A.C Zettlemoyer

Water at interfaces: Molecular structure and dynamics

Abstract Recent advances in molecular mechanics and dynamics of water on inorganic surfaces are reviewed. The structures of hydroxylated surfaces and water-hydroxyl adducts, determined by LEED-Auger studies and infrared spectroscopy, are described to a detail only recently resolved. New mechanistic concepts of nucleation and freezing have ensued from the analysis of time-correlation functions, showing that low-frequency angular perturbations dominate the different behavior of water in bulk phases and at surfaces. While the rotational motion of water molecules in the liquid phase can be interpreted as rotation modulated by making and breaking hydrogen bonds with the neighbors, water adsorbed on nucleating catalysis undergoes an irreversible and complete reorientation in a fraction of the rotational period.

Water on Organosilane-Treated Silica Surfaces

Abstract Both a fully hydroxylated silica (HiSil 233) and a partially hydroxylated silica (Cab-o-Sil M-5) were treated with organosilanes and a silazane. Water adsorption isotherms were then monitored on the resulting products. Reflectance IR spectroscopic measurements were used to follow the reactions of the surface hydroxyls with the silanes and to examine the subsequent water adsorption. The main objective was to ascertain the water susceptibilities of the different functional groups and the availability to water of any residual hydroxyls. The silazane and silanes employed were: hexamethyldisilazane, vinyl-trichlorosilane, and the alkoxysilanes: γ-glycidoxypropyltrimethoxysilane, 3-mercaptopropyltrimethyoxysilane, and amino-propyltriethoxysilane. The latter three were reacted from the liquid phase, first hydrolyzing to the silanetriols, with additional complications from condensation reactions. The vinyl compound and the disilazane were reacted from the vapor phase. Sharp differences between the results for the fully hydroxylated and the partially (25%) hydroxylated silica were found. The silanetriols upon heat treatment formed a multilayer polymer network on the latter and only a two-dimensional network on the former apparently due to hydrogen bonding to residual hydroxyls. The reacted silanes formed umbrellas over reacted hydroxyls on the former which were nevertheless available to water molecules. With the latter, however, a much more hydrophobic surface was produced. A number of details were also provided by the reflectance IR measurements.

Adsorption studies on hydrated and dehydrated silicas

Abstract Dehydration of colloidal silicas by heating at elevated temperatures improves their ice nucleating ability. Adsorption studies were performed to study their surface characteristics, on both hydroxylated and dehydroxylated samples. To monitor surface areas, argon was found to be a more reliable adsorbent than nitrogen. The latter displayed a specificity depending on hydroxyl concentration. The highly dehydrated amorphous silicas gave the same surface areas if 16.2 A2 was used for the co-area of nitrogen and 16.6 A2 for argon. Water adsorption was used to assess the surface concentration of hydroxyls and isopropanol adsorption to determine their steric distribution. The dehydrated silica prepared by heating a wet, precipitated variety hydrated partially on cooling in air. Contrary to earlier reports, boiling in water did not yield a fully hydroxylated surface.

Adsorption studies on ice-nucleating substrates. Hydrophobed silicas and silver iodide

Abstract The properties of solid surfaces required for effective heterogeneous nucleation of ice from the vapor were investigated. Amorphous silicas were modified in various ways and tested in a cloud chamber. The surface properties of these substrates were then determined by gas adsorption techniques. The spatial distribution of the polar adsorption sites on the surface was examined by a steric hindrance method employing isopropanol adsorption. It was found that silicas heat treated in the presence of sodium chloride had satisfactory nucleating ability in the cloud chamber. Furthermore, the polar adsorption sites on these good nucleants were found to cover approximately half the available surface area and were distributed in clusters on a low-energy surface. Adsorption studies of several silver iodide preparations, on the other hand, showed the polar adsorption sites to be related to the impurity content of the substrates, which also yield polar sites in a hydrophobic matrix. A comparison of the nucleating properties of silver iodide preparations and modified silicas in water vapor and in bulk water indicated that good crystallographic match between substrate and ice is of primary importance, especially in bulk nucleation. In the absence of a favorable substrate structure, heterogeneous nucleation from the vapor is largely controlled by the water adsorption properties of the substrate. A partially hydrophilic substrate, which is an effective ice-nucleating agent in water vapor, is less effective in bulk water possibly because the site adsorption and clustering mechanism is rendered inoperative when the surface is covered with water.

Entropy of adsorption and the mobility of water vapor on α-Fe2O3

The physically adsorbed monolayer of water vapor on hydroxylated α-Fe2O3 is shown to be immobile through application of adsorption thermodynamics and statistical mechanics. The integral entropy of adsorption obtained from multitemperature isotherms at constant spreading pressure is −33.2 eu for half a monolayer. This value compares favorably with the theoretical value of −35.8 eu calculated from statistical mechanics for immobile adsorption, but not with the value −15.9 eu similarly derived for mobile adsorption. The experimental and theoretical entropies for immobile adsorption agree quite well over the entire monolayer. Spreading pressures required to evaluate the experimental integral entropies were obtained in two ways: (1) graphically; and (2) analytically, with a two-part integration of the Gibbs equation based on Dubinin-Radushkevich behavior in the monolayer and on Frenkel-Halsey-Hill multilayers. Both methods gave the same spreading pressure for a given coverage. The mechanism by which the monolayer is localized is presumably hydrogen bonding of a single water molecule to two underlying hydroxyl groups.

Ice nucleation by hydrophobic substrates

ZusammenfassungDie Fähigkeit, Eiskeime zu bilden, konnte bei Silikaproben durch Aktivierung derjenigen von Silberjodid angenähert werden. Zwei Methoden sind angewandt worden: die eine beruht auf dem Ioneneinbau hydrophober Materialien, die andere auf Hydrophobierung durch Wärmebehandlung der ursprünglichen Silikaproben. Das Prinzip der Aktivierung besteht darin, dass eine zur Hauptsache wasserabstossende Oberfläche mit getrennten hydrophilen Stellen geschaffen wird. Wenn Silberjodid als Modellsubstanz betrachtet wird, dann sollte nur ungefähr ein Viertel der Oberfläche Wasser adsorbieren. Es wird ein molekulares Modell der Adsorption auf einer solchen Oberfläche beschrieben.

Surface properties of Ni(OH)2 and NiO. I. Water adsorption and heat of immersion of Ni(OH)2

Nickelous hydroxide samples were prepared from a nickelous nitrate solution with distilled ammonia gas. Particle sizes by X-ray diffraction and electron microscopy ranged from 40 to 400 nm. Water adsorption isotherms, measured gravimetrically, indicated that hydrophilicity depends upon sample preparation. The isosteric heats of adsorption calculated for one sample from multi-temperature isotherms at 25 and 39°C showed high heats of adsorption, 69 kJ/mole, at coverages below 4 molecule/nm2. The heat of immersion of the same sample of Ni(OH)2, measured by means of an adiabatic calorimeter as a function of water precoverage, showed that the heat decreased linearly from an initial zero water coverage value of 320 to 190 mJ/m2 at a coverage of 2.9 molecule/nm2. It is concluded that the surface of Ni(OH)2 is predominately hydrophobic with a limited concentration of polar sites on the crystal edges available for hydrogen bonding with polar adsorbates.

The effect of alkali halides and silver nitrate on the crystallization of silica powders

The effect of alkali halides on the crystallization of amorphous silica powders has been investigated. Normally crystallization does not occur below 1000°C, but in the presence of about 20% by weight of lithium chloride, α-quartz forms within 2 h at 700°C. In the presence of sodium and potassium salts, α-cristobalite forms at about 800 to 850°C. It appears that quartz is formed in the presence of the lithium salt because Li+ can enter the quartz lattice, whereas Na+ and K+ can penetrate the more expanded polymorphic form, cristobalite. It is possible that the cations are accompanied by oxide ions in all cases to maintain electrical neutrality. Limited penetration of the amorphous particles is sufficient to nucleate the new phase. Other properties of silica products treated with salts are reviewed.

Water on silica and silicate surfaces. iii. hexamethyldisilazane-treated silica surfaces

Abstract Both a fully hydroxylated (HiSil) and a partially hydroxylated silica (Cab-O-Sil) were titrated to increasing coverage by HMDS. The hydrophobicity induced by the chemisorption of the (CH3)3 Si-groups was then monitored by water vapor adsorption isotherms as a function of ligand coverage. The Cab-O-Sil was made strongly hydrophobic whereas the underlying, unreacted OHs on the HiSil, even at full coverage of one ligand per 40 A2, remained available to the water molecules. Reflectance ir was employed to examine the HMDS and the water interaction with the modified silica surfaces; a new major revelation was the N—H band indicating conclusively that ammonia from the disilazane was also adsorbed.

The rheology of printing inks. III. Studies of simple dispersions

One of the most important factors underlying the rheological properties of printing inks is the interaction between the pigment and the vehicle. The complicated formulations of most inks, however, make it difficult to make a detailed study of this factor, and very little attention has been given it in the past. For this reason a simple system of calcium carbonate in polybutene oil was chosen for the study reported here. With this system, quantitative information on the nature of the pigment-vehicle interaction has been obtained. An understanding of simple systems, such as this one, is essential before we can hope to understand the much more complicated systems present in commercial printing inks. The particle size of the pigment has been found to have a marked effect on the viscosity: i.e., the smaller the particle size, the greater the viscosity at a given loading. This effect has been explained on the basis that there is a shell of immobilized vehicle on the surface of each pigment particle and in the narrow portions of the interstices of agglomerates. This immobilized vehicle increases the effective pigment volume to an extent proportional to the amount of pigment surface present. This increase of effective pigment volume can be treated quantitatively so that data on similar pigments of different particle size can be correlated. Particle-size distribution in the range of 0.1 to 0.4 μ has no effect on the viscosity. The addition of a surface-active agent, i.e., stearic acid, has been found to reduce the viscosity by localizing the pigment-vehicle interaction and thus decreasing the thickness of the shell of adsorbed vehicle. These results add to our understanding of the many factors which contribute to the rheological properites of printing inks and which, in turn, control their printing properties.