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 [ similar to 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.