Water Mobility: A Bridge Between the Hofmeister Series and the Friction of Zwitterionic Surfaces In Aqueous Solutions

In this work, we systematically studied the effects of ions in the Hofmeister series on the friction of zwitterionic surfaces with carboxybetaine self-assembled monolayers (CB-SAMs) as model surfaces using molecular dynamics simulations. The friction coefficients between two CB-SAM surfaces under shear were calculated in the presence of LiCl, NaCl, KCl, CsCl, CsF, CsBr, or CsI solutions at 1M. Results show that there is a strong correlation between the order of ions in the Hofmeister series and the friction of the CB-SAM surfaces. For salt solutions with the same anion, the friction of the surfaces in a solution with kosmotropic cations has larger friction than that in a solution with chaotropic ones. The same relationship between the order of ions in the Hofmeister series and the friction of the surfaces can be found for the salt solutions with the same cation but different anions. The analysis of water near the surfaces further suggests that these surfaces are hydrated with a similar amount of water molecules and the mobility of interfacial water is the key factor which bridges the relationship between the order of ions in the Hofmeister series and the friction of zwitterionic surfaces. High water mobility promotes the lubrication of zwitterionic surfaces.