Polymer Surface Transport Is a Combination of in-Plane Diffusion and Desorption-Mediated Flights

Abstract

Previous studies of polymer motion at solid/liquid interfaces described the transport in the context of a continuous time random walk (CTRW) process, in which diffusion switches between desorption-mediated “flights” (i.e., hopping) and surface-adsorbed waiting-time intervals. However, it has been unclear whether the waiting times represented periods of complete immobility or times during which molecules engaged in a different (e.g., slower or confined) mode of interfacial transport. Here we designed high-throughput, single-molecule tracking measurements to address this question. Specifically, we studied polymer dynamics on either chemically homogeneous or nanopatterned surfaces (hexagonal diblock copolymer films) with chemically distinct domains, where polymers were essentially excluded from the low-affinity domains, eliminating the possibility of significant continuous diffusion in the absence of desorption-mediated flights. Indeed, the step-size distributions on homogeneous surfaces exhibited an additional diffusive mode that was missing on the chemically heterogeneous nanopatterned surfaces, confirming the presence of a slow continuous mode due to 2D in-plane diffusion. Kinetic Monte Carlo simulations were performed to test this model and, with the theoretical in-plane diffusion coefficient of D 2D = 0.20 μm2/s, we found a good agreement between simulations and experimental data on both chemically homogeneous and nanopatterned surfaces.

Three-Dimensional Tracking of Interfacial Hopping Diffusion

“…the distributions were broader) on surfaces corresponding to a stronger adsorbate-surface interaction. Previous work has suggested that surface diffusion was influenced by multiple effects, e.g., adsorbate-surface interactions (desorption rate), re-adsorption probability, physical obstacles, chemical surface heterogeneity, etc [24,[39][40][41][42][43]. The results presented here provide a comprehensive picture of how and why adsorbate-surface interactions influence interfacial motion.…”

“…While interfacial diffusion is nominally two-dimensional (2D) and conventionally described in terms of 2D Brownian motion, longstanding theoretical models [7][8][9][10][11][12][13][14][15][16] have predicted that interfacial mass transport could actually be dominated by "flights" through an adjacent liquid phase, which would dramatically alter the nature of interfacial molecular motion; an understanding of this process is necessary in order to rationally control mass transport at surfaces. Recent experimental results indirectly support these predictions, by measuring the 2D projection of trajectories for atoms, molecules, polymers, and nanoparticles, in thin films, at solid/liquid interface, and on lipid bilayers, which can be represented as an intermittent process, with periods of apparent immobility alternating with long "flights" comprising a heavy-tailed distribution [17][18][19][20][21][22][23][24][25][26]. However, the evidence for the presence of three-dimensional (3D) hops remains indirect, and critical aspects of the proposed "hopping" process remain a mystery.…”

3D Imaging of Hopping Molecules

“…[24,[39][40][41][42][43]. The results presented here provide a comprehensive picture of how and why adsorbate-surface interactions influence interfacial motion.…”

“…While interfacial diffusion is nominally two dimensional (2D) and conventionally described in terms of 2D Brownian motion, longstanding theoretical models [7][8][9][10][11][12][13][14][15][16] have predicted that interfacial mass transport could actually be dominated by "flights" through an adjacent liquid phase, which would dramatically alter the nature of interfacial molecular motion; an understanding of this process is necessary in order to rationally control mass transport at surfaces. Recent experimental results indirectly support these predictions by measuring the 2D projection of trajectories for atoms, molecules, polymers, and nanoparticles, in thin films, at solid-liquid interface, and on lipid bilayers, which can be represented as an intermittent process with periods of apparent immobility alternating with long flights comprising a heavy-tailed distribution [17][18][19][20][21][22][23][24][25][26]. However, the evidence for the presence of three-dimensional (3D) hops remains indirect, and critical aspects of the proposed "hopping" process remain a mystery.…”

Estimating and leveraging protein diffusion on ion-exchange resin surfaces

“…Surface diffusion of small molecules (10)(11)(12) and polymers (13)(14)(15)(16), unlike that of proteins, has been widely reported in liquid-solid systems. In reverse-phase liquid chromatography, hydrocarbons have been reported to diffuse at the interfacial region near the end of the alkyl chains of the stationary phase (17)(18)(19)(20)(21).…”