Exploiting Heteroaggregation to Quantify the Contact Angle of Charged Colloids at Interfaces

Abstract

We exploit the aggregation between oppositely charged particles to visualize and quantify the equilibrium position of charged colloidal particles at the fluid−water interface. A dispersion of commercially available charge-stabilized nanoparticles was used as the aqueous phase to create oil−water and air−water interfaces. The colloidal particles whose charge was opposite that of the nanoparticles in the aqueous phase were deposited at the chosen fluid−water interface. Heteroaggregation, i.e., aggregation between oppositely charged particles, leads to the deposition of nanoparticles onto the larger particle located at the interface; however, this only occurs on the surface of the particle in contact with the aqueous phase. This selective deposition of nanoparticles on the surfaces of the particles exposed to water enables the distinct visualization of the circular three-phase contact line around the particles positioned at the fluid−water interface. Since the electrostatic association between the nanoparticles and the colloids at interfaces is strong, the nanoparticle assembly on the larger particles is preserved even after being transferred to solid substrates via dip-coating. This facilitates the easy visualization of the contact line by electron microscopy and the determination of the equilibrium contact angle of colloidal particles (θ) at the fluid−water interface. The suitability of the method is demonstrated by the measurement of the three-phase contact angle of positively and negatively charged polystyrene particles located at fluid−water interfaces by considering particles with sizes varying from 220 nm to 8.71 μm. The study highlights the effect of the size ratio between the nanoparticles in the aqueous phase and the colloidal particles on the accuracy of the measurement of θ.

Electrostatic Heteroaggregation: Fundamentals and Applications in Interfacial Engineering

“…The electrostatic heteroaggregation between the particles adsorbed at the interface and the particles dispersed in the bulk has been studied when the diameter of the particles at the interface ( D i ) and the diameter of the particles in the bulk ( D b ) are significantly different, i.e., D i ≫ D b . , The first step in such studies involves creating a fluid–water interface by taking an aqueous dispersion of charged nanoparticles or a reaction mixture that leads to the in situ formation of charged nanoparticles in a suitable container such as a beaker or a Perti dish. For the latter, any metal salt along with an appropriate quantity of reducing agent can be used.…”

“…Influence of the particle size ratio ( P SR ), defined as the ratio of the diameter of nanoparticles in the bulk to the diameter of particles adsorbed at the interface, on the magnitude of the three-phase contact angle (θ) measured by the heteroaggregation approach. Reprinted with permission from ref .…”