Electrostatic Adhesion of Nanosized Particles: The Cohesive Role of Water

Adhesion between chemically dissimilar solids is not often observed for fundamental reasons, especially the large solid−solid interfacial tensions involved that, in turn, derive from the fundamental characteristics of van der Waals and other intermolecular interactions. However, this difficulty can be overcome in many cases by mixing particulate solids within aqueous media and drying the resulting dispersion. In this work, transmission electron microscopy (TEM) and scanning probe microscopy (SPM) were used to obtain evidence for strong adhesion between the following pairs of organic and inorganic nanoparticles: Stöber silica and poly(styrene-co-butyl acrylate-co-acrylic acid) (SA) latex, calcium montmorillonite and the same latex, and titanium dioxide and another SA latex. Adhesion was observed even though the particles in each pair are highly dissimilar and thus are expected to have a high interfacial tension. Bulk or aggregate particle nanohybrids were obtained by drying mixed aqueous dispersions at different particle concentrations and were examined using bright-field and energy-filtered imaging in TEM, as well as intermittent-contact and phase-contrast SPM. Association between silica, clay, or TiO2 and the latex particles was observed under several conditions, and partial particle segregation was also observed. A general mechanism for the formation of hybrid or composite monoliths is proposed, based on the action of capillary forces during the drying process followed by electrostatic interactions within the dry solid, between negative particles and cationic domains formed by dry serum solutes. Using this model, calculated electrostatic adhesion energy between dissimilar particles can be used to explain previous literature data. This mechanism is suitable for making hybrid monoliths from nanosized particles.