354 CARBON

molecular lateral interactions in two dimensions. Some of the calculations of the properties of various isolated molecules on graphitic surfaces will be discussed. In addition, several theoretical approaches to the interacting two dimensional system will be presented, including: the virial equation of state; Lennard- Jones-Devonshire cell theory; Eyring’s significant structures theory; and an equation of state for hard spheres with weak attractive interactions. Wherever possible, comparisons between theory and experi- ment will be given, as well as a discussion of the assumptions involved in the calculations.

102. A radioactive tracer method for the determination of low pressure adsorption isotherms*

H. C. Friedman, A. Weinstein,? W. S. Diethorn and P. L. Walker, Jr. (Pennsylvania State University, University Park, Pennsylvania). A versatile radioactive tracer technique is described for the determination of adsorption isotherms at very low relative pressures. Development of this technique was motivated by the interest in correlating low-coverage isotherms with the density of defects in carbons. Our first application of this technique is the determination of xenon isotherms on a graphitized (3000°C) Sterling MT carbon black, which has a very homogeneous surface. Five day xenon-133 is used as the radioactive tracer. Xenon adsorption isotherms below - 100°C for this carbon are reported at relative pressures Cp/&) extending from 10m3 to lo- ’ 3, the lower limit of sensitivity. At this latter pressure, the fractional surface coverage is 7 x 10-i* and the volume of xenon adsorbed is 10-l ’ cm3/gram. The isotherms are linear below a relative pressure of lo- 8. Heats of adsorption versus surface coverage, found from these isotherms, provide information on surface homogeneity of the carbon.

‘Supported by the U.S. Atomic Energy Commission on Contract AT(30-l)-1710. iPresent address: E. I. du Pont Co., Gibbstown, N.J.

103. The determination of adsorption parameters for graphitic surfaces

R. A. Pierotti and J. J. McAlpin (Georgia Institute of Technology, Atlanta, Georgia). A modified form of the significant structure theory of physical adsorption is used to analyze isotherm data for gases adsorbed on graphite substrates. Since the significant structure two-dimensional equation of state reduces to that of an ideal two-dimensional gas at low surface coverages, it is possible to use a modified form of the treatment of Ross and Olivier to determine the parameters associated with the gas-solid interaction energy and with the vibrations of the gas relative to the surface of the solid. These quantities are evaluated from the temperature dependence of the Henry Law constants determined from low coverage data. The parameters in the theory which govern the departure of the isotherm from the Henry Law region are the Lennard-Jones (6-12) energy and distance parameters for the gas-gas interactions in the vicinity of the solid. The distance parameter can be evaluated in a number of independent ways, leaving only the energy parameter to be evaluated by fitting the theoretical isotherm equation to experimental data. A sensitive test of the consistence of the evaluated parameters is the prediction of isotherms at a number of temperatures or its equivalent, the prediction of the isosteric heat of adsorption. A number of adsorbates including argon, krypton, carbon tetrachloride, and benzene will be considered. Data for these gases adsorbed on both graphite and boron nitride will be presented.

104. The adsorption of carbon dioxide on carbons at 0°C and the presence of dual-energy adsorption sites

V. R. Deitz (U.S. Naval Research Laboratory, Washington, D.C.). Since graphite is an anisotropic crystal, the boundary carbon atoms of graphitic solids may be located either in the basal planes or in some combination of prism faces. The actual number in each category will vary with the structure assumed by the particular solid. Experimental evidence indicates that the adsorption of carbon dioxide at low coverage (less than 1%) may provide a means to differentiate between the two kinds of sites. The isotherm at 0°C consists of two distinct parts: that at very low pressures corresponds to the strong adsorption on edge atoms, and at higher pressures, additional adsorption takes place on sites of weaker

ABSTRACTS 3.55

adsorption energy located in the more abundant basal planes present in most materials. Results will be presented for a graphitized carbon black (FT), mineralogical graphite, pyrolitic graphite, coconut shell charcoal, and natural-diamond fragments.

105. Adsorption isotherms of n-butane at -78.5” and 0°C on ultrapure mineralogical graphite: a suggested method for surface-area determination

J. de D. Lopez-Gonzalez and M. A. Banares (University of Granada, Granada, Spain). Adsorption isotherms of n-butane on ultrapure mineralogical graphite at 0°C and -78.5”C have been determined from about 0.0002 up to about 0.8 relative pressures. Both isotherms cross at one point as it happened in a previous work when N, and Ar were adsorbed on the same graphite at 77°K and 90°K. The surface area of the graphite used was determined by different methods and a new method for surface-area determination is suggested, based on the crossing-point of the two isotherms. The values of surface area obtained from this method, which is believed to be satisfactory for solids with low surface energy, are in accordance with the values obtained by conventional methods. From the data of the two isotherms differential and integral heats and entropies of adsorption have been calculated as a function of the frac- tional surface coverage. The values so obtained have been used to justify the suggested “crossing- point” method for surface-area determination in solids with low surface energy.

106. The heats of wetting of carbons in relation to their specific surface areas

R. I. Razouk, Sh. Nashed and (Mrs.) Wedad E. Mourad (Chemistry Department, Faculty of Science, Ain Shams University, Abbassai, Cairo). Adsorption and heat of wetting measurements have been made on 30 carbons prepared from corn cobs, date pits and rice husks under varying conditions of carboniza- tion. Both the heat of wetting in methanol and the specific surface area calculated from the adsorption of methanol vapor vary with the thermal and chemical treatment of the charcoal, but no simple correlation is found between the two sets of values even for carbons prepared from the same material. Actually, the heat of wetting per unit area varies several folds. Still greater variations are encountered when cyclohexane, benzene and carbon tetrachloride are used as adsorbates and wetting liquids. This divergence is ascribed to changes in the nature and energetics of the surface of the carbons produced by the thermal and chemical treatment, and which might occur besides the variations in the specific surface area. It is concluded that the use of the heat of wetting as a measure of the specific surface area of carbons has to be adopted with much reserve, and is only warranted if there is independent proof that the heat of wetting per unit area is the same for the various carbons.

107. Effect of heat treatment on the adsorption of surfactants from aqueous solution on Spheron 6 and Sterling MT

F. G. Greenwood, G. D. Parfitt and D. G. Wharton (University of Nottingham, England). The adsorption isotherms for sodium dodecyl sulphate and dodecyl trimethylammonium bromide from aqueous solution on Spheron 6, Graphon and samples of Spheron 6 heat treated at 1000, 1400, 2000 and 27OO”C, have been determined at 25°C. Also for sodium dodecyl sulphate on Sterling MT heated under the same conditions as the Spheron 6. For the carbon blacks heated at 1400” and above, a marked point of inflection was observed in the isotherms at equilibrium concentrations below the critical micelle concentration, but not for the original and 1000” samples. The surfaces of the carbons have been characterized by measurements of surface area and heats of immersion in water. The adsorption of the surfactants, both in degree and character, is associated with the surface structure of the carbons in terms of hydrophilic sites and homogeneity.

108. Surface areas of non-porous carbons by gas adsorption

D. M. Young (Dow Chemical of Canada, Ltd., Sarnia, Ontario). A literature survey on the estimation of surface areas of non-porous carbons leads to the conclusion that surface areas determined from nitrogen adsorption isotherms at - 195°C are usually consistent with values obtained by non-adsorptive

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