Martín Pérez-Rodríguez

MD simulations of the formation of stable clusters in mixtures of alkaline salts and imidazolium-based ionic liquids.

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 1 atm were investigated by means of molecular dynamics simulations. The effect of the size of the salt cation was also analyzed by comparing these results with those for mixtures of [BMIM][PF6] with NaPF6. Lithium/sodium solvation and ionic mobilities were analyzed via the study of radial distribution functions, coordination numbers, cage autocorrelation functions, mean-square displacements (including the analysis of both ballistic and diffusive regimes), self-diffusion coefficients of all the ionic species, velocity and current autocorrelation functions, and ionic conductivity in all the ionic liquid/salt systems. We found that lithium and sodium cations are strongly coordinated in two different positions with the anion present in the mixture. Moreover, [Li](+) and [Na](+) cations were found to form bonded-like, long-lived aggregates with the anions in their first solvation shell, which act as very stable kinetic entities within which a marked rattling motion of salt ions takes place. With very long MD simulation runs, this phenomenon is proved to be on the basis of the decrease of self-diffusion coefficients and ionic conductivities previously reported in experimental and computational results.

STATIC STRUCTURE OF ELECTROLYTE SYSTEMS AND THE LINEAR RESPONSE FUNCTION ON THE BASIS OF A DRESSED-ION THEORY

The static structure of a bulk electrolyte solution or colloid system is investigated in the framework of a dressed-ion theory (DIT). The number–number, charge–number, and charge–charge static structure factors are calculated and are seen to depend only on the linear response function of the DIT α(k), the α function therefore determining the charge structure of the fluid in what is an expression of the fluctuation–dissipation theorem. The expression of the static structure factors for one-component charged spheres (OCCS) is evaluated in the random-phase approximation and in a modified version of the mean-spherical approximation (MSA), using the hard-sphere fluid as a reference system, and an explicit expression for the linear response function and dielectric function is obtained. The effective screening length (κ−1) and the transition from monotonic exponential to oscillatory behavior obtained from the modified MSA expression of the α function are seen to improve the ones derived from the second moment c...

Relaxation of the ionic cloud on the basis of a dressed-ion theory

The dynamic response of a bulk electrolyte or colloid solution to an external field is investigated on the basis of a dressed-ion theory (DIT) in a hydrodynamic point of view. The radial part of the perturbed electric field acting on a given ion is explicitly calculated in terms of the DIT quantities derived from the linear response function calculated from the modified mean-spherical aproximation (MMSA) and its static and frequency-dependent limits are analyzed. In the static case, the asymptotic behavior is analyzed and Onsager’s result is reformulated in terms of the effective charges and effective screening length and in the limit of vanishing concentration Debye–Falkenhagen–Onsager results are recovered. In the frequency-dependent DIT transport theory a relation between the field frequency and the time of relaxation of the ionic atmosphere is shown to be needed in order to get real renormalized charges and screening lengths. A decay of the perturbed electrostatic field as the inverse square root of t...

Thermodynamics of electrolyte solutions in the modified mean spherical approximation

The exact Ornstein–Zernike formalism for ionic fluids is seen to be equivalent to the dressed-ion theory (DIT), therefore proving the exact character of this mean-field formalism. The modified mean spherical approximation (MMSA) [Varela et al., J. Chem. Phys. 109, 1930 (1998)] is a modified version of the mean spherical approximation, which corrects some of the deficiencies of the original version of this closure relation in the prediction of the effective screening length. The MMSA effective non-Debye decay length, developed in the framework of the dressed-ion transport theory (DIT) of Kjellander and Mitchell, is an improvement on those of other theoretical and numerical schemes, which include self-consistent second moment approaches, asymptotic expansions, and nonlinear Debye–Huckel approximations. The MMSA screening length is used to analyze thermodynamic magnitudes of the charged fluid such as the internal energy and the osmotic coefficient and the results are seen to fit accurately to hypernetted cha...

Multilayer adsorption model for the protein–ligand interaction

In the present work we present a theoretical formalism based on the combination of the Brunauer–Emmet–Teller multilayer adsorption model with an electrolytic adsorbate, and the results are used to predict binding isotherms of several synthetic penicillin drugs onto human serum albumin. The occurrence of adsorption maxima in these binding processes is correctly predicted by this noncooperative binding model and it is demonstrated to be due to the ionic character of the adsorbate. The effect of the hydrophobic interactions between adsorbate monomers on the value of the maximum number of adsorbed particles is also a matter of study, and it is proven that this number increases with increasing hydrophobic character of the adsorbate.

Conductance of symmetric electrolyte solutions: Formulation of the dressed-ion transport theory (DITT)

The concentration dependence of the electrical conductance of several symmetrical electrolytes is studied using the dressed-ion transport theory (DITT) formalism. This theoretical scheme is constructed from the Fuoss–Onsager theory using the equilibrium dressed-ion theory (DIT) distribution functions for electrolyte and colloid media. The conductance equation is constructed from previous results for the relaxation of the ionic atmosphere [Varela et al., J. Chem. Phys. 110, 4483 (1999)], now completed with the calculation of the concentration-dependent electrophoretic velocity of ions on the basis of a DIT in a hydrodynamic point of view. Onsager’s limiting law for the electrophoretic velocity based on the Debye–Huckel (DH) equilibrium distribution function is reformulated in terms of the DIT renormalized quantities, effective charges and effective screening length. The electrophoretic correction to the velocity calculated in the framework of this DITT is shown to exhibit a nontrivial dependence on the ren...

Permitivity of penicillin V: a thermodynamic study

Abstract A method of determining critical concentrations for aggregation in self-associating systems of low aggregation number by dielectric constant measurement has been applied in a study of the self-association of penicillin V as a function of temperature. Dielectric properties of penicillin V are more sensitive to structural modifications in bulk amphiphilic solutions than the electrical conductivity and provide a simpler method for the determination of critical concentrations of such systems without the necessity of the complex numerical data analysis required in the determination of inflection points in electrical conductivity data, which show a more gradual change throughout the measured concentration. Thermodynamic parameters of micelle formation were derived from the variation of the first critical concentration using a modified form of the mass action model applicable to systems of low aggregation number. The good agreement between the experimental and theoretical standard enthalpies of aggregation,ΔH°m, supports the validity of the proposed model to predict thermodynamic parameters of aggregation.

Cooperative binding of drugs on human serum albumin

In order to explain the adsorption isotherms of the amphiphilic penicillins nafcillin and cloxacillin onto human serum albumin (HSA), a cooperative multilayer adsorption model is introduced, combining the Brunauer–Emmet–Teller (BET) adsorption isotherm with an amphiphilic ionic adsorbate, whose chemical potential is derived from Guggenheims theory. The non-cooperative model has been previously proved to qualitatively predict the measured adsorption maxima of these drugs [Varela, L. M., García, M., Pérez-Rodríguez, M., Taboada, P., Ruso, J. M., and Mosquera, V., 2001, J. chem. Phys., 114, 7682]. The surface interactions among adsorbed drug molecules are modelled in a mean-field fashion, so the chemical potential of the adsorbate is assumed to include a term proportional to the surface coverage, the constant of proportionality being the lateral interaction energy between bound molecules. The interaction energies obtained from the empirical binding isotherms are of the order of tenths of the thermal energy, therefore suggesting the principal role of van der Waals forces in the binding process.