Hydration force due to the reduced screening of the electrostatic repulsion in few-nanometer-thick films

Experiments with foam films from solutions of 1 mM SDS + 100 mM electrolyte (LiCl, NaCl and CsCl) were carried out by a thin-film-pressure balance. The measured dependences of disjoining pressure versus film thickness exhibit a steep increase when the thickness of the film's water core becomes smaller than 3.7 nm. This behavior can be interpreted as a manifestation of the hydration force. We unsuccessfully tried to interpret the data with different available theoretical models. Eventually, we found that a relatively simple model of “reduced screening” can fit the data. Such reduced screening of the electric field could exist only in a narrow range of film thicknesses, which practically coincides with the region where the hydration repulsion is acting. This model and its experimental verification are described in the article.

At larger thicknesses, the disjoining pressure in foam films corresponds to the conventional double-layer repulsion: The electrostatic repulsion is screened by counterions dissociated from the film surfaces and ions from the electrolyte (Debye screening). A considerably stronger repulsion is observed in few-nanometer-thick films, which can be explained with a reduced screening due only to counterions dissociated from the film surfaces.Figure optionsDownload high-quality image (280 K)Download as PowerPoint slideResearch highlights
► Different models of the hydration repulsion were tested against experimental data.
► A new model with reduced screening of the electric field complies with the data.
► This model assumes that at small thicknesses the film contains only counterions.
► Then, the screening of the electric field is weaker and the repulsion is stronger.
► The regime of charge regulation, related to counterion condensation, takes place.