Optical imaging of surface chemistry and dynamics in confinement

We imaged the interfacial structure and dynamics of water in a microscopically confined geometry in three dimensions and on millisecond time scales with a 3D structurally illuminated wide-field second harmonic microscope. Image contrast reports on the orientational order of interfacial water, induced by charge-dipole interactions between water molecules and surface charges. The images were converted into surface potential maps. Spatially resolved surface acid dissociation constant pKa,s values were determined for the silica deprotonation reaction by following pH induced chemical changes on the curved and confined surfaces of a glass micro-capillary immersed in aqueous solutions. These values ranged from 2.3 to 10.7 along the wall of a single capillary, because of surface heterogeneities. Water molecules that rotate along an oscillating external electric field were also imaged.