Simple Method for Controlled Association of Colloidal-Particle Mixtures using pH-Dependent Hydrogen Bonding

We describe a simple method for the controlled mixing of particles that could be used to produce materials with new properties. We demonstrate the procedure with sets of silica particles that have each been coated with one of two different organic thin films. One set of particles is functionalized with carboxylic acid groups and the other with ethylene oxide. Each of these sets of particles is stable in solution. When mixed, heteroaggregation can be induced reversibly and on demand simply by changing the pH. We provide evidence that control over aggregation is achieved by the ability to alter the number of hydrogen bonds between different types of particles and thus the strength of the attraction between different particles. We provide support for this mechanism by measuring the forces between a plate coated in a thin film of carboxylic groups and particles coated in ethylene oxide using colloid probe AFM. At pH 9, where we expect most of the acidic groups to be deprotonated, there is a strong repulsion between the particle and plate. However, at pH 3, the force is attractive, which we assign to the hydrogen bonding between the ether oxygen of the PEO and the hydrogen of the carboxylic acid group. Heteroflocculation occurred in the pH range of 3−4.5. At pH 5 and above, no flocculation was observed. Because the number of hydrogen bonds per surface area and therefore the strength of binding between dissimilar particles can be titrated through control of the pH, we can control the surface forces and avoid rapid coagulation that produces low density and disorganized particle arrangements. The control of interaction forces and therefore the approach of particles should allow the production of composite materials having mixture or product properties.