Immersion enthalpy

Table 1 also reports the specific enthalpies of immersion (J g- ) of the different CMS into liquids with different molecular size dicholomethane (CH2CI2, 0.33 nm), benzene (CeHs, 0.37 nm), cyclohexane (CeHi2, 0.48 nm), 2,2-dimethylbutane (2,2-DMB, 0.56 nm) and a-pinene (0.70 nm). These values can be analysed in different ways to obtain the pore size distribution of the CMS. On one hand, the areal enthalpy of wetting (per square meter of surface) of a given liquid for a carbon surface can be obtained by using a nonporous carbon of well-known surface area as reference. Theoretical and experimental evidence has been given to support the assumption that the immersion enthalpy is simply proportional to the surface area available to the immersion liquid, irrespective of the micropore... 

Data from immersion enthalpies into liquids whose vapour is adsorbed according to the Dubinin-Radushkevich (D-R) equation can also be analysed following the approach suggested by Stoeckli and Kraehenbuehl [5]. They showed that the immersion enthalpy of a solid into a given liquid is related to the... 

If there is chemical interaction, as with water, then the heat of immersion gives the integral heat of this interaction, i.e., the integral enthalpy of hydration. The initial state of a nanophase material, clay, or zeolite is difficult to define in the latter case because it is not easy to prepare a totally anhydrous sample. Measurements of immersion enthalpies of... 

The changes in molar immersion enthalpies, AwHm = A h, upon the surface modification in methanol and benzene are presented in Fig. 5. In methanol, parallel with an increasing organophilicity of the surface, AwHni decreases nearly exponentially, while in benzene it increases (to a significantly lesser extent). Our experimental data can be approximated by the following exponential function[22 ... 

Figure 12.3 Integral adsorption enthalpy and corresponding immersion enthalpy as a function of pore size, as calculated by DFT for the filling of a sUt-shaped pore by a monoatomic fluid. (Adapted fi om [72].)...

Rodriguez-Reinoso, F., Molina Sabio, M., and Gonzalez, M.T. (1997). Effect of oxygen surface groups on the immersion enthalpy of activated carbons in liquids of different polarity. Langmuir, 13, 2354-8. 

Denoyel et al. [45] derived the pore size distributions of two sets of activated carbons (one activated in water vapor and the other activated with phosphoric acid) using immersion calorimetric data. They concluded that immersion calorimetry is a convenient technique to assess the total surface area available for a given molecule and the micropore size distribution. More recently, Villar-Rodil et al. [46] have followed this approach to characterize the porous texture of a series of NomexO-derived carbon fibers activated to various bum-offs using liquids with different molecular dimensions as well as N2 and CO2 adsorption Isotherms. Table 3 includes the immersion enthalpies and corresponding surface areas. Relative changes in surface area accessible to the different adsorbates were ascribed to... 

Immersion Enthalpy (AH, ) of Fumed Oxides (with Different Specific Surface Area Sret and Bulk Density p,) in Water... 

De Salazar et al. prepared CMS by carbon deposition by the pyrolysis of benzene on an activated carbon. The carbon was prepared by the carbonization of peach stone followed by its activation with CO2. The effect of preparation variables such as temperature, time, and benzene partial pressure on the characteristics of MSC was studied. The MSC obtained in this study were completely accessible to both dichloromethane and benzene as shown by similar values of immersion enthalpy for both liquids. However, the accessibility of a particular liquid into the MSC was found to depend upon the deposition time. A linear relationship was found between the decrease of accessibility of these two liquids to the microporosity of the MSC and the deposition time. The separation ability of different MSC for N2/O2 and CO2/CH4 mixtures was analyzed by measuring the adsorption kinetics of pure gases. It was observed that the carbon sample without any deposition was not selective for the separation of these mixtures, but the separation could be obtained after pyrolytic... 

The changes in molar immersion enthalpies, AwHm, that accompany siuface modification in toluene are presented in Fig. 17 for increasing organophilicity of... 

The enthalpy of immersion, AA/in,m is defined as the enthalpy change, at constant temperature, that takes place when a solid is immersed into a wetting liquid in which it does not dissolve nor react. The immersion enthalpy of a given solid can be measured starting from different initial states, which must be specified. A higher value is obtained when the solid surface is essentially pristine, such that a liquid-solid interface is formed during the measurement. 

In other cases, the surface may be pre-covered, totally or partially, by a liquid film prior to the heat of wetting measurement. The sample is pre-covered by the vapor of the immersion liquid at different partial pressures, and this sample is immersed in the liquid. At low partial pressures, the vapor adsorbs on high-energy sites, and the enthalpy of immersion will be lower as compared with that obtained with the outgassed solid. The enthalpy of immersion continues to decrease as the coverage becomes complete (this is achieved by increasing the partial pressure of the vapor when the sample is treated). At this point, the immersion enthalpy obtained corresponds to the formation of an adsorbed film-liquid interface. 

One of the first attempts to relate the immersion enthalpy of a given solid-liquid system to the surface area of the solid (as distinct from earlier work with coals) was accomplished by Harkins et al. (1944). As a first approach, it can be assumed that, in the absence of complicating effects such as micropore filling, the energy of immersion is directly proportional to the surface area available to the liquid ... 

Stoeckli and Krachenbuehl (1981) proposed an alternative approach (more developed) to the interpretation of immersion enthalpy data, which is based on the Dubinin model of micropore volume filling. They obtained a mathematical relationship between the enthalpy of immersion of a microporous solid into a liquid whose vapor is adsorbed according to the DR equation and the parameters of the DR equation (Equation (4.12)). 

Table 5.3 contains the micropore volumes of selected samples, determined by application of the DR equation to CO2 adsorption data at 273 K, as well as the surface area accessible to dichloromethane, evaluated from the immersion enthalpy measurements and using a graphitized carbon black (V3G) as a non-porous reference. The CO2 adsorption at 273 K, on the PS char, was lower than the other oxidized samples, and its surface area, as measured by immersion into dichloromethane, was almost zero, thus indicating the very narrow microporosity developed in this char (see also De Salazar et al., 2000). 

Frictional properties Functional groups Galvanomagnetic properties Gas storage Heat of adsorption Immersion enthalpy Intercalation reactions Interfacial properties Lattice constant Lattice dynamics Luminescence Magnetic properties Mechanical properties Microporosity... 

Where AHi is the enthalpy of immersion of a microporous solid into organic liquids, P is the affinity coefficient, Vo is the micropore volume of the solid, Vn, is the molar volume of the liquid filling the micropore system, a is the thermal expansion coefficient and Eq is the characteristic energy of the adsorbent. In general the experimental immersion enthalpy, AHi(exp), includes the contribution of micropores, AHi(mic), and of the external surface, S [3] ... 

Denoyel et al [4] proposed a different approximation focused on the determination of the surface area of porous materials fi om immersion calorimetry measurements. It is based on geometrical considerations so that they conclude that the immersion enthalpy is proportional to the surface accessible to the liquid probe and that it is independent of the shape and size of the ptores. The areal enthalpy of immersion, hi (J.m ), for a non-porous carbon black into each wetting liquid is used as reference to determine the surface area wetted hy the liquid probe in a porous carbon. 

The aim of this paper is to determine the textural charaeteristics of several activated carbons by using immersion calorimetry measurements and to relate these results with the information obtained fi om other experimental techniques. The textural characteristics of the adsorbents obtained fi om the immersion enthalpies by the application of the several approaches, already commented, are discussed. 

It has been reported [4] that the immersion heats can be used to determine the surface area of porous materials. For this purpose it is necessary to measure the immersion heat in a non-porous adsorbent of known surface area, as V3G, which is used as reference. Thus it is accepted that the immersion enthalpy of the porous adsorbent in a liquid is simply... 

Interlayer water orientation, immersion enthalpy, clay swelling... 

The results of adsorption from solution are comparable with those of immersion enthalpy measurements when they may be carried out with the pure liquid solute. 

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