Specific anion effects on the hydration and tribological properties of zwitterionic phosphorylcholine-based brushes

Polymer brushes are known to form highly efficient lubricating layers for a broad range of contacting surfaces in water. Here we focus on brushes of poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) polymers that are end-anchored to the substrate at a high density. We show that the friction coefficient, which is measured upon sliding atomic force microscopy colloid probes on PMPC brushes at different normal loads, decreases when salts are added (1 M sodium fluoride (NaF), sodium chloride (NaCl), sodium iodide (NaI), cesium chloride (CsCl) and cesium iodide (CsI)). Interestingly, we observe that the reduction in the friction coefficient increases with increasing anionic size in the solution, while the size of the cation has no measureable effect. We relate the reduction in the friction coefficient to a decrease in the compliance of the brush. In pure water the brush is swollen and soft, resulting in a large contact area between colloids and brushes and, therefore, a relatively “high” friction coefficient. In salt solutions, the stiffness of the brush increases with increasing anionic size resulting in a lower contact area and, therefore, lower friction coefficient.