Specific surfaces determination

One of the most important uses of specific surface determination is for the estimation of the particles size of finely divided solids the inverse relationship between these two properties has already been dealt with at some length. The adsorption method is particularly relevant to powders having particle sizes below about 1 pm, where methods based on the optical microscope are inapplicable. If, as is usually the case, the powder has a raiige of particle sizes, the specific surface will lead to a mean particle size directly, whereas in any microscopic method, whether optical or electron-optical, a large number of particles, constituting a representative sample, would have to be examined and the mean size then calculated. 

Value of fl = S ISf, for a number of solids and various vapours (S and S p are the specific surfaces determined by use of nitrogen and of another vapourt respectively]... 

Filter aids should have low specific surface, since hydraulic resistance results from frictional losses incurred as liquid flows past particle surfaces. Specific surface is inversely proportional to particle size. The rate of particle dispersity and the subsequent difference in specific surface determines the deviations in filter aid quality from one material to another. For example, most of the diatomite species have approximately the same porosity however, the coarser materials experience a smaller hydraulic resistance and have much less specific surface than the finer particle sizes. 

The Sorptometer appears to offer several ad, vantages for use in determining the surface area of fine powdered proplnt ingredients. By comparing the specific surface determined by N2 adsorption with the specific surface determined by other methods, an estimate of the deviation of the particles from a sphere can be made. For many very fine powders, N2 adsorption is one of the few methods available for describing the, size of the particles... 

Information about the pore structure in the coals can be obtained by comparison of the specific surfaces determined by the three techniques. Before discussing the detailed results, however, we would like to point out some general properties of the different methods of determining specific surfaces. 

Despite all simplifications the model of particle in the rectangular potential well, extended to include the population of excited le els. describes quite well the dependence of ortho-positronium lifetime on the pore radius. In this model the o-Ps lifetime is ruled entirely by geometrical factors, however, maybe the chemical composition of the medium should be taken into account. The lifetime vs. average radius dependence is particularly steep below 5 nm. and in this range the positron annihilation method can be useful for determination of average pore radii. The specific surface determines the distribution of o-Ps between small voids in the bulk and pores. 

D is the surface fractal dimension (2 < D < 3), S(cr ) - the specific surface determined by molecules with the cross-section (Ti. D is close to two for the geometrically smooth, uniform surface and it is close to three for geometrically nonuniform, narrow-pored surface. 

The specific surface determined by any of the above methods can be converted into an equivalent mean spherical diameter xsv using the following simple equation xsv = 6/Sv, where Sv is volume specific surface. The equivalent mean diameter xsv is the size of a spherical particle which, if the powder consisted of only such particles, would have specific surface area the same as the actual sample. 

GOST 10898.5-84 ionites. Method of specific surface determination. 

Santamarina, J. C., K. A. Klein, Y. H. Wang, andE.Prencke. 2002. Specific surface Determination and relevance. Canadian GeotechnicalJoumal 39, no. 1 233-241. 

Specific surfaces for cellulose fibres calculated by eqn (5.53) are listed in Table 5.8 [157] the specific surface determined by adsorption of nitrogen at —196°C is given for comparison. The values of specific surfaces obtained with different injected substances and at different temperatures are very close to each other, which proves that the gas chromatographic method can be applied successfully to the determination of relatively low specific surface areas of low energy solids. The small discrepancy in the case of nitrogen adsorption (1.9 against the average value 1.61 m -g" ) may be... 

Later work with various binary liquid mixtures and different types of solids indicated that the flow calorimeter methods of specific surface determination can also be applied to homogenous surfaces represented by graphitised carbon blacks. In this case strong preferential adsorption of long-chain paraffins from n—heptane was used to produce a monolayer and the corresponding heat of adsorption measured the total graphitic basal plane in such carbons [11]. 

The fineness of grinding of cements may be determined by sieving or a separation method (the fineness being expressed as a certain percentage by weight above a certain size, e. g., as residue retained on a standard sieve), but is more usually based on the specific surface determined by the Blaine method (air permeability of a bed of cement, the result being expressed in cm /g the finer the cement, the higher the specific surface) (see also Section X.1). 

As specified in Part 1 of DIN 1164, a cement conforming to this Standard must not leave a residue of more than 3% by weight on the 0.2 mm test sieve (DIN 4188). The specific surface determined by the air permeability method should be not less than 2200cm /g (in special cases not less than 2000cm /g). 

An important criterion for the performance of blends is the specific surface of the disperse phases, i.e., the shape and size of the dispersed particles. In heterogeneous polymer blends, interactions are possible only through the interfaces. Besides the thermodynamic interactions, the specific surface determines the adhesion between the phases and therefore the properties of the blend. 

The physics of X-ray refraction are analogous to the well known refraction of light by optical lenses and prisms, governed by Snell s law. The special feature is the deflection at very small angles of few minutes of arc, as the refractive index of X-rays in matter is nearly one. Due to the density differences at inner surfaces most of the incident X-rays are deflected [1]. As the scattered intensity of refraction is proportional to the specific surface of a sample, a reference standard gives a quantitative measure for analytical determinations. 

There are complexities. The wetting of carbon blacks is very dependent on the degree of surface oxidation Healey et al. [19] found that q mm in water varied with the fraction of hydrophilic sites as determined by water adsorption isotherms. In the case of oxides such as Ti02 and Si02, can vary considerably with pretreatment and with the specific surface area [17, 20, 21]. Morimoto and co-workers report a considerable variation in q mm of ZnO with the degree of heat treatment (see Ref. 22). 

The specific surface area of a solid is one of the first things that must be determined if any detailed physical chemical interpretation of its behavior as an adsorbent is to be possible. Such a determination can be made through adsorption studies themselves, and this aspect is taken up in the next chapter there are a number of other methods, however, that are summarized in the following material. Space does not permit a full discussion, and, in particular, the methods that really amount to a particle or pore size determination, such as optical and electron microscopy, x-ray or neutron diffraction, and permeability studies are largely omitted. 

From the arguments of the present section it is clear that an inverse relationship holds between the specific surface and the particle size, and if the particles are long or thin it is the minimum dimension, the thickness of the plates or of the rods, which mainly determines the magnitude of the specific surface. 

So far in this section, the specific surface has been taken as the dependent variable and the particle size as the independent variable. In practice one is often more concerned with the converse case where the specific surface of a disperse solid has been determined directly (by methods which will be explained in the subsequent chapters) and one wishes to calculate a particle size from it. 

The numerical values of and a, for a particular sample, which will depend on the kind of linear dimension chosen, cannot be calculated a priori except in the very simplest of cases. In practice one nearly always has to be satisfied with an approximate estimate of their values. For this purpose X is best taken as the mean projected diameter d, i.e. the diameter of a circle having the same area as the projected image of the particle, when viewed in a direction normal to the plane of greatest stability is determined microscopically, and it includes no contributions from the thickness of the particle, i.e. from the dimension normal to the plane of greatest stability. For perfect cubes and spheres, the value of the ratio x,/a ( = K, say) is of course equal to 6. For sand. Fair and Hatch found, with rounded particles 6T, with worn particles 6-4, and with sharp particles 7-7. For crushed quartz, Cartwright reports values of K ranging from 14 to 18, but since the specific surface was determined by nitrogen adsorption (p. 61) some internal surface was probably included. f... 

A Type II isotherm indicates that the solid is non-porous, whilst the Type IV isotherm is characteristic of a mesoporous solid. From both types of isotherm it is possible, provided certain complications are absent, to calculate the specific surface of the solid, as is explained in Chapter 2. Indeed, the method most widely used at the present time for the determination of the surface area of finely divided solids is based on the adsorption of nitrogen at its boiling point. From the Type IV isotherm the pore size distribution may also be evaluated, using procedures outlined in Chapter 3. 

To obtain the monolayer capacity from the isotherm, it is necessary to interpret the (Type II) isotherm in quantitative terms. A number of theories have been advanced for this purpose from time to time, none with complete success. The best known of them, and perhaps the most useful in relation to surface area determination, is that of Brunauer, Emmett and Teller. Though based on a model which is admittedly over-simplified and open to criticism on a number of grounds, the theory leads to an expression—the BET equation —which, when applied with discrimination, has proved remarkably successful in evaluating the specific surface from a Type II isotherm. 

Specific surface area of carbon blacks before and after graphitization, determined by electron microscopy (A,) end by nitrogen adsorption (A ]t... 

Comparison of specific surface of anatase and zinc oxide determined by electron microscopy A ) and by nitrogen adsorption A )... 

Following the pioneer work of Beebe in 1945, the adsorption of krypton at 77 K has come into widespread use for the determination of relatively small surface areas because its saturation vapour pressure is rather low (p° 2Torr). Consequently the dead space correction for unadsorbed gas is small enough to permit the measurement of quite small adsorption with reasonable precision. Estimates of specific surface as low as 10 cm g" have been reported. Unfortunately, however, there are some complications in the interpretation of the adsorption isotherm. 

Carbon dioxide cannot be recommended for routine determinations of specific surface on the other hand, it should be particularly suitable for the study of the polarity of surfaces in systems where chemisorption can be excluded from consideration. 

General conclusions determination of specific surface from adsorption isotherms... 

When other adsorptives, such as those detailed in Section 2.9, are employed for surface area determination, calibration against nitrogen or argon is strongly recommended, so long as the specific surface exceeds lm g . For areas below this figure the calibration becomes too inaccurate, and an alternative adsorptive, usually krypton, has to be used. 

The degree of uncertainty of 10 per cent or more, inseparable from estimates of specific surface from adsorption isotherms, even those of nitrogen, may seem disappointing. In fact, however, attainment of this level of accuracy is a notable achievement in a field where, prior to the development of the BET method, even the order of magnitude of the specific surface of highly disperse solids was in doubt. The adsorption method still provides the only means of determining the specific surface of a mass of non-... 

The evaluation of pore size distribution by application of the Kelvin equation to Type IV isotherms has hitherto been almost entirely restricted to nitrogen as adsorptive. This is largely a reflection of the widespread use of nitrogen for surface area determination, which has meant that both the pore size distribution and the specific surface can be derived from the same isotherm. 

It follows therefore that the specific surface of a mesoporous solid can be determined by the BET method (or from Point B) in just the same way as that of a non-porous solid. It is interesting, though not really surprising, that monolayer formation occurs by the same mechanism whether the surface is wholly external (Type II isotherm) or is largely located on the walls of mesopores (Type IV isotherm). Since the adsorption field falls off fairly rapidly with distance from the surface, the building up of the monolayer should not be affected by the presence of a neighbouring surface which, as in a mesopore, is situated at a distance large compared with the size of a molecule. 

Striking confirmation of the conclusion that the BET area derived from a Type IV isotherm is indeed equal to the specific surface is afforded by a recent study of a mesoporous silica, Gasil I, undertaken by Havard and Wilson. This material, having been extensively characterized, had already been adopted as a standard adsorbent for surface area determination (cf. Section 2.12). The nitrogen isotherm was of Type IV with a well defined hysteresis loop, which closed at a point below saturation (cf. F, in Fig. 3.1). The BET area calculated from it was 290 5 0 9 m g , in excellent agreement with the value 291 m g obtained from the slope of the initial region of the plot (based on silica TK800 as reference cf. p. 93). 

At the point where capillary condensation commences in the finest mesopores, the walls of the whole mesopore system are already coated with an adsorbed film of area A, say. The quantity A comprises the area of the core walls and is less than the specific surface A (unless the pores happen to be parallel-sided slits). When capillary condensation takes place within a pore, the film-gas interface in that pore is destroyed, and when the pore system is completely filled with capillary condensate (e.g. at F in Fig. 3.1) the whole of the film-gas interface will have disappeared. It should therefore be possible to determine the area by suitable treatment of the adsorption data for the region of the isotherm where capillary condensation is occurring. 

Values of specific surface of alumina gels determined by nitrogen adsorption and by mercury porosimetry ... 

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