1 research outputs found
pH-Controlled Microbubble Shell Formation and Stabilization
We report on microbubbles
with a shell self-assembled from an anionic
perfluoroalkylated surfactant, perfluorooctylÂ(ethyl)Âphosphate (<i>F</i>8<i>H</i>2Phos). Microbubbles were formed and
effectively stabilized from aqueous solutions of <i>F</i>8<i>H</i>2Phos at pH 5.6–8.5. This range overlaps
the domains of existence of the monosodic and disodic salts. The shell
morphology of microbubbles formed spontaneously by heating aqueous
solutions of <i>F</i>8<i>H</i>2Phos was monitored
during cooling, directly on the microscope’s stage. At pH 5.6,
the shell collapses through nucleation of folds, as typical for insoluble
surfactants. At pH 8.5, no folds were seen during shrinking. At higher
pH, the microbubbles rapidly adsorb on the glass. The effect of pH
(from 5.6 to 9.7) on adsorption kinetics of <i>F</i>8<i>H</i>2Phos at the air/water interface, and on the elasticity
of its Gibbs films, was determined. At low pH, <i>F</i>8<i>H</i>2Phos is highly surface active. The interfacial film undergoes
a dilute-to-condensed phase transition and a dramatic increase of
elastic module, leading to extremely high values (up to 500 mN m<sup>–1</sup>). At high pH, the surfactant’s adsorption
is quasi-instantaneous, but interfacial tension lowering is limited,
leading to very low elastic module (∼5 mN m<sup>–1</sup>). At pH 5.6 and 8.5, the interfacial tension of <i>F</i>8<i>H</i>2Phos adsorbed on millimetric bubbles and compressed
at a rate similar to that exerted on micrometric bubbles during deflation
is lower than the equilibrium interfacial tension. Langmuir monolayers
of <i>F</i>8<i>H</i>2Phos are highly stable at
low pH and feature a liquid expanded/liquid condensed transition;
at high pH, they do not withstand compression. Both mono- and disodic <i>F</i>8<i>H</i>2Phos salts are needed to effectively
stabilize microbubbles: the rapidly adsorbed disodic salt stabilizes
a newly created air/water interface; the more surface active monosodic
salt then replaces the more water-soluble disodic salt at the interface.
During deflation, the surfactant shell undergoes a transition toward
a highly elastic phase, which further contributes to bubble stabilization