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

• Published in: Journal of colloid and interface science Vol. 426; no. 426; pp. 9 - 21
• Main Authors: Lesov, I., Tcholakova, S., Denkov, N.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 15.07.2014; Elsevier
• Subjects: Chemistry; Colloidal state and disperse state; Drainage channels; Drainage measurement; Drying; Emulsions. Microemulsions. Foams; Exact sciences and technology; Foam rheology; Foam stability; Foamability; Foaming; Foams; General and physical chemistry; Particle stabilized foams; Porous materials; Solid foams; Stability; Surfactants; Suspension rheology; Foam stability; Suspension rheology; Foamability; Porous materials; Particle stabilized foams; Solid foams; Foam rheology; Stability; Rheology; Suspension; Porous material; Precursor; Solid; Particle; Foam
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2014.03.067

[Display omitted] •Appropriate conditions are found to obtain porous materials from foamed suspensions.•Theoretical analysis reveals the key characteristics of the foamed suspensions.•These characteristics can be used to predict the stability of wet and drying foams.•Short-chain surfactants adsorb on bubble surface and increase the foamability.•Long-chain surfactants adsorb on particle surface and increase foam stability. 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 10Pa 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.