We report the experimental characterization and molecular dynamics simulations of the frictional properties of monolayer films derived from n-alkanethiols on gold and n-alkyltrichlorosilanes on silicon. The coefficient of friction, as determined by a nanotribometer, is strongly dependent on the surface group of the monolayer and the chain length of the adsorbate. For a homologous series of n-alkanethiols on gold, the coefficient of friction is constant at ~0.07 for carbon chain lengths (n) of 8 or greater but increases sharply for thinner monolayers. This critical chain length is reduced to 6 for monolayers derived from n-alkyltrichlorosilanes on silicon. This observation of a critical chain length is attributed to the low cohesional energy of these thin monolayers that allows the probe to interact with the underlying metal (oxide) surface. Molecular simulations enable visualization of the effect of the sliding probe on the structure of the alkyl chains in the monolayer films. Monolayers that terminate in hydroxyl or carboxylic acid groups exhibit higher coefficients of friction (~0.25) than those of CH3-terminated surfaces. Nonetheless, these higher energy surfaces enable the deposition of ionic liquids of 1-n-butyl-3-methylimidazolium triflate to form lubricating, mobile films that reduce frictional forces, as shown by both experiments and simulations, and improve interfacial stability against the sliding forces.