Measurements of screening length in salt solutions by total internal reflection microscopy: Influence of van der Waals forces and instrumental noise

Total internal reflection microscopy (TIRM) measurements of colloidal spheres in aqueous electrolyte solutions have been carried out with focus on screening properties of various electrolytes and methods and analyses for reliably extracting them from TIRM measurements. Approximations based on Lifshitz theory of flat plates are found to yield van der Waals interactions that are too strong compared to measured interaction potentials. Allowing for some attenuation of these interactions due to surface roughness, a consistent set of screening lengths can be extracted by fitting a model of the interaction, comprising screened Coulomb, van der Waals, and gravitational interactions, to the TIRM data. With the exception of the 2:2 electrolytes, the screening lengths extracted from the TIRM measurements are well described by Debye–Hückel theory, including some surfactants below their critical micelle concentration and electrolytes with large size asymmetries between anion and cation. In 2:2 electrolytes the screening lengths are found to be larger than the corresponding Debye length by as much as 50% at the highest salt concentrations studied. While this deviation is significantly larger than predictions of theory based on the primitive model of electrolytes, similar magnitudes are found from analysis of BD simulations with added noise at realistic levels in the incident intensity. The work shows that care has to be taken when extracting parameters governing potentials in the presence of noise, particularly at high ionic strengths when potentials become steep at short separations.