Combining Force Fields and Neural Networks for an Accurate Representation of Bonded Interactions.

We present a formalism of a neural network encoding bonded interactions in molecules. This intramolecular encoding is consistent with the models of intermolecular interactions previously designed by this group. Variants of the encoding fed into a corresponding neural network may be used to economically improve the representation of torsional degrees of freedom in any force field. We test the accuracy of the reproduction of the ab initio potential energy surface on a set of conformations of two dipeptides, methyl-capped ALA and ASP, in several scenarios. The encoding, either alone or in conjunction with an analytical potential, improves agreement with ab initio energies that are on par with those of other neural network-based potentials. Using the encoding and neural nets in tandem with an analytical model places the agreements firmly within "chemical accuracy" of ±0.5 kcal/mol.

Combining Force Fields and Neural Networks for an Accurate Representation of Chemically Diverse Molecular Interactions.

A key goal of molecular modeling is the accurate reproduction of the true quantum mechanical potential energy of arbitrary molecular ensembles with a tractable classical approximation. The challenges are that analytical expressions found in general purpose force fields struggle to faithfully represent the intermolecular quantum potential energy surface at close distances and in strong interaction regimes; that the more accurate neural network approximations do not capture crucial physics concepts, e.g., nonadditive inductive contributions and application of electric fields; and that the ultra-accurate narrowly targeted models have difficulty generalizing to the entire chemical space. We therefore designed a hybrid wide-coverage intermolecular interaction model consisting of an analytically polarizable force field combined with a short-range neural network correction for the total intermolecular interaction energy. Here, we describe the methodology and apply the model to accurately determine the properties of water, the free energy of solvation of neutral and charged molecules, and the binding free energy of ligands to proteins. The correction is subtyped for distinct chemical species to match the underlying force field, to segment and reduce the amount of quantum training data, and to increase accuracy and computational speed. For the systems considered, the hybrid ab initio parametrized Hamiltonian reproduces the two-body dimer quantum mechanics (QM) energies to within 0.03 kcal/mol and the nonadditive many-molecule contributions to within 2%. Simulations of molecular systems using this interaction model run at speeds of several nanoseconds per day.

Carbon-based tribofilms from lubricating oils.

Moving mechanical interfaces are commonly lubricated and separated by a combination of fluid films and solid 'tribofilms', which together ensure easy slippage and long wear life. The efficacy of the fluid film is governed by the viscosity of the base oil in the lubricant; the efficacy of the solid tribofilm, which is produced as a result of sliding contact between moving parts, relies upon the effectiveness of the lubricant's anti-wear additive (typically zinc dialkyldithiophosphate). Minimizing friction and wear continues to be a challenge, and recent efforts have focused on enhancing the anti-friction and anti-wear properties of lubricants by incorporating inorganic nanoparticles and ionic liquids. Here, we describe the in operando formation of carbon-based tribofilms via dissociative extraction from base-oil molecules on catalytically active, sliding nanometre-scale crystalline surfaces, enabling base oils to provide not only the fluid but also the solid tribofilm. We study nanocrystalline catalytic coatings composed of nitrides of either molybdenum or vanadium, containing either copper or nickel catalysts, respectively. Structurally, the resulting tribofilms are similar to diamond-like carbon. Ball-on-disk tests at contact pressures of 1.3 gigapascals reveal that these tribofilms nearly eliminate wear, and provide lower friction than tribofilms formed with zinc dialkyldithiophosphate. Reactive and ab initio molecular-dynamics simulations show that the catalytic action of the coatings facilitates dehydrogenation of linear olefins in the lubricating oil and random scission of their carbon-carbon backbones; the products recombine to nucleate and grow a compact, amorphous lubricating tribofilm.

On the importance of accounting for nuclear quantum effects in ab initio calibrated force fields in biological simulations.

In many important processes in chemistry, physics, and biology the nuclear degrees of freedom cannot be described using the laws of classical mechanics. At the same time, the vast majority of molecular simulations that employ wide-coverage force fields treat atomic motion classically. In light of the increasing desire for and accelerated development of quantum mechanics (QM)-parameterized interaction models, we reexamine whether the classical treatment is sufficient for a simple but crucial chemical species: alkanes. We show that when using an interaction model or force field in excellent agreement with the "gold standard" QM data, even very basic simulated properties of liquid alkanes, such as densities and heats of vaporization, deviate significantly from experimental values. Inclusion of nuclear quantum effects via techniques that treat nuclear degrees of freedom using the laws of classical mechanics brings the simulated properties much closer to reality.

Editorial Commentary: Fluoroscopy Is Seldom Required During Knee Posterolateral Reconstruction.

Identifying the structures of the lateral knee is critical during knee posterolateral corner reconstruction. Several methods exist that can help estimate the femoral insertions of these structures on lateral radiographs. However, it is important to recognize the limitations of these methods and that anatomic visualization is often more practical and more accurate. Until percutaneous or more minimally invasive techniques become standardized, intraoperative fluoroscopy is seldom needed or used for posterolateral corner reconstruction, whereas radiographic analysis of lateral knee structures could be of benefit in cases of failed reconstruction to assess tunnel placement.

Trends in open shoulder surgery among early career orthopedic surgeons: who is doing what?

BACKGROUND: The incidence of various open shoulder procedures has changed over time. In addition, various fellowships provide overlapping training in open shoulder surgery. There is a lack of information regarding the relationship between surgeon training and open shoulder procedure type and incidence in early career orthopedic surgeons. METHODS: The American Board of Orthopaedic Surgery Part-II database was queried from 2002 to 2016 for reported open shoulder procedures. The procedures were categorized as follows: arthroplasty, revision arthroplasty, open instability, trauma, and open rotator cuff. We evaluated procedure trends as well as their relationship to surgeon fellowship categorized by Sports, Shoulder/Elbow, Hand, Trauma, and "Other" fellowship as well as no fellowship training. We additionally evaluated complication data as it related to procedure, fellowship category, and volume. RESULTS: Over the 2002-2016 study period, there were increasing cases of arthroplasty, revision arthroplasty, and trauma (P < .001). There were decreasing cases in open instability and open rotator cuff (P < .001). Those with Sports training reported the largest overall share of open shoulder cases. Those with Shoulder/Elbow training reported an increasing overall share of arthroplasty cases and higher per candidate case numbers. The percentage of early career orthopedic surgeons reporting 5 or more arthroplasty cases was highest among Shoulder/Elbow candidates (P < .001). Across all procedures, those without fellowship training were least likely to report a complication (odds ratio [OR], 0.76; 95% confidence interval, 0.67-0.86; P < .001). Shoulder/Elbow candidates were least likely to report an arthroplasty complication (OR, 0.84, P = .03) as was any surgeon reporting 5 or more arthroplasty cases (OR, 0.81; 95% confidence interval, 0.70-0.94; P = .006). CONCLUSION: The type and incidence of open shoulder surgery procedures continues to change. Among early career surgeons, those with more specific shoulder training are now performing the majority of arthroplasty-related procedures, and early career volume inversely correlates with complications.

Modulating Enzyme Activity by Altering Protein Dynamics with Solvent.

Optimal enzyme activity depends on a number of factors, including structure and dynamics. The role of enzyme structure is well recognized; however, the linkage between protein dynamics and enzyme activity has given rise to a contentious debate. We have developed an approach that uses an aqueous mixture of organic solvent to control the functionally relevant enzyme dynamics (without changing the structure), which in turn modulates the enzyme activity. Using this approach, we predicted that the hydride transfer reaction catalyzed by the enzyme dihydrofolate reductase (DHFR) from Escherichia coli in aqueous mixtures of isopropanol (IPA) with water will decrease by ∼3 fold at 20% (v/v) IPA concentration. Stopped-flow kinetic measurements find that the pH-independent khydride rate decreases by 2.2 fold. X-ray crystallographic enzyme structures show no noticeable differences, while computational studies indicate that the transition state and electrostatic effects were identical for water and mixed solvent conditions; quasi-elastic neutron scattering studies show that the dynamical enzyme motions are suppressed. Our approach provides a unique avenue to modulating enzyme activity through changes in enzyme dynamics. Further it provides vital insights that show the altered motions of DHFR cause significant changes in the enzyme's ability to access its functionally relevant conformational substates, explaining the decreased khydride rate. This approach has important implications for obtaining fundamental insights into the role of rate-limiting dynamics in catalysis and as well as for enzyme engineering.

Defect-Based Modulation of Optoelectronic Properties for Biofunctionalized Hexagonal Boron Nitride Nanosheets.

Defect engineering potentially allows for dramatic tuning of the optoelectronic properties of two-dimensional materials. With the help of DFT calculations, a systematic study of DNA nucleobases adsorbed on hexagonal boron-nitride nanoflakes (h-BNNFs) with boron (VB ) and nitrogen (VN ) monovacancies is presented. The presence of VN and VB defects increases the binding strength of nucleobases by 9 and 34 kcal mol-1 , respectively (h-BNNF-VB >h-BNNF-VN >h-BNNF). A more negative electrostatic potential at the VB site makes the h-BNNF-VB surface more reactive than that of h-BNNF-VN , enabling H-bonding interactions with nucleobases. This binding energy difference affects the recovery time-a significant factor for developing DNA biosensors-of the surfaces in the order h-BNNF-VB >h-BNNF-VN >h-BNNF. The presence of VB and VN defect sites increases the electrical conductivity of the h-BNNF surface, VN defects being more favorable than VB sites. The blueshift of absorption peaks of the h-BNNF-VB -nucleobase complexes, in contrast to the redshift observed for h-BNNF-VN -nucleobase complexes, is attributed to their observed differences in binding energies, the HOMO-LUMO energy gap and other optoelectronic properties. Time-dependent DFT calculations reveal that the monovacant boron-nitride-sheet-nucleobase composites absorb visible light in the range 300-800 nm, thus making them suitable for light-emitting devices and sensing nucleobases in the visible region.

The effect of defect types on the electronic and optical properties of graphene nanoflakes physisorbed by ionic liquids.

Defect engineering and non-covalent interaction strategies allow for dramatically tuning the optoelectronic properties of graphene. Using ab initio density functional theory (M06-2X/cc-pVDZ), we find that the nature of defects on the graphene nanoflakes (GNFs) and the size of defective GNF (DGNF) surfaces affect the binding energy (ΔEb) of ionic liquids (ILs) and the UV-Vis absorption spectra of DGNFIL complexes. Further, our results indicate that increasing the size of DGNFs affects the geometrical structure of the surfaces and increases the binding energy of ILs by about 10%. Analysis based on AIM and EDA shows that the interactions between ILs and DGNFs are non-covalent in nature (dispersion energy being dominant) and associated with charge transfer between the IL and nanoflakes. A comparison between the ΔEb values of ILs on DGNFs, GNFs, and h-BN nanoflakes (h-BNNF) shows that the presence of defects on the GNF surfaces increases the binding energy values as follows: DGNFIL > pristine GNFIL > h-BNNFIL. Our calculations indicate that increasing the size of DGNF surfaces leads to a decrease in the HOMO-LUMO energy gap (Eg) of the DGNF surfaces. Orbital energy and density of state calculations show that the Eg of DV(SW)-GNFs decreases upon IL adsorption and their Fermi energy level is shifted depending on the type of IL, thus enabling better conductivity. Reactivity descriptors generally indicate that the chemical potential (µ) and chemical hardness (η) of nanoflakes decrease upon IL adsorption, whereas the electrophilicity index (ω) increases. The UV-Vis absorption spectrum of DV-GNF and SW-GNF shows four bands in the visible spectrum which correspond to π → π* transitions with the absorption bands of SW-GNF appearing at higher wavelengths than those of DV-GNF. The most intense absorption bands in DV-GNF (λ = 348 nm) and SW-GNF (λ = 375 nm) are associated with electronic transitions HOMO-1 → LUMO+2 and HOMO → LUMO+1, respectively. In addition, these absorption bands undergo a red-shift by both increasing the size of the DV(SW)-GNF surfaces and IL adsorption. We also observe that the energy gaps and absorption spectra can be altered by varying the defect types and the type of IL adsorbate, where the defect types affect the spectral shapes of the bands and adsorbates at the first absorption peak, thus having potential application for light-emitting devices.

Subnanometre ligand-shell asymmetry leads to Janus-like nanoparticle membranes.

Self-assembly of nanoparticles at fluid interfaces has emerged as a simple yet efficient way to create two-dimensional membranes with tunable properties. In these membranes, inorganic nanoparticles are coated with a shell of organic ligands that interlock as spacers and provide tensile strength. Although curvature due to gradients in lipid-bilayer composition and protein scaffolding is a key feature of many biological membranes, creating gradients in nanoparticle membranes has been difficult. Here, we show by X-ray scattering that nanoparticle membranes formed at air/water interfaces exhibit a small but significant ∼6 Šdifference in average ligand-shell thickness between their two sides. This affects surface-enhanced Raman scattering and can be used to fold detached free-standing membranes into tubes by exposure to electron beams. Molecular dynamics simulations elucidate the roles of ligand coverage and mobility in producing and maintaining this asymmetry. Understanding this Janus-like membrane asymmetry opens up new avenues for designing nanoparticle superstructures.

Surface Charge-Transfer Doping of Graphene Nanoflakes Containing Double-Vacancy (5-8-5) and Stone-Wales (55-77) Defects through Molecular Adsorption.

The adsorption of six electron donor-acceptor (D/A) organic molecules on various sizes of graphene nanoflakes (GNFs) containing two common defects, double-vacancy (5-8-5) and Stone-Wales (55-77), are investigated by means of ab initio DFT [M06-2X(-D3)/cc-pVDZ]. Different D/A molecules adsorb on a defect graphene (DG) surface with binding energies (ΔEb ) of about -12 to -28 kcal mol-1 . The ΔEb values for adsorption of molecules on the Stone-Wales GNF surface are higher than those on the double vacancy GNF surface. Moreover, binding energies increase by about 10 % with an increase in surface size. The nature of cooperative weak interactions is analyzed based on quantum theory of atoms in molecules, noncovalent interactions plot, and natural bond order analyses, and the dominant interaction is compared for different molecules. Electron density population analysis is used to explain the n- and p-type character of defect graphene nanoflakes (DGNFs) and also the change in electronic properties and reactivity parameters of DGNFs upon adsorption of different molecules and with increasing DGNF size. Results indicate that the HOMO-LUMO energy gap (Eg ) of DGNFs decreases upon adsorption of molecules. However, by increasing the size of DGNFs, the Eg and chemical hardness of all complexes decrease and the electrophilicity index increases. Furthermore, the values of the chemical potential of acceptor-DGNF complexes decrease with increasing size, whereas those of donor-DGNF complexes increase.

Evaluating the Free Energies of Solvation and Electronic Structures of Lithium-Ion Battery Electrolytes.

Adaptive biasing force molecular dynamics simulations and density functional theory calculations were performed to understand the interaction of Li(+) with pure carbonates and ethylene carbonate (EC)-based binary mixtures. The most favorable Li carbonate cluster configurations obtained from molecular dynamics simulations were subjected to detailed structural and thermochemistry calculations on the basis of the M06-2X/6-311++G(d,p) level of theory. We report the ranking of these electrolytes on the basis of the free energies of Li-ion solvation in carbonates and EC-based mixtures. A strong local tetrahedral order involving four carbonates around the Li(+) was seen in the first solvation shell. Thermochemistry calculations revealed that the enthalpy of solvation and the Gibbs free energy of solvation of the Li(+) ion with carbonates are negative and suggested the ion-carbonate complexation process to be exothermic and spontaneous. Natural bond orbital analysis indicated that Li(+) interacts with the lone pairs of electrons on the carbonyl oxygen atom in the primary solvation sphere. These interactions lead to an increase in the carbonyl (C=O) bond lengths, as evidenced by a redshift in the vibrational frequencies [ν(C=O)] and a decrease in the electron density values at the C=O bond critical points in the primary solvation sphere. Quantum theory of atoms in molecules, localized molecular orbital energy decomposition analysis (LMO-EDA), and noncovalent interaction plots revealed the electrostatic nature of the Li(+) ion interactions with the carbonyl oxygen atoms in these complexes. On the basis of LMO-EDA, the strongest attractive interaction in these complexes was found to be the electrostatic interaction followed by polarization, dispersion, and exchange interactions. Overall, our calculations predicted EC and a binary mixture of EC/dimethyl carbonate to be appropriate electrolytes for Li-ion batteries, which complies with experiments and other theoretical results.

Prediction of cyclohexane-water distribution coefficient for SAMPL5 drug-like compounds with the QMPFF3 and ARROW polarizable force fields.

We present the performance of blind predictions of water-cyclohexane distribution coefficients for 53 drug-like compounds in the SAMPL5 challenge by three methods currently in use within our group. Two of them utilize QMPFF3 and ARROW, polarizable force-fields of varying complexity, and the third uses the General Amber Force-Field (GAFF). The polarizable FF's are implemented in an in-house MD package, Arbalest. We find that when we had time to parametrize the functional groups with care (batch 0), the polarizable force-fields outperformed the non-polarizable one. Conversely, on the full set of 53 compounds, GAFF performed better than both QMPFF3 and ARROW. We also describe the torsion-restrain method we used to improve sampling of molecular conformational space and thus the overall accuracy of prediction. The SAMPL5 challenge highlighted several drawbacks of our force-fields, such as our significant systematic over-estimation of hydrophobic interactions, specifically for alkanes and aromatic rings.

Risk Factors for Infection After Knee Arthroscopy: Analysis of 595,083 Cases From 3 United States Databases.

PURPOSE: To identify and quantify patient- and procedure-related risk factors for post-arthroscopic knee infections using a large dataset. METHODS: An administrative health care database including 8 years of records from 2 large commercial insurers and Medicare (a 5% random sample) was queried to identify all knee arthroscopies performed on patients aged at least 15 years using Current Procedural Terminology (CPT) codes. Each CPT code was designated as a high- or low-complexity procedure, with the former typically requiring accessory incisions or increased operative time. Deep infections were identified by a CPT code for incision and drainage within 90 days of surgery. Superficial infections were identified by International Classification of Diseases, Ninth Revision infection codes without any record of incision and drainage. Patients were compared based on age, sex, body mass index, tobacco use, presence of diabetes, and Charlson Comorbidity Index. RESULTS: A total of 526,537 patients underwent 595,083 arthroscopic knee procedures. Deep postoperative infections occurred at a rate of 0.22%. Superficial infections occurred at a rate of 0.29%. Tobacco use and morbid obesity were the largest risk factors for deep and superficial infections, respectively (P < .001; relative risk of 1.90 and 2.19, respectively). There were also higher infection rates among patients undergoing relatively high-complexity arthroscopies, men, obese patients, diabetic patients, and younger patients (in order of decreasing relative risk). Increased Charlson Comorbidity Index was associated with superficial and total infections (P < .001). CONCLUSIONS: Post-arthroscopic knee infections were more frequent among morbidly obese patients, tobacco users, patients undergoing relatively complex procedures, men, obese patients, diabetic patients, relatively young patients, and patients with increased comorbidity burdens in this study population. This knowledge may allow more informed preoperative counseling, aid surgeons in patient selection, and facilitate infection prevention by targeting individuals with higher inherent risk. LEVEL OF EVIDENCE: Level IV, cross-sectional study.

Effects of Hand Fellowship Training on Rates of Endoscopic and Open Carpal Tunnel Release.

PURPOSE: To investigate rates, trends, and complications for carpal tunnel release (CTR) related to fellowship training using the American Board of Orthopaedic Surgery Part II Database. METHODS: We searched the American Board of Orthopaedic Surgery database for patients with carpal tunnel syndrome who underwent either open carpal tunnel release (OCTR) or endoscopic (ECTR) from 2003 to 2013. Cases with multiple treatment codes were excluded. Data were gathered on geographic location, fellowship, and surgical outcomes. Data were then divided into 2 cohorts: hand fellowship trained versus non-hand fellowship trained. We performed analysis with chi-square tests of independence and for trend. RESULTS: Overall, 12.4% of all CTRs were done endoscopically. Hand fellowship-trained orthopedists performed about 4.5 times the number of ECTR than did non-hand fellowship-trained surgeons. An increasing trend over time of ECTR was seen only among the hand fellowship cohort. The northwest region of the United States had the highest incidence (23.1%) of ECTR, and the Southwest the lowest incidence (5.9%). The complication incidence associated with CTR overall was 3.6%, without a significant difference between ECTR and OCTR. Within the hand fellowship cohort the complication incidence for ECTR was significantly less than for OCTR. There was no difference in overall complication rates with ECTR and OCTR between the 2 cohorts. Wound complications were higher with OCTR (1.2% vs 0.25%) and nerve palsy with ECTR (0.66% vs 0.27%); with postoperative pain equivalent between techniques independent of fellowship training. CONCLUSIONS: Within the United States from 2003 to 2013, the rate of ECTR increased, as did complications. However, complication rates remained low in the first 2 years of practice. Hand fellowship-trained surgeons performed more ECTR than did non-hand fellowship-trained orthopedic surgeons, and both groups had similar complication rates. TYPE OF STUDY/LEVEL OF EVIDENCE: Therapeutic IV.

Initial medical management of rotator cuff tears: a demographic analysis of surgical and nonsurgical treatment in the United States Medicare population.

BACKGROUND: Rotator cuff tears have a lifetime incidence between 25% and 40% in the United States, but optimum treatment strategies and protocol have not yet been widely accepted. This study evaluated the proportions of patients treated with nonoperative and operative modalities and how this proportion has changed during an 8-year period (2005-2012) among patients with Medicare. METHODS: Using the PearlDiver patient record database, we identified Medicare patients having been diagnosed with a rotator cuff tear. These patients were then stratified on the basis of treatment with physical therapy, subacromial/glenohumeral injection, or rotator cuff repair. We analyzed the data in regard to standard demographic information, comorbidities, and the Charlson Comorbidity Index. RESULTS: During the study period, 878,049 patients were identified and 397,116 patients had rotator cuff repair. The proportion of patients treated initially with physical therapy dropped from 30.0% in 2005 to 13.2% in 2012, and the subacromial/glenohumeral injection proportion decreased from 6.00% to 4.19% (P < .001). The proportion of patients who had rotator cuff repair increased from 33.8% to 40.4% from 2005 to 2012 (P < .001). Charlson Comorbidity Indexes were significantly lower in operative patients compared with each nonoperative treatment examined. DISCUSSION: This analysis demonstrates a significant decrease in the initial trial of nonoperative treatment and an increase in the rate of surgery. Patients undergoing rotator cuff repair had fewer comorbidities than those undergoing nonoperative treatments. It also demonstrates that patients who had a trial of injection had a higher incidence of eventual rotator cuff repair compared with the patients with an initial trial of physical therapy.

Electronic Structure Insights into the Solvation of Magnesium Ions with Cyclic and Acyclic Carbonates.

A computational framework to rank the solvation behavior of Mg(2+) in carbonates by using molecular dynamics simulations and density functional theory is reported. Based on the binding energies and enthalpies of solvation calculated at the M06-2X/6-311++G(d,p) level of theory and the free energies of solvation from ABF-MD simulations, we find that ethylene carbonate (EC) and the ethylene carbonate:propylene carbonate (EC:PC) binary mixture are the best carbonate solvents for interacting with Mg(2+) . Natural bond orbital and quantum theory of atoms in molecules analyses support the thermochemistry calculations with the highest values of charge transfer, perturbative stabilization energies, electron densities, and Wiberg bond indices being observed in the Mg(2+) (EC) and Mg(2+) (EC:PC) complexes. The plots of the noncovalent interactions indicate that those responsible for the formation of Mg(2+) carbonate complexes are strong-to-weak attractive interactions, depending on the regions that are interacting. Finally, density of state calculations indicate that the interactions between Mg(2+) and the carbonate solvents affects the HOMO and LUMO states of all carbonate solvents and moves them to more negative energy values.

Effect of nanoscale confinement on freezing of modified water at room temperature and ambient pressure.

Understanding the phase behavior of confined water is central to fields as diverse as heterogeneous catalysis, corrosion, nanofluidics, and to emerging energy technologies. Altering the state points (temperature, pressure, etc.) or introduction of a foreign surface can result in the phase transformation of water. At room temperature, ice nucleation is a very rare event and extremely high pressures in the GPa-TPa range are required to freeze water. Here, we perform computer experiments to artificially alter the balance between electrostatic and dispersion interactions between water molecules, and demonstrate nucleation and growth of ice at room temperature in a nanoconfined environment. Local perturbations in dispersive and electrostatic interactions near the surface are shown to provide the seed for nucleation (nucleation sites), which lead to room temperature liquid-solid phase transition of confined water. Crystallization of water occurs over several tens of nanometers and is shown to be independent of the nature of the substrate (hydrophilic oxide vs. hydrophobic graphene and crystalline oxide vs. amorphous diamond-like carbon). Our results lead us to hypothesize that the freezing transition of confined water can be controlled by tuning the relative dispersive and electrostatic interaction.

Comparison of the interfacial dynamics of water sandwiched between static and free-standing fully flexible graphene sheets.

Classical molecular dynamics simulations are used to present a detailed atomistic picture of the instantaneous local structures of water and the structural evolution of stationary and dynamically evolving graphene-water interfaces. The confinement effects are strongly coupled to the nature of the interface, which eventually governs its nanoscopic structural arrangements and interface dynamics. We show that the structure, transport properties, and vibrational densities of states of proximal water molecules are strongly correlated with the nature of the graphene-water interface. We identify and correlate features in vibrational spectra with characteristic structural features observed at the atomic scale for the confined water molecules near a stationary and dynamically evolving hydrophobic surface such as graphene. Our simulations indicate that the local orientation, ordering, and solvation dynamics of interfacial water molecules are a strong function of the graphene slit-width, which is controlled by the nature of the interface (fully flexible vs. static). A monotonic decrease in local ordering with increasing slit-width was observed for the static graphene-water interface, whereas a non-monotonic variation was seen for its fully flexible counterpart. The simulation results offer useful insights into the effect of interfacial dynamics in defining the structure and transport properties at graphene-aqueous media interfaces. Finally these simulations provide a molecular level interpretation of the differential confinement effects arising from the dynamically evolving graphene-water interface.

Non-equilibrium effects evidenced by vibrational spectra during the coil-to-globule transition in poly(N-isopropylacrylamide) subjected to an ultrafast heating-cooling cycle.

Molecular dynamics simulations in conjunction with finite element calculations are used to explore the conformational dynamics of a thermo-sensitive oligomer, namely poly(N-isopropylacrylamide) (PNIPAM), subjected to an ultra-fast heating-cooling cycle. Finite element (FE) calculations were used to predict the temperature profile resulting from laser-induced heating of the polymer-aqueous system. The heating rate (∼0.6 K ps(-1)) deduced from FE calculations was used to heat an aqueous solution of PNIPAM consisting of 30 monomeric units (30-mer) from 285 K to 315 K. Non-equilibrium effects arising from the ultra-fast heating-cooling cycle results in a hysteresis during the coil-to-globule transition. The corresponding atomic scale conformations were characterized by monitoring the changes in the vibrational spectra, which provided a reliable metric to study the coil-to-globule transition in PNIPAM and vice-versa across the LCST. The vibrational spectra of bonds involving atoms from the oligomer backbone and the various side-groups (amide I, amide II, and the isopropyl group of PNIPAM) of the oligomers were analyzed to study the conformational changes in the oligomer corresponding to the observed hysteresis. The differences in the vibrational spectra calculated at various temperatures during heating and cooling cycles were used to understand the coil-to-globule and globule-to-coil transitions in the PNIPAM oligomer and identify the changes in the relative interactions between various atoms in the backbone and in the side groups of the oligomer with water. The shifts in the computed vibrational spectral peaks and the changes in the intensity of peaks for the different regions of PNIPAM, seen across the LCST during the heating cycle, are in good agreement with previous experimental studies. The changes in the radius of gyration (Rg) and vibrational spectra for amide I and amide II regions of PNIPAM suggest a clear coil-to-globule transition at ∼301 K during the heating cycle from 285 K to 315 K. During the heating cycle, a comparison of the vibrational spectra of isopropyl groups in PNIPAM at 285 K and 315 K suggests dehydration of the isopropyl moieties at 315 K. This implies that the oligomer-water interactions are dominant below the LCST whereas oligomer-oligomer interactions pre-dominate above the LCST. On the other hand, during the cooling cycle minor changes in the Rg and vibrational spectra of the PNIPAM oligomer in going from 315 K to 285 K indicate that the interactions between oligomer-oligomer and between the oligomer and water are less perturbed during the cooling cycle. Our simulations suggest that the observed hysteresis is a consequence of ultrafast heating-cooling kinetics, which allows insufficient relaxation times for the solvated oligomer.