Behaviour of wetting films of sodium hyaluronate saline solutions.

The behaviour of wetting films of sodium hyaluronate (NaHA) saline solutions, at physiological conditions, is investigated using interferometry. Concentrations in the range of dilute and semi-dilute regimes were chosen to assess the role of bulk molecular interactions on the surface forces. The transition from dilute to semi-dilute solutions is known to occur at c*=0.59 mg mL(-1). Dilute solutions form stable films whose behaviour is explained by van der Waals interactions. For semi-dilute solutions, the disjoining pressure vs film thickness isotherms show branches attributed to stepwise thinning or film stratification, which is interpreted in terms of a network as predicted by the theory of semi-dilute polyelectrolyte solutions. The distance between branches is 16 nm for the NaHA concentration of 2 mg mL(-1) and 12 nm for the concentration of 3.5 mg mL(-1), in agreement with the calculated correlation lengths of the polymeric network. Since no experimental evidence exists for the presence of a stable network in the bulk, this tendency for self-association should be the result of the liquid confinement. To our knowledge, the existence of oscillatory disjoining pressure isotherms in the presence of excess salt was never observed for other polyelectrolyte solutions.

Stability of triglyceride liquid films on hydrophilic and hydrophobic glasses.

Wetting and dewetting of solid surfaces by oily fluids were investigated in terms of the stability of the liquid film formed between an air bubble and the solid surface. With the objective of understanding how molecules with low polarity but relatively complex molecular structure behave at the solid/liquid interface, three liquid triglycerides with different chain length and saturation were chosen, namely, tributyrin, tricaprylin, and triolein. Tributyrin and tricaprylin exist in milkfat while triolein is present in vegetable oils. The stability of the liquid films may be inferred from the shape of the disjoining pressure isotherms, which represent the dependence of the disjoining pressure on the film thickness. Disjoining pressure isotherms for films of the three triglycerides on hydrophilic and hydrophobic glasses were obtained using a recently developed apparatus, based on the interferometric technique. The experimental curves are compared with the theoretical predictions of London-Hamaker. The deviations between theory and experiment are interpreted in terms of a structural component of the disjoining pressure. All triglycerides form metastable films on both hydrophilic and hydrophobic glasses which means that for disjoining pressures higher than a critical value, pi(c), a wetting transition occurs and the film ruptures. The mechanisms for film rupture are discussed and a correlation between film stability and the apolar (Lifshitz-van der Waals) and the polar components of the spreading coefficient is proposed.