RESUMO
The adsorption of bovine serum albumin (BSA) onto relatively hydrophobic TiO(2) surfaces was studied by ellipsometry as a function of pH and BSA concentration. Titanium oxide layers were electrochemically grown on Ti disc electrodes. When fast attachment of BSA onto TiO(2) takes place, the adsorption can be considered as occurring in two different steps. The first step is fast and is the result of the direct adsorption of the protein molecules that attach to the surface without changing their conformation. The second process is slow and lasts for several hours. In this process, the adsorbed amount remains constant, whereas the thickness of the layer increases and its refractive index decreases with time. The changes in this second step are due mainly to rearrangements in the adsorbed layer produced by variations in the conformation and structure of the adsorbed molecules. The main conformational changes take place in the direction normal to the surface because lateral molecule-molecule interactions impede significant lateral expansion. Adsorption from BSA solutions of low concentration does not appear to lead to significant reconformation of the protein layer. Comparison with adsorption on powdered TiO(2) indicates that the adsorbed amount and the effective area occupied by an adsorbed BSA molecule can remain about constant even when strong surface reconformation takes place. Copyright 1999 Academic Press.
RESUMO
The proton adsorption behavior of eight humic acids (HA) and a fulvic acid (FA) was studied as a function of pH and KNO(3) concentration. The emphasis is on the comparison of the different humics with respect to their ion binding properties and on the comparison of two different models to describe the electrostatic interactions: the Donnan model and the impermeable sphere (IS) model. Viscosimetric data were used to estimate the hydrodynamic volumes and radii of the HA molecules. These data were incorporated in the electrostatic models and calculations could be carried out without any adjustable parameter. The Donnan model in combination with hydrodynamic volumes obtained by viscometry cannot adequately describe the electrostatic effects related to changes of the electrolyte concentration. This model leads to good prediction of the HA behavior if unrealistically large volumes are used for fulvics and unrealistically large volume-salt concentration dependencies are used for humics. The IS model can successfully reproduce experimental proton adsorption data with physically realistic radii. The good performance of the IS model and the poor performance of the Donnan model is directly related to the fact that the hydrodynamic volumes of the molecules are too small to allow for charge compensation within the molecular limits. The combination of viscometry with the IS model leads to a consistent description of the electrostatic in humics and to a consistent way of positioning the master curves. Therefore, the electrostatic potentials and the intrinsic affinity distributions of the different samples can be compared on an equal basis. The similarities in the intrinsic affinity distributions give faith for the possibility to develop a generic model to describe the ion binding to humics. Copyright 1999 Academic Press.
