Single Molecule Observations of Desorption-Mediated Diffusion at the Solid-Liquid Interface

Abstract

By directly observing molecular trajectories on a chemically heterogeneous surface, we have identified two distinct modes of diffusion involving (1) displacements within isolated surface islands (crawling mode), and (2) displacements where a molecule desorbs from an island, diffuses through the adjacent liquid phase, and readsorbs on another island (flying mode). The diffusion coefficients corresponding to these two modes differ by an order of magnitude, and both modes are also observed on chemically homogeneous surfaces. Comparison with previous results suggested that desorption-mediated diffusion is the primary transport mechanism in self-assembled monolayer formation.

Diffusion mechanisms at the Pb solid–liquid interface: Atomic level point of view

“…That is, the molecules first desorb from the surface, and then undergo Fickian diffusion in the bulk liquid, and re-adsorb surface lastly [12]. This diffusion process has been verified in the experiments, such as the diffusion of polymers at heterogeneous S-L interfaces [13,14]. Recently, it has been shown that the movement of the solute molecules on a solid surface involves two diffusion mechanisms, the immobilization (oscillates around the equilibrium site) and the desorption-mediated jumps [14,18].…”

“…Close to S-L interfaces, however, the diffusion coefficient is not sufficient to describe the dynamic property of particles. Schwartz et al tracked the motion of molecules at S-L interfaces and they found that some molecular trajectories appeared immobilized while others were highly mobile [14,18]. We will discuss the dynamic properties at the Pb solid-melt interface in more detail below.…”

“…For a regular 2D random walk, the formula of this distribution can be expressed by C(r 2 , t) = exp(−r 2 /4Dt), where D is the diffusion coefficient. At the S-L interfaces [13,14], however, the distribution has been described by a dual-exponential function, which elucidates two independent diffusive modes. Figure 4 shows the C(r 2 , t) curve of the liquid atoms on the surface, where t is 0.9 ns.…”

“…This diffusion process has been verified in the experiments, such as the diffusion of polymers at heterogeneous S-L interfaces [13,14]. Recently, it has been shown that the movement of the solute molecules on a solid surface involves two diffusion mechanisms, the immobilization (oscillates around the equilibrium site) and the desorption-mediated jumps [14,18]. With the continuous time random walk model and two-dimensional periodic potential theory model, Schwartz and co-workers [18,19] reasonably elucidated the experimental phenomena and the diffusion mechanisms at the S-L interfaces, and the results show that two different kinds of diffusion modes result in the deviation of the self-part of van Hove function from Guassian [18].…”

“…However, the mechanism of the liquid molecules/atoms moving on solid surfaces remains mysterious and a matter of debate. Now there are lots of experiments and theories which have contributed to fundamental studies on the mobility of the molecules at heterogeneous S-L interfaces [7][8][9][10][11][12][13][14][15][16][17]. Because of the significant challenges inherent in measuring dynamic properties for homogeneous S-L interfaces, experimental data are much limited in this case.…”