Surface area of powders

One of the most important characteristics of fine powders is the area of the surface of the solids this is usually expressed as specific surface area. This is a measure of the fineness of the powder (the specific surface increases with decreasing particle size) as well as of its porosity but it is unable to discriminate between a monodisperse powder and one containing a wide range of particle sizes. 

Surface area is important in all applications where the process is surface-dependent like in mass and heat transfer, flow through packed beds or fluidization. Activity of drugs, setting time of cement and effectiveness of cracking catalysts are just three examples of direct dependence on specific surface. Some such materials, like fillers or catalysts, are often specified in units of specific surface rather than in particle size and its distribution. Specific surface also offers some practical advantages, in favourable cases, in the ease and speed of measurement and also in that it gives a 

There are many methods of surface area measurement any surface-dependent phenomenon can be used for such measurement. The following is an incomplete list of some of the available methods  

The different methods rarely agree because the values obtained depend on the procedures used and also on the assumptions made in the theory relating the specific surface to the phenomena measured. The actual method to be used for surface area measurement is selected depending on the purpose of the measurement. 

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. 

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The surface area of powders is determined by subsieve-sizer which is designed for measurement of average particle sizes in the range of 0.2 to 50 microns. The relationship between average particle diameter and specific surface area (SSA) is given by the following expression ... 

How is the BET isotherm used to determine the surface area of powders ... 

The most well-known method for determining the specific surface area of powders is based on a theory of multimolecular adsorption of gases developed by Brunauer, Emmett, and Teller (BET) (1). The BET method involves the determination of the quantity of a gas which, when adsorbed on the surface of the solid, would completely cover the solid with a monolayer of the gas. 

Half an hour should be sufficient time for extraction offlavonol glycosides because of the high surface area of powdered material and the large volume of solvent. Another method of extraction is to add solvent to the sample in two to three aliquots, decant each through Whatman no. 1 filter paper at the end of a specified time, and combine the filtrates. 

Many of the most popular methods for determining the surface area of powders and porous materials depend on the measurement of adsorption. The early work of Paneth (1922) involved the use of radioactive indicators and dye adsorption. It had been noted by Marc (1911) that the uptake of dye by an inorganic powder tended to approach a certain saturation level. Paneth set out to show that this level of maximum adsorption corresponded to the formation of a unimolecular layer. In spite of a number of unsolved problems, dye adsorption soon became one of the most widely used techniques for die study of fine powders. 

The surface area of particles is related to particles size, as discussed previously. The surface area of powders affects the drug dissolution rate, powder flow, cohesiveness, and adsorption. Furthermore, the surface area of solid materials may also... 

Table 4.3 Variation of pH with Surface Area of Powders...

The cancrinite product has a low leach rate. Values obtained by leaching with distilled water are in the range of lO to 10" g of sample/ cm -day based on BET surface areas of powdered samples. Diffusion of cesium ion from the product into the leachant is slower than for the sodium ion. 

The surface areas of powders, Sbet, are commonly measured for catalysis by the... 

BET surface area of powder changes with the system of precursor and fuel. Synergetic effect of the decomposition of precursor and the combustion of fuel leads to foam-structured powder with high BET surface area. The presence of urea and nitric acid is very important... 

BS 4359 1994. Determination of the specific surface area of powders. Part 4 Recommendations for methods of determination of metal surface area using gas adsorption techniques. ... 

As the name suggests, gas adsorption methods measure the surface area of powders from the amount of gas adsorbed onto the powder surface. The methods measure both external and internal surfaces (including open pores in the particles) and can, therefore, yield physically meaningful average particle sizes only with non-porous materials. 

As permeability depends on specific surface area of the powder, it is used for measuring surface area of powders - see section 2.5 for details. Permeability is also highly sensitive to porosity (voidage) of the powder, as can be seen from the Carman-Kozeny equation, Fig. 4 section 2.5, and the state of compaction is,... 

The relation between the drying method and the surface area of powders is important. There are many drying methods, such as air, vacuum, and freeze drying. This point is illustrated with the examples of (Bao.2Pbo.8)Ti03. In powder preparation, the surface reactivity, that is, the specific surface area, plays a very important role. Specific surface areas are often determined by the BET (Brunauer, Eminett, and Teller) method. Typical BET plots of the sol-gel, CMO (calcined mixed oxide method), and MSS (molten salt synthesis method) [15] derived powders are shown in Fig. 1. 

One of the earliest applications of radioactive tracer techniques to surface investigations is that of Paneth and Vorwerk/ who used the Pb isotope, thorium B, to determine the surface area of powdered PbS04 by exchange between the surface Pb ions of the solid and the radioactive Pb in solution. Since then a vast number of studies of catalytic systems involving the use of radioisotopes have been reported. As the earlier literature has been extensively reviewed by Campbell and Thomson, the present Report is mainly concerned with the literature published between 1970 and mid-1982, particular emphasis being placed on new developments in experimental methods. 

Powders Estimation of the effective surface area of powders with an indeterminate number and size of particles is a complicated problem. This quantity can, in principle, be determined by the BET method. However, decomposition of a powder is not uniform in space. Because of self-cooling, internal layers of powdered samples are heated more weakly than the surface layers and, therefore, make a lower contribution to the total decomposition effect. 

BET (Braunner-Emmet-Teller)—measures the surface area of powdered solids using the physical absorption of a gas, usually molecular nitrogen. The analysis uses the Braunner, Emmet, and Teller model of isothermal adsorption. 

Determination of the Specie Surface Areas of powders, BS4359 Part 4 (1995). (Recommendations for Methods of Determination of Metal Surface Area using Gas Adsorption Techniques)... 

The theory of physical adsorption of gas molecules on solid surfaces was derived by Brunauer, Emmett, and Teller (BET). The theory serves as the basis for the most widely used technique to assess specific surface area of powders and solids. It extends the Langmuir isotherm concept. 

Surface area of powders is another important parameter that can be used to determine the average particle size, together with the consideration of shape of the particles and the presence or absence of pores. Surface area is usually measured basing on the gas adsorption phenomenon [34]. Adsorption means that gas molecules as adsorbate are condensed on the free surfaces of the solid powders as adsorbent, which is different from absorption, where the gas molecules penetrate into the mass of the absorbing solids. There are two types of adsorption (i) physical adsorption and (ii) chemical adsorption. 

The BET surface area of powder samples was determined by applying the standard Brunauer-Emmett-Teller method to the nitrogen adsorption isotherm obtained at -196°C using a Quantachrome, Nova 2000e (Boynton Beach, FL) instrument... 

Specific surface area of powder (m g ) Oxygen content (in wt%) of ... 

Surface area of powders collected on the auxiliary stainless steel strips. 

Vitreous silica has the same solubility as other amorphous silica. Because of the small specific surface area of powdered silica glass in comparison with that of microamorphous or colloidal silicas, workers found it difficult to establish solubility equilibrium. Stober (144) found that at pH 8.4 in Ringer s solution (0.9% NaCI, 0.1% NaHCOj) at 25 C, at least 15 days was required to reach equilibrium when 20 m of silica surface was exposed per liter, regardless of particle size Without the use of this solution, which has an optimum catalytic effect, it would probably have been impossible to establish equilibrium. The solubility was found to be about 100 ppm. 

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