In situ observations of adsorbed microgel particles

The formation and morphological changes of a pH-responsive microgel layer on silica and mica were studied by tapping mode atomic force microscopy. First, lightly cross-linked, sterically-stabilised poly(2-vinylpyridine) (P2VP) particles were adsorbed in their non-solvated latex form at pH 4.8 to produce a structurally-disordered monolayer that covers the entire substrate. Addition of acid to this particulate film induces a latex-to-microgel transition at pH 3.0, causes particle swelling (and also some desorption) and produces a uniform, swollen film with localised hexagonal packing. Returning to pH 4.8 causes partial microgel deswelling to form individual oblate spheroidal P2VP latex particles, which retain the localised order previously induced by swelling. The swelling and collapse of this P2VP film was reversible during subsequent pH cycles, with no further desorption observed. The adsorbed amount of P2VP microgel/latex was quantified at each pH by determining the surface density and dimensions of the adsorbed particles. These measurements allow the microgel surface excess to be calculated for the first time. The initial adsorbed amount is less than that predicted by the standard Random Sequential Adsorption model (RSA) for hard particle adsorption, and is explained by the unexpected deformation of these high Tg particles due to their strong electrostatic attraction to the solid-liquid interface.