Individual adsorption of low volatility pheromones: Amphiphilic molecules on a clean water-air interface.

Environmental conditions can alter olfactory scent and chemical communication among biological species. In particular, odorant molecules interact with aerosols. Thermodynamics variables governing the adsorption from air to water surface of bombykol, the most studied pheromone, and of three derivative molecules, bombykal, bombykoic acid, and bombykyle acetate, are computed by steered and un-biased molecular dynamics in order to compare the role of their polar head group on adsorption on aqueous aerosols. When adsorbed, the molecule center of mass stands at about 1.2 Å from the interface and oscillates on the same length scale, trapped in an energy well. Gibbs energy of adsorption and desorption time of bombykol are found to be 9.2 kBT and 59 µs, respectively. The following ordering between the molecules is observed, reading from the more to the least adsorbed: bombykoic acid > bombykol > bombykoic acetate > bombykal. It originates from a complex interplay of entropy and enthalpy. The entropy and enthalpy of adsorption are discussed in the light of structural arrangement, H-bonding, and hydrophilic tail positioning of the molecules at the interface. Our results show that, when dispersed in the air, pheromones adsorb on aqueous aerosols. However, the individual residence time is quite short on pure water surfaces. Aerosols can, therefore, only have a decisive influence on chemical communication through collective effects or through their chemical composition that is generally more complex than that of a pure water surface.

Selective layer-free blood serum ionogram based on ion-specific interactions with a nanotransistor.

Despite being ubiquitous in the fields of chemistry and biology, the ion-specific effects of electrolytes pose major challenges for researchers. A lack of understanding about ion-specific surface interactions has hampered the development and application of materials for (bio-)chemical sensor applications. Here, we show that scaling a silicon nanotransistor sensor down to ~25 nm provides a unique opportunity to understand and exploit ion-specific surface interactions, yielding a surface that is highly sensitive to cations and inert to pH. The unprecedented sensitivity of these devices to Na+ and divalent ions can be attributed to an overscreening effect via molecular dynamics. The surface potential of multi-ion solutions is well described by the sum of the electrochemical potentials of each cation, enabling selective measurements of a target ion concentration without requiring a selective organic layer. We use these features to construct a blood serum ionogram for Na+, K+, Ca2+ and Mg2+, in an important step towards the development of a versatile, durable and mobile chemical or blood diagnostic tool.

Thermodynamic Description of Synergy in Solvent Extraction: II Thermodynamic Balance of Driving Forces Implied in Synergistic Extraction.

In the second part of this study, we analyze the free energy of transfer in the case of synergistic solvent extraction. This free energy of the transfer of an ion in dynamic equilibrium between two coexisting phases is decomposed into four driving forces combining long-range interactions with the classical complexation free energy associated with the nearest neighbors. We demonstrate how the organometallic complexation is counterbalanced by the cost in free energy related to structural change on the colloidal scale in the solvent phase. These molecular forces of synergistic extraction are driven not only by the entropic term associated with the tight packing of electrolytes in the solvent and by the free energy cost of coextracting water toward the hydrophilic core of the reverse aggregates present but also by the entropic costs in the formation of the reverse aggregate and by the interfacial bending energy of the extractant molecules packed around the extracted species. Considering the sum of the terms, we can rationalize the synergy observed, which cannot be explained by classical extraction modeling. We show an industrial synergistic mixture combining an amide and a phosphate complexing site, where the most efficient/selective mixture is observed for a minimal bending energy and maximal complexation energy.

Thermodynamic Description of Synergy in Solvent Extraction: I. Enthalpy of Mixing at the Origin of Synergistic Aggregation.

Revisiting aggregation of extractant molecules into water-poor mixed reverse micelles, we propose in this paper to identify the thermodynamic origins of synergy in solvent extraction. Considering that synergistic extraction properties of a mixture of extractants is related to synergistic aggregation of this mixture, we identify here the elements at the origin of synergy by independently investigating the effect of water, acid, and extracted cations. Thermodynamic equations are proposed to describe synergistic aggregation in the peculiar case of synergistic solvent extraction by evaluating critical aggregation concentration (CAC) as well as specific interactions between extractants due to the presence of water, acid and cations. Distribution of two extractant molecules in the free extractants and in reverse micelles was assessed, leading to an estimation of the in-plane interaction parameter between extractants in the aggregates as introduced by Bergström and Eriksson ( Bergström, M.; Eriksson, J. C. A Theoretical Analysis of Synergistic Effects in Mixed Surfactant Systems . Langmuir 2000 , 16 , 7173 - 7181 ). Based on this model, we study the N,N'-dimethyl-N,N'-dioctylhexylethoxymalonamide (DMDOHEMA) and di(2-ethylexyl) phosphoric acid (HDEHP) mixture and show that adding nitric acid enhances synergistic aggregation at the equimolar ratio of the two extractants and that this configuration can be related to a favored enthalpy of mixing.

Water electrolysis and energy harvesting with zero-dimensional ion-sensitive field-effect transistors.

The relationship of the gas bubble size to the size distribution critically influences the effectiveness of electrochemical processes. Several optical and acoustical techniques have been used to characterize the size and emission frequency of bubbles. Here, we used zero-dimensional (0D) ion-sensitive field-effect transistors (ISFETs) buried under a microbath to detect the emission of individual bubbles electrically and to generate statistics on the bubble emission time. The bubble size was evaluated via a simple model of the electrolytic current. We suggest that energy lost during water electrolysis could be used to generate electric pulses at an optimal efficiency with an array of 0D ISFETs.

Synergism by coassembly at the origin of ion selectivity in liquid-liquid extraction.

In liquid-liquid extraction, synergism emerges when for a defined formulation of the solvent phase, there is an increase of distribution coefficients for some cations in a mixture. To characterize the synergistic mechanisms, we determine the free energy of mixed coassembly in aggregates. Aggregation in any point of a phase diagram can be followed not only structurally by SANS, SAXS, and SLS, but also thermodynamically by determining the concentration of monomers coexisting with reverse aggregates. Using the industrially used couple HDEHP/TOPO forming mixed reverse aggregates, and the representative couple U/Fe, we show that there is no peculiarity in the aggregates microstructure at the maximum of synergism. Nevertheless, the free energy of aggregation necessary to form mixed aggregates containing extracted ions in their polar core is comparable to the transfer free energy difference between target and nontarget ions, as deduced from the synergistic selectivity peak.

Two-scale Brownian dynamics of suspensions of charged nanoparticles including electrostatic and hydrodynamic interactions.

We propose here a multiscale strategy based on continuous solvent Brownian dynamics (BD) simulations to study the dynamical properties of aqueous suspensions of charged nanoparticles. We extend our previous coarse-graining strategy [V. Dahirel et al., J. Chem. Phys. 126, 114108 (2007)] to account for hydrodynamic interactions between solute particles. Within this new procedure, two BD simulations are performed: (1) The first one investigates the time scales of the counterions and coions (the microions) with only one nanoparticle in the simulation box but explicit microions, (ii) the second one investigates the larger time scale of the nanoparticles with numerous nanoparticles in the simulation box but implicit microions. We show how individual and collective transport coefficients can be computed from this two-scale procedure. To ensure the validity of our procedure, we compute the transport coefficients of a 10-1 model electrolyte in aqueous solution with a 1-1 added salt. We do a systematic comparison between the results obtained within the new procedure and those obtained with explicit BD simulations of the complete system containing several nanoparticles and explicit microions. The agreement between the two methods is found to be excellent: Even if the new procedure is much faster than explicit simulations, it allows us to compute transport coefficients with a good precision. Moreover, one step of our procedure also allows us to compute the individual transport coefficients relative to the microions (self-diffusion coefficients and electrophoretic mobility).

Ion-mediated interactions between charged and neutral nanoparticles.

Monte-Carlo simulations are used to study the ion-mediated effective interaction between weakly charged and highly charged nanoparticles in an implicit solvent. Three models of nanoparticles are successively studied, from crude charged hard spheres to dipolar and non-spherical nanoparticles. The analysis of the effective potential revealed that in an electrolyte solution, even a neutral nanoparticle feels an important repulsive force in the presence of a charged nanoparticle, with a typical range similar to the Debye length. When the two nanoparticles carry charges of opposite sign, we have shown that this repulsion can reverse the effect of the direct attractive electrostatic potential at short distances. This also yields the change of sign of the effective potential as a function of the relative orientations of two anisotropic nanoparticles. Moreover, we found that the 3-body terms of the effective potentials can overcome the 2-body terms, which is not observed in the case of symmetrically charged nanoparticles.

Electrostatic relaxation and hydrodynamic interactions for self-diffusion of ions in electrolyte solutions.

The concentration dependence of self-diffusion of ions in solutions at large concentrations has remained an interesting yet unsolved problem. Here we develop a self-consistent microscopic approach based on the ideas of mode-coupling theory. It allows us to calculate both contributions which influence the friction of a moving ion: the ion atmosphere relaxation and hydrodynamic interactions. The resulting theory provides an excellent agreement with known experimental results over a wide concentration range. Interestingly, the mode-coupling self-consistent calculation of friction reveal a nonlinear coupling between the hydrodynamic interactions and the ion atmosphere relaxation which enhances ion diffusion by reducing friction, particularly at intermediate ion concentrations. This rather striking result has its origin in the similar time scales of the relaxation of the ion atmosphere relaxation and the hydrodynamic term, which are essentially given by the Debye relaxation time. The results are also in agreement with computer simulations, with and without hydrodynamic interactions.

How the excluded volume architecture influences ion-mediated forces between proteins.

The effective interactions between model proteins of various shapes are computed by means of Monte Carlo simulations. In particular, we determine how the modification of the excluded volume architecture influences both entropic and purely electrostatic ion-mediated forces between proteins. We find that interprotein interactions are strongly affected by protein shape, which results in a high decrease of electrostatic screening for typical active site geometries. Effective interactions are then closer to the direct Coulombic interactions, and both affinity and selectivity are enhanced by several orders of magnitude.

Analytical theories of transport in concentrated electrolyte solutions from the MSA.

Ion transport coefficients in electrolyte solutions (e.g., diffusion coefficients or electric conductivity) have been a subject of extensive studies for a long time. Whereas in the pioneering works of Debye, Hückel, and Onsager the ions were entirely characterized by their charge, recent theories allow specific effects of the ions (such as the ion size dependence or the pair association) to be obtained, both from simulation and from analytical theories. Such an approach, based on a combination of dynamic theories (Smoluchowski equation and mode-coupling theory) and of the mean spherical approximation (MSA) for the equilibrium pair correlation, is presented here. The various predicted equilibrium (osmotic pressure and activity coefficients) and transport coefficients (mutual diffusion, electric conductivity, self-diffusion, and transport numbers) are in good agreement with the experimental values up to high concentrations (1-2 mol L(-1)). Simple analytical expressions are obtained, and for practical use, the formula are given explicitly. We discuss the validity of such an approach which is nothing but a coarse-graining procedure.

An analytical model for probing ion dynamics in clays with broadband dielectric spectroscopy.

A simple two-state model is proposed to explicitly derive the ionic contribution to the frequency-dependent dielectric permittivity of clay. This model is based on a separation of time scales and accounts for two possible solvation modes (inner/outer-sphere complexes) for ions in the interlayer spacing and a possible chemical exchange between both forms. The influence on the permittivity of thermodynamic (distribution constant K(d)) and dynamic (diffusion coefficient, chemical relaxation rate) parameters is discussed. In turn, this model is used to analyze experimental data obtained with Na-montmorillonite for two relative humidities. The values of the parameters extracted from these measurements, and their variation with water content, show that the proposed model is at least reasonable.

Ionic self-diffusion in concentrated aqueous electrolyte solutions.

A self-consistent microscopic theory is developed to understand the anomalously weak concentration dependence of ionic self-diffusion coefficient D(ion) in electrolyte solutions. The self-consistent equations are solved by using the mean spherical approximation expressions of the static pair correlation functions for unequal sizes. The results are in excellent agreement both with the known experimental results for many binary electrolytes and also with the new Brownian dynamics simulation results. The calculated velocity time correlation functions also show quantitative agreement with simulations. The theory also explains the reason for observing different D(ion) in recent NMR and neutron scattering experiments.

Ab initio study of deuterium in the dissociating regime: sound speed and transport properties.

The sound speed and the transport properties of dense hydrogen (deuterium) are computed from local spin-density approximation molecular-dynamics simulations in the dissociating regime. The sound speed c(s) is evaluated from the thermodynamical differentiation of the equation of state in the molecular phase and is in very good agreement with recent experiments. The diffusion constant D and the viscosity eta are extracted from simulations performed at V=6, 4, and 2.7 cm(3)/mole, corresponding, respectively, for deuterium at rho=0.672, 1.0, and 1.5 g/cm(3) in a range of temperatures 1000 K