McMillan–Mayer theory for solvent effects in inhomogeneous systems: Calculation of interaction pressure in aqueous electrical double layers

We demonstrate how to use the McMillan–Mayer theory to include solvent effects in effective solute–solute interactions for inhomogeneous systems, extending a recent derivation [S. Marčelja, Langmuir 16, 6081 (2000)] for symmetric planar double layers to the general case. In the exact treatment, the many-body potential of mean force between the solute molecules can be evaluated for an inhomogeneous reference system in equilibrium with pure bulk solvent. The reference system contains only solvent and a finite number, n, of fixed solute molecules and it has an external potential that in some cases is different from that of the original system. It is discussed how the n-body potential of mean force between the ions for the relevant cases of large n values can be approximated by a sum of effective singlet and pair interactions evaluated in the presence of, on average, all n ions, i.e., at finite concentration. In examples considered in this work we use effective interionic pair potentials evaluated from bulk electrolyte calculations at finite electrolyte concentrations. We calculate the contribution to the double layer interaction pressure arising from the interaction between ions dissolved in aqueous electrolyte. In cases of moderate or high surface charge, calculations show several new effects. At small surface separations one finds attractive and then strongly repulsive contributions. For surface charge density around one negative charge per 70 Å2 the full results for pressures resemble “secondary hydration force” measured in classical experiments in 1980s. When there is a tendency for ions to adsorb at the surfaces there is a marked change in behavior. The force is then oscillatory, reminiscent of results obtained with the surface force apparatus at low electrolyte concentration

Density fluctuations of hard-sphere fluids in narrow confinement

Spatial confinement induces microscopic ordering of fluids, which in turn alters many of their dynamic and thermodynamic properties. However, the isothermal compressibility has hitherto been largely overlooked in the literature, despite its obvious connection to the underlying microscopic structure and density fluctuations in confined geometries. Here, we address this issue by probing density profiles and structure factors of hard- sphere fluids in various narrow slits, using x-ray scattering from colloid-filled nanofluidic containers and integral-equation-based statistical mechanics at the level of pair distributions for inhomogeneous fluids. Most importantly, we demonstrate that density fluctuations and isothermal compressibilities in confined fluids can be obtained experimentally from the long-wavelength limit of the structure factor, providing a formally exact and experimentally accessible connection between microscopic structure and macroscopic, thermodynamic properties. Our approach will thus, for example, allow direct experimental verification of theoretically predicted enhanced density fluctuations in liquids near solvophobic interfaces

Surface interactions in simple electrolytes

We present some new results and concepts which have emerged from the rigorous application of the integral equation methods of liquid state physics to the problem of inhomogeneous ionic fluids in the vicinity of charged surfaces. The report summarizes our recent results for surface interactions in mono- and divalent electrolytes obtained using the anisotropic Hypemetted Chain approximation. Compared to the Poisson-Boltzmann mean-field theory the major differences are an attractive contribution to the surface interaction due to ion-ion correlations and a repulsion from the ionic cores. The attraction is particularly strong in divalent electrolytes, where the double-layer repulsion is dramatically weakened or turned into an attraction. Some consequences of this pressure contribution and its connection to Van der Waals interactions are pointed out. The size of the ions affects the pressure predominantly at short surface separations, and then to a substantial extent for large ion radii or at high surface charge.Nous présentons des résultats et des concepts nouveaux qui proviennent de l'application rigoureuse des méthodes d'équation intégrale de la physique des liquides au problème des fluides ioniques qui sont inhomogènes au voisinage de surfaces chargées. Cet article rassemble nos résultats récents obtenus par l'approximation HNC anisotrope pour les interactions entre surfaces dans les électrolytes mono- et divalents. Par comparaison avec la théorie de champ moyen de Poisson-Boltzmann, les différences principales sont une contribution attractive aux interactions entre surface due aux corrélations ion-ion et une répulsion causée par les cœurs durs des ions. L'attraction est particulièrement forte dans les électrolytes divalents, où la répulsion de double couche est spectaculairement réduite ou transformée en attraction. Nous indiquons quelques conséquences de cette contribution aux pressions ainsi que son lien avec les interactions de Van der Waals. La taille des ions intervient dans la pression surtout pour les faibles distances entre surfaces, et principalement pour les ions de grands rayons ou à forte densité de charge

Controlled deposition of highly ordered soluble acene thin films:Effect of morphology and crystal orientation on transistor performance

\u3cp\u3e(Figure Presented) Controlling the morphology of soluble small molecule organic semiconductors is crucial for the application of such materials in electronic devices. Using a simple dip-coating process we systematically vary the film drying speed to produce a range of morphologies, including oriented needle-like crystals. Structural characterization as well as electrical transistor measurements show that intermediate drying velocities produce the most uniformly aligned films.\u3c/p\u3