Arman Seyyedi

I have delayed answering your question to see if others would post a response. If your question was 'What is the best life lesson you have ever learned:

https://www.researchgate.net/post/What_is_the_best_life_lesson_you_have_ever_learned

then you may be expecting over 10000 answers. I suspect that there are only a small number of people (on RG) that actually read and understand your question. Then they have to take the time to reply which can be a lengthy process in comparison to a trite one liner in a non-scientific question as above. This, for me is the major problem with RG and the reason that I will delete my account. There is no research or science in formulating a response to the 'life lesson' question.

I will answer your question in parts below. Be aware that by answering your question my RG score will reduce...

Arman Seyyedi

Surface area can be measured in various ways using various techniques. The (SI) units of surface area are m2. The units (area per unit volume) of specific surface area are m2/m3, or if we know the true density of the material, we can convert this to m2/kg. The usual method for SA determination is with BET which is physisorption of N2 gas at its boiling point of – 1960C. This measures all the external and internal surface accessible to the N2 molecule (assumed area of 0.162 nm²). Change the molecule or atom (e.g. to Kr or Ar or He. The latter is often used to calculate the dead space in a system) and the calculated surface area will change because there’s different accessibility of the surface for other gases. The BET equation is the mathematical engine or algorithm that converts the experimental data to a SA or SSA.

In permeability measurements, such as Blaine (extensively used in cement) we time the passage of a fluid (gas or liquid) through a packed bed of the material – the smaller the particles then the more convoluted is the passage of the fluid and thus the longer time it takes. The algorithm used here is the Carmen-Kozeny equation. See, for example: https://en.wikipedia.org/wiki/Kozeny%E2%80%93Carman_equation

This will also generate a (pseudo-) specific surface area, of course in the same units that specific surface area is expressed in.

In light scattering we indirectly measure the size distribution of particles by means of the angular scattering of light intensity. We use an algorithm (Mie theory usually) to generate sizes including mean values such as the De Brouckère Mean (D[4,3]) and the Sauter Mean Diameter, the surface area moment mean, D[3,2]. The basic link between the D[3,2} and the SSA is:

SSA = 6/D[3,2]

See also: Basic Principles of Particle Size Analysis http://tinyurl.com/zo6mfgz

So, again we generate another surface area based on this light scattering technique. As the majority of scattering is around the contour of the particle, we can’t examine the internal pore structure as perhaps BET would do. So, although we’re measuring surface areas in all cases the technique of measurement and equations are different so different values are generated. I’ve explored this in a paper where I looked at BET SSA’s and laser diffraction SSA’s for barium ferrite. See: ‘Micron sized nano-materials’ Powder Technology 174(1):6-9 · May 2007 DOI: 10.1016/j.powtec.2006.10.012

Here we see a good linear link between SSA generated by the 2 routes, but the values are not the same. I point out the convenience of the light scattering measurement in comparison to BET. So, in summary there's usually a good correspondence between the different techniques but they cannot be assumed to give the same values. Of course, all the techniques are verified by the use of an appropriate standard material with the specified technique such as the NIST 114q cement.

Arman Seyyedi

In many areas of work, e.g. catalysis, the SI system is an unwieldy set of units. We rarely will use one kg of material to measure any property. Thus, other types of specific units have been used because of convenient aspects. We take the example of the convenient m2/g for catalysts combined with microns (= micrometers = 10-6 m). Expressed in this manner, then SSA’s when converted to size via the formula D[3,2] = 6/SSA we have convenient units in place. Thus, we can derive and understand that, for unit density, an SSA of 60 m2/cm3 (= m2/g for unit density) will imply an average surface area moment size (Sauter Mean Diameter) of 0.1 microns (6/60) or 100 nm. If we were working with the SI system, we’d end up with particle sizes in meters and thus a lot of negative exponents which can induce mathematical difficulties and errors.