Dynamic Molecular Behavior on Thermoresponsive Polymer Brushes

The surface dynamics of individual surfactant and polymer molecules on thermally responsive polymer brushes (poly­(<i>N</i>-isopropylacrylamide), PNIPAAM) were studied using high throughput single molecule tracking microscopy. The probe molecules universally exhibited intermittent hopping motion, in which the diffusion switched between mobility and confinement with a broad distribution of waiting times; this was analyzed in the context of a continuous time random walk (CTRW) model described using “waiting time” and “flight length” distributions. We found that the surface mobility, which was affected by waiting times and flight lengths, of both probe molecules increased abruptly with temperature above the 32 °C lower critical solution temperature (LCST) transition of the PNIPAAM brush. In particular, above the LCST, where the polymer brush collapsed into a more hydrophobic dense polymer film, the effective diffusion coefficients and mobile fraction of probe molecule increased, suggesting that mobility was inhibited by penetration into the brush at lower temperatures. Waiting times at lower temperature were twice as long as at higher temperatures, and the longest flight length increased from 0.9 to 1.8 μm. Moreover, we found that the high density of strong binding sites available on the swollen PNIPAAM brush led to long waiting times and a high probability of readsorption, which resulted in short flight lengths, while the absence of strong binding sites on collapsed PNIPAAM films led to short waiting times and long flights

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

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 <i>D</i>_2D = 0.20 μm²/s, we found a good agreement between simulations and experimental data on both chemically homogeneous and nanopatterned surfaces

Prediction of Phase Equilibrium of Methane Hydrates in the Presence of Ionic Liquids

In this work, a predictive method is applied to determine the vapor–liquid-hydrate three-phase equilibrium condition of methane hydrate in the presence of ionic liquids and other additives. The Peng–Robinson–Stryjek–Vera Equation of State (PRSV EOS) incorporated with the COSMO-SAC activity coefficient model through the first order modified Huron–Vidal (MHV1) mixing rule is used to evaluate the fugacities of vapor and liquid phases. A modified van der Waals and Platteeuw model is applied to describe the hydrate phase. The absolute average relative deviation in predicted temperature (AARD-T) is 0.31% (165 data points, temperature ranging from 273.6 to 291.59 K, and pressure ranging from 1.01 to 20.77 MPa). The method is further used to screen for the most effective thermodynamic inhibitors from a total of 1722 ionic liquids and 574 electrolytes (combined from 56 cations and 41 anions). The valence number of ionic species is found to be the primary factor of inhibition capability, with the higher valence leading to stronger inhibition effects. The molecular volume of ionic liquid is of secondary importance, with the smaller size resulting in stronger inhibition effects