Quality of adsorption data. In order to get reliable experimental results from adsorption data, several conditions should be respected. In the following paragraphs, the experimental results quality and reliability are discussed. The reliability of the VSSA approach first depends on the quality of the adsorption data. As a first step, the reproducibility and repeatability of the surface areas obtained by applying the BET and the t-plot transformations of the nitrogen adsorption isotherms were studied in three laboratories. The repeatability of the BET and the total surface area (St) values are quite satisfactory within each laboratory, while the reproducibility between laboratories strongly depends on the degassing conditions applied to the samples. For a comparable degassing conditions, an acceptable reproducibility is obtained i.e. about 7%. This corresponds to the generally accepted margin of error for adsorption data. For porous, nanostructured or coated nanoparticles, the t-plot transformation should be used because only the external surface of the particle should be considered to calculate the VSSA. The internal porosity or the porosity created by the coating can be determined by analysing the low pressure area of the t-plot (low value of t). This requires to have not only good degassing conditions but also a sufficient number of experimental points in the low pressure region and to use the appropriate standard isotherm. In these conditions, the reliability of the t-plot analysis is very good. To get reliable information on nanomaterials, the quality of adsorption data requires: o a good degassing of the sample, under high vacuum, preferably in the adsorption apparatus for a significant number of hours, at room temperature to avoid modification of the sample o to reach adsorption equilibrium at each experimental point o to have enough number of experimental points in the low pressure region of the isotherm, in order to identify the internal porosity or the porosity generated by the coating o ideally a complete adsorption isotherm or at least a sufficient number of experimental points to cover the relative pressure range from 0 up to 0.65. If these conditions are respected, then the analysis of adsorption isotherms via the BET and the t-plot transformations will give reliable results.
Quality of adsorption data. The procedure for measuring the surface area by nitrogen adsorption isotherms is according to ISO (2006, 2007 and 2010) and IUPAC (1985). The most critical step in the measurement is the outgassing stage, i.e. to clear the adsorbent surface to any adsorbed molecules before the measurements. The outgassing process of the solid sample can be performed at room temperature, or at a higher temperature to promote desorption of strongly adsorbed species. The pre-treatment conditions must remain sufficiently mild to avoid any modification of the textural characteristics of the sample but severe enough to obtain a good surface clean-up. It is recommended to perform the pre-treatment in the cell used to carry out the adsorption. Whatever the temperature, the solid must be submitted to a minimum vacuum of 10-2 Nm-2 i.e. 10 mPa during at least one hour. Such conditions are readily achieved with the aid of conventional vacuum equipment, usually a combination of a rotary and diffusion pump. In general, the lower the temperature, the longer the outgassing process should be. A larger the numbers of experimental points are always recommended to measure during the measurements. For nanomaterials and nanostructured materials, it is recommended to multiply the number of points in the low-pressure region of the isotherm. As a consequence, to get appropriate information on nanomaterials, the quality of adsorption data requires o a good degassing of the sample, under vacuum in the adsorption apparatus, during a few hours, at room temperature to avoid modification of the sample o to reach adsorption equilibrium at each experimental point
