Wetting films stabilized by block-copolymers

Thin aqueous films formed on a solid surface play an important role in adhesion, spreading, and colloidal stability. These phenomena are all relevant for paint systems. Measuring surface forces in these films is an experimental challenge, and over the years several techniques have been developed to measure the interaction forces as a function of the thickness of the film, the so-called disjoining pressure isotherms. A thin film balance technique (TFB) in combination with an ellipsometer offers the possibility to study the surface forces in aqueous films formed at a silica surface.The aim of this project was twofold. Firstly, we wanted to set up the technique and test it by investigating interaction forces in thin aqueous films formed between an air interface and a solid interface (air/aqueous film/silica). Secondly, we intended to apply this technique for investigating interaction forces in aqueous films formed between an oilinterfaceand a solid interface (oil/ aqueous film/ silica). As happens so often in research, we were 'immersed' in the first system, and as a result of it, we did not reach the second aim. Hence, this thesis deals with the experimental investigations of the interaction forces in thin aqueous films formed between an air phase and a solid phase. A non-ionic polymeric surfactant is used as a stabilizer of these films. In addition, we have also used colloidal probe atomic force microscopy (CP-AFM) in order to see if phenomena similar to those in wetting films might occur in the interactions between two silica surfaces, immersed in solutions of the surfactant.In chapter 1 an introduction is given to wetting films, to the surfactant used and to the technique used in the present study. A short overview of the interaction forces in wetting films and the drainage of these films is presented. As foam films, which are aqueous films between two air phases (air/ aqueous film/ air), have much more extensively been studied than wetting films, we also discussed drainage of foam films, as well as and an interesting finding in foam films stabilized by similar surfactants.In chapter 2 the solution behaviour of the surfactant and its adsorption on silica-water and air-water interfaces is studied using light scattering, surface tensiometry and ellipsometry. Interaction forces in wetting films stabilized by the surfactant are measured and are interpreted in terms of a disjoining pressure as a function of a thickness of the film. Parameters of the systems are the concentration of the surfactant and the concentration of NaCl. The main findings are that addition of NaCl lowers the cmc, diminishes adsorption of the surfactant on silica, and increases the thickness of the wetting film stabilized by the surfactant.In chapter 3 we investigate the thinning of wetting films formed from aqueous solution of the surfactant on silica using a TFB and an ellipsometer. In addition, imaging ellipsometry is used to visualize the film structures at subsequent stages of their development. The main findings are that the thick films observed in chapter 2 are metastable. The time required for the formation of steady films strongly increases with the concentration of electrolyte. For sufficiently large amounts of salt, two characteristic relaxation regimes can be clearly identified: after initial quick thinning, further thinning slows down enormously. The interpretation we give of the observed features is that these typical kinetic regimes are the result of coupled dependencies of the bulk and interfacial properties of the surfactant on salt concentration.In chapter 4 the time-dependent thickness of wetting films formed from aqueous solutions of NaCl on a silica surface modified with the surfactant is revisited, because of a rather surprising finding: the thickness as a function of time could go through a maximum. It turned out that that the thickness of the films depends strongly on whether or not the surfactant was applied over the entire substrate or only at a spot coinciding with the investigated wetting film. In the former case, the thickness shows a transient thickness maximum, in the latter case, it equilibrates quickly. A possible interpretation of the transient thicknesses is given, and the equilibrium values of the thickness are discussed.In chapter 5 the effects of pH and different additives (NaCl, Na 2 SO 4 , NaSCN and urea) on the adsorption of the surfactant at the interfaces of wetting films, on film drainage and on the interaction forces in these films are examined using an ellipsometer and a TFB. These additives reduce the adsorption of the surfactant on silica and all retard the drainage of the films stabilized by the surfactant. Moreover, the film destabilizes at high pH values if 0.1 M NaCl is present. The reduction of the adsorbed amount is correlated to a dramatic slow down of film drainage. We show that these effects must be attributed to bridging attraction between a densely covered air-water interface and a very sparsely covered silica-water interface.In chapter 6 the interactions between two silica surfaces, immersed in solutions of the surfactant is studied by CP-AFM. The main findings are that (i) there are long ranged attractive forces between bare hydrophobic and hydrophilic surfaces in aqueous solution; and that (ii) a weak adhesion occurs between a hydrophobic and a hydrophilic surface in the presence of the surfactant in 0.1 M NaCl. Results (i) can be attributed to capillary bridging and (ii) to observation bridging of the surfactant between a densely covered hydrophobic surface and a sparsely covered hydrophilic surface, thus confirming the effect found in chapter 5.