Macroporous gibbsite foams prepared from particle-stabilized emulsions using corn starch and agar as binders

Corn starch and agar were used independently as a water-uptake binder in combination with anionic sodium dodecul sulfate (SDS) which modifies the hydrophobicity of cationic gibbsite platelets for preparation of air-in-water (a/w) gibbsite foams via a simple mechanical frothing. Contact-angle (θ) measurements revealed that the apparent θ decreased from partially hydrophobic (θ ∼ 62°) to more hydrophilic (θ ∼ 47°) when the corn starch was first added, leading to foams with a reduced stability. As the concentration of corn starch reached above ∼8 vol.%, θ returned back to greater than 60°, rendering then a stable foam. On the other hand, θ was found to decrease from 57° to 50° when the agar concentration increased above a mere 0.16 vol.%. This gave rise to a pronounced drainage and coalescence of the foam, and a further increase of the agar concentration only led to a quick disappearance of the a/w bubbles. By tailoring the binder concentration, macroporous gibbsite foams were produced from the air- or freeze-dried wet foams, which typically consisted of packing void cells over a cell-size distribution of 50–400 μm, a porosity ranging from 77% to 86%, and a three-point (green) rupture strength of up to 240 kPa.