pH-responsive liquid marbles stabilized with poly(2-vinylpyridine) particles

Submicron-sized sterically-stabilized lightly cross-linked poly(2-vinylpyridine) latexes were synthesized by non-aqueous dispersion polymerization using a hydrophobic polydimethylsiloxane macromonomer and used to prepare millimetre-sized ‘liquid marbles’. These ‘liquid marbles’ exhibited long-term stability when transferred onto the surface of liquid water provided that the solution pH was pH 4.9 or above. In contrast, the use of more acidic solutions (pH < 2.9) led to immediate ‘liquid marble’ disintegration and spontaneous dispersal of the poly(2-vinylpyridine) particles in their protonated microgel form. Moreover, the liquid marbles on the surface of water immediately disintegrated on addition of acid. Thus the critical minimum pH required for long-term ‘liquid marble’ stability correlates closely with the known pK_a value of 4.7 for poly(2-vinylpyridine) chains.