Factors controlling the formation and stability of foams used as precursors of porous materials I. Lesov, S. Tcholakova ⇑ , N. Denkov Department of Chemical Engineering, Faculty of Chemistry and Pharmacy, Sofia University, 1 James Bourchier Ave., 1164 Sofia, Bulgaria article info Article history: Received 11 November 2013 Accepted 28 March 2014 Available online 3 April 2014 Keywords: Foamability Foam stability Foam rheology Suspension rheology Particle stabilized foams Porous materials Solid foams abstract 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 foam- ing 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. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Particle-stabilized foams [1–26] and emulsions [25–36] have attracted considerable research interest recently, due to their unique properties and potential technological applications. Among the most interesting applications are those related to the possibil- ity for fabrication of porous materials with complex hierarchical structure, by using particle-stabilized drops or bubbles as tem- plates [18–28]. Several groups described procedures for generation and stabilization of foams by appropriate particle–surfactant mix- tures [1–17]. In some of these studies it was shown that inorganic colloidal particles can be hydrophobized in situ, by adsorption of short-chain amphiphilic molecules, and these surface-modified particles can act as very efficient foam stabilizers [2,3,9,10,14,23]. The effect of the composition of the initial colloidal suspension on the air volume fraction, average bubble size, and bubble size distribution in the formed wet foams was investigated [23]. In other studies, [12–14,16] aqueous foams prepared by dispersions of disk-shaped Laponite particle and containing the cationic surfac- tant cetyltrimethylammonium bromide (CTAB), were studied. All these studies showed that the foams could be stabilized by two basic, qualitatively different mechanisms: (a) adsorption of surfac- tant-modified particles on the bubble surface and the related sta- bilization of the foam films by the particles; (b) formation of a network by aggregated particles in the foaming phase [12,17]. The particle modification by adsorbing surfactant molecules is highly specific and depends on the type and concentration of sur- factants and particles [2,10,12,15,16]. Therefore, it is still very dif- ficult (or impossible) to predict what would be the foamability of such particle–surfactant mixtures and what would be the stability of the generated foams. For example, Sepulveda and Binner [19] claimed that suspension viscosity had a little effect on the final vol- ume of the generated foam, whereas Dhara and Bhargava [22] observed that the total porosity of the final material decreased lin- early with the increase of suspension viscosity. Thus we see that even the ‘‘simple’’ effect of suspension viscosity, which is governed http://dx.doi.org/10.1016/j.jcis.2014.03.067 0021-9797/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author. Fax: +359 2 962 5643. E-mail address: SC@LCPE.UNI-SOFIA.BG (S. Tcholakova). Journal of Colloid and Interface Science 426 (2014) 9–21 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis