Double Layer Relaxation Measurements Using Atomic Force Microscopy

Abstract

The force acting on the tip during its immersion in the double layer region was measured for various tip-approaching velocities. The double layer electric field acting on the tip results in a repulsive force component when the tip is immersed in the diffuse layer corresponding to a region far away from the liquid/solid interface (∼100 nm) and an attraction when it is immersed in the near-surface layer (∼2 nm). For higher values than 10 μm/s (in water), the transient force on the tip results from the interaction of two double layers (Si3N4 tip and mica substrate) with their charge distribution in a nonequilibrium situation. In the inner layer, the thickness of the interaction region as well as the force on the tip decrease with an increasing approaching speed while in the diffuse layer there is a force increase with an approaching velocity. Dimethyl sulfoxide shows only a repulsive force, indicating that the attraction in the near-surface layer in water is associated with a variable water dielectric constant at the liquid/solid interface. In water the measured transient forces as a function of the tip/substrate distance show a relaxation time of ∼1.1 ms when the tip is immersed in the inner layer while the diffuse layer shows a relaxation time of ∼5.6 ms. The measured diffuse layer relaxation time for dimethyl sulfoxide is ∼0.5 ms.