Effect of esterically bonded agents on the monolayer structure and foamability of nano-silica

The use of nanoparticles as foam stabilisers has gained increasing attention in recent years, for applications in a number of process industries, such as food colloids, metal and ceramics production and mineral flotation. Here we consider the foaming behaviour of 270 nm silica particle suspensions, hydrophobised to varying extents via esterification with butanol, octanol and dodecanol. The foaming behaviour was related to the compressed two-dimensional monolayer behaviour from Langmuir-Blodgett trough and SEM studies. The octanol esterified species performed best as both dynamic and static foaming agents, indicating particles of moderate hydrophobicity may work best as stabilisers. Dynamic foam stability results were linked to the balancing of interfilm drainage and steric stabilisation mechanisms. No firm conclusions could be drawn on the total effect of hydrophobicity on static foams, as particles esterified in dodecanol shear-flocculated under the stresses of foam production. Foam stability correlated well to compressed monolayer behaviour, with two-dimensional stability being influenced by the competing effects of a more disordered network structure and higher interfacial strength, as particle hydrophobicity was increased. It is noted that even with the best performing particles, less than 10% of a given dispersion interacted under shaking to form bulk foam. (C) 2008 Elsevier B.V. All rights reserved.