Super stable foams have been recently generated from aqueous solutions of hydrophobic, charge-stabilized ethyl cellulose (EC) nanoparticles produced by a nano-precipitation (anti-solvent) method. The exceptional stability of these foams has been broadly attributed to irreversible adsorption of EC nanoparticles at the gas-liquid interface, but the details of the adsorption process are not completely understood yet. We address in this work a number of fundamental questions related to nanoparticle-interactions controlling colloidal stability and nanoparticle-interface interactions during the course of EC nanoparticle adsorption.  Using dynamic surface tension (DST) as a probe of EC nanoparticle adsorption, we show that irreversible adsorption kinetics is unequivocally characterized in terms of the adsorption energy and rate constant, and the maximum (jamming) coverage. Strong adsorption of EC nanoparticles is observed at neutral pH and in the absence of any ions, suggesting the parallel action of an attractive non-DLVO force. Consideration of a hydrophobic force explains strong adsorption for all ionic strengths and leads to quantitative predictions of colloidal stability. The former is revealed by the interpretation of DST measurements at different ionic strengths using a novel model. It is shown that the adsorption kinetics of EC nanoparticles are not affected by changes of the ionic strength, but colloidal stability is. Through a series of zeta-potential measurements and DLVO calculations, the transition from colloidal stability to coagulation with increasing ionic strength is defined. This transition is also determined experimentally by measuring the nanoparticle size distribution over time using dynamic light scattering and good agreement with theory is observed. These findings have significant implications for the fundamental understanding of colloidal systems and the utilization of nanoparticle colloids for the stabilization of emulsions and foams.

Colloidal stability and adsorption-driven self-assembly of ethyl cellulose nanoparticles at the air-water interface

248th National Meeting (2014)

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