Water Contact Angle On Polyelectrolyte‐Coated Surfaces: Effects of Film Swelling and Droplet Evaporation

The wettability by water of polyelectrolyte mono‐ and multilayers with different polycations or polyanions as the exposed layer is studied by contact angle measurements using the sessile drop technique. Measurements in a water‐saturated atmosphere and in ambient conditions [∼40% relative humidity (r.h.)] are made to study the effect of swelling and evaporation on the contact angle. It is found that these effects depend on the outermost layer of the polyelectrolyte coating. For a coating with Poly(styrene sulfonate) (PSS) as the exposed layer the water contact angle also indicates a change in the swelling state when the pre‐equilibrated film is directly in contact with liquid water. Droplet evaporation from polyelectrolyte coatings leads to a linear decrease of the contact angle with time. A significant enhancement of the droplet evaporation rate is found for a coating with Poly(diallyldimethylammonium bromide) (PDADMAC) as the exposed layer, which is attributed to the existence of a surface energy related driving force to the swelling of this polyelectrolyte coating. The studies show that a highly sophisticated interplay between hydrophobic backbone and charge density determines the wetting behavior, irrespective of the sign of surface charge.