroups. The effect of number of headgroups (N HG) and the types of interactions between them (short-range versus long-range) has been examined on the surfactant aggregation. The simulations show that the aqueous solutions of surfactant chains with N HG > 1 and a short-range head−head interactions behave in a manner similar to that of the solutions of surfactant chains with N HG = 1 and long-range electrostatic interactions between the headgroups. The similarity of the results in the two models is discussed on the basis of free energy of micellization and is shown to be an effect of similar excluded volume of headgroups in the two models.

Alkylsilane monolayer films are used in the fabrication of microelectromechanical systems (MEMS) as anti-stiction coatings. Recently, these films are also being considered as anti-friction coating for MEMS parts that operate under conditions of friction, such as gears, motors, etc. Study of frictional behaviour of these films is of significant interest from the performance point of view. In this work we use the method of molecular dynamics (MD) to study the friction between two rigid silica substrates coated with alkylsilane monolayers. The friction coefficient, friction force and normal force on the films are obtained as a function of separation between the substrates, temperature of the films and velocity of the substrates. The results of simulations are compared with the thermal activation model of Briscoe and Evans (1982 Proc. R. Soc. Lond. A 380 389). The frictional behaviour of the films as a function of separation between the substrates follows the thermal activation model. The MD simulations in the present work show that the normal force (or pressure) on the films is dependent on the temperature and the substrate velocity. This is in contrast to the thermal activation model in which the pressure on the films is assumed to be independent of these variables. The simulation results are seen to be in agreement with a modified thermal activation model which takes into account the dependence of pressure on the temperature and substrate velocity.

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