Direct Observation of the Dynamics of Self-Assembly of Individual Solvation Layers in Molecularly Confined Liquids

Confined liquids organize in solidlike layers at the liquid-substrate interface. Here we use force-clamp spectroscopy AFM to capture the equilibrium dynamics between the broken and reformed states of an individual solvation layer in real time. Kinetic measurements demonstrate that the rupture of each individual solvation layer in structured liquids is driven by the rupture of a single interaction for 1-undecanol and by two interactions in the case of the ionic liquid ethylammonium nitrate. Our results provide a first description of the energy landscape governing the molecular motions that drive the packing and self-assembly of each individual liquid layer.

Figure 1

Solvation layers in 1-undecanol revealed by constant velocity AFM. (a) Force vs distance plot of the AFM tip approaching towards (red line) and, subsequently, retracting from (blue line) the HOPG surface in 1-undecanol at constant velocity, showing the rupture and reversible reformation of a total of 5 confined layers. (b) Rupturing and reforming forces ($n=90$ individual trajectories) for the first four measurable solvation layers.

Figure 2

The equilibrium dynamics of an individual solvation layer in 1-undecanol captured by force-clamp AFM. (a) Applying a constant force of 430 pN samples the rupture and reformation dynamics of layer 3. The equilibrium is readily displaced upon changing the applied force. (b) Plot of the abundance of the reformed state as a function of the applied force. (c) The dwell times of rupture and reformation [(a), inset] depend exponentially on the applied force (red and blue dots, respectively). The error has been calculated using the bootstrap method [17]. (d) Repeating the process for each individual layer (Figs. S6–S8) allows the full 1D reconstruction of the energy landscape of confined 1-undecanol.

Figure 3

The solvation dynamics of EAN. (a) Force extension trajectories showing the rupture and reformation of 3 confined EAN solvation layers. (b) The system hops between layers 1 and 2 as the applied constant force varies from 70–40 pN. (c) Relative abundances of the reformed (green) and ruptured state (red) of layer 1 and the ruptured state of layer 2 as a function of the applied force. Global fit of Eq. (2) yields $\mathrm{\Delta }l=4.3\pm 0.4\AA$ and $\mathrm{\Delta }G=(5.9\pm 0.6)kT$ (layer 2, red line); $\mathrm{\Delta }l=6.2\pm 0.1\AA$ and $\mathrm{\Delta }G=(4.5\pm 1)kT$ (layer 3, gray line). (d) The cumulative density function reconstructed from the transition dwell times is fitted, as a first approximation, to an exponential function to obtain the rates of rupture (red line, $\tau =29\pm 4\mathrm{ms}$) and reformation (blue line, $\tau =500\pm 100\mathrm{ms}$) for $F=100\mathrm{pN}$. (e) Fitting of Eq. (4) yields $\mathrm{\Delta }G=(4.5\pm 1)kT$ and $N=2.0\pm 0.8$ (EAN, red solid line). For comparison, the same equation was fitted to 1-undecanol results (gray solid line) yielding $\mathrm{\Delta }G=(31\pm 2)kT$ and $N=0.9\pm 0.1$.

Figure 4

Schematics of the rupturing process for (a) 1-undecanol ($N=1$) and (b) EAN ($N=2$).