Water on Oxide Surfaces

Most oxidE.. surfaces interact with ambient water vapor to form a layer of chemisorbed hydroxyls. Physical adsorption of multilayer water readily starts by hydrogen bonding onto the hydroxyl array. The normally hydrophilic surface 0£ silica can be modified by heating to produce a predominantly hydrophobic matrix which contains but a few isolated hydrophilic sites, around which water adsorbs in clusters. The authors\u27 results on both hydrophilic and hydrophobed oxides are discussed and compared to results in the literature

Adsorption from Solution

A major problem of the thermodynamic theory of adsorption at the solid/liquid interface is concerned with the definition of the heterogeneous surface in terms of mathematically treatable model. The paper gives a review of the theoretical approaches applied to studies of adsorption from concentrated and diluted binary mixtures. Experimental work has enjoyed much success since the uniform surface, graphitized carbon blacks became available. Results are described and discussed of measurements of heats of immersion on such surfaces

Adsorption from Solution

A major problem of the thermodynamic theory of adsorption at the solid/liquid interface is concerned with the definition of the heterogeneous surface in terms of mathematically treatable model. The paper gives a review of the theoretical approaches applied to studies of adsorption from concentrated and diluted binary mixtures. Experimental work has enjoyed much success since the uniform surface, graphitized carbon blacks became available. Results are described and discussed of measurements of heats of immersion on such surfaces

Sorption of Water and Polar-Nonpolar Organic Vapors on Microporous Chromia

High surface area, narrow particle size distribution chromic;t was investigated for its microporosity. Adsorption studies with argon, water vapor, methanol, isopropanol, butane, isobutane, neopentane, and heptane indicated alternative approaches to the determination of micropore volume

Water on Oxide Surfaces

Most oxidE.. surfaces interact with ambient water vapor to form a layer of chemisorbed hydroxyls. Physical adsorption of multilayer water readily starts by hydrogen bonding onto the hydroxyl array. The normally hydrophilic surface 0£ silica can be modified by heating to produce a predominantly hydrophobic matrix which contains but a few isolated hydrophilic sites, around which water adsorbs in clusters. The authors\u27 results on both hydrophilic and hydrophobed oxides are discussed and compared to results in the literature

Gas-Liquid Nucleation in Two Dimensional System

We study the nucleation of the liquid phase from a supersaturated vapor in two dimensions (2D). Using different Monte Carlo simulation methods, we calculate the free energy barrier for nucleation, the line tension and also investigate the size and shape of the critical nucleus. The study is carried out at an intermediate level of supersaturation(away from the spinodal limit). In 2D, a large cut-off in the truncation of the Lennard-Jones (LJ) potential is required to obtain converged results, whereas low cut-off (say, $2.5\sigma$ is generally sufficient in three dimensional studies, where $\sigma$ is the LJ diameter) leads to a substantial error in the values of line tension, nucleation barrier and characteristics of the critical cluster. It is found that in 2D, the classical nucleation theory (CNT) fails to provide a reliable estimate of the free energy barrier. It underestimates the barrier by as much as 70% at the saturation-ratio S=1.1 (defined as S=P/PC, where PC is the coexistence pressure at reduced temperature $T^{\star}= 0.427$). Interestingly, CNT has been found to overestimate the nucleation free energy barrier in three dimensional (3D)systems near the triple point. In fact, the agreement with CNT is worse in 2D than in 3D. Moreover, the existing theoretical estimate of the line tension overestimates the value significantly.Comment: 24 pages, 8 figure

Characterization of the Surface of High-Area Ni(OH)2 and NiO

The surface propertij:ls of Ni(OH)2, and the mechanism of decomposition of Ni(OH)2 to NiO were investigated by a variety , of experimental approaches including gas adsorption, heats of immersion and diffuse ir reflectance. The N~(OHh samples were prepared by bubbling ammonia gas through a Ni(N03)2 solution at different temperatures. Ni(OHh is relatively hydrophobic while NiO is completely hydrophilic. A relatiQIIlShip was estabhlshed between the specific surface areas of rthe parent Ni(OH)2 and Its decomposition product NiO which supports a mechanism of dehydration at 200 °c involving separation along the hexagona~ planes. This mechanism was confirmed by electron micrographs during decomposmon of Ni(OHh to NiO. Gravimetric studies of the decomposition of Ni(OH)2 at 200 °c iindicated that approximately 14°/o of the hydroxyl groups are not removed. Diffuse ir reflectance studies showed that surface hydroxyls were not removed. Physical adsorption of water vapor on Ni(OHh as a function of surface area supports the hypo·thesis that the basal planes are hydrophobic and the crystal edges are hydrophilic

Simulation of fluid-solid coexistence in finite volumes: A method to study the properties of wall-attached crystalline nuclei

The Asakura-Oosawa model for colloid-polymer mixtures is studied by Monte Carlo simulations at densities inside the two-phase coexistence region of fluid and solid. Choosing a geometry where the system is confined between two flat walls, and a wall-colloid potential that leads to incomplete wetting of the crystal at the wall, conditions can be created where a single nanoscopic wall-attached crystalline cluster coexists with fluid in the remainder of the simulation box. Following related ideas that have been useful to study heterogeneous nucleation of liquid droplets at the vapor-liquid coexistence, we estimate the contact angles from observations of the crystalline clusters in thermal equilibrium. We find fair agreement with a prediction based on Young's equation, using estimates of interface and wall tension from the study of flat surfaces. It is shown that the pressure versus density curve of the finite system exhibits a loop, but the pressure maximum signifies the "droplet evaporation-condensation" transition and thus has nothing in common with a van der Waals-like loop. Preparing systems where the packing fraction is deep inside the two-phase coexistence region, the system spontaneously forms a "slab state", with two wall-attached crystalline domains separated by (flat) interfaces from liquid in full equilibrium with the crystal in between; analysis of such states allows a precise estimation of the bulk equilibrium properties at phase coexistence

Curvature Dependence of Surface Free Energy of Liquid Drops and Bubbles: A Simulation Study

We study the excess free energy due to phase coexistence of fluids by Monte Carlo simulations using successive umbrella sampling in finite LxLxL boxes with periodic boundary conditions. Both the vapor-liquid phase coexistence of a simple Lennard-Jones fluid and the coexistence between A-rich and B-rich phases of a symmetric binary (AB) Lennard-Jones mixture are studied, varying the density rho in the simple fluid or the relative concentration x_A of A in the binary mixture, respectively. The character of phase coexistence changes from a spherical droplet (or bubble) of the minority phase (near the coexistence curve) to a cylindrical droplet (or bubble) and finally (in the center of the miscibility gap) to a slab-like configuration of two parallel flat interfaces. Extending the analysis of M. Schrader, P. Virnau, and K. Binder [Phys. Rev. E 79, 061104 (2009)], we extract the surface free energy gamma (R) of both spherical and cylindrical droplets and bubbles in the vapor-liquid case, and present evidence that for R -> Infinity the leading order (Tolman) correction for droplets has sign opposite to the case of bubbles, consistent with the Tolman length being independent on the sign of curvature. For the symmetric binary mixture the expected non-existence of the Tolman length is confirmed. In all cases {and for a range of radii} R relevant for nucleation theory, gamma(R) deviates strongly from gamma (Infinity) which can be accounted for by a term of order gamma(Infinity)/gamma(R)-1 ~ 1/R^2. Our results for the simple Lennard-Jones fluid are also compared to results from density functional theory and we find qualitative agreement in the behavior of gamma(R) as well as in the sign and magnitude of the Tolman length.Comment: 25 pages, submitted to J. Chem. Phy