Polysaccharides at interfaces 1. Adsorption of cholesteryl-pullulan derivatives at the solution-air interface. Kinetic study by surface tension measurements

The surface properties of a series of cholesteryl-pullulan (CHP) derivatives have been assessed by surface tension measurements at the solution-air interface. The results reveal that these properties are related to the nature of the hydrophobic cholesteryl group substituted in pullulan, and that the unsubstituted polysaccharide does not display any surface activity. The adsorption kinetics of such an amphiphilic macromolecule has been shown to be diffusion controlled, obeying the Ward and Tordai¨diffusional model only at low solution concentrations. In the 2 × 10⁻⁷–5 × 10⁻⁶ mol l⁻¹ concentration range for which this model is verified, the calculated diffusion coefficients are concentration dependent. The non-ideality of the system at higher concentrations may be explained both by the presence of solute/solute interactions in solution and in adsorbed monolayers, and by the existence of an adsorbed layer, even at time t₀, which prevents the process of adsorption from being governed only by diffusion.     

Polysaccharides at interfaces 2. Surface potential of adsorbed cholesteryl-pullulan monolayers at the solution-air interface

The surface potential of adsorbed monolayers of cholesteryl-pullulan (CHP) derivatives has been determined by the ionizing differential electrode method. It has been found that this potential is highly dependent on the degree of cholesterol grafted onto pullulan, and that the native polysaccharide displays neither surface activity nor surface potential. As the disordered structure of the non-ionic polysaccharide unit generates a random orientation of intrinsic dipole moments, it has been considered that its contribution to the measured surface potential is rather small, compared to the cholesteryl group dipolar contribution. The surface densities of cholesteryl groups of adsorbed CHP molecules have been determined from the relationship between the surface potential and the surface density of spread cholesterol molecules. The assessment of these quantities was essential, as the determination of the surface tension data for the CHP derivatives with low cholesteryl content (CHP_45−0.6 and CHP_50−0.9) was difficult to achieve (Part I of this work [B. Deme´, V. Rosilio and A. Baszkin, Colloids Surfaces B: Biointerfaces, 4 (1995) 357]). These results complement those from the surface tension measurements, and confirm that in the surface layer of the adsorbed polysaccharide the ordered cholesteryl groups are oriented towards the air phase and the disordered polysaccharide is immersed in the aqueous subphase. Proposed models for semi-organized adsorbed CHP layers are discussed.