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_2D. 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_2D, 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^–, HCO₃^–, CO₃^2–, Cl^–, and F^–) 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 CO₂ on ionic conductivity in ammonium hydroxide solutions. CO₂ readily reacts with OH^– to form HCO^–₃ and is found to lower the solution ionic conductivity by almost 50%

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