Hydrophobic interaction and patch charge attraction in α-Al₂O₃ dispersions under the influence of adsorbed low molecular-weight polyacrylic acid sodium salt and poly(methacrylic acid) sodium salt: yield stress and AFM force study

Maximum yield stress data showed that low molecular-weight (Mw) (∼7 kDa) poly(methacrylic acid) sodium salt (PMA-Na) additive at low surface coverage displayed significant patch charge attraction in contrast to polyacrylic acid sodium salt (PAA-Na) additive of similar Mw and surface coverage. Intramolecular hydrophobic interaction between CH₃ groups in the polymer molecule during adsorption produced a much more compact patch with a higher negative charge density giving rise to the stronger patch charge attraction. At high surface coverage, intermolecular hydrophobic interaction between CH₃ groups on the adsorbed layer of the interacting particles was not observed from maximum yield stress data. Such interaction was, however, observed in AFM force-distance characterization data for interaction between spherical alumina particles and sapphire plates coated with PMA-Na in retraction mode. The compression of the adsorbed layers at contact during the approach mode was postulated to deform and breakup the intramolecular interaction between the CH₃ groups and promoted intermolecular interaction between these groups in the layer coating the particle and plate. This resulted in a strong adhesion force seen in the retraction mode after contact at low pH near the point of zero charge.