Generation of ultra-stable Pickering microbubbles via poly alkylcyanoacrylates

A range of solution conditions (pH, surfactant concentration and type) have been tested for the polymerization of alkyl cyanoacrylates (ethyl (ECA), butyl (BCA) and octyl (OCA)) into nanoparticles (NPs) potentially capable of stabilizing highly unstable microbubbles (MBs) of air in aqueous solutions. The optimum system was butyl cyanoacrylate (BCA) polymerized into PBCA particles at pH 4 in the presence of 1 wt.% Tyloxapol surfactant. These PBCA particles were highly effective at stabilizing MBs of only a few microns in size for at least 2 months. Microscopy over a range of length scales clearly indicated that these particles were stabilized via a Pickering mechanism. Only a relatively low volume fraction (ca. 1 vol.%) of MBs could be obtained via a single aeration step of a 0.7 wt.% dispersion of PBCA particles in a high shear mixer. Although this could be increased to 2 and 3 vol.% by second and third aerations, this reflects the difficulty of obtaining and maintaining rapid enough particle coverage of small bubbles even under turbulent conditions. Similar sizes and yields of PBCA particles could be obtained in the absence of surfactants, but these particles, with or without addition of surfactant afterwards, could not stabilize MBs. We estimate that approximately one quarter of the Tyloxapol when present during polymerization is incorporated into the particles on polymerization, which somehow imparts the correct surface hydrophobicity and contact angle to the particles at the A/W interface, making such particles so very effective as Pickering MB stabilizers.