Polyelectrolyte multilayers under compression: concurrent osmotic stress and colloidal probe atomic force microscopy

Abstract

Colloidal interactions have been characterised using both osmotic stress and surface forces. Here these methods are employed concurrently to measure the interaction forces of polyelectrolyte multilayers that when cross-linked form a dextran impermeable membrane. The force data, corrected for the thickness of the polyelectrolyte multilayer film, has been expressed as pressure versus separation enabling the interaction from osmotic stress measurements to be compared to the measured interaction from the colloid probe technique. The combined technique is valuable in evaluating the interaction forces associated with compression of polymer films at different rates and over a wide range of pressure and demonstrates features that are not revealed when just one technique is employed. The combination of the techniques allows both attractive forces and strongly repulsive forces to be measured and shows that the measured repulsion is greater in the force data than in the osmotic data. This is due to insufficient equilibration time in the AFM measurements, even at the slowest approach rates available, indicating that AFM force measurements between polyelectrolytes will always contain a dynamic component. That is we demonstrate that colloid probe measurements between polymer surfaces overestimate the equilibrium repulsive interaction due to the rate at which the measurement is performed.

Quantifying Double-Layer Potentials at Liquid–Gas Interfaces from Vibrational Sum-Frequency Generation

“…Results and Discussion. In our procedure, we have recorded SFG spectra for different [C 16 ). As ionic strengths we have chosen low 0.7 mM and high 500 mM values for each surfactant mixing ratio.…”

“…However, for investigations of aqueous interfaces, there exist only a few experimental techniques that can probe their charging state and provide quantitative information on ϕ 0 . For solid/liquid interfaces, classical methods such as the surface-force apparatus and (colloidal probe) atomic force microscopy − have been used to provide information on ϕ 0 by an analysis of force–distance curves and the application of the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. For liquid/gas interfaces, the use of the thin-film pressure balance (TFPB) technique , is so far the only commonly accepted method for quantitative analysis but is also constrained to stable foam or emulsion thin films.…”

Direct Measurement of Interaction Forces between Surfaces in Liquids Using Atomic Force Microscopy

“…McLean et al (2005) reported that the Milner, Witten, and Cates (MWC) model (Milner et al, 1988) gives a better fitting than the AdG theory for the interaction between hydrophobic surfaces with adsorbed triblock copolymer layers, presumably because the segment density profile on the adsorbed polymer on the surfaces is in line with the assumptions of the MWC model. Further, it is theoretically and experimentally difficult to decouple steric forces from hydrodynamic forces associated with the expulsion of solvent during compression of a polymer brush or film (Wu et al, 2018). The steric force also acts between surfaces with adsorbed aggregate structures, such as micelles.…”