Interfacial water at hydrophilic surfaces: Measurement of force at the Nafion-water interface

Abstract

On the microscopic scale, water is complex due to its unique molecular structure and bonding capabilities. This interfacial water has been studied for nearly 200 years, since Helmholtz first proposed his model of water at a charged surface (1850s). In biology, charged surfaces are everywhere and in addition, are hydrophilic, rough and irregular in geometry. These features contribute to interfacial water's spectroscopic, electrical and mechanical distinctiveness from bulk water. While the ordering and extent of interfacial water from a surface has a long, controversial history, long-range water ordering on the hundreds of nanometers to hundreds of microns scale has been confirmed and measured at a variety of biological interfaces and next to many hydrogels and hydrophilic surfaces. One hypothesis for such long-range ordering is called the exclusion zone (EZ). EZs are found next to many hydrophilic surfaces and are characterized by the exclusion of microsphere suspensions, colloids, dyes and some solutes from boundary layers 100-300 μm in thickness. In this dissertation, the EZ phenomenon was established to be based on the separation of charge between the (negatively charged) interfacial EZ water and the bulk (proton-enriched) water zone beyond the EZ (PEZ). We hypothesize, that based on charge separation, there is a significant electrostatic force that exists between the EZ and PEZ. A Nafion tube, which has previously demonstrated robust EZs, was attached to the tip of a custom-fabricated ribbon-like beam sensor. The geometry of the ribbon blocked one side of the Nafion tube and obstructed proton diffusion, thus unilateral force would develop on the contralateral side and be measured by the sensor. When water was added, the tube hydrated and the sensor measured a force of 22 ± 2 μN. pH-sensitive dye measurements of the region confirmed an electrostatic interaction since proton distributions were confirmed to be heavily weighted on the unblocked side of sensor and in the same direction as the measured deflection. Significant electrostatic forces at hydrophilic interfaces in water may play a role in a diverse array of processes in nature, such as protein folding, adhesion and water harvesting.