Excess chemical potential of thiophene in [C4MIM] [BF4, Cl, Br, CH3COO] ionic liquids, determined by molecular simulations.

The excess chemical potential of thiophene in imidazolium-based ionic liquids [C4mim][BF4], [C4mim][Cl], [C4mim][Br], and [C4mim][CH3COO] were determined by means of molecular dynamics in conjunction with free energy perturbation techniques employing non-polarizable force fields at 300 K and 343.15 K. In addition, energetic and structural analysis were performed such as: interaction energies, averaged noncovalent interactions, radial, and combined distribution functions. The results from this work revealed that the ionic liquids (ILs) presenting the most favorable excess chemical potentials ([C4mim][BF4], [C4mim][CH3COO]) are associated with the strongest energetic interaction between the thiophene molecule and the ionic liquid anion, and with the weakest energetic interaction between the thiophene molecule and the ionic liquid cation.

Los grupos informales y su relación con el rendimiento de las MiPyMES en México / Informal groups and their relationship with the performance of MSMEs in Mexico / Grupos informais e sua relação com a atuação das MPMEs no México

Resumen Introducción: en la actualidad, el liderazgo, la cultura y el clima organizacional que ejercen los grupos informales representan algunos de los factores clave en el rendimiento empresarial de las MiPyMES debido a que fortalece el incremento de la productividad en un mercado altamente competitivo. Objetivo: el propósito de la investigación es identificar la influencia conjunta del liderazgo transformacional, la cultura clan y el clima organizacional que presentan los grupos informales de trabajo sobre el rendimiento empresarial de las MiPyMES de la región del bajío del estado de Guanajuato, México. Materiales y métodos: el diseño metodológico fue cuantitativo, explicativo, observacional y transversal. Se empleó una muestra de 225 participantes. Para la comprobación de hipótesis se desarrolló un modelo de ecuación estructural (SEM). En cuanto a los índices de bondad y ajuste del SEM (χ2=146.10 gl=52; CFI=0.969 y TLI=0.960; RMSEA=.06) fueron absolutamente aceptables. Resultados: de acuerdo con los resultados obtenidos a través del SEM, se demuestra que el liderazgo transformacional influye positiva y significativamente sobre el clima organizacional (β1=0.26; p<0.001) como también en la cultura clan (β2=0.81; p<0.001) de los grupos informales. Asimismo, la cultura clan de los grupos informales influye (β3=0.61; p<0.001) positiva y significativamente sobre el clima organizacional. Finalmente, el clima organizacional de los grupos informales influye (β4=0.68; p<0.001) positiva y significativamente sobre el rendimiento empresarial de las MiPyMES. Conclusión: Los grupos informales representan un recurso promueve en el éxito de las MiPyMES y con ello, potencializar el rendimiento del sector empresarial de las MiPyMES en estado de Guanajuato.

Abstract Introduction: At present, the leadership, culture and organizational climate exercised by informal groups represent some of the key factors in the business performance of MSMEs because it strengthens the increase in productivity in a highly competitive market. Objective: the purpose of the research is to identify the joint influence of transformational leadership, clan culture and organizational climate presented by informal work groups on the business performance of MSMEs in the Bajio region of the state of Guanajuato, Mexico. Materials and methods: the methodological design was quantitative, explanatory, observational and transversal. A sample of 225 participants was used. For hypothesis testing, a structural equation model (SEM) was developed. Regarding the goodness and fit indices of the SEM (χ2 = 146.10 gl = 52; CFI = 0.969 and TLI = 0.960; RMSEA = 0.06) they were absolutely acceptable. Results: according to the results obtained through the SEM, it is shown that transformational leadership positively and significantly influences the organizational climate (β1 = 0.26; p <0.001) as well as clan culture (β2 = 0.81; p <0.001) of informal groups. Likewise, the clan culture of informal groups influences (β3 = 0.61; p <0.001) positively and significantly on the organizational climate. Finally, the organizational climate of informal groups has a positive and significant influence (β4 = 0.68; p <0.001) on the business performance of MSMEs. Conclusions: informal groups represent a resource that promotes the success of MSMEs and with it, potentiates the performance of the business sector of MSMEs in the state of Guanajuato.

Resumo Introdução: atualmente, a liderança, a cultura e o clima organizacional exercido pelos grupos informais representam alguns dos fatores-chave no desempenho empresarial das MPMEs, pois fortalecem o aumento da produtividade em um mercado altamente competitivo. Objetivo: o objetivo da pesquisa é identificar a influência conjunta da liderança transformacional, cultura de clã e clima organizacional apresentada por grupos de trabalho informais sobre o desempenho empresarial das MPMEs na região de Bajío, no estado de Guanajuato, México. Materiais e métodos: o desenho metodológico foi quantitativo, explicativo, observacional e transversal. Uma amostra de 225 participantes foi utilizada. Para o teste de hipótese, um modelo de equação estrutural (SEM) foi desenvolvido. Em relação aos índices de bondade e ajuste do SEM (χ2 = 146,10 gl = 52; CFI = 0,969 e TLI = 0,960; RMSEA = 0,06), eles foram absolutamente aceitáveis. Resultados: de acordo com os resultados obtidos por meio do SEM, mostra-se que a liderança transformacional influência positiva e significativamente o clima organizacional (β1 = 0,26; p <0,001) e também a cultura do clã (β2 = 0,81; p <0,001) de grupos informais. Da mesma forma, a cultura de clã dos grupos informais influência (β3 = 0,61; p <0,001) positiva e significativamente no clima organizacional. Por fim, o clima organizacional dos grupos informais influência (β4 = 0,68; p <0,001) de forma positiva e significativa no desempenho empresarial das MPMEs. Conclusões: os grupos informais representam um recurso que promove o sucesso das MPMEs e com isso potencializa a atuação do setor empresarial das MPMEs no estado de Guanajuato.

Thermodynamic, structural and dynamic properties of ionic liquids [C4mim][CF3COO], [C4mim][Br] in the condensed phase, using molecular simulations.

In this work a series of thermodynamic, structural, and dynamical properties for the 1-butyl-3-methylimidazolium trifluoroacetate ([C4mim][CF3COO]) and 1-butyl-3-methylimidazolium bromide, ([C4mim][Br]) ionic liquids (ILs) were calculated using Non-polarizable Force Fields (FF), parameterized using a methodology developed previously within the research group, for condensed phase applications. Properties such as the Vapor-Liquid Equilibrium (VLE) curve, critical points (ρ c, T c), Radial, Spatial and Combined Distribution Functions and self-diffusion coefficients were calculated using Equilibrium Molecular Dynamics simulations (EMD); other properties such as shear viscosities and thermal conductivities were calculated using Non-Equilibrium Molecular Dynamics simulations (NEMD). The results obtained in this work indicated that the calculated critical points are comparable with those available in the literature. The calculated structural information for these two ILs indicated that the anions interact mainly with hydrogen atoms from both the imidazolium ring and the methyl chain; the bromide anion displays twice the hydrogen coordination number than the oxygen atoms from the trifluoroacetate anion. Furthermore, Non-Covalent interactions (NCI index), determined by DFT calculations, revealed that some hydrogen bonds in the [C4mim][Br] IL displayed similar strength to those in the [C4mim][CF3COO] IL, in spite of the shorter O--H distances found in the latter IL. The majority of the calculated transport properties presented reasonable agreement with the experimental available data. Nonetheless, the self-diffusion coefficients determined in this work are under-estimated with respect to experimental values; however, by escalating the electrostatic atomic charges for the anion and cation to ±0.8e, only for this property, a remarkable improvement was obtained. Experimental evidence was recovered for most of the calculated properties and to the best of our knowledge, some new predictions were done mainly in thermodynamic states where data are not available. To validate the FF, developed previously within the research group, dynamic properties were also evaluated for a series of ILs such as [C4mim][PF6], [C4mim][BF4], [C4mim][OMs], and [C4mim][NTf2] ILs.

Molecular dynamics simulations of 2-(dimethylamino)ethanol (DMEA).

We develop a multipurpose force field to investigate the properties of the condensed phases of 2-(dimethylamino)ethanol (DMEA). We use ab initio computations at the HF/6-311++G(2d,2p) level to derive partial charges, obtain force constants, and compute the electrostatic potential of the DMEA. We find that the HF predictions for the dipole moment are in excellent agreement with the experimental result (2.6 D). The computations also show the strong preference of DMEA to form intramolecular hydrogen bonds between the hydrogen in the alcohol group and nitrogen. We have tested the accuracy of our force field by computing coexistence and interfacial properties as well as thermal conductivities in wide range of thermodynamic states. In all these instances we find excellent agreement with the available experimental data. We have further investigated the structure of the liquid by computing pair correlations. Our results indicate a clear preference for DMEA to form low-dimensional structures, such as linear and bifurcated chains, which are driven by the association of the DMEA molecules via the alcohol group. Overall, our force field provides a good basis to compute the bulk and interfacial properties of DMEA.

Interfacial and coexistence properties of soft spheres with a short-range attractive Yukawa fluid: molecular dynamics simulations.

Molecular dynamics simulations have been carried out to obtain the interfacial and coexistence properties of soft-sphere attractive Yukawa (SAY) fluids with short attraction range, κ = 10, 9, 8, 7, 6, and 5. All our simulation results are new. These data are also compared with the recently reported results in the literature of hard-core attractive Yukawa (HAY) fluids. We show that the interfacial and coexistence properties of both potentials are different. For the surveyed systems, here we show that all coexistence curves collapse into a master curve when we rescale with their respective critical points and the surface tension curves form a single master curve when we plot γ* vs. T/T(c).

The Wolf method applied to the liquid-vapor interface of water.

The Wolf method for the calculation of electrostatic interactions is applied in a liquid phase and at the liquid-vapor interface of water and its results are compared with those from the Ewald sums method. Molecular dynamics simulations are performed to calculate the radial distribution functions at room temperature. The interface simulations are used to obtain the coexisting densities and surface tension along the coexistence curve. The water model is a flexible version of the extended simple point charge model. The Wolf method gives good structural results, fair coexistence densities, and poor surface tensions as compared with those obtained using the Ewald sums method.

Effect of flexibility on surface tension and coexisting densities of water.

Molecular dynamics simulations of pure water at the liquid-vapor interface are performed using direct simulation of interfaces in a liquid slab geometry. The effect of intramolecular flexibility on coexisting densities and surface tension is analyzed. The dipole moment profile across the liquid-vapor interface shows different values for the liquid and vapor phases. The flexible model is a polarizable model. This effect is minor for liquid densities and is large for surface tension. The liquid densities increase from 2% at 300 K to 9% at 550 K when the force field is changed from a fully rigid simple point charge extended (SPCE) model to that of a fully flexible model with the same intermolecular interaction parameters. The increases in surface tension at both temperatures are around 11% and 36%, respectively. The calculated properties of the flexible models are closer to the experimental data than those of the rigid SPCE. The effect of the maximum number of reciprocal vectors (h(z) (max)) and the surface area on the calculated properties at 300 K is also analyzed. The coexiting densities are not sensitive to those variables. The surface tension fluctuates with h(z) (max) with an amplitude larger than 10 mN m(-1). The effect of using small interfacial areas is slightly larger than the error in the simulations.

Stress anisotropy induced by periodic boundary conditions.

Finite size effects due to periodic boundary conditions are investigated using computer simulations in the canonical ensemble. We study liquids with densities corresponding to typical liquid coexistence densities, and temperatures between the triple and critical points. The components of the pressure tensor are computed in order to analyze the finite size effects arising from the size and geometry of the simulation box. Two different box geometries are considered: cubic and parallelepiped. As expected the pressure tensor is isotropic in cubic boxes, but it becomes anisotropic for small noncubic boxes. We argue this is the origin of the anomalous behavior observed recently in the computation of the surface tension of liquid-vapor interfaces. Otherwise, we find that the bulk pressure is sensitive to the box geometry when small simulation boxes are considered. These observations are general and independent of the model liquid considered. We report results for liquids interacting through short range forces, square well and Lennard-Jones, and also long range Coulombic interactions. The effect that small surface areas have on the surface tension is discussed, and some preliminary results at the liquid vapor-interface for the square well potential are given.

Oscillatory surface tension due to finite-size effects.

The simulation results of surface tension at the liquid-vapor interface are presented for fluids interacting with Lennard Jones and square-well potentials. From the simulation of liquids we have reported [M. González-Melchor et al., J. Chem. Phys. 122, 4503 (2005)] that the components of pressure tensor in parallelepiped boxes are not the same when periodic boundary conditions and small transversal areas are used. This fact creates an artificial oscillatory stress anisotropy in the system with even negative values. By doing direct simulations of interfaces we show in this work that surface tension has also an oscillatory decay at small surface areas; this behavior is opposite to the monotonic decay reported previously for the Lennard Jones fluid. It is shown that for small surface areas, the surface tension of the square-well potential artificially takes negative values and even increases with temperature. The calculated surface tension using a direct simulation of interfaces might have two contributions: one from finite-size effects of interfacial areas due to box geometry and another from the interface. Thus, it is difficult to evaluate the true surface tension of an interface when small surface areas are used. Care has to be taken to use the direct simulation method of interfaces to evaluate the predicted surface tension as a function of interfacial area from capillary-wave theory. The oscillations of surface tension decay faster at temperatures close to the critical point. It is also discussed that a surface area does not show any important effect on coexisting densities, making this method reliable to calculate bulk coexisting properties using small systems.