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.
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.
The surface dynamics of individual
surfactant and polymer molecules
on thermally responsive polymer brushes (poly(N-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.
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