Molecular Conformational Manifolds between Gas-Liquid Interface and Multiphasic

The analysis of conformational changes of hydrocarbon molecules is imperative in the prediction of their transport properties in different phases, such as evaporation/condensation coefficients (b) in the gas-liquid interface and evaporation rates of fuel droplets (k) in multiphases. In this letter, we analyze the effects of entropic contributions (TDSev(T)) to DGev(T) during the evaporation/condensation of chain conformers at the interface with a modified version of the solvation model SMD/!B97X-D/cc-pVTZ in which the temperature dependency of surface tension and the interfacial flow density of the conformers is taken into account. The evaporation/condensation coefficient (b) and evaporation rate (k) are respectively calculated using the statistical associating fluid theory (SAFT) and a combined quantum-classical reaction rate theory named quantum transition state theory-classical kinetic gas theory (QTST-CKGT). The detailed analyses show the importance of internal entropic states over the interfacial layer induced by meso-confinement phenomena in the very vicinity of fuel droplets surfaces

Revisiting kinetic boundary conditions at the surface of fuel droplet hydrocarbons: An atomistic computational fluid dynamics simulation

The role of boundary conditions at the interface for both Boltzmann equation and the set of Navier-Stokes equations have been suggested to be important for studying of multiphase flows such as evaporation/condensation process which doesn't always obey the equilibrium conditions. Here we present aspects of transition-state theory (TST) alongside with kinetic gas theory (KGT) relevant to the study of quasi-equilibrium interfacial phenomena and the equilibrium gas phase processes, respectively. A two-state mathematical model for long-chain hydrocarbons which have multi-structural specifications is introduced to clarify how kinetics and thermodynamics affect evaporation/condensation process at the surface of fuel droplet, liquid and gas phases and then show how experimental observations for a number of n-alkane may be reproduced using a hybrid framework TST and KGT with physically reasonable parameters controlling the interface, gas and liquid phases. The importance of internal activation dynamics at the surface of n-alkane droplets is established during the evaporation/condensation process

On Maximum Signless Laplacian Estrada Indices of Graphs with Given Parameters

Signless Laplacian Estrada index of a graph $G$, defined as $SLEE(G)=\sum^{n}_{i=1}e^{q_i}$, where $q_1, q_2, \cdots, q_n$ are the eigenvalues of the matrix $\mathbf{Q}(G)=\mathbf{D}(G)+\mathbf{A}(G)$. We determine the unique graphs with maximum signless Laplacian Estrada indices among the set of graphs with given number of cut edges, pendent vertices, (vertex) connectivity and edge connectivity.Comment: 14 pages, 3 figure

Molecular conformational effects in H + n -heptane reaction rate calculations

Accurate fuel combustion modelling is a matter of immense importance to design clean combustors and reduce greenhouse gas emissions and pollutants. In this Brief Communication, we present the effects of internal dynamics of one n-heptane molecule which are controlling chemical kinetics of hydrogen abstraction reactions through multi-pathway reaction dynamics. It is established that the slope of Arrhenius plots dramatically changes in comparison with the harmonic single static pathway approach in the temperature range of 200–3000 K. We apply a combination of the multiple conformation statistical thermodynamic approach and variational transition-state theory (VTST) to obtain dynamic multi-path rate coefficients (kMP-T-VTST and kMP-LH-VTST). Compared with single-path VTST (kSP-H-VTST) results, the thermal reaction rate coefficients obtained from our MP-VTST calculations differ considerably due to the fact that tunnelling and cross-conformational effects in the reactions, and the anharmonic and quasi-harmonic contributions in multiple conformer molecules cannot be ignored or simplified

Specificity Switching Pathways in Thermal and Mass Evaporation of Multicomponent Hydrocarbon Droplets: A Mesoscopic Observation

For well over one century, the Hertz–Knudsen equation has established the relationship between thermal – mass transfer coefficients through a liquid – vapour interface and evaporation rate. These coefficients, however, have been often separately estimated for one-component equilibrium systems and their simultaneous influences on evaporation rate of fuel droplets in multicomponent systems have yet to be investigated at the atomic level. Here we first apply atomistic simulation techniques and quantum/statistical mechanics methods to understand how thermal and mass evaporation effects are controlled kinetically/thermodynamically. We then present a new development of a hybrid method of quantum transition state theory/improved kinetic gas theory, for multicomponent hydrocarbon systems to investigate how concerted-distinct conformational changes of hydrocarbons at the interface affect the evaporation rate. The results of this work provide an important physical concept in fundamental understanding of atomistic pathways in topological interface transitions of chain molecules, resolving an open problem in kinetics of fuel droplets evaporation

Effects of the surroundings and conformerisation of n-dodecane molecules on evaporation/condensation processes.

The evaporation/condensation coefficient (β) and the evaporation rate (γ) for n-dodecane vs. temperature, gas pressure, gas and liquid density, and solvation effects at a droplet surface are analysed using quantum chemical density functional theory calculations of several ensembles of conformers of n-dodecane molecules in the gas phase (hybrid functional ωB97X-D with the cc-pVTZ and cc-pVDZ basis sets) and in liquid phase (solvation method: SMD/ωB97X-D). It is shown that β depends more strongly on a number of neighbouring molecules interacting with an evaporating molecule at a droplet surface (this number is estimated through changes in the surface Gibbs free energy of solvation) than on pressure in the gas phase or conformerisation and cross-conformerisation of molecules in both phases. Thus, temperature and the surrounding effects at droplet surfaces are the dominant factors affecting the values of β for n-dodecane molecules. These values are shown to be similar (at reduced temperatures T/Tc 0.8) than the values of β calculated by the molecular dynamics force fields (MD FF) methods. This endorses the reliability of the previously developed classical approach to estimation of β by the MD FF methods, except at temperatures close to the critical temperature

Constraints on scalar-tensor theories from observations

We study the dynamical description of scalar-tensor gravity by performing the best-fit analysis for two cases of exponential and power-law form of the potential and scalar field function coupled to the curvature. The models are then tested against observational data. The results show that in both scenarios the Universe undergoes an acceleration expansion period and the geometrical equivalent of dark energy is associated with a time-dependent equation of state.Comment: 16 pages, 32 figure

Quantum Mechanical effects in n-alkane droplets

According to quantum chemical (QC) calculations of a series of n-alkanes (CnH2n+2 at n = 1-8) the gap ΔHL between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases with increasing methylene group (-CH2-) number in the n-alkane chain. While the LUMO level appears to be relatively unchanged, in good agreement with experimental results, the HOMO level is unstabilized and ΔHL decreases from approximately 11 eV (methane) to 6.5 eV (n-octane). Comparative ab initio calculations show not only the quantum confinement effects (QCE) in n-alkanes in the gas phase due to the reduction of the ΔHL value, but also enabled selection of cost-effective methodologies for modelling long-chain n-alkanes (n > 8). The used methodologies include ‘Local Spin Density Approximation’, combining special exchange functional with suitable correlation functional. Electronic structures and energetics of n-pentane (C5H12) in the evaporation/condensation processes are studied to understand the molecular mechanism for these processes. Two main step processes, liquid (L) 1 liquid-gas (L-G) interface 2 gas (G), are analysed. While phase transformations between L and L-G is estimated as activation processes, it is found that there is no energy barrier in the transitions between L-G and

Corticosteroid Administration to Prevent Complications of Anterior Cervical Spine Fusion: A Systematic Review.

Study Design: Systematic review. Objectives: Anterior cervical approach is associated with complications such as dysphagia and airway compromise. In this study, we aimed to systematically review the literature on the efficacy and safety of corticosteroid administration as a preventive measure of such complications in anterior cervical spine surgery with fusion. Methods: Following a systematic literature search of MEDLINE, Embase, and Cochrane databases in July 2016, all comparative human studies that evaluated the effect of steroids for prevention of complications in anterior cervical spine surgery with fusion were included, irrespective of number of levels and language. Risk of bias was assessed using MINORS (Methodological Index for Non-Randomized Studies) checklist and Cochrane Back and Neck group recommendations, for nonrandomized and randomized studies, respectively. Results: Our search yielded 556 articles, of which 9 studies (7 randomized controlled trials and 2 non-randomized controlled trials) were included in the final review. Dysphagia was the most commonly evaluated complication, and in most studies, its severity or incidence was significantly lower in the steroid group. Although prevertebral soft tissue swelling was less commonly assessed, the results were generally in favor of steroid use. The evidence for airway compromise and length of hospitalization was inconclusive. Steroid-related complications were rare, and in both studies that evaluated the fusion rate, it was comparable between steroid and control groups in long-term follow-up. Conclusions: Current literature supports the use of steroids for prevention of complications in anterior cervical spine surgery with fusion. However, evidence is limited by substantial risk of bias and small number of studies reporting key outcomes

Ultrasound - guided access during percutaneous nephrolithotomy: Entering desired calyx with appropriate entry site and angle

Objectives: To evaluate the success of ultrasonography directed renal access in entering the target calyx from proper entry site and in the direction of renal pelvis during percutaneous nephrolithotomy (PCNL). Materials and Methods: PCNL cases who were operated on by one fellow from May- June 2014 were included in this study. A vertically placed ultrasound probe on the patient flank in prone position was used to identify the preselected target calyx. Needle was advanced through needle holder and fluoroscopy was used to document the entered calyx, site and angle of entry. Results: Successful entering to the target calyx was achieved in 43 cases (91). Successful entry with appropriate entry site and angle was observed in 34 cases (72). Reasons for failure were minimal hydronephrosis, upper pole access and high lying kidneys. Conclusions: Although it is feasible to access a preselected calyx by ultrasonography guidance during PCNL, but entry to the calyx from the appropriate site and direction is another problem and needs more experience. In cases of minimal hydronephrosis, superior pole access or high lying kidneys, ultrasonography is less successful and should be used with care