Electrolytes related dynamic mechanical properties of nanoconfined water

Charged surfaces and electrolytes play an important role in many scientific and technological applications such as molecular friction, transport through bio-membranes, and generation of electrochemical energy. Using a home-built ultra-sensitive atomic force microscope, we have measured the stiffness, damping coefficient, and Maxwell's relaxation time of a few molecular layers thin film of electrolytic water against atomically smooth mica surface. Water containing NaCl and CsCl have been compared with ultrapure water. The viscoelastic properties of the nanoconfined film exhibit a strong dependence on the compression rate of the film as well as the type of electrolyte present. The increase in Maxwell's relaxation time as a function of NaCl concentration gives rise to a solidlike dynamic mechanical response under quasi-static conditions (compression rate = 0.2 nm/s). In contrast, the CsCl containing electrolytic water seems to suppress the solidlike dynamic mechanical behavior, which results from an apparent jamming of the molecules during a squeeze-out, as suggested elsewhere (Khan et al., Phys. Rev. Lett. 2010). These results suggest that the size and hydration number of ions play an important role in nanoconfined water.