NaCl Inducing Phase Transformation in Interfacial Water Under Quasistatic-Loading Conditions

From clay swelling to geological transformations, from the structure to functioning of living organism, and from metallic corrosion to the recovery of oil from deeply lying pores, the presence of ions in water at the interface plays an important role. Many studies have been performed to understand the role of ions in water at the interface between solid to solid, solid to liquid and soft matter to liquid media. However, the dynamics and mechanical properties of electrolytic water at the interface have not yet been completely understood. The present day literature still consists controversial results with some time contradictory explanations. Here we present a study of the effects of the concentration of NaCl on the stiffness and damping properties of interfacial water confined between mica substrate and an oxidized silicon cantilever. We have used a home-built Scanning Probe Microscope (SPM), which uses an optical fiber, based on Fabry-Perot interferometer principle, to monitor the amplitude and phase of the cantilever with <0.1 Å precision. Here we present the behavior of electrolytic water of various NaCl concentrations (0.001-1M) near the interface under different dynamical conditions. We have observed that even under nearly static condition (a compression rate of 2 Angstrom per second), the presence of NaCl introduces peaks in the relaxation time which were not seen in the case of pure water, suggesting a phase transformation of the electrolytic water  from a liquid-like to more solid-like behavior at the interface. The critical concentration of NaCl suggesting the phase transformation seems to be lying around 0.1 M NaCl. Our detailed and systemic study of the effects of NaCl ions in pure water under different dynamical conditions may contribute to better understanding of the role of electrolyte in interfacial water as well as may help to clear some of the underlying controversies in its physics and chemistry.