The (de)stability and functionality of thin liquid films plays an important role in processes and technical applications like foaming, emulsification and flotation. Within this work the stability of single films is tuned by the addition of different electrolytes. The films are either free-standing (foam films) or supported by a solid substrate (wetting films) and they are formed from aqueous surfactant solutions. The interactions between film surfaces are determined in a thin film pressure balance (TFPB). By this method equilibrium measurements of a disjoining pressure isotherm (disjoining pressure vs. film thickness) are determined. The disjoining pressure is a quantitative measure for the interactions between the two opposing film interfaces, including repulsive electrostatic, attractive van der Waals and steric contributions .
Usually the addition of low molecular salt like NaCl leads to a thinning of the aqueous film due to screening of the electrostatic interactions. But not only the ionic strength, also the type of added ion has a pronounced effect on the film thickness and stability. For instance a “simple” water wetting film becomes thicker and more stable by adding NaI instead of NaF at a fixed ionic strength [2,3]. It is explained by a stronger adsorption of I- ions at the film surface in comparison to F- ions due to differences in the hydration shell. Similar trends are observed for foam films and wetting films formed from surfactant solutions. Surprinsingly, also an SDS film is thicker and more stable after addition of NaI instead of NaF, which means that negatively precharged surfaces can be even more negatively charged by the addition of chaotropc (water breaking) anions .
In the second part of the talk the effect of organic ions like alkanoates is discussed and their properties as salt and co-surfactant .
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