5 research outputs found
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Coexistence of Multilayered Phases of Confined Water: The Importance of Flexible Confining Surfaces
Flexible nanoscale
confinement is critical to understanding the
role that bending fluctuations play on biological processes where
soft interfaces are ubiquitous or to exploit confinement effects in
engineered systems where inherently flexible 2D materials are pervasively
employed. Here, using molecular dynamics simulations, we compare the
phase behavior of water confined between flexible and rigid graphene
sheets as a function of the in-plane density, ρ<sub>2D</sub>. We find that both cases show commensurate mono-, bi-, and trilayered
states; however, the water phase in those states and the transitions
between them are qualitatively different for the rigid and flexible
cases. The rigid systems exhibit discontinuous transitions between
an (<i>n</i>)-layer and an (<i>n</i>+1)-layer
state at particular values of ρ<sub>2D</sub>, whereas under
flexible confinement, the graphene sheets bend to accommodate an (<i>n</i>)-layer and an (<i>n</i>+1)-layer state coexisting
in equilibrium at the same density. We show that the flexible walls
introduce a very different sequence of ice phases and their phase
coexistence with vapor and liquid phases than that observed with rigid
walls. We discuss the applicability of these results to real experimental
systems to shed light on the role of flexible confinement and its
interplay with commensurability effects
Role of Hydrophilicity and Length of Diblock Arms for Determining Star Polymer Physical Properties
We
present a molecular simulation study of star polymers consisting
of 16 diblock copolymer arms bound to a small adamantane core by varying
both arm length and the outer hydrophilic block when attached to the
same hydrophobic block of poly-δ-valerolactone. Here we consider
two biocompatible star polymers in which the hydrophilic block is
composed of polyethylene glycol (PEG) or polymethyloxazoline (POXA)
in addition to a polycarbonate-based polymer with a pendant hydrophilic
group (PC1). We find that the different hydrophilic blocks of the
star polymers show qualitatively different trends in their interactions
with aqueous solvent, orientational time correlation functions, and
orientational correlation between pairs of monomers of their polymeric
arms in solution, in which we find that the PEG polymers are more
thermosensitive compared with the POXA and PC1 star polymers over
the physiological temperature range we have investigated
Insights into the Transport of Aqueous Quaternary Ammonium Cations: A Combined Experimental and Computational Study
This study focuses on understanding
the relative effects of ammonium
substituent groups (we primarily consider tetramethylammonium, benzyltrimethylammonium,
and tetraethylammonium cations) and anion species (OH<sup>–</sup>, HCO<sub>3</sub><sup>–</sup>, CO<sub>3</sub><sup>2–</sup>, Cl<sup>–</sup>, and F<sup>–</sup>) on ion transport
by combining experimental and computational approaches. We characterize
transport experimentally using ionic conductivity and self-diffusion
coefficients measured from NMR. These experimental results are interpreted
using simulation methods to describe the transport of these cations
and anions considering the effects of the counterion. It is particularly
noteworthy that we directly probe cation and anion diffusion with
pulsed gradient stimulated echo NMR and molecular dynamics simulations,
corroborating these methods and providing a direct link between atomic-resolution
simulations and macroscale experiments. By pairing diffusion measurements
and simulations with residence times, we were able to understand the
interplay between short-time and long-time dynamics with ionic conductivity.
With experiment, we determined that solutions of benzyltrimethylammonium
hydroxide have the highest ionic conductivity (0.26 S/cm at 65 °C),
which appears to be due to differences for the ions in long-time diffusion
and short-time water caging. We also examined the effect of CO<sub>2</sub> on ionic conductivity in ammonium hydroxide solutions. CO<sub>2</sub> readily reacts with OH<sup>–</sup> to form HCO<sup>–</sup><sub>3</sub> and is found to lower the solution ionic
conductivity by almost 50%
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Effect of Hydrophobic Core Topology and Composition on the Structure and Kinetics of Star Polymers: A Molecular Dynamics Study
We
present a molecular dynamics study of the effect of core chemistry
on star polymer structural and kinetic properties. This work serves
to validate the choice of a model adamantane core used in previous
simulations to represent larger star polymeric systems in an aqueous
environment, as well as to explore how the choice of size and core
chemistry using a dendrimer or nanogel core may affect these polymeric
nanoparticle systems, particularly with respect to thermosensitivity
and solvation properties that are relevant for applications in drug
loading and delivery
Structural transition of nanogel star polymers with pH by controlling PEGMA interactions with acid or base copolymers
<p>We use small angle X-ray scattering (SAXS) to characterise a class of star diblock polymers with a nanogel core on which the outer block arms are comprised of random copolymers of temperature sensitive PEGMA with pH sensitive basic (PDMAEMA) and acidic (PMAA) monomers. The acquired SAXS data show that many of the nanogel star polymers undergo a sharp structural transition over a narrow range of pH, but with unexpectedly large shifts in the apparent pKa with respect to that of the acidic or basic monomer unit, the linear polymer form or even an alternate star polymer with a tightly cross-linked core chemistry. We have demonstrated a distinct and quantifiable structural response for the nanogel star copolymers by altering the core or by pairing the monomers PDMAEMA–PEGMA and PMAA–PEGMA to achieve structural transitions that have typically been observed in stars through changes in arm length and number.</p> <p></p