Force measurements between mica surfaces in concentrated electrolyte solutions

The force-distance profiles are in agreement with Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in dilute solutions. However as solute concentration being elevated, the deviations from DLVO theoretical prediction are observed with the experiment. In this work, the force as a function of distance between two molecularly smooth mica sheets immersed in concentrated NaCl solutions is measured using the surface forces apparatus (SFA). In 0.1 M solution, the long-range force shows no deviation from DLVO force, while at short distance the discrepancies between experiment and theory can be explained by introducing a hydration force. The fitted surface potential Ψ_d consists with ion-exchange model prediction by shifting the diffuse layer on the mica surface at a distance of a diameter of hydrated ion. In the more concentrated solutions, the measured results indicate that an extra diffuse layer is formed. Here, the cause of the unexpected phenomenon is due to the effect of crowing and overadsorption by considering co-ions, which result in Na⁺ and Cl^- alternated layering distributions at the interface. Therefore, solidification areas are more likely to be formed under the condition of the confined space, especially in the highly concentrated 3 M and 5 M solutions.