
Luis Miguel VarelaUniversity of Santiago de Compostela | USC · Department of Particle Physics
Luis Miguel Varela
Physics
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Publications (120)
La mecánica estadística constituye, sin duda alguna, una de las ramas centrales de la física contemporánea, conectando la turbulenta y aleatoria dinámica microscópica de los sistemas físicos con su descripción macroscópica, propia de la termodinámica. Este manual transita algunos de los principales resultados de esta disciplina, comenzando con la i...
The most currently used ionic liquids (ILs) are protic ionic liquids (PILs), subject to extensive investigation regarding their physical properties. These compounds along with their mixtures with other substances such as salts and solvents, serve as electrolytes in next generation electrochemical smart devices, and emerge as viable candidates to re...
Phase transition temperatures of three ionic liquids (ILs) with ammonium cations of different alkyl chain length and nitrate common anion were determined in this paper by differential scanning calorimetry. Thermal stability of these ILs in air and nitrogen atmosphere and the maximum operation temperatures of the compounds were also studied using a...
The application of ionic liquids in lithium ion batteries has increased in recent years in order to find new electrolytes that improve both performance and safety. The gathering of experimental data in order to understand the behaviour of these systems will help us design electrolytes that can be optimized for a specific objective.
Data on electrol...
We have built an experimental device with the aim to measure the expected electrical conductivity anisotropy in a liquid crystal, obtained from the gelation of the ionic liquid 1-ethyl-3-methyl imidazolium decyl sulfate, [EMIM][DSO4]. This ionic liquid has the particularity that it transits to a semi-solid gel state at room temperature when it cont...
15 Thermophysical properties of the ternary liquid mixtures IL + lithium salt + carbonate were studied 16 in this work in terms of thermal transition temperatures, thermal stability and electrical conductiv-17 ity of mixtures against pure compounds. The selected ionic liquid is the N-butyl-pyrrolidinium 18 bis(trifluoromethanesulfonyl)imide; the li...
In this work, viscosity, density and electrical conductivity of the ionic liquid, Ethylimidazolium Nitrate pure and doped with lithium nitrate salt were determined in order to check the properties’ improvement of the ionic liquid with the salt addition to consider these mixtures as potential electrolytes. Viscosity decreases exponentially as temper...
The thermal behavior of the mixtures of two protic ionic liquids (ethylammonium nitrate (EAN) and ethylimidazolium nitrate ([EIm][NO3])) with lithium nitrate salt (LiNO3), and one aprotic ionic liquid (butyl methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMpyrr][TFSI)) with lithium bis(trifluoromethylsulfonyl)imide salt (LiTFSI), at differ...
The ionic liquids (ILs) doped with metal salts have become a real alternative as electrolytes for batteries, but the right choice of these compounds for reaching the adequate properties and performance is still a challenge, and strategies are therefore needed for achieving it. The thermophysical properties of IL 1-butyl-1-methylpyrrolidinium bis[(t...
The effect of the nitrate salt addition on the liquid range of the protic ionic liquid ethylammonium nitrate (EAN) was studied in this work. The salts used were monovalent, LiNO3; divalent, Mg(NO3)2 and Ca(NO3)2, and trivalent, Al(NO3)3; inorganic salts containing metals relevant for electrochemical applications. Solid-liquid transitions, which def...
In this paper we show the synthesis, structural characterization, phase diagram and physical properties of the ionic liquid 1-ethyl-3-methyl imidazolium decyl sulfate [EMIm][DSO4] for the first time. At 25 ºC it is a crystalline solid or a liquid depending on the thermal history, being its melting point about 33 ºC and its solidification one about...
Ionic liquids are attractive because they offer versatility in the design of organic salts. As ion-rich media, ionic liquids can control the systems properties by tuning the size, charge, and shape of the composing ions. Whilst the focus has mainly been on the potential applications of ionic liquids as solvents, they also provide innovative opportu...
We report measurements of density, viscosity and electrical (ionic) conductivity of three aqueous mixtures of 1-ethyl-3-methyl imidazolium octyl-sulfate ([EMIm][OSO4] or C2C1Im-C8SO4) saturated with univalent sulfate salts in the temperature range 10 °C to 90 °C. This ionic liquid presents a lyotropic mesophase about room temperature when mixed wit...
The structural and dynamical properties of bulk mixtures of long-chained primary and secondary alcohols (propanol, butanol, and 2-pentanol) with protic ionic liquids (ethylammonium and butylammonium nitrate) were studied by means of molecular dynamics simulations and small angle X-ray scattering (SAXS). Changes in the structure with the alcohol con...
The objective of this work was to study the effect of the temperature and the lengthening of the linear alkyl chain of the anion in the transport physical properties of the pure ionic liquids 1-ethyl-3-methyl imidazolium n-alkyl sulphate (being n = 0, 1, 2, 4, 6 and 8). Density, viscosity and electrical conductivities were measured at atmospheric p...
A molecular dynamics study of mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]) with magnesium tetrafluoroborate (Mg[BF4]2) confined between two parallel graphene walls is reported. The structure of the system is analyzed by means of ionic density profiles, lateral structure of the first layer close to the graphene surface and...
We report a molecular dynamics study on the effect of electronic polarization on the structure and single-particle dynamics of mixtures of the aprotic ionic liquid 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)-imide ([EMIM][TFSI]) doped with a lithium salt with the same anion at 298 K and 1 bar. In particular, we analyze the effect of e...
This volume sheds light on the current state of complex networks and nonlinear dynamics applied to the understanding of economic and social phenomena ranging from geographical economics to macroeconomics and finance, and its purpose is to give readers an overview of several interesting topics for research at an intermediate level. Three different a...
This chapter aims at reviewing complex network and nonlinear dynamical models and methods that were either developed for or applied to socioeconomic issues, and pertinent to the theme of New Economic Geography. After an introduction to the foundations of the field of complex networks, the present summary introduces some applications of complex netw...
This work describes the behaviour of water molecules in 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid under nanoconfinement between graphene sheets. By means of molecular dynamics simulations, an adsorption of water molecules at the graphene surface is studied. A depletion of water molecules in the vicinity of the neutral and negativel...
Molecular dynamics simulations of mixtures of the protic ionic liquid ethylammonium nitrate (EAN) and the aprotic 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) are reported and the results are compared with experimental density and electrical conductivity measurements. Essentially ideal mixing of the ionic liquids is seen to take plac...
The structure, dynamics, and phase behavior of a binary mixture based on the protic ionic liquid 1-ethylimidazolium bis(trifluoromethanesulfonyl)imide (C2HImTFSI) and imidazole are investigated by (1)H NMR spectroscopy, vibrational spectroscopy, diffusion NMR, calorimetric measurements, and molecular dynamics simulations. Particular attention is gi...
This work is a contribution to the knowledge of the surface and bulk behaviour of mixtures containing an ammonium based protic ionic liquid (PIL) and a molecular solvent. To achieve our goal, we have measured the surface tension and the density of the binary mixtures {ethylammonium nitrate, propylammonium nitrate or butylammonium nitrate + water or...
In this paper we apply spectrally resolved white light interferometry to measure refractive and group index over a wide spectral band from 400 to 1000 nm. The output of a Michelson interferometer is spectrally decomposed by a homemade prism spectrometer with a high resolution camera. The group index is determined directly from the phase extracted f...
By means of density-functional-theoretic calculations, we investigate the structural and electronic properties of a hexagonal InP sheet and of hydrogen-passivated zigzag InP nanoribbons (ZInPNRs) with Stone-Wales (SW)-type defects. Our results show that the influence of this kind of defect is not limited to the defected region but it leads to the f...
In this work we study mechanisms of solvent-mediated ion interactions with charged surfaces in ionic liquids by molecular dynamics simulations, in an attempt to reveal the main trends that determine ion-electrode interactions in ionic liquids. We compare the interfacial behaviour of Li(+) and K(+) at a charged graphene sheet in a room temperature i...
We report a molecular dynamics study of the structure and single-particle dynamics of mixtures of a protic (ethylammonium nitrate) and an aprotic (1-butyl-3-methylimidazolium hexaflurophosphate [BMIM][PF6]) room-temperature ionic liquids doped with magnesium and calcium salts with a common anion at 298.15 K and 1 atm. The solvation of these divalen...
En esta comunicación se presenta las medidas de conductividad eléctrica de disoluciones
binarias acuosas conteniendo 1-etil-3-metilimidazolio decil sulfato, 1-etil-2,3-metilimidazolio decil sulfato y 1-etil-1-metilpirrolidinio decil sulfato realizadas comprendiendo un amplio intervalo de temperaturas. La elección de estos LIs responde a nuestro int...
Trying to contribute to an overall picture of solvation in ionic liquids, we report a review of the main recent results in the field of molecular dynamics simulations of structural and dynamic properties of mixtures of ionic liquids with molecular cosolvents or salts. The majority of reported results indicate that solvation of neutral molecules and...
The bulk structure of mixtures of two protic ionic liquids, propylammonium nitrate and butylammonium nitrate, with a salt with a common anion, is analyzed at room temperature by means of small angle X-ray scattering and classical molecular dynamics simulations. The study of several structural properties, such as density, radial distribution functio...
In this communication the synthesis and physical properties of the new ionic liquid 1-ethyl-3-methyl imidazolium decyl sulfate, are shown. The interest of this new salt lies in its tendency to jellify when it is exposed to the atmosphere by absorbing water from moisture up to about a 10% of its own mass; then it keeps the mass ±2% stable depending...
The thermal conductivities of nine protic ionic liquids (ILs) have been investigated between 293 and 340 K. Within this range, the thermal conductivities are between 0.18 W•m-1•K-1 and 0.30 W•m-1•K-1. These values are higher than those typically associated with oils and aprotic ILs, but lower than those of strongly hydrogen bonding solvents like wa...
The Z-scan technique is used to characterize the nonlinear refraction induced by a train of ultrashort laser pulses (80 fs, repetition rate 80.75 MHz) for a set of ionic liquids as a function of their structural parameters such as cation, anion type and alkyl chain length in the cation. The nonlinear refractive index change is originated by linear...
We report the existence of hydrophobically driven lyotropic rigid gel phase in aqueous mixtures of the ionic liquid (IL) 1-ethyl-3-methyl imidazolium octyl sulfate (EMIM-OS), and we characterize the physical properties of the gel phase by means of density, electrical conductivity and viscosity measurements. Also we include polarized microscopy imag...
The structural properties of a mixture of ethylammonium nitrate (EAN), a protic ionic liquid, and lithium nitrate are explored using X-ray diffraction and reverse Monte Carlo simulation. The system is characterised by the amphiphilically driven polar-apolar domains segregation that is peculiar of EAN, whilst the lithium ions distribute in the charg...
In this work, the effect of molecular cosolvents (water, ethanol, and methanol) on the structure of mixtures of these compounds with a protic ionic liquid (ethylammonium nitrate) is analyzed by means of classical molecular dynamics simulations. Included are as-yet-unreported measurements of the densities of these mixtures, used to test our paramete...
We performed molecular dynamics simulations of mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate with lithium tetrafluoroborate and potassium tetrafluoroborate between two charged and uncharged graphene walls, in order to analyze the structure of the well-known formation of layers that takes place on liquids under confinement. For this purp...
This chapter aims at reviewing complex networks models and methods that were
either developed for or applied to socioeconomic issues, and pertinent to the
theme of New Economic Geography. After an introduction to the foundations of
the field of complex networks, the present summary adds insights on the
statistical mechanical approach, and on the mo...
The structure of solutions of lithium nitrate in a protic ionic liquid with a common anion, ethylammonium nitrate, at room temperature is investigated by means of molecular dynamics simulations. Several structural properties, such as density, radial distribution functions, hydrogen bonds, spatial distribution functions, and coordination numbers, ar...
Structural and dynamical properties of room-temperature ionic liquids containing the cation 1-butyl-3-methylimidazolium ([BMIM]+) and three different anions (hexafluorophosphate, [PF6]-, tetrafluoroborate, [BF4]-, and bis(trifluoromethylsulfonyl)imide, [NTf2]-) doped with several molar fractions of lithium salts with a common anion at 298.15 K and...
In this study, the thermo--hydrodynamic behaviour of the heat transfer fluid in a storage tank connected to a domestic solar collector is investigated. In order to do this, a numerical simulation of the fluid inside the storage tank is performed using Ansys Fluent. The numerical results are then compared with experimental measurements, taken from a...
We present experimental measurements of specific electrical (or ionic) conductivity of seven binary systems of 1-ethyl-3-methyl imidazolium alkyl sulfate (EMIM-CnS) with water or ethanol. Electrical conductivity was measured at 298.15 K in all range of concentrations, and selected mixtures also at 288.15 K, 308.15 K and 318.15 K. The alkyl chain of...
Bahe–Varela (B–V) theory, based in the pseudo-lattice model, explains the thermodynamics of electrolyte solutions over the whole composition range. Thus, Bahe in 1972 extended the Debye–Huckel theory, developed for the most diluted electrolytes, to more concentrated solutions taking into account long range inter-ionic Coulombian forces. The introdu...
We report a systematic study of the effect of the cationic chain length and degree of hydrogen bonding on several equilibrium and transport properties of the first members of the alkylammonium nitrate protic ionic liquids (PILs) family (ethylammonium, propylammonium, and butylammonium nitrate) in the temperature range between 10 and 40 °C. These pr...
We present in this paper experimental measurements of density for pure ionic liquids of the 1-ethyl-3-methylimidazolium alkyl sulfate family, with the alkyl group being ethyl ([EMIM][ES]), butyl ([EMIM][BS]), hexyl ([EMIM][HS]), and octyl ([EMIM][OS]), and their binary mixtures with water and with ethanol in the whole composition range, at 288 K, 2...
In this work, we present surface tension experimental measurements for eight binary systems containing water or ethanol and an ionic liquid (IL) of the 1-ethyl-3-methyl imidazolium alkyl sulphate family, being the alkyl chain of the anion: ethyl, butyl, hexyl and octyl. Measurements were performed at the temperature of 25.0 °C and atmospheric press...
In this work, we used molecular dynamics simulations to analyze in detail the spatial distributions of the different constituents in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate with three polar molecular species: water and two alcohols of different chain lengths (methanol and ethanol). In particular, we report results regarding the in...
The existence of imitative behavior among consumers is a well-known phenomenon in the field of Economics. This behavior is especially common in markets determined by a high degree of innovation, asymmetric information and/or price-inelastic demand, features that exist in the pharmaceutical market. This paper presents evidence of the existence of im...
This paper extends previous ionic conductivity measurements to the phase transition between liquid to solid states, where we present measurements for increasing and decreasing temperatures. Data show the existence of hysteresis loops for some ILs, while others do not present any transition (at least it is not measurable). Seven of the studied ILs h...
In this contribution we will show that the mainstream of contemporary economics, after its initial development in X/X th century analytical and conceptual framework of classical Physics, is still anchored to its conceptual core, the general equilibrium theory, neglecting the evolution of physics itself towards the treatment of complex systems, and...
In this paper, we present a model for the thermal conductivity of ionic liquids and their mixtures with solvents based on the pseudolattice framework developed by Bahe and Varela. To this end, we first discuss the possible interpretation of some of the fundamental elements of lattice dynamics in the context of a liquid and show computational result...
In this work, extensive molecular dynamics simulations of the dynamics of mixtures of ionic liquids (ILs) composed of the cation 1-hexyl-3-methylimidazolium and several anions of different hydrophobicity degrees (Cl(-), BF(4)(-), PF(6)(-)) with alcohols of different chain lengths (methanol and ethanol) are reported. We evaluated the influence of th...
In this work, extensive molecular dynamics simulations of mixtures of alcohols of several chain lengths (methanol and ethanol) with the ionic liquids (ILs) composed of the cation 1-hexyl-3-methylimidazolium and several anions of different hydrophobicity degrees (Cl(-), BF(4)(-), PF(6)(-)) are reported. We analyze the influence of the nature of the...
The existence and properties of mesoscopic self-assembly structures formed by surfactants in protic ionic liquid solutions are reported. Micellar aggregates of n-alkyltrimethylammonium (n = 10, 12, 14, 16) chlorides and bromides and of n-alkylpyridinium (n = 12, 16) chlorides in ethylammonium nitrate and propylammonium nitrate were observed by mean...
We have performed extensive molecular dynamic simulations to analyze the influence of cation and anion natures, and of water concentration, on the structure and dynamics of water-1-alkyl-3-methylimidazolium ionic liquid mixtures. The dependence on water concentration of the radial distribution functions, coordination numbers, and hydrogen bonding d...
By explicitly considering surface roughness at the atomic level, we
quantitatively show that the thermal conductivity of Si nanowires can be lower
than Casimir's classical limit. However, this violation only occurs for deep
surface degradation. For shallow surface roughness, the Casimir formula is
shown to yield a good approximation to the phonon m...
Good thermal insulation is much harder to achieve than electrical insulation. Thus, the astonishingly low thermal conductivities recently reported on Si nanowires came as a surprise, since the displayed values were an order of magnitude lower than predicted by the diffuse boundary limit of Casimir's theory. Recent theoretical work has employed the...
In patchy particle systems where there is a competition between the self-assembly of finite clusters and liquid-vapor phase separation, re-entrant phase behavior can be observed, with the system passing from a monomeric vapor phase to a region of liquid-vapor phase coexistence and then to a vapor phase of clusters as the temperature is decreased at...
The adsorption of atmospheric water on the air–liquid interface of ionic liquids is analyzed by means of a modified version of the Brunauer–Emmet–Teller (BET) multilayer adsorption isotherm including lateral interactions between adsorbed molecules, treated in a mean-field fashion. Recently reported experimental results of water adsorption on hydrop...
In this paper, we demonstrate that it is possible to know the refractive index, nD, of every given mixture of 1-alkyl-3methyl imidazolium tetrafluoroborate with water and ethanol just from the knowledge of the refractive index and density of pure components. To do that, we measured nD for seven different mixtures in all range of existing concentrat...
In this work, it is reported viscosity, density, sound velocity and electrochemical impedance spectroscopy (EIS) measurements of calf thymus DNA in aqueous solutions, in order to investigate the overlapping C* and entanglement concentrations C e . These critical concentrations demark the conditions at which start appearing interactions and entangle...
A statistical mechanical framework for charge transport in ionic liquid–solvent mixtures based on the existence of a statistical lattice structure (pseudolattice) throughout the whole range of concentration is reported. The ion distribution is treated in a mean-field Bragg–Williams-like fashion, and the ionic motion is assumed to take place through...
In a recent paper we presented experimental measurements of the dynamic viscosity and specific electrical conductivity, for binary systems of 1-alkyl-3-methyl imidazolium tetrafluoroborate, [CnMIM][BF4], with water. In this paper we complete those measurements with four systems of [CnMIM][BF4] + ethanol, the alkyl chains of the ionic liquids (ILs)...
In the present paper we present an experimental analysis and a thermal and hydrodynamic modelling of a newly designed flat-plate solar collector characterized by its corrugated channel and by the high surface area directly in contact with the heat transport fluid. The thermal and hydrodynamic modelling of the collector has been performed by means o...
A mathematical model of a serpentine flat-plate solar collector using finite elements is presented. The numerical simulations focus on the thermal and hydrodynamic behaviour of the collector. The energy equation is numerically solved, as are the Navier-Stokes equations for an incompressible fluid. Numerical results involving one part of the serpent...
We present new experimental data of the surface tension and density in binary mixtures of the ionic liquid (IL) family 1-alkyl-3-methyl imidazolium tetrafluoroborate with water and ethanol. Measurements were performed at 25.0 °C and atmospheric pressure, covering all range of possible concentrations. From these data we calculate the surface tension...
Dynamic viscosity and specific electrical conductivity on three binary systems of 1-alkyl-3-methyl imidazolium tetrafluoroborate [CnMIM][BF4] with water covering all ranges of concentrations are presented. Viscosity was measured at 15.0 °C, 25.0 °C, 35.0 °C, and 45.0 °C, while conductivity was measured at those four temperatures for equimolar mixtu...
We investigated the thermal conductivity of InP nanowires with diameters $d=0.83$, 1.66, 2.49, 3.32, 4.15, and 4.97 nm by means of molecular-dynamics simulations using a potential with two-body and three-body contributions. In the temperature range of 100\char21{}700 K, thermal conductivities of approximately $1.0\char21{}2.5\text{ }\text{W}/(\text...
A theoretical model for ionic liquids (ILs) based on a pseudoreticular structural model for the bulk mixture is reported. The original Bahe-Varela pseudolattice theory of concentrated ionic solutions is modified and the short-range interactions modeled by a Lennard-Jones potential. In this framework, the surface tension of the pure IL is calculated...