Forces between a monolayer at an air/water interface and a particle in solution: influence of the sign of the surface charges and the subphase salt concentration

Abstract

The way the rigidity, molecular packing density, and charge of an insoluble monolayer at an air/aqueous interface and the type of substrate affect the deposition of a monolayer on a substrate is still unclear. In this study, we aimed to better understand the forces involved in the deposition process, when a solid surface is brought near a monolayer. We achieved this by using the Monolayer Particle Interaction Apparatus to directly measure the interaction forces between a monolayer at an air/aqueous interface and a hard particle in solution. The monolayer surface pressure, the monolayer and particle surface charge, and the concentration of salt in the subphase were varied. An adhesion was observed between a particle and a monolayer in an unlike-charged monolayer–particle system, or when a particle with a non-zero contact angle was used in the measurement. The addition of salt changed the packing of the monolayer, increasing the stiffness of the monolayer and generally decreasing the adhesion. Increasing the surface pressure of the monolayer decreased the interfacial stiffness and generally decreased the adhesion.