Lateral Force Microscopy Study on the Shear Properties of Self-Assembled Monolayers of Dialkylammonium Surfactant on Mica

Abstract

Frictional forces between self-assembled surfactant monolayers on mica and a silicon tip were measured under dynamic shear, using a lateral force microscope. Molecular friction was resolved on the surfactantcoated surfaces. Dependence of friction upon load and shear velocity was also studied. The results are compared with those for bare substrate mica under similar conditions. We found that at a fixed shear velocity, friction increases with applied load in a nearly linear fashion for both the substrate mica and the monolayer-coated surfaces. Within the velocity range of 0.01 to 120 pra/s, friction increases monotonically with shear velocity for bare mica; however, for the monolayer surfaces, friction initially increases with velocity and then decreases. Similar behavior was also found for monolayers in hexane-saturated atmosphere, but with the maximum friction occurring at a higher velocity than that in dry air.

The effect of chain density on the frictional behavior of surfaces modified with alkylsiloxanes and immersed in n-alcohols

“…These maxima shifted to lower sliding velocity with increasing n and increasing applied normal load. These maxima were similar to maxima in the friction force reported under dry conditions for surface forces apparatus 14 and LFM 13,15 studies of monolayers of amorphous dialkylammonium surfactants, as well as for LFM studies of LangmuirBlodgett films of stearic acid in the liquid expanded state 16 and monolayers of unsymmetrical n-dialkyl sulfides on gold. 17 The behavior of the maxima that we observed for SAMs of OTS in n-alcohols was consistent with a model of the energy dissipation in the film based on the characteristic relaxation time of the alkyl chains that is analogous to viscoelastic behavior in bulk polymer systems.…”

“…We used the method of calibration presented by Noy and co-workers to estimate the lateral spring constant, k lat . 30 S lat was obtained from friction traces over very small scan sizes, as described by Liu and co-workers, 15 where the torsion of the cantilever is a linear function of the lateral movement of the tip for small twisting angles. The values of the friction force reported in this paper are averages of five separate data points for each specific sliding velocity and applied normal load.…”

Molecular Friction as a Tool to Identify Functionalized Alkanethiols

“…9 In addition, the nature of the supernatant solution, or the absence of it (i.e., ambient or vacuum), can influence the surface tension between end groups in a SAM and therefore also the friction. 17, 31 Friction mapping can thus provide valuable information about the composition and specific chemical [32][33][34][35][36][37] and biological 26,38 properties of a surface layer at the nanoscale.…”

Direct Observation of Amyloid Nucleation under Nanomechanical Stretching

“…[32] Earlier work in 19's focused on trying to explain the friction of rubbers on smooth surface. Modern work extended the model to other soft matter such as surfactant [33], alkanothiol monolayer [34], and protein films [35] [36]. In the theory, two mechanisms are incorporated.…”