A Brief Evaluation of Pore Structure Determination for Bioaerogels

Abstract

This review discusses the most commonly employed methods for determining pore size and pore size distribution in bioaerogels. Aerogels are materials with high porosity and large surface areas. Most of their pores are in the range of mesopores, between 2 and 50 nm. They often have smaller or larger pores, which presents a significant challenge in determining the exact mean pore size and pore size distribution in such materials. The precision and actual value of the pore size are of considerable importance since pore size and pore size distribution are among the main properties of aerogels and are often directly connected with the final application of those materials. However, many recently published papers discuss or present pore size as one of the essential achievements despite the misinterpretation or the wrong assignments of pore size determination. This review will help future research and publications evaluate the pore size of aerogels more precisely and discuss it correctly. The study covers methods such as gas adsorption, from which BJH and DFT models are often used, SEM, mercury porosimetry, and thermoporometry. The methods are described, and the results obtained are discussed. The following paper shows that there is still no precise method for determining pore size distribution or mean pore size in aerogels until now. Knowing that, it is expected that this field will evolve in the future.

Keywords: aerogel; gas adsorption; pore size; pore size distribution; thermoporometry.

Figure 1

Pore size classification.

Figure 2

Pore types.

Figure 3

(a) The IUPAC Classification of adsorption isotherms for gas–solid equilibria. (b) The classification of hysteresis loops (reprinted from [9] with permission, © IUPAC, De Gruyter, 2015).

Figure 4

SEM micrographs of cross-sections of (a) S-CNC and (b) P-CNC aerogels. Both types of aerogels have similar morphology with cross-linked CNC sheets (or flakes) separated by macropores. Insets at higher magnification show similar mesoporous structures of the CNC sheets. Reprinted with permission from [15].

Figure 5

Effect of mercury pressure on specific cumulative pore volume of (a) X (80:20 ethanol/H₂O) and (b) Y (50:50 ethanol/H₂O SiO₂ aerogels. Reprinted with permission from [74].

Figure 6

(a) Pore size distribution for aeropectins from 3 wt% citrus pectin solution (1, BJH approach; 2, Mercury porosimetry) and from 5 wt% citrus pectin solution (3, Mercury porosimetry). (reprinted from [2] with permission). (b) Pore size distribution from thermoporometry (filled square) and from Hg intrusion (open square) for silica gel. (reprinted from [81] with permission).

Figure 7

DSC thermogram of aerogels obtained for the determination of pore size by thermoporometry (reprinted from [86] with permission).