Boundary lubrication by sodium salts: A Hofmeister series effect

► Aqueous fluids of water with salts and guar gum solutions with salts were tested tribologically. ► Both systems show enhanced lubrication with salts. ► Friction coefficients depend on position within Hofmeister series. ► Surface-bound hydrated ions provide aqueous lubrication in salt and water. ► Salt-promoted surface-bound hydrated polymers lubricate in salt and guar gum solutions.