Frictional properties of alkanethiols

Lio, A., Charych, D.H., Salmeron, M. Comparative atomic force microscopy study of the chain length dependence of frictional properties of alkanethiols on gold and alkylsilanes on mica. J. Phys. Chem. B 101, 3800-3805 (1997)... 

Lee, S., Shon, Y.-S., Colorado, R. Ir., Guenard, RJ.., Lee, T.R., Perry, S.S. The influence of packing denaties and surface order on the frictional properties of alkanethiol self-assembled mono-layers (SAMs) on gold a comparison of SAMs derived from normal and spiroalkanedithiols. Langmuir 16, 2220-2224 (2000)... 

We present a quantitative study of frictional properties of pure self-assembled monolayers (SAMs) of alkanethiols as a function of chain length and mixed SAMs of dodecanethiol and 11-mercapto-l-undecanol as a function of surface composition on Au (111) using atomic force/friction force microscopy (AFM/FFM). The lateral and normal forces were calibrated in situ using a combined two-slope and added-mass method. Molecular dynamics simulations were also carried out to interpret the chain length dependence of frictional properties of alkanethiols. We then extended the in situ force calibration method to the mixed SAMs and investigated the effects of chemical nature and relative humidity on the frictional properties. Friction coefficients were plotted as a function of surface composition with different relative humidity. Such a plot could serve as a reference in determining surface composition in a nanoscale domain by measuring its friction coefficient. 

Quantitative measurements of nano-scale frictional properties of pure and mixed SAMs on Au (111) were achieved by in situ normal and lateral force calibration of AFM/FFM. For pure SAMs, the friction coefficients for the same alkanethiol system but with different tips, differ by less than 15%, indicating the reliability of nano-scale frictional and normal force measurements using a scanning force microscope. The friction coefficient increases as the chain length decreases as also found previously by other workers. Tip-based molecular dynamics simulations were carried out to interpret the chain length dependence on frictional properties of alkanethiols. Simulation results show that AFM/FFM tip penetrates deeper into films formed by shorter chain SAMs, causing higher friction. 

The lubricant properties of alkanethiols and fluorinated alkanes have been studied extensively by scanning probe techniques [163]. In agreement with experiments on LB monolayers it was found that the fluorocarbon monolayers show considerably higher friction than the corresponding hydrocarbon monolayers [164, 165 and 166] even though the fluorocarbons are known to have the lowest surface free energy of all organic materials. 

In their study, Park et al.100 investigated the frictional properties of fluorine-terminated alkanethiol SAMs grafted to gold surfaces. The frictional properties of the system were investigated by sliding two SAMs past one another at velocities in the stick-slip regime under various external loads. The simulations yield the shear stress as and the kinetic friction coefficient pk can be estimated from the slope of a plot of as versus load, using the relationships contained in Eqs. [4] and [5]. 

Nanoscale Frictional Properties of Pure and Mixed Alkanethiols on Au(lll) by Scanning Force Microscopy... 

Understanding molecular-scale adhesion, friction, lubrication, and wear is crucial to modern technologies, such as microelectromechanical systems (MEMS) and hard disk drives.With atomic force/friction force microscopy (AFM/FFM),several studies have shown the correlation of frictional properties of self-assembled monolayers (SAMs) of alkanethiols with their chain lengths and terminal groups. The long chain monolayers (more than 12 carbon units) have lower friction coefficients compared with their short chain counterparts. Recently, Kim et al investigated the effect of the size of chain termination on frictional properties and found that the difference in friction arises predominantly from the difference in the size of the terminal groups. The AFM/FFM has also been applied to study frictional... 

The combined added-mass and two-slope approach is further used in this work to obtain quantitative frictional forces in mixed SAMs in nano-scale domains. There are a number of important questions to be answered. First, how does the chemical nature (hydrophobic or hydrophilic) of terminal groups of alkanethiol monolayers affect friction Second, is there any relationship between surface composition and frictional properties Third, how does the relative humidity (RH) affect frictional properties In order to address these issues, a comparative study of frictional properties of dodecanethiol, 11-mercapto-l-undecanol, and their mixtures (3 1, 1 1, 1 3) on Au (111) is carried out. It is expected that relative humidity will affect frictional properties, particularly for hydrophilic surfaces. Thus, all our experiments were carried out in a homebuilt environment-controlled chamber. 

Friction and wear properties of alkanethiols on Au and alkylsilane monolayers on both Si and mica using AFM have been examined by a number of groups (1, 3, 6, 9). Short-chain alkanethiols with less than 8 carbon atoms, are less densely packed than the longer chains and thus are likely to have more defects. For both types of monolayers, these studies have shown that the friction decreases as the length of the alkyl portion of the chain increases. Adhesion between the AFM tip and the monolayer decreased as the chain length increased (1), presumably due to the decreased contact area that arises from the decreased compressibility of the films. 

Houston IE Kim HI, Adhesion friction, and mechanical properties of functionalized alkanethiol self-assembled monolayers, Acc. Chem. Res., 2002, 35, 547-553. 

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