Surface area Harkins-Jura method

Equations (33) and (34) form the theoretical basis for the absolute Harkins-Jura (HJ) method [76,94] to estimate the solid surface area. However, in the earlier calorimetric experiments applying the Harkins-Jura principle, the term QjJJ, was always neglected. Neglecting it may lead to certain discrepancies between the surface areas determined by the Harkins-Jura and BET methods in the case of water adsorbed on oxides. 

Harkins and Jura (Ref 5) have described an improved modification of the basic B.E.T. method for the calculation of surface area. Eigsti Dwiggins(Ref 16) evaluated this method for the determination of the particle size of chemical delay compns vs other methods, as described earlier in this article (see Table 9)... 

Data taken from the adsorption leg of the isotherm of Figure 17.11 are listed in the first two columns of the following table. Test the applicability of the following equilibrium theories (a) Langmuir (b) infinite BET and (c) Harkins and Jura. From (a) and (b) obtain estimates of the surface area of the adsorbent and compare the values with that obtained by the point B method. One molecule of nitrogen adsorbed on alumina occupies 0.162 nm2. 

In addition to the relative method, Harkins and Jura have also developed an absolute method for surface area measurement which is independent of... 

An important contribution made by the Harkins and Jura absolute method, however, must not be overlooked. Their measurements of some specific surface areas give confirmation to the value of 16.2 for the cross-sectional area of nitrogen. This value, when employed with the BET theory, gave exactly the same specific surface area as the HJ absolute method. 

Loeser and Harkins (45) have made a critical comparison of the Brunauer-Emmett-Teller and Harkins-Jura methods of calculating the surface area of graphite by adsorption of n-heptane at 25° C. The lower limit of surface area which can be measured by the Harkins-Jura method using n-heptane is given as 2500 sq. cm. 

Figure 6.2. Principle of the Harkins and Jura absolute method for determining the surface area.

The total surface area needs to be known to determine the change of internal surface energy from the heat of immersion. This is often done by adsorption measurements [94,83]. An alternative method was suggested by Harkins and Jura 92. They proposed not to immerse a clean solid but to expose the solid first to the vapor of the liquid. If the liquid wets the solid at the saturating vapor... 

Textural Parameters. Adsorption-desorption isotherms of N2 at 77K were determined in a Micromeritics ASAP 2010 with a micropore system. Prior to measurement, the samples were outgassed at 140 C for at least 16 h. The specific surface area was determined by the BET method, assuming that the area of a nitrogen molecule is 0.162 nm [12]. Micropore volume was calculated by the t-plot method using the Harkins and Jura [13] thickness. We used model isotherms calculated from density functional theory (DFT) to determine the pore size distributions and cumulative pore volume of the pillared samples by taking the adsorption branch of the experimental nitrogen isotherm, assuming slit-like pores [14]. 

The derived values of specific surface area, a, and micropore volume, Vp, have been obtained from t-plots, as-plots and DA plots by the well known procedures described in the literature [1,3]. The Harkins-Jura (HJ) form of standard multilayer thickness curve was used to construct the t-plots. In our view, this approach is of limited value since it does not make allowance for the dependence of the standard isotherm on the surface structure of the adsorbent. For this reason, we prefer to adopt the empirical as-method, but this still leaves open the choice of the standard isotherm for nitrogen on an appropriate type of nonporous carbon. 

The heat measured in step (3) corresponds to the adsorption of an unknown amount of vapor on the carbon surface. Now, in case of a wetting system, we know that the conditions are then fulfilled to have a multilayer adsorbed. This means that the heat measured in step (4) corresponds to the immersion energy of a precovered solid, so that it can be used for the surface area determination by the modified Harkins and Jura method [7]. The addition of the heats measured in steps (3) and (4) after suitable correction leads to the usual immersion energy. If the full wetting is not reached at saturation pressure it is then needed to proceed to step (5). More details about this procedure and the way to get experimental data are described elsewhere [9]. 

The extent of the solid surface for a given liquid—solid system, the immersion energy increases with the surface area (applications measurement of the surface area either by comparison, using a reference material, or by applying a modified absolute Harkins and Jura method). 

The BET theory, of course, is still the basis for the most widely used method of determining surface areas. Especially after the confirmatory work of Harkins and Jura (46), it now seems clear that the determination of surface areas from the BET theory is fortunately not very sensitive to the simplifying assumptions in the BET model, and that BET surface areas are the best that can be had at the present time. 

Nitrogen physisorption measurements were performed on a Micromeritics Tristar 3000 apparatus at -196 °C. Prior to analysis the samples were dried in a helium flow for 14 horns at 120 °C. Surface areas (St), and micropore (Vmicro) and mesopore (Vmeso) volumes were determined using the t-method [13] with the Harkins-Jura thickness equation. There is no standard method for the determination of blocked mesopore volume (Vmeso,bi)- For this we used the pore size distribution from the desorption branch of the isotherm calculated using BJH theory [14]. The total amoimt of Vmeso,bi was determined considering that the volume in pores with a diameter of 2 - 5 run is (partially) blocked. 

The adsorption isotherm yields the amount of gas adsorbed on the surface. Unless the molecular area occupied by the adsorbed gas is known, the adsorption isotherm yields only relative surface areas rather than the absolute values. This is the reason for using only one gas (nitrogen or krypton) to determine the surface areas of different solids. However, Harkins and Jura [45] developed an absolute method of... 

The surface areas of all the samples were measured using the B.E.T. method with nitrogen adsorption at 77 K and a Micromeritics ASAP 2000 for the determination of the pore size distribution for the most interesting ones. Mesopore size distributions were calculated using the Barrett, Joyner and Halenda (BJH) method, assuming a cylindrical pore model (IS). In the analysis of micropore volume and area, the t-plot method is used in conjunction with the Harkins-Jura thickness equation (16). 

The Harkins and Jura (36) absolute method of calculating specific surface area from adsorption data apparently gives more consistent results than the BET method when different adsorbates are used on dinerent kinds of solids. It is based on an empirical equation ... 

Many workers subsequently verified agreement between the Harkins and Jura and the BET methods of calculating surface area from adsorption data. According to Dreving et al. (37) the BET equation is obeyed on silica gels at p/p, from 0.03S to 0.33, and the Harkins and Jura equation is obeyed from 0.075 to 0.58. 

The method proposed by Harkins et al. (1944) which they called the absolute method , included the previous coverage of the outgassed sample with an adsorbed film (five to seven molecular layers) of the immersion liquid. During the immersion experiment, the liquid sees a surface with an extent equivalent to that of the solid, but with a chemical nature corresponding to that of the bulk liquid. An improvement of this method was later proposed by Partyka et al. (1979) who deduced that, for a number of non-porous solids, the coverage with just 1.5 molecular layers was enough to screen the solid surface without reducing the available surface area. In this modified Harkins-Jura technique, water was used as the immersion liquid for solids with hydrophilic surfaces and pentanol for solids with hydrophobic surfaces. 

This is fortunately offered by the calorimetric experiments which suggest that the BET monolayer content physically corresponds to an energetically strong retention. This quantity, provided by the BET equation, could therefore be called safely the BET strong retention capacity This quantity includes two parts, which are the micropore capacity and the monolayer content on the non-microporous portions of the surface. The latter, which provides the external (/.e. non-microporous) surface area is easily assessed by means of the as or t methods, without even requesting the very low part of the adsorption isotherm. The as method is to be preferred when one wishes to carry out a more detailed analysis of the micropores and when the low pressure range of the adsorption isotherm is available. Conversely, if one only wishes to assess a reliable external surface area, he will probably find it simpler to use the t method this can indeed easily be done in a software, after introducing the appropriate multilayer equation, like the Harkins and Jura t-curve equation [13]. The recommended succession of calculations is therefore ... 

Another method to determine the surface area comes from / theory. The values obtained by this method (as analyzed by Condon [13]) seem to agree with some other methods, such as the absolute method of Harkins and Jura [14] and the conclusions by Kaganer [15, 16], It also consistent with X-ray analysis for some porous samples. For a non-porous, single energy surface the following equation holds according to / theory [17] ... 

Harkins and Jura [7] described a method of obtaining the surface area in an absolute way from a calorimetric measurement. They addressed many of the concerns regarding the method [8] but one must still qualify the method as being very limited. Porosity of any type would significantly alter the answer. 

Harkins and Jura [20] describe a method to obtain the absolute surface area of a solid by the following method. Firstly, the powder is exposed to a high vapor pressure of water. Indeed it is best to expose it in a high-sensitivity calorimeter over a reservoir of water. The powder is then allowed to fall into the reservoir and the amount of heat produced is measured. By doing so, one eliminates the outer surface of the adsorbed film releasing the energy associated with the liquid-gas interface surface tension. Since the liquid-gas surface tension energy is known one may then calculate from the amount of heat released the area of the powder (or at least the outer surface area of the adsorbed film before immersion). 

A remarkable attempt was made by Harkins and Jura nearly fifty years ago to overcome some of these difficulties. Their idea was to cover a non-porous adsorbent with a multilayer thick enough to have a liquid-like surface. The pre-coated adsorbent should therefore exhibit a surface enthalpy identical to that of the liquid and immersion in the same liquid should liberate an amount of energy equivalent to the removal of this surface. In principle, therefore, it would seem a fairly simple matter to calculate the surface area from the heat of immersion of the coated solid. The measurements Harkins and Jura appeared to indicate that 5-7 molecular layers were required to overcome the influence of the solid surface and reduce the surface energy to that of the liquid. Since this layer thickness would correspond to a very high relative pressure, it would be difficult to avoid some capillary condensation -even with non-porous powdered materials-and therefore the method appeared to have very limited applicability. 

A more recent investigation has revealed that this problem can be overcome by using water as the liquid since two molecular layers are sufficient to effectively screen the underlying surface of many adsorbents. These results have led to a modification of the original Harkins-Jura "absolute method for surface area determination and they make it possible to apply the technique to mesoporous solids (by avoiding the complication of capillary condensation). Obviously, the approach cannot be used in isolation to study micropore filling, activated entry or molecular sieving, but it becomes a powerful tool when combined with gas adsorption. 

If the solid is first equilibrated with saturated vapor, then immersed in pure liquid adsorbate, the solid-vapor interface is destroyed and the heat liberated should correspond to /,. die surface energy of the pure liquid. The above assumption is made in what is termed the absolute method of Harkins and Jura (HJa) [107] who obtained a heat of immersion of 1.705 kJ kg-1 for titanium dioxide which, when divided by the surface energy of the adsorbent, water (11.8 kJ kg-1) gave a surface area of 14.4 m2 g-1 in agreement with the BET value. For a comprehensive bibliography and description of the calorimeter used, readers are referred to Adamson [30]. The validity of the HJa method may be questioned because exposure to a saturating vapor causes capillary condensation which reduces the available surface. A correction is also required for the thickness of the adsorbed film. 

W. Harkins and G. Jura. An adsorption method for the determination of the area of a solid without the assumption of a molecular area and the area occupied by nitrogen molecules on the surface of solids, j. amer. chem. soc. 66 (1944) 1366. surface of solids, part xiii. J. Chem. Phys., 11 431-442, 1943. 

The Harkins-Jura method for estimating the specific surface area Shj of a solid available to a given immersional liquid is based on Eq. 6.22 [20, 54, 55]. In practice, water is by far the most frequently used liquid because of the small size of its molecules (the van der Waals diameter of a water molecule is about 0.28 nm). The specific surface area of a hydrophilic solid is thus calculated as follows ... 

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