Squeeze-Out of Branched Alkanes on Graphite

We study squalane and heptamethylnonane (HMN) confined between a conducting atomic force microscope tip and a graphite surface. Solvation layering occurs for both liquids but marked differences in the squeeze out mechanics are observed for ordered or disordered monolayers. The squalane monolayer at $25°\mathrm{C}$ is an ordered solid, as verified by direct imaging, and the squeeze out can be modeled using elastic continuum mechanics. HMN is in a disordered state at $25°\mathrm{C}$ and cannot be modeled as a single elastic asperity even in solid-solid contact because HMN liquid is trapped in the contact zone.

Figure 1

Data showing force as a function of the tip-sample separation for squalane and HMN on HOPG. Clear solvation jumps are observed in squalane indicated by $n=0–5$ ($n=0$ is the graphite surface). HMN shows less defined jumps indicated by $n=0–3$ ($n=0$ is the graphite surface) with several kinks in the force curve (shown with unlabelled arrows).

Figure 2

Current vs force curve for the tip in contact with the first squalane layer ($n=1$). There are distinct “slow varying current” and “fast varying current” regions (the latter being close to the $n=1\to 0$ layer transition at 8–10 nN force). The inset shows data, taken with the same tip, as the tip is pulled off the first layer. The variation of current with force follows a DMT model.

Figure 3

(a) STM topographic image of the squalane monolayer on HOPG showing ordered domains. $\mathrm{\text{Image size}}=60\mathrm{nm}\times 60\mathrm{nm}$. Tunnel conditions: $V=600\mathrm{mV}$ (sample positive), $i_t=10\mathrm{pA}$. (b) Molecular resolution STM topographic image revealing individual squalane molecules. $\mathrm{\text{Image size}}=13\mathrm{nm}\times 13\mathrm{nm}$. Tunnel conditions: $V=1.0\mathrm{V}$ (sample positive), $i_t=12\mathrm{pA}$.

Figure 4

Current vs force curve for HMN at low force. The tip is within the monolayer ($n=1$) over most of this force region (there is uncertainty at the higher forces on whether the tip is within $n=1$ or $n=0$). The current variation with force is highly variable.

Figure 5

Current vs force curve at high force. The arrows indicate the direction of the force curve cycle. The tip-sample approach speed is $10\mathrm{nm}/\mathrm{s}$. (a) Data for HMN at $25°\mathrm{C}$. The curve does not show a sharp squeeze out of the monolayer ($n=1\to 0$ transition), although consideration of the current magnitude indicates that some part of the tip apex is in contact with HOPG at forces $\ge 10\mathrm{nN}$. (b) Data for squalane at $65°\mathrm{C}$. The curve does not show a sharp squeeze out of the monolayer ($n=1\to 0$ transition) and cannot be fitted to a continuum elastic model. (c) Data for squalane at $25°\mathrm{C}$. The monolayer is squeezed out suddenly ($n=1\to 0$ transition) and the tip contacts the HOPG when the force reaches $\sim 12.5\mathrm{nN}$ (squares). The variation in current while pulling the tip off the HOPG surface (circles) is fitted with the DMT model (solid curve) to give the contact area.