Measuring Colloidal Forces With Atomic Force Microscopy 1: Salt Influence on Hydrophobic and Hydrophilic Interactions

Colloidal forces are essential for maintaining the stability and functionality of colloidal systems, affecting various industrial, biological, and environmental processes. They play an important role in determining the behavior of particles in suspensions, including stability, aggregation, and surface interactions. In this primer, we present basic concepts and protocols for studying colloidal interactions at different salt concentrations using atomic force microscopy (AFM). Following this methodology, hydrophilic substrates (i.e., silica) were easily functionalized with a hydrophobic fluorocarbon (1H,1H,2H,2H-Perfluorooctyltrimethoxysilane, FOTS) via chemical vapor deposition (CVD) and characterized by the sessile drop method, electrophoretic light scattering, AFM imaging, and scanning electron microscopy (SEM) to determine parameters such as contact angle, zeta potential, and surface roughness, respectively. Thus, after the preparation and characterization of a well-defined colloidal system, force-distance experiments using AFM allowed for the measurement of hydrophobic and hydrophilic interactions in salt solutions. Furthermore, we describe in detail the processing and fitting of the experimental data with an extended DLVO model.