Boundary lubrication by sodium salts: a Hofmeister series effect

Boundary lubrication plays an important role in the function of sliding surfaces in contact. Of particular interest in this study, boundary regime tribology is relevant for understanding textural attributes perceived during oral consumption of food, where the tongue squeezes and slides against the hard palate. This work investigates aqueous lubrication of a sliding/rolling ball-on-disc contact by sodium anions of the Hofmeister series in both water and guar gum solutions. Low concentrations (0.001 M) of strongly kosmotropic salts provide reduced friction coefficients in both systems (water and guar gum solutions), although a different mechanism prevails in each. Surface-bound hydrated ions are responsible in the case of water, and salt-promoted adsorption of hydrated-polymer chains dominate with guar gum. In each system, friction decreases in accordance with the Hofmeister series: iodide, nitrate, bromide, chloride, fluoride, phosphate and citrate. The addition of salt has little impact on solution of bulk viscosity, and so this work demonstrates that significant boundary lubrication can be provided without surface modification and with lubricants of viscosity similar to that of water.