No abstract

The adsorption behavior of positively charged poly(propylene imine) dendrimers on glass has been studied by scanning angle reflectometry as a function of generation, pH, and ionic strength. The results indicate that the adsorption is controlled by properties of both the dendrimers (size and charge) and the surface layer (distribution of glass surface sites). At constant pH and ionic strength the collector properties remain the same, and the adsorption scales with dendrimer size. Because of the limited number of glass surface sites there is only 15% surface coverage at maximum. Adsorption increases with decreasing pH and increasing ionic strength, and the effect of pH is much more pronounced for the smaller dendrimers. To explain the adsorption results, not only the size and charge of the dendrimer have to be taken into account, but also the charge density and the roughness of the surface. This implies that the dendrimer size is comparable to the characteristic length scale of the surface roughness.

Comblike poly(ethyleneimine) (PEI) with -(CH2CH2)N(CH2CH2NH2)-as the repeating unit has been synthesized by employing a protecting group strategy, and analyzed by potentiometric titration. If the degree of protonation is plotted as a function of pH, the titration curve shows three protonation steps separated by two intermediate plateaus, namely one at a degree of protonation of 1 /2 and a second one at 3 /4. The first protonation step occurring at pH around 9.5 corresponds to the protonation of the primary amine groups on the side chains. During the second protonation step at pH around 4.5 every second tertiary amine protonates. The final protonation step, where the remaining tertiary amines protonate, is suspected to occur in a pH range near 0. The latter point and the detailed protonation mechanism are deduced on the basis of a site-binding model. The model, which has been independently calibrated on titration data of low molecular weight amines, is capable to predict the titration curve quantitatively to a good degree of accuracy.

Five generations of carboxylate-functionalized poly(propyleneimine) dendrimers have been synthesized starting from a double Michael addition of amine-functionalized poly(propyleneimine) dendrimers to methyl acrylate followed by basic hydrolysis using LiOH in a water/methanol mixture. The dendritic compounds have been characterized using 1H NMR, 13C NMR, IR, and ESI−MS spectroscopy. Subsequently, the protonation behavior of these potential complexing agents is studied with potentiometric titrations in 0.1 and 1.0 M KCl solutions. The carboxylate-functionalized dendrimers show a characteristic onionlike shell protonation behavior. The titration curves for different ionic strengths cross at pH 8.5 which is exactly after protonation of the outermost shell of amines (θ ∼ 0.25). This crossing point is found not to be identical to the point of zero charge θ ∼ 0.5, which is observed at pH 4.5. The Ising model was used to rationalize the complete set of titration data simultaneously for five generations with only one microscopic pK value for each additional shell and a set of three nearest neighbor pair interaction parameters.

The adsorption of Rayleigh particles is analyzed in terms of particle radius and surface coverage with thin island film theory with both scanning angle light reflectometry and ellipsometry around the Brewster angle. A comparison between both techniques shows that an additional uniformity parameter can be extracted out of the experimental reflectivity data. This gives information about the distribution of the adsorbed mass normal to the surface. Fixed angle reflectometry is less sensitive to surface properties than fixed angle ellipsometry. This is closely related to the fact that ellipsometry measurements provide an extra measurable physical quantity, the change in ellipticity at the surface, which has a weaker but different dependence on surface coverage and layer thickness. This enables ellipsometry to distinguish between a broad range of combinations of surface coverage and particle radii that give similar reflectivity. Fixed angle reflectometry can therefore only lead to an interpretation in terms of adsorbed mass. Scanning angle reflectometry measurements, on the contrary, can easily be interpreted in terms of surface concentration and thickness and make further ellipsometry measurements unnecessary.