Hiromu Asada

Phase transition of nitrogen adsorbed on benzene-preplated graphite at 77 K

Abstract Volumetric adsorption isotherms of nitrogen have been measured on Grafoil preplated with various amounts of benzene at 77 K. Preplated benzene as small as ~ 1 4 monolayer prevents post-adsorbed nitrogen from undergoing the fluid-solid phase transition on the graphite basal plane. When more benzene is preplated, a new step appears on the isotherm at higher pressures. The new step is assigned to a phase transition of nitrogen adsorbed onto a monolayer or single-layer islands of benzene.

Monte Carlo study of multilayer adsorption on surfaces with preadsorbed particles

Abstract The Monte Carlo simulation method is used to investigate multilayer adsorption of a volatile gas in the presence of a preadsorbed, nonvolatile, layer. It is shown that the proposed algorithm leads to results demonstrating that the predicted mechanism of multilayer formation is in a good qualitative agreement with experimental data obtained for krypton adsorption on graphite preplated with carbon tetrachloride. The influence of the relative magnitudes of the parameters describing various types of molecular interactions in the system on the mechanism of multilayer formation is also discussed.

The phase of condensate coexisting with the solid phase of nitrogen on Grafoil, studied by volumetric adsorption measurements of nitrogen

Abstract Volumetric adsorption isotherms of nitrogen at 77 K have been measured on Grafoil (compressed exfoliated graphite) preadsorbed with various amounts of cyclohexane or n -hexane. For cyclohexane preadsorption, the fluid-solid phase transition of post-adsorbed nitrogen is observed as a step on the isotherm until cyclohexane completes its monolayer, indicating that post-adsorbed nitrogen compresses cyclohexane to convert its submonolayer phases into the monolayer phase. On the other hand, n -hexane preadsorbed at submonolayer coverage is found, just like CCl 4 studied previously, to prevent nitrogen from undergoing the fluid-solid phase transition. It is argued that n -hexane islands coexisting with pure nitrogen domains are in the higher-order commensurate submonolayer phase with nitrogen involved along its domain walls.

A New Statistical Aspect of the Cluster Variation Method for Lattice Systems

This paper presents an alternative statistical way to derive the cluster variation method (CVM) for lattice systems. The formulation is developed for a series of different clusters, each of which is the largest overlap cluster between two clusters of the next larger type. We arrive at the CVM expression of the lattice configuration factor by deriving the number of different ways of distributing clusters of a selected type in the lattice so that they overlap each other at the largest overlap clusters in a physically correct manner. The essential assumption employed is that individual overlapping events are statistically independent of each other. This reveals a new statistical aspect of the CVM: The CVM is based on a Bethe tree of clusters of the selected type.

Displacement transition in Kr/cyclohexane adsorbed on graphite: what is the driving force?

Abstract The adsorption of Kr on cyclohexane-preplated graphite has been studied volumetrically at 77K. A displacement transition and layer-by-layer condensation transitions occur as the amount of Kr increases. Through quantitative analysis of adsorption isotherms, compositions and organizations of adsorbed phases have been derived. It has been found that as many Kr atoms as cyclohexane molecules reside on top of a cyclohexane monolayer prior to the displacement transition. It is argued that the displacement transition is driven by the energy released when kr atoms adsorbed on the cyclohexane monolayer are transferred onto the graphite surface to create a monolayer phase of Kr.

EXPLOSIVE VAPORIZATION OF CRYSTAL WATER ON TEMPERATURE-PROGRAMMED HEATING OF A THIN FILM OF HYDRATED CALCIUM NITRATE

Abstract The temperature-programmed vaporization of crystal water in vacuum was studied for a thin film of Ca(NO3) · nH2O prepared on a nichrome ribbon heater. The mass-spectrometric signal intensity of vaporized water measured as a function of the film temperature reveals a spectrum consisting of two sharp peaks, in remarkable contrast to the broad spectra observed for other kinds of hydrated inorganic salts. The low-temperature peak exhibits normal kinetic features which seem to be consistent with direct emission of water molecules from the crystalline lattice of the hydrated salt. However, the high-temperature peak appears in an explosive manner, and its intensity relative to the low-temperature peak increases film thickness or with increasing heating rate. These features of the high-temperature peak suggest the occurence of a kinetic phase transition of the hydrated salt from a crystalline state to a non-crystalline state, rather than step-wise dehydration of the crystalline salt. in an explosive manner, and its intensity relative to the low-temperature peak increases with increasing film thickness or with

Cluster variation theory of the condensation of atoms in a honeycomb lattice gas with first nearest neighbor exclusion

Abstract The condensation of atoms in the honeycomb lattice gas is studied by the cluster variation method. It is assumed that atoms exclude each other from the first nearest neighbor sites and attract each other at farther neighbor sites. When the two (1×1) sublattices have the same potential energy for atoms, the condensation is characterized by the order–disorder phase transition of the first order. Also the second order phase transition takes place at hypercritical temperatures to give rise to a tricritical point. The effect of the third nearest neighbor interaction on the transition behavior is investigated in detail. When the two sublattices give different potential energies to atoms, the second order phase transition is deteriorated to a continuous phase transition between the poorly ordered and the highly ordered phase. The critical temperature as well as the density of the low-density phase for the two-dimensional condensation are found to be higher than those estimated by using a conventional (1×1) triangular lattice gas model.

Abnormal effect of conjugated molecules on the fluid-solid phase transition of N2 and CO adsorbed on graphite

Adsorption isotherm measurements were carried out for N2 on the basal plane of graphite held at a temperature 77 K and preplated with n-alkanes or conjugated species such as mono-substituted benzenes, anthracene, 9,10-dimethylanthracene, and phthalocyanine copper. The isotherm step arising from the fluid–solid phase transition has been found to be abnormally depressed by the conjugated species, whereas the n-alkanes have no effect on the phase behavior of N2. A similar phase transition for CO has also been found to be depressed by preadsorbed phthalocyanine copper. It is suggested that a small amount of these conjugated species dissolved in N2 and CO locally induce the 3×3 commensurate structure of N2 and CO to increase the molecular density in the fluid phase and, therefore, to depress abnormally the fluid–solid phase transition of N2 and CO.

Adsorption of Kr and CH4 on n-hexane-preplated graphite : displacement of and layering on the condensate

Abstract Adsorption of Kr and CH 4 on graphite preplated with n -hexane has been studied volumetrically at 77 K. Both gases undergo layering transitions not only on the graphite surface but also on the n -hexane layer. Even when more than one monolayer of n -hexane covers the surface, the layering occurs on the graphite surface, indicating that Kr and CH 4 displace part of preadsorbed n -hexane. This is the first observation where both the displacement of and the layering on the preadsorbed condensate occur in the same system, although the latter predominates. We examine the present systems as well as those studied previously in terms of the monolayer displacement energy of the adsorbate. We find that the monolayer displacement energy difference between the two co-adsorbates can successfully predict which of the two modes of coadsorption, the layer-on-layer mode and the two-phase mode, appears in the system.

Displacement transition in CH4/cyclohexane adsorbed on graphite

The adsorption of methane on cyclohexane-preplated graphite has been studied volumetrically at 77 K. Methane undergoes a displacement transition as well as layer-by-layer condensation transitions, similarly to Kr studied previously. The isotherm step of the displacement transition is clear and sharp, being indicative of a first-order phase transition. The transition takes place when the adsorbed methane molecules exceed the pre-adsorbed cyclohexane in number. The results support our previous interpretation that the displacement transition is a change from the layer-on-layer mode to the two-phase mode of two-component adsorption.