Factors controlling the formation and stability of foams used as precursors of porous materials.

The remarkable stability of particle-stabilized foams and the opportunity to use them for production of novel porous materials have been attracting the researchers' attention in the recent years. The major aim of the current study is to clarify the factors, controlling the foamability and stability of foams, formed from concentrated silica suspensions in the presence of the amphoteric surfactant CAPB. The experiments showed that: (1) two regions can be defined with respect to suspension foaminess: Region 1 - good foaming and Region 2 - strongly suppressed foaming. The foam volume decreased linearly with the increase of suspension viscosity, so that Region 2 appears as a result of the excessively high suspension viscosity. (2) Based on foam stability four sub-regions were observed in Region 1: region 1S - stable foams, which can be dried to form stable porous materials; 1UD - foams which are unstable with respect to water drainage and collapse upon drying; 1UC - foams which are stable to drainage, but are unstable to drying, due to crack formation; 1UF - unstable foams which completely fall apart upon drying. (3) Foams in Regions 1S and 1UC had yield stress above 10 Pa which prevented liquid drainage, while foams from Region 1UD drained because of their lower yield stress. (4) The particles in the foams assembled in a 3D network in the Plateau channels and the nodes, while surfactant stabilized the foam films between the bubbles. These results define the conditions, appropriate for formation of stable, highly porous silica materials with low mass density, which can be further modified (sintered, impregnated, hydrophobized, etc.) to serve as catalyst supports, porous filters, insulating materials, etc.