The properties of water adsorption on both aircraft-combustor and laboratory-made soot particles are studied within a wide range of temperatures characteristic of the troposphere. Depending on the chemistry of the soot surface and the degree of its hydrophobicity, the humidity dependence of water adsorption may decrease with a temperature fall. The oxidation of the soot surface due to its prolonged exposure to exhaust results in an increase of the heat of water adsorption and may change the temperature dependence of water adsorption. Soot particles whose relatively hydrophobic surface is characterized by a low isosteric heat of adsorption as compared to the heat of ice sublimation are most effective for ice nucleation. With a temperature decrease to -40degreesC, almost complete crystallization of adsorbed water, except for the water condensed in micropores, is observed. It is assumed that the phenomena under consideration are an efficient mechanism of soot hydration and determine the formation of ice particles in a contrail and in cirrus clouds of the upper troposphere.

The hygroscopic properties of the soot produced in a discharge between graphite electrodes were studied to gain insights into the behavior of soot under atmospheric conditions. Adsorption, calorimetric, and spectroscopic measurements show that this soot exhibits hydrophilic properties. The measured adsorption-desorption isotherms and the differential heat of adsorption suggest that the humidification of the surface is accompanied by changes in the structure of the aggregates of particles under the action of water vapor. The specific surface area ofthe soot changes when it is subiected to a treatment with multiple adsorption-desorption cycles. This may be attributed to the formation of mesopores around interparticle contacts or to the swelling of the sample. The structure of the soot was found to be stabilized in the presence of saturated water vapor. A comparative analysis of the heat of wetting shows that, when brought into contact with saturated water vapor, the soot becomes highly hydrophilic. Such hydrophilic particles may act as nucleation centers in cloud and aircraft-contrail.

The morphology, microstructure, surface area, porosity, and chemical composition of soot particles emitted from a gas-turbine combustor have been studied. Combustor soot particles appear to be chemically and structurally unstable and present microporosity and a strongly disordered graphitic structure. Discrepancies between the properties of combustor-generated soots arise from their postformation conditions, quenching environments, and sampling conditions. During long accumulation time, soot collected on the combustor walls experiences graphitization and oxidation. Upon long processing by hot exhaust gases, soot particles collected far from the combustor exit exhibit significant transformations (1) from paracrystalline to nodular amorphous microstructure and (2) from spherical particles to fused agglomerates following the increase in microporosity and oxygen content. Combustor-generated soot properties are compared to those of kerosene flame soot, which is produced in the laboratory and is proposed to be a surrogate of combustor soot for atmospheric studies. It is shown that the chemical composition, porosity, and extent of graphitization of combustor-generated and kerosene flame soots influence their electrical and hygroscopic properties. The characteristics of kerosene soot appear to be close to those of combustor soot but exhibit a number of specific feature variations such as graphitized nanostructure and ultramicroporosity, which influence its water adsorbability.

The isotherms and differential heats of benzene adsorption at 291, 303, 315, and 333 K, water adsorption at 291 and 303 K, and acetonitrile adsorption at 303 K were measured on a high-silica zeolite, silicalit. At the initial stage of pore filling, high differential heats of water, benzene, and acetonitrile adsorption were observed, and the isotherms were convex toward the pressure axis. This is characteristic of zeolites and is evidence of the existence of hydrophilic centers. These are residual Na cations and OH groups of structure defects, which influence pore filling but are absent on the outside surface. The hydrophilic properties also depended on the sieve action of micropores, and the silicalit capacity with respect to water noticeably increased as the duration and temperature of sample pretreatment grew. The ratio between the adsorption volumes of main pores with respect to water, benzene, and acetonitrile equaled 1 : 3 : 4.5. The biporous structure of the surface was observed. In conformity with this observation, step isotherms of benzene adsorption were recorded at different temperatures, and the dependence of the differential heats of adsorption on filling at 303 K contained two peaks.

Adsorption isotherms measured by different methods but under identical conditions are compared with each other. Discrepancies associated with systematic errors in the calibration of the setup and in sample preparation, as well as variations in adsorption temperature and in adsorptive concentration, are discussed. Positive and negative deviations of adsorption values determined by the volumetric and gravimetric methods from those obtained by the gravimetric method were within 2.3-13.4%. Serious difficulties encountered during comparison of results of static and dynamic measurements are largely associated with the problem of attainment of equilibrium and with the uncertainty in adsorbent temperature. The scatter in adsorption values may be as high as 30%. Gravimetric measurements with proper corrections for buoyancy are most accurate, especially with the use of an electromagnetic balance, which permits the weighing of large amounts of adsorbent.

The adsorption of triethylamine at 30 and 100°C on γ-Fe 2 O 3 samples used in the production of magnetic discs has been studied by adsorption calorimetry. The thermokinetic curves show that the adsorption of triethylamine takes place in two stages. The differential heats of adsorption have been determined, and found to decrease (as far as the heat of condensation) with increasing surface concentration of the adsorbate, indicating non-uniformity of the γ-Fe 2 O 3 surface