M.J.B. Evans

Acidic and basic sites on the surface of porous carbon

Abstract Thermal desorption, acid-base depletion, mass titration and calorimetric measurements on a series of porous BPL carbons are interpreted as indicating that acid surface sites are oxygen based while basic sites are associated with the carbon itself. Both acidic and basic sites were found on all carbons whether they were heat treated to 900 °C in nitrogen or oxidized, chemically with nitric acid or electrochemically. For the most extensively oxidized sample, the number of basic sites is approximately zero, while for a heat treated carbon the number of acidic sites is very small. An analysis for the wholly acid carbon and the entirely basic carbon yields p K a and p K b values which are characteristic of molecular keto or hydroxy carboxylic acids and Lewis bases, respectively.

Water and cyclohexane vapour adsorption on oxidized porous carbon

Abstract Changes in cyclohexane and water vapour adsorptions and enthalpies of immersion for a series of progressively oxidized porous carbons have been measured. The adsorption and thermal results are discussed in terms of the pore-filling mechanism of Dubinin and the influence of surface oxides on the water adsorption is examined. It is concluded that the important enthalpic contribution for cyclohexane comes from pore-filling while for water, the hydrogen bonding-like interaction around surface oxide sites plays the major role.

The adsorption of water vapor by porous carbon

Abstract Water vapor adsorption isotherms on active carbon and graphite have been measured and analyzed by means of the two Dubinin-Serpinski equations and a third similar equation, based on the same principles. Surface oxide content and enthalpy of immersion measurements are shown to give support to the validity of these equations.

Surface studies on carbon: Water adsorption on polyvinylidene chloride carbon

Abstract The adsorption of water on polyvinylidene chloride carbon has been studied, with particular emphasis being placed on the contribution of the surface oxides on the adsorption process. The extent of the surface oxides has been measured by thermal desorption techniques and has been related to the adsorption isotherm and the immersional energetics. It has been found that the surface oxides desorbing as CO are probably the sites for the initial adsorption of water at low relative pressures (0.0–0.2), and are situated within the pores of the carbon. These sites present a relatively uniform surface for water adsorption with a net heat of adsorption of 5.7 kcal/mole. The water molecules adsorbed on these sites then act as centers for further adsorption, in a multilayer type of process with a net heat of adsorption of only 0.40 kcal/mole, until pore filling is complete.

An investigation of the pore structure and molecular sieve properties of polyvinylidene chloride carbons

Abstract Straight and branched chain alkanes have been used as molecular probes in a study of the molecular sieve properties of carbons obtained by fast and slow pyrolysis of polyvinylidene chloride. The uniformity of micropore size in both types of carbon is demonstrated by the sharp reduction in their capacity to adsorb alkanes having two or more methyl groups as side chains, particularly when two of the methyl groups are attached to the same carbon atom. Heats of immersion of the carbons in the same alkanes show an even sharper reduction for branched chain alkanes, presumably as a consequence of the constraints imposed on molecules adsorbed in narrow pores resulting in lower net integral heats of adsorption. It is concluded that the carbons contain a uniform array of micropores 5.5–6.0Ain width.

Volume changes in saran charcoal caused by the adsorption of water, methanol and benzene vapours

Abstract Using a novel dilatometric technique the authors have measured volume changes produced in Saran charcoal by the adsorption of water, methanol and benzene vapours. With all three adsorbates, contraction in volume amounting to approximately 0.1% was observed during adsorption, followed by a larger expansion as saturation was approached. Maximum contraction occurred when the water uptake was about 75% of the saturation value, while for methanol and benzene it occurred at less than half the saturation value. These observations are explained on the basis of the “bridging” of pores by adsorbed molecules suggested by Lakhanpal and Flood, assuming that the adsorbent has slot-like pores between 6 and 7 A in width.

Dynamic adsorption of water-soluble and insoluble vapours on activated carbon

Abstract Dynamic adsorption of organic vapours on humidified carbon beds was studied in order to examine the effect of water-solubility on the adsorption. The carbon beds were pre-loaded with water and the adsorption was carried out at two relative humidities (RH), that is, 73 and 89%. At the higher humidity, the water content of the carbon was ca 40 wt% and there was > 90% drop in the service time for each adsorbate as compared to dry carbon. At RH of 73%, where the amount of water was ca 20 wt%, the drop in service time changed with the type of the adsorbate. Molecules with high affinity to the carbon surface, whether soluble or not, were less influenced by the presence of water. Solubility was not found to be a prominent factor which affects the adsorption on a wet carbon bed. It is suggested that with carbon treated at 73% RH, adsorption starts on dry surface and proceeds by replacing adsorbed water, rather than by direct diffusion through condensed water in the pores.

Surface oxides on porous carbon

Wet oxidation of BPL porous carbon by H2O2 and HNO3 changes the pore structure and also increases the concentration of surface oxides. KOH uptake is increased by oxidation and is seen to be dependent on KOH concentration and to a lesser extent on inert electrolyte concentration. Enthalpy of immersion measurements have allowed the estimation of the enthalpy of hydration of the surface oxides and the enthalpy of neutralization of the easily accessible acid oxides.

Capillary phenomena: XX. Fluid bridges between horizontal solid plates in a gravitational field

Abstract The capillary properties of fluid bridges formed, in a gravitational field, between two large horizontal solid plates were elucidated by methods used previously. Meridian curves which on rotation give the fluid bridges were computed for a wide range of bridge volumes and bridge heights for which the contact angles at the upper and lower solids were 0, 52, 90, 139, and 180°. Plots of the dependence of bridge volume on bridge height were constructed for several values of the lower contact radius between bridge and plate for each angle: these show contact-angle dependence. More striking differences among the systems with different contact angles are revealed by plots of the force exerted by the bridge on the upper plate, and by the pressure difference across the curved interface where it meets the upper plate, as functions of bridge volume (for constant plate separation), and as functions of bridge height (at constant bridge volume). The considerations reveal many, but probably not all, of the instabilities which these bridges experience when manipulated along equilibrium paths. Comparison with a previous report (J. Colloid Interface Sci.75, 409 (1980)) shows the different features and the similar aspects of bridges constrained at the edges of axially aligned vertical rods.

Capillary phenomena. XII. Properties of fluid bridges between solids in a gravitational field

Capillary phenomena associated with fluid bridges between the ends of two vertically positioned cylindrical rods with a common axis have been elucidated by using the methods used previously for pendent and sessile drops, emergent and captive bubbles, and holms. Meridian curves which on rotation give the bridges have been found meeting the boundary conditions at the solid edges. Bridge volumes Vα as a function of bridge height Z⦵ have been obtained for rods of equal reduced radius, R = 0.5, 1, 2, and 5; the closed loops give the possible equilibrium configurations whose stability has also been examined. Stability partly depends on the method of manipulation of the system. Equilibrium paths have been obtained for the variation of the bridge height, i.e., rod separation, at constant bridge volume, and for the variation of bridge volume at a chosen rod separation distance. The applied force on the upper rod (taken as a thin disk) and the pressure difference across the interface have been evaluated for these systems. The equilibrium paths cover a range where boundary instability is not expected for contact angles θ = 0° at the ends and sides of the rods.