9 research outputs found

    Molecular Dynamics Study of the Diffusivity of a Hydrophobic Drug Cucurbitacin B in Pseudo-poly(ethylene oxide‑<i>b</i>‑caprolactone) Micelle Environments

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    Isobaric–isothermal molecular dynamics simulation was used to study the diffusion of a hydrophobic drug Cucurbitacin B (CuB) in pseudomicelle environments consisting of poly­(ethylene oxide-<i>b</i>-caprolactone) (PEO-<i>b</i>-PCL) swollen by various amounts of water. Two PEO-<i>b</i>-PCL configurations, linear and branched, with the same total molecular weight were used. For the branched configuration, the block copolymer contained one linear block of PEO with the same molecular weight as that of the PEO block used in the linear configuration but with one end connecting to three PCL blocks with the same chain length, hereafter denoted PEO-<i>b</i>-3PCL. Regardless of the configuration, the simulation results showed that the diffusivity of CuB was insensitive to the water concentration up to ∼8 wt % while that of water decreased with an increasing water concentration. The diffusivity of CuB (10<sup>–8</sup> cm<sup>2</sup>/s) was 3 orders of magnitude lower than that of water (10<sup>–5</sup> cm<sup>2</sup>/s). This is attributed to the fact that CuB relied on the wiggling motion of the block copolymers to diffuse while water molecules diffused via a hopping mechanism. The rates at which CuB and water diffused into PEO-<i>b</i>-PCL were twice those in PEO-<i>b</i>-3PCL because the chain mobility and the degree of swelling are higher and there are fewer intermolecular hydrogen bonds in the case of PEO-<i>b</i>-PCL. The velocity autocorrelation functions of CuB show that the free volume holes formed by PEO-<i>b</i>-3PCL are more rigid than those formed by PEO-<i>b</i>-PCL, making CuB exhibit higher-frequency collision motion in PEO-<i>b</i>-3PCL than in PEO-<i>b</i>-PCL, and the difference in frequency is insensitive to water concentration

    Prediction of the Active Layer Nanomorphology in Polymer Solar Cells Using Molecular Dynamics Simulation

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    Active layer nanomorphology is a major factor that determines the efficiency of bulk heterojunction polymer solar cells (PSCs). Synthesizing diblock copolymers in which acceptor and donor materials are the constituent blocks is the most recent method to control the structure of the active layer. In the current work, a computational method is proposed to predict the nanomorphology of the active layer consisting of a diblock copolymer. Diblock copolymers have a tendency to self-organize and form well-defined nanostructures. The shape of the structure depends on the Flory–Huggins interaction parameter (i.e., χ), the total degree of polymerization (<i>N</i>) and volume fractions of the constituent blocks (φ<sub>i</sub>). In this work, molecular dynamics (MD) simulations were used to calculate χ parameters for two different block copolymers used in PSCs: P3HT-<i>b</i>-poly­(S<sub>8</sub>A<sub>2</sub>)-C<sub>60</sub> and P3HT-<i>b</i>-poly­(n-butyl acrylate-<i>stat</i>-acrylate perylene) also known as P3HT-<i>b</i>-PPerAcr. Such calculations indicated strong segregation of blocks into cylindrical structure for P3HT-<i>b</i>-poly­(S<sub>8</sub>A<sub>2</sub>)-C<sub>60</sub> and intermediate segregation into cylindrical structure for P3HT-<i>b</i>-PPerAcr. Experimental results of P3HT-<i>b</i>-poly­(S<sub>8</sub>A<sub>2</sub>)-C<sub>60</sub> and P3HT-<i>b</i>-PTP4AP, a diblock copolymer having very similar structure to P3HT-<i>b</i>-PPerAcr, validate our predictions

    Effect of Inorganic Salt Contaminants on the Dissolution of Kaolinite Basal Surfaces in Alkali Media: A Molecular Dynamics Study

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    Owing to the increasing popularity of using waste disposal as a source material, inorganic salts such as CaCl<sub>2</sub> and MgCl<sub>2</sub> are frequently present in geopolymerization typically taking place in alkali media. Such contaminants influence the dissolution of the aluminosilicate source materials and consequently the properties of the geopolymers made. This work is particularly aimed at elucidating the dissolution mechanism of a well-known clay, namely, kaolinite, in alkali media with the presence of two aforementioned aqueous medium contaminants using molecular dynamics (MD) simulation. A series of MD simulations was carried out on model kaolinite, with its tetrahedral and deprotonated octahedral surfaces exposed to the alkali solutions containing neat Na<sup>+</sup> or neat K<sup>+</sup> cations at two concentrations, 3 and 5 M. Different concentrations of CaCl<sub>2</sub> and MgCl<sub>2</sub> contaminants (i.e., 0.1, 0.3, and 0.5 M) were added to such alkali solutions. Atomic density profiles show that all cations, including those from the contaminants adsorbed on the two basal surfaces, intensify the dissociation of the aluminate groups from the deprotonated octahedral surface. The dissociation mechanism is somewhat similar to that of the alkali media without contaminants, in which cations weaken the interaction between the aluminum and bridging oxygen atoms. The number of the aluminate groups dissociated decreased with increasing contaminant concentration. In fact, at the highest contaminant concentration used (i.e., 0.5 M), the number of dissociated aluminate groups was even lower than that of the system without contaminants. This observation was attributed to the fact that most of chloride anions remained in the bulk solution at 0.1 M but an increasing amount of chloride ions started to cluster around the cations at 0.5 M, thereby screening the interaction between cations and the deprotonated octahedral surface. As a result, the screening effect reduced the number of aluminate groups dissociated from the surface. Structural analyses of the deprotonated octahedral surface indicated that the crystallinity of the surface decreased with increasing simulation time and alkali solution concentration. No dissolution of the tetrahedral surface was observed for all systems studied

    Molecular Dynamics Study of the Role of Water in the Carbon Dioxide Intercalation in Chloride Ions Bearing Hydrotalcite

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    Molecular dynamics simulation was used to study the role of water in the intercalation of CO<sub>2</sub> with a model Mg–Al–Cl-hydrotalcite mineral at ambient pressure and temperature. The ClayFF force field was used along with a model Mg–Al–Cl-hydrotalcite containing different amounts of water (H<sub>2</sub>O) and carbon dioxide (CO<sub>2</sub>) molecules in its interlayer spacing. It was observed that high CO<sub>2</sub> content, say 3.85 mmol g<sup>–1</sup>, could be achieved at low water concentrations or even without the presence of water. However, high water concentrations (e.g., 2 H<sub>2</sub>O molecules/hydrotalcite unit cell, the maximum allowed water concentration observed experimentally) could also yield similar CO<sub>2</sub> content, but in this case, the presence of water led to a significant interlayer spacing expansion (from 23.0 Å (no water) to 28.5 Å). The expansion was likely due to the change in the orientation distribution of the CO<sub>2</sub> molecules. Analyzing the orientation of CO<sub>2</sub> molecules revealed that they preferred to orientate parallel to the mineral surface at low water concentrations. However, as water concentration increased, CO<sub>2</sub> molecules exhibited a wider range of orientations with a significant fraction of them orienting more or less perpendicular to the mineral surface, especially at high CO<sub>2</sub> contents. The observed change in the orientation of CO<sub>2</sub> was attributed to the dipole interaction between H<sub>2</sub>O and CO<sub>2</sub> molecules and the reduced interaction between CO<sub>2</sub> and the hydroxyl groups on hydrotalcite. Also, it was observed that water molecules formed extensive hydrogen bond networks. All of the above findings seem to explain the contradicting results reported in the literature that water is needed under certain conditions to increase the amount of CO<sub>2</sub> captured by hydrotalcites. Here, we showed that high amounts of CO<sub>2</sub> can be intercalated with the presence of water

    Underwater Adhesion of a Stimuli-Responsive Polymer on Highly Oriented Pyrolytic Graphite: A Single-Molecule Force Study

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    Exploration aiming to understand how a single polymer chain interacts with interested solid–water interface and its dependence on the environmental stimulus is critical, as it is important for a variety of scientific research and practical applications, such as underwater adhesives or smart systems for controlled attachment/detachment. Here, by single molecule force spectroscopy (SMFS), results on underwater adhesion of a single stimuli-responsive polymer chain on a model hydrophobic surface are reported. Salt concentration was used as an effective stimulus for the transition of the stimuli-responsive polymer from hydrophilic (solvophilic) hydrated state to hydrophobic (solvophobic) state in SMFS experiment. The probed equilibrium single-molecule adhesion force that changed from 45 to 76 pN with increasing salt concentration were free of other interactions that are responsible for cohesions, indicating that solvent quality is critical in determining the strength of single-molecule interfacial adhesion. Moreover, the derived simple quantitative thermodynamic model by combining single-molecule detachment mechanics with hydration free energy illustrated that higher single-molecule adhesion force was resulted from higher solvation/hydration free energy. The experimental study and theoretical analysis presented in this work quantitatively revealed the role of hydrophobic and more general solvophobic interactions in single-molecule level and shed light on the molecular structure optimization for desired applications, such as underwater adhesives or smart interfaces

    Probing Single-Molecule Adhesion of a Stimuli Responsive Oligo(ethylene glycol) Methacrylate Copolymer on a Molecularly Smooth Hydrophobic MoS<sub>2</sub> Basal Plane Surface

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    Molybdenum disulfide (MoS<sub>2</sub>) has been receiving increasing attention in scientific research due to its unique properties. Up to now, several techniques have been developed to prepare exfoliated nanosize MoS<sub>2</sub> dispersions to facilitate its applications. To improve its desired performance, as-prepared MoS<sub>2</sub> dispersion needs further appropriate modification by polymers. Thus, understanding polymer–MoS<sub>2</sub> interaction is of great scientific importance and practical interest. Here, we report our results on molecular interactions of a biocompatible stimuli-responsive copolymer with the basal plane surface of MoS<sub>2</sub> determined using single molecule force spectroscopy (SMFS). Under isothermal conditions, the single-molecule adhesion force of oligo­(ethylene glycol) methacrylate copolymer was found to increase from 50 to 75 pN with increasing NaCl concentration from 1 mM to 2 M, as a result of increasing hydrophobicity of the polymers. The theoretical analysis demonstrated that single-molecule adhesion force is determined by two contributions: the adhesion energy per monomer and the entropic free energy of the stretched polymer chain. Further data analysis revealed a significant increase in the adhesion energy per monomer with a negligible change in the other contribution with increasing salt concentration. The hydrophobic attraction (HA) was found to be the main contribution for the higher adhesion energy in electrolyte solutions of higher NaCl concentrations where the zero-frequency of van der Waals interaction were effectively screened. The results illustrate that oligo­(ethylene glycol) methacrylate copolymer is a promising polymer for functionalizing MoS<sub>2</sub> and that one can simply change the salt concentration to modulate the single-molecule interactions for desired applications

    Competitive Adsorption of Naphthenic Acids and Polyaromatic Molecules at a Toluene–Water Interface

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    The early-stage competitive co-adsorption of interfacially active naphthenic acids (NAs) and polyaromatic (PA) molecules to a toluene–water interface from the bulk toluene phase was studied using molecular dynamics (MD) simulation. The NA molecules studied had the same polar functional group but different cycloaliphatic nonpolar tails, and a perylene bisimide (PBI)-based molecule was used as a representative PA compound. The results from our simulations suggest that the size and structural features of NA molecules greatly influence the interfacial activity of PA molecules and partitioning of NA molecules at the toluene–water interface. At low concentrations of PA (∼2.3 wt %) and NA (∼0.4 wt %) molecules, NA molecules containing large cycloaliphatic rings (e.g., four rings) or with a very long aliphatic tail (e.g., carbon chain length of 14) were observed to impede the migration of PA molecules to the interface, whereas small NA molecules containing two cycloaliphatic rings had little effect on the adsorption of PA molecules at the toluene–water interface. At high NA concentrations, the adsorption of PA molecules (∼5.75–17.25 wt %) was greatly hindered by the presence of small NA molecules (∼1.6–4.8 wt %) due to the solvation of PA nanoaggregates in the bulk. Adsorption mechanisms of PA and NA molecules at toluene–water interfaces were clarified through a detailed analysis on the interactions among different species in the system. The results obtained from this work provide insights into designing appropriate chemical demulsifiers or co-demulsifiers for breaking water-in-oil emulsions of great industrial applications

    Role of Naphthenic Acids in Controlling Self-Aggregation of a Polyaromatic Compound in Toluene

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    In this work, a series of molecular dynamics simulations were performed to investigate the effect of naphthenic acids (NAs) in early stage self-assembly of polyaromatic (PA) molecules in toluene. By exploiting NA molecules of the same polar functional group but different aliphatic/cycloaliphatic nonpolar tails, it was found that irrespective of the presence of the NA molecules in the system, the dominant mode of π–π stacking is a twisted, offset parallel stacking of a slightly larger overlapping area. Unlike large NA molecules, the presence of small NA molecules enhanced the number of π–π stacked PA molecules by suppressing the hydrogen bonding interactions among the PA molecules. Smaller NA molecules were found to have a higher tendency to associate with PA molecules than larger NA molecules. Moreover, the size and distribution of π–π stacking structures were affected to different degrees by changing the size and structural features of the NA molecules in the system. It was further revealed that the association between NA and PA molecules, mainly through hydrogen bonding, creates a favorable local environment for the overlap of PA cores (i.e., π–π stacking growth) by depressing the hydrogen bonding between PA molecules, which results in the removal of some toluene molecules from the vicinity of the PA molecules

    Single-Molecule MoS<sub>2</sub>–Polymer Interaction and Efficient Aqueous Exfoliation of MoS<sub>2</sub> into Single Layer

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    Single molecule force spectroscopy (SMFS) was utilized to study single-molecule interactions between synthesized polymers and MoS<sub>2</sub> surfaces that provide the guidance to determine candidate polymers for efficient aqueous exfoliation of bulk MoS<sub>2</sub> into single-layer nanosheets. The technique allowed the direct quantification of the adhesion force of single polymer molecules with both basal and edge surfaces of MoS<sub>2</sub>. Compared with two studied neutral polymers, highly water-soluble cationic poly­(vinylbenzyl trimethyl ammonium chloride) (PVBTA) was shown to be more promising for MoS<sub>2</sub> exfoliation as a result of strong single-molecule interactions with both basal and edge surfaces of MoS<sub>2</sub>. In 1 mM NaCl solution of pH around 5.5, the measured single-molecule adhesion forces on basal and edge surfaces were around 59 and 51 pN, respectively. Such strong adhesion force led to high performance of PVBTA in exfoliating MoS<sub>2</sub> bulk material into single-layer sheets. Compared with reported approaches in the literatures, an order of magnitude higher concentration of single-layer MoS<sub>2</sub> sheets in water was prepared using an order of magnitude less treatment time in this study. This research demonstrated that SMFS is a powerful tool to select appropriate surface-active polymers for effective exfoliation of MoS<sub>2</sub>, which could be applied to a variety of practical applications, such as water adhesion  and dispersing
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