Hi All,
BET SSA was measured for a pigment (TiO2) using both octane sorption at 30 C (on an IGC system) and N2 sorption. Octane sorption measurement gave a BET SSA three times higher than that obtained through N2 sorption. Although I am aware that it is not straightforward to compare the two measurements mostly due to the different nature/size of the two adsorptive probes and to the different analysis conditions, is there any particular reason why octane sorption would give a BET SSA overestimation in comparison to N2 sorption. Should I not expect the other way around?
Thanks,
Antonio
Hi, Antonio,
was N2 sorption measured classically at low temperature? Two (hypothetical) reasons: (1) octane-based SSA, even determined from an apparent (?) BET behavior, (was it?) involved in fact formation of multiple layers ..I would check with a great attention how well the BET-like trend looks like BET...Was it indeed BET or measuring distribution coefficients at few concentrations?
(2) In some cases nitrogen-based SSA is known to be less than the CO2-based SSA, due to the differences in temperature and strongly retarded diffusion of N2 at low temperature in small pores, as compared with CO2. Octane is obviously large nolecule (but not when it becomes a chain, at least in one direction), yet, here there might be also a similar effect involving an effective penetration of octane into relatively small pores at 30 C, ad compared with retarded diffusion of N2 at lowe temperatures. How porose is the material and how hysteretic is gas sorption? This could give an idea about role of small pores in accomodation of different size gases measured at different temperatures...
Please compare molecules (octane and n2) volume. Also, you will not obtain capillary condensation of N2 at 30°C.
Best Regards
SAM
The difference is really large. What values did you use for cross-sectionnal area of n-octane and N2 ? Could you show us your data (N2 and octane sorption isotherms) and summarize your experimental conditions (especially the amount of sample used for a measurement and the N2 surface area of the TiO2 sample)?
Mikhail Borisover, N2 sorption was carried out at -196C. Octane sorption seems to follow a type II isotherm. I have no information about neither the porosity of the material nor the type of hysteresis.
Juliette Blanchard,
- N2
-196 C
cross sectional area = 0.162 nm2
mass=560 mg
- Octane
30 C
cross sectional area = 0.63 nm2
mass= 10 mg
From what I can see, there is no reason to discuss the value obtained from N2 sorption measurement.
I'm not familiar with IGC. Did you use a protocol similar to the one described in the link below? If yes, did you consider the surface area developed by the glass beads?
https://www.atascientific.com.au/wp-content/uploads/2017/02/APP225IGC-BET-Surface-Area.pdf
What is the particle size and nature of your adsorbent?
Can you show the adsorption isotherms obtained? (With the raw data is even better).
Also... how did you get the cross sectional surface area of octane?