Abnormal thermally stabilized switchable aqueous foams

Abstract

Aqueous foams generally decay upon increasing temperature because of deterioration of surface activities. However, there is a need for developing switchable foams that can be stabilized at high temperature but degraded when temperature is decreased. We reported in this work the synthesis of N-(polyoxyethylene stearate)-N,N-dimethylglycine carboxylic acid betaine (ESAB) and poly(N-isopropylacrylamide)-modified cellulose nanocrystals (MCNC). Unprecedent thermally stabilized foams were developed with ESAB alone and co-stabilized with ESAB and MCNC in 100,000 mg⋅L−1 NaCl at different temperature and different pressure. Surface tension and interfacial viscoelasticity of ESAB solutions were examined, and the morphologies of the foams were visualized with fluorescence and confocal microscopies. The foamability and stability of ESAB-foams are boosted when temperature is increased. Introduction of MCNC as a co-stabilizer further improves the stability of the foams. Both foam systems are stable at 120 °C, but burst at room temperature, the foaming and defoaming properties can be switched between these two temperatures. Reduced critical micellar concentration and surface tension, as well as the thickened liquid film with increasing temperature are ascribed to the thermal stabilization of the foams and switchabaility. More uniform and denser bubbles at higher temperature slow down the coarsening process of the ESAB foams, and the thickened liquid film of ESAB/MCNC foams further retards the liquid drainage.