I am looking to compare water adsorption isotherm for silica from two sources:
Article Water Adsorption Kinetics and Contact Angles of Silica Particles
Article Determination of Physisorbed Waters on Silica Gel and Porous...
The first source uses units of mg of water per m^2 of silica surface, while the second uses the more common units of ml of water at stp per gram of silica.
My initial thought was to use dimensional analysis with molar volume of water vapor and the given specific surface area of the silica used in the first study, but this gives unreasonably low values that do not agree with the paper's own literature comparison.
Is there a conversion factor other than specific surface area that I'm missing?
Dear Dr. Bronson Hausmann,
these data are often espressed in cc STP per gram (or per unit surface). STP is at 1 atm and 273 K. 1 mol of gas at STP occupies 22.4 L. Then 1 mmol occupies 22.4 mL.
The adsorbed volume is a gas volume at STP which can be converted into mmoles (divide cm3(SPT) by 22.414) which can thus be converted to mg (multiply with “molecular weight of gas” in mg/mmol, e.g. 28 for N2) and to a corresponding liquid volume by multiplying with the liquid density at the boiling point (e.g. 958 kg/m3 for H2O).
Best regards, Pierluigi Traverso
Bronson Hausmann
Usually, when calculating the specific surface (m2 / g), they mean that the particles have a spherical shape, but they can be cylindrical, conical, or other shape. Under equilibrium conditions, the nanoparticles take a spherical shape with a minimum Gibbs surface energy. When calculating ml / g, it is necessary to take into account the shape factor or how the specific surface of nanoparticles of different shapes changes with the same mass (volume).
The per gram of the powder can be converted to per m^2 through the specific surface area of the powder. For the same synthesis conditions of powder, the adsorption of a gas/vapour on that powder should be normalized based on the m^2 of it.
In our paper, we convert these entities using the SSA.
Article Specific Surface Area From Nitrogen Adsorption Data at 77 K ...
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