Water–solid interactions on NaCl: Tracking adsorption to deliquescence with in situ scanning probe microscopy under a wide range of water vapor pressure

Water adsorption and phase transitions on NaCl surfaces before dissolution play a crucial role in understanding interfacial water-solid interactions. In this study, we employ variable-pressure scanning probe microscopy (SPM) to systematically investigate nanoscale morphological and tribological changes across a wide range of relative humidity (RH). At extremely low RH (<10-2%), water shows a strong affinity for Na+ ions, leading to increased friction, particularly at surface defects such as step edges. As RH increases to several tens of percent, this high-friction region expands across entire terrace areas. Below ∼40% RH, hydrated ion clusters form, locally reducing friction due to their liquid-like nature. Above ∼40% RH, these hydrated ion clusters disperse, resulting in a global decrease in surface friction. At higher RH levels, increased lubrication facilitates NaCl nanostructure movement, reducing pre-existing surface anisotropy and accelerating dissolution dynamics until deliquescence (∼75% RH). Our findings indicate that Cl- ion release is enhanced by water clusters, while strongly bonded Na+ ions remain exposed, acting as preferential sites for further adsorption. By utilizing SPM across a broad RH spectrum (10-7 to ∼75%), this study provides new insights into the fundamental nanoscale mechanisms governing water adsorption, phase changes, and dissolution at the NaCl-water interface.