The origin of the density scaling exponent for polyatomic molecules and the estimation of its value from the liquid structure.

In this article, we unravel the problem of interpreting the density scaling exponent for the polyatomic molecules representing the real van der Waals liquids. Our studies show that the density scaling exponent is a weighted average of the exponents of the repulsive terms of all interatomic interactions that occur between molecules, where the potential energy of a given interaction represents its weight. It implies that potential energy is a key quantity required to calculate the density scaling exponent value for real molecules. Finally, we use the well-known method for potential energy estimation and show that the density scaling exponent could be successfully predicted from the liquid structure for fair representatives of the real systems.

Outdoor exposure in children from Buenos Aires Province, Argentina.

OBJECTIVES: To evaluate myopia risk factors, mainly outdoor exposure and reading habits, in a country with low prevalence of myopia (Buenos Aires Province, Argentina). METHODS: Consecutive children interviewed in a clinical private practice setting were autorefracted under cycloplegia with cyclopentolate 1%. Their parents consented to fill a questionnaire about schooling, tutorial classes, outdoor exposure, reading habits, and cellphone use, both on weekdays and weekends. The Spanish questionnaire was based on past English questionnaires of myopia clinical trials. The spherical equivalent of the right eye was used for the refractive distribution. The average daily hours spent for each activity were calculated. RESULTS: This study involved 115 children aged 10.48 ±â€¯3.65 years (range 5-18 years), with 56.5% being girls. Children had 8 h of schooling per day in 62.6% of cases, and only 14.8 % had tutorial classes after school. There were 38.3% myopes (<-0.50 D), 24.3% hyperopes (>+2.00 D) and the rest were emmetropes. The mean time that these children spent outdoors per day was 3.94 ±â€¯1.45 h (27.60 ±â€¯10.16 h per week). The total mean time spent reading and writing per day was 1.50 ±â€¯0.98 h, and that spent using cellphones and tablets was 2.43 ±â€¯1.66 h. CONCLUSION: In an environment with low myopia prevalence, children spend almoast 4 h per day outdoors, much more than the usual recommendation of 2 h a day for myopia prevention.

Heart Transplantation as a Result of Pediatric Inflammatory Multisystem Syndrome in an Adolescent.

Several reviews have shown that COVID-19 in children is a relatively mild disease. However, a rare complication affecting children and adolescents after COVID-19 has been identified. Pediatric inflammatory multisystem syndrome temporally associated with COVID-19 (PIMS-TS), which in some cases manifests itself as a hyperinflammatory syndrome with a multiorgan failure, may lead to death. We report a case of a 17-year-old patient who was admitted to the hospital with cardiogenic shock of unknown etiology. The disease was life-threatening, thus necessitating mechanical ventilation, circulatory support, and extracorporeal therapy due to renal and liver dysfunction. The patient tested negative for SARS-CoV-2 Reverse Transcription Polymerase Chain Reaction. Other infectious causes of illness were excluded. However, the patient had a positive IgG antibody test result and high levels of interleukin-6, which helped to diagnose PIMS-TS. Intravenous immunoglobulin and steroid therapies were initiated, unfortunately, with poor outcome. The patient's critical condition, particularly end-stage heart failure, led to mechanical circulatory support implantation and finally orthotopic heart transplantation. After the surgery, the patient's condition improved gradually. PIMS-TS manifests itself with different clinical images and as a state of varying severity, ultimately causing multiorgan dysfunction with shock resembling toxic shock syndrome. Ultimately, myocardial complications of PIMS-TS necessitated heart transplantation in the described patient.

Corneal endothelial cell density during diabetes mellitus and ocular diabetes complications treatment.

Diabetes mellitus may affect the cornea at various levels. Ocular surface changes and dry eye had been studied. Researchers are concerned that medical treatment of diabetes or retinal complications may result in endothelial damage and cell loss. This report summarizes the possibility of endothelial cell loss in diabetic patients. A decrease in endothelial cell density (ECD) in diabetic patients has been reported. In addition, corneal thickness may increase in diabetic patients. Significant endothelial cell loss has been demonstrated in long-term disease and in cases of poor metabolic control. No association between the use of oral hypoglycemics and ECD has been reported. There is also no evidence of an association between the use of insulin and corneal endothelial damage. No difference in ECD among the various degrees of retinopathy or with a history of photocoagulation has been shown. Regarding the studies comparing diabetic and non-diabetic patients undergoing cataract surgery, in all cases, the decrease in ECD is higher in diabetic patients than that seen in non-diabetic patients. However, there is no evidence of increased endothelial damage in diabetics compared to non-diabetics during vitreo-retinal surgery in phakic eyes. No significant changes in corneal endothelium after intravitreal anti-VEGF injections have been referenced.

Exploring the connection between the density-scaling exponent and the intermolecular potential for liquids on the basis of computer simulations of quasireal model systems.

In this paper, based on the molecular dynamics simulations of quasireal model systems, we propose a method for determination of the effective intermolecular potential for real materials. We show that in contrast to the simple liquids, the effective intermolecular potential for the studied systems depends on the thermodynamic conditions. Nevertheless, the previously established relationship for simple liquids between the exponent of the inverse power law approximation of intermolecular potential and the density-scaling exponent is still preserved when small enough intermolecular distances are considered. However, our studies show that molecules approach each other at these very short distances relatively rarely. Consequently, only sparse interactions between extremely close molecules determine the value of the scaling exponent and then strongly influence the connection between dynamics and thermodynamics of the whole system.

Virial-potential-energy correlation and its relation to density scaling for quasireal model systems.

In this paper, we examine the virial- and the potential-energy correlation for quasireal model systems. This correlation constitutes the framework of the theory of the isomorph in the liquid phase diagram commonly examined using simple liquids. Interestingly, our results show that for the systems characterized by structural anisotropy and flexible bonds, the instantaneous values of total virial and total potential energy are entirely uncorrelated. It is due to the presence of the intramolecular interactions because the contributions to the virial and potential energy resulting from the intermolecular interactions still exhibit strong linear dependence. Interestingly, in contrast to the results reported for simple liquids, the slope of the mentioned linear dependence is different than the values of the density scaling exponent. However, our findings show that for quasireal materials, the slope of dependence between the virial and potential energy (resulting from the intermolecular interactions) strongly depends on the interval of intermolecular distances that are taken into account. Consequently, the value of the slope of the discussed relationship, which enables satisfactory density scaling, can be obtained. Interestingly, this conclusion is supported by the results obtained for analogous systems without intermolecular attraction, for which the value the slope of the virial-potential-energy correlation is independent of considered intermolecular distances, directly corresponds to the exponent of the intermolecular repulsion, and finally leads to accurate density scaling.

The effect of molecular architecture on the physical properties of supercooled liquids studied by MD simulations: Density scaling and its relation to the equation of state.

Theoretical concepts in condensed matter physics are typically verified and also developed by exploiting computer simulations mostly in simple models. Predictions based on these usually isotropic models are often at odds with measurement results obtained for real materials. One of the examples is an intriguing problem within the density scaling idea that has attracted attention in recent decades due to its hallmarks of universality, i.e., the fact that the difference between the density scaling exponent and the exponent of the equation of state is observed for real materials, whereas it has not been reported for the model system. In this paper, we use new model molecules of simple but anisotropic architecture to study the effect of molecular anisotropy on the dynamic and thermodynamic properties of the system. We identify the applicable range of intermolecular interactions for a given physical process, and then we explain the reason for observed differences between the behavior of the model and real systems. It demonstrates that the new model systems open broad perspectives for simulation and theoretical research, for example, into unifying concepts in the glass transition physics.

Activation volume of selected liquid crystals in the density scaling regime.

In this paper, we demonstrate and thoroughly analyze the activation volumetric properties of selected liquid crystals in the nematic and crystalline E phases in comparison with those reported for glass-forming liquids. In the analysis, we have employed and evaluated two entropic models (based on either total or configurational entropies) to describe the longitudinal relaxation times of the liquid crystals in the density scaling regime. In this study, we have also exploited two equations of state: volumetric and activation volumetric ones. As a result, we have established that the activation volumetric properties of the selected liquid crystals are quite opposite to such typical properties of glass-forming materials, i.e., the activation volume decreases and the isothermal bulk modulus increases when a liquid crystal is isothermally compressed. Using the model based on the configurational entropy, we suggest that the increasing pressure dependences of the activation volume in isothermal conditions and the negative curvature of the pressure dependences of isothermal longitudinal relaxation times can be related to the formation of antiparallel doublets in the examined liquid crystals. A similar pressure effect on relaxation dynamics may be also observed for other material groups in case of systems, the molecules of which form some supramolecular structures.

Molecular Factors Governing the Liquid and Glassy States Recrystallization of Celecoxib in Binary Mixtures with Excipients of Different Molecular Weights.

Transformation of poorly water-soluble crystalline pharmaceuticals to the amorphous form is one of the most promising strategies to improve their oral bioavailability. Unfortunately, the amorphous drugs are usually thermodynamically unstable and may quickly return to their crystalline form. A very promising way to enhance the physical stability of amorphous drugs is to prepare amorphous compositions of APIs with certain excipients which can be characterized by significantly different molecular weights, such as polymers, acetate saccharides, and other APIs. By using different experimental techniques (broadband dielectric spectroscopy, differential scanning calorimetry, X-ray diffraction) we compare the effect of adding the large molecular weight polymer-polyvinylpyrrolidone (PVP K30)-and the small molecular weight excipient-octaacetylmaltose (acMAL)-on molecular dynamics as well as the tendency to recrystallization of the amorphous celecoxib (CEL) in the amorphous solid dispersions: CEL-PVP and CEL-acMAL. The physical stability investigations of the binary systems were performed in both the supercooled liquid and glassy states. We found that acMAL is a better inhibitor of recrystallization of amorphous CEL than PVP K30 deep in the glassy state (T < Tg). In contrast, PVP K30 is a better crystallization inhibitor of CEL than acMAL in the supercooled liquid state (at T > Tg). We discuss molecular factors governing the recrystallization of amorphous CEL in examined solid dispersions.

Universal Behavior of Dielectric Responses of Glass Formers: Role of Dipole-Dipole Interactions.

From an exhaustive examination of the molecular dynamics in practically all van der Waals molecular glass formers ever probed by dielectric spectroscopy, we found that the width of the α-loss peak at or near the glass transition temperature T_{g} is strongly anticorrelated with the polarity of the molecule. The larger the dielectric relaxation strength Δε(T_{g}) of the system, the narrower is the α-loss peak. This remarkable property is explained by the contribution from the dipole-dipole interaction potential V_{dd}(r)=-Dr^{-6} to the attractive part of the intermolecular potential, making the resultant potential more harmonic, and the effect increases rapidly with the dipole moment µ and Δε(T_{g}) in view of the relation, D∝(µ^{4}/kT_{g})∝kT_{g}[Δε(T_{g})]^{2}. Since the novel correlation discovered encompasses practically all van der Waals molecular glass formers studied by dielectric spectroscopy, it impacts the large dielectric research community as well as those engaged in solving the glass transition problem.

Ionic liquids and their bases: Striking differences in the dynamic heterogeneity near the glass transition.

Ionic liquids (ILs) constitute an active field of research due to their important applications. A challenge for these investigations is to explore properties of ILs near the glass transition temperature Tg, which still require our better understanding. To shed a new light on the issues, we measured ILs and their base counterparts using the temperature modulated calorimetry. We performed a comparative analysis of the dynamic heterogeneity at Tg for bases and their salts with a simple monoatomic anion (Cl(-)). Each pair of ionic and non-ionic liquids is characterized by nearly the same chemical structure but their intermolecular interactions are completely different. We found that the size of the dynamic heterogeneity of ILs near Tg is considerably smaller than that established for their dipolar counterparts. Further results obtained for several other ILs near Tg additionally strengthen the conclusion about the relatively small size of the dynamic heterogeneity of molecular systems dominated by electrostatic interactions. Our finding opens up new perspectives on designing different material properties depending on intermolecular interaction types.

Effects of dynamic heterogeneity and density scaling of molecular dynamics on the relationship among thermodynamic coefficients at the glass transition.

In this paper, we define and experimentally verify thermodynamic characteristics of the liquid-glass transition, taking into account a kinetic origin of the process. Using the density scaling law and the four-point measure of the dynamic heterogeneity of molecular dynamics of glass forming liquids, we investigate contributions of enthalpy, temperature, and density fluctuations to spatially heterogeneous molecular dynamics at the liquid-glass transition, finding an equation for the pressure coefficient of the glass transition temperature, dTg/dp. This equation combined with our previous formula for dTg/dp, derived solely from the density scaling criterion, implies a relationship among thermodynamic coefficients at Tg. Since this relationship and both the equations for dTg/dp are very well validated using experimental data at Tg, they are promising alternatives to the classical Prigogine-Defay ratio and both the Ehrenfest equations in case of the liquid-glass transition.

Role of entropy in the thermodynamic evolution of the time scale of molecular dynamics near the glass transition.

In this paper, we investigate how changes in the system entropy influence the characteristic time scale of the system molecular dynamics near the glass transition. Independently of any model of thermodynamic evolution of the time scale, against some previous suppositions, we show that the system entropy S is not sufficient to govern the time scale defined by structural relaxation time τ. In the density scaling regime, we argue that the decoupling between τ and S is a consequence of different values of the scaling exponents γ and γ(S) in the density scaling laws, τ=f(ρ(γ)/T) and S=h(ρ(γ(S))/T), where ρ and T denote density and temperature, respectively. It implies that the proper relation between τ and S requires supplementing with a density factor, u(ρ), i.e., τ=g(u(ρ)w(S)). This meaningful finding additionally demonstrates that the density scaling idea can be successfully used to separate physically relevant contributions to the time scale of molecular dynamics near the glass transition. The relation reported by us between τ and S constitutes a general pattern based on nonconfigurational quantities for describing the thermodynamic evolution of the characteristic time scale of molecular dynamics near the glass transition in the density scaling regime, which is a promising alternative to the approaches based as the Adam-Gibbs model on the configurational entropy that is difficult to evaluate in the entire thermodynamic space. As an example, we revise the Avramov entropic model of the dependence τ(T,ρ), giving evidence that its entropic basis has to be extended by the density dependence of the maximal energy barrier for structural relaxation. We also discuss the excess entropy S(ex), the density scaling of which is found to mimic the density scaling of the total system entropy S.

General rules prospected for the liquid fragility in various material groups and different thermodynamic conditions.

The fragility parameter has been acknowledged as one of the most important characteristics of glass-forming liquids. We show that the mystery of the dramatic change in molecular dynamics of systems approaching the glass transition can be better understood by the high pressure study of fragility parameters defined in different thermodynamic conditions. We formulate and experimentally confirm a few rules obeyed by the fragility parameters, which are also rationalized by the density scaling law and its modification suggested for associated liquids. In this way, we successfully explore and gain a new insight into the pressure effect on molecular dynamics of van der Waals liquids, polymer melts, ionic liquids, and hydrogen-bonded systems near the glass transition.

The complex, non-monotonic thermal response of the volumetric space of simple liquids.

In this paper, an intricate effect of the isothermal compression of simple liquids on their volumetric response is reported for α,ω-halogenoalkanes as examples. We apply an accurate experimental technique, scanning transitiometry, to directly measure isobaric thermal volume expansivities αp of the liquids in a wide density range. To thoroughly analyze the observed intersection of the experimental isothermal pressure dependences of αp, we develop a class of equations of state derived in the density scaling regime for molecular dynamics, finding successful temperature parameterizations of an isothermal equation of state (EOS) intrinsically adapted to describe volumetric data in an extremely wide density range. The EOS based analyses of the scanning transitiometry data as a function of temperature T and pressure p undoubtedly show that the previously considered crossing point of the isothermal dependences αp(p) is in general represented by a non-linear and non-monotonic curve in the (T-p) phase diagram.

On the scaling behavior of electric conductivity in [C4mim][NTf2].

In this work we examine, for the first time, the molar conductivity behavior of the deeply supercooled room temperature ionic liquid [C4mim][NTf2] in the temperature, pressure and volume thermodynamic space in terms of density scaling (TV(γ))(-1) combined with the equation of state (EOS). The exponent γσ determined from the Avramov model analysis is compared with the coefficient obtained from the viscosity studies carried out at moderate temperatures. Therefore, the experimental results presented herein provide the answer to the long-standing question regarding the validity of thermodynamic scaling of ionic liquids over a wide temperature range, i.e. from the normal liquid state to the glass transition point. Finally, we investigate the relationship between the dynamic and thermodynamic properties of [C4mim][NTf2] represented by scaling exponent γ and Grüneisen constant γG, respectively.

Equation of state in the generalized density scaling regime studied from ambient to ultra-high pressure conditions.

In this paper, based on the effective intermolecular potential with well separated density and configuration contributions and the definition of the isothermal bulk modulus, we derive two similar equations of state dedicated to describe volumetric data of supercooled liquids studied in the extremely wide pressure range related to the density range, which is extremely wide in comparison with the experimental range reached so far in pressure-volume-temperature measurements of glass-forming liquids. Both the equations comply with the generalized density scaling law of molecular dynamics versus h(ρ)/T at different densities ρ and temperatures T, where the scaling exponent can be in general only a density function γ(ρ) = d ln h/d ln ρ as recently argued by the theory of isomorphs. We successfully verify these equations of state by using data obtained from molecular dynamics simulations of the Kob-Andersen binary Lennard-Jones liquid. As a very important result, we find that the one-parameter density function h(ρ) analytically formulated in the case of this prototypical model of supercooled liquid, which implies the one-parameter density function γ(ρ), is able to scale the structural relaxation times with the value of this function parameter determined by fitting the volumetric simulation data to the equations of state. We also show that these equations of state properly describe the pressure dependences of the isothermal bulk modulus and the configurational isothermal bulk modulus in the extremely wide pressure range investigated by the computer simulations. Moreover, we discuss the possible forms of the density functions h(ρ) and γ(ρ) for real glass formers, which are suggested to be different from those valid for the model of supercooled liquid based on the Lennard-Jones intermolecular potential.