Adsorption of Poly(amido amine) (PAMAM) Dendrimers on Silica:  Importance of Electrostatic Three-Body Attraction

Abstract

Adsorption of poly(amido amine) (PAMAM) dendrimers to silicon oxide surfaces was studied as a function of pH, ionic strength, and dendrimer generation. By combining optical reflectometry and atomic force microscopy (AFM), the adsorbed layers can be fully characterized and an unequivocal determination of the adsorbed mass becomes possible. For early stages, the adsorption process is transport limited and of first order with respect to the dendrimer solution concentration. For later stages, the surface saturates and the adsorbed dendrimers form loose but correlated liquidlike surface structures. This correlation is evidenced by a peak in the pair correlation function determined by AFM. The maximum adsorbed amount increases with increasing ionic strength and pH. The increase with the ionic strength is explained by the random sequential adsorption (RSA) model and electrostatic repulsion between the dendrimers. The adsorbing dendrimers interact by the repulsive screened Coulomb potential, whose range decreases with increasing ionic strength and thus leads to increasing adsorbed densities. The pH increase is interpreted as an effect of the substrate and is quantitatively explained by the extended three-body RSA model. This model stipulates the importance of a three-body interaction acting between two adsorbing dendrimers and the charged substrate. The presence of the charged substrate weakens the repulsion between the adsorbing dendrimers and thus leads to higher surface densities. This effect can be interpreted as an additional attractive three-body interaction, which acts in addition to the usual two-body repulsion and originates from the additional screening of the Coulomb repulsion by the counterions accumulating in the diffuse layer.

The Effects of Surface Charging Properties on Colloid Transport in Porous Media

“…While the θmax of the sulfate latex is constant, that of carboxyl latex decreases with pH. Since the positive charge of zirconia increases at low pH, one may expect the increase of θ max at low pH due to the strong attraction between highly charged zirconia and oppositely charged sulfate latex as found for the deposition of dendritic polyelectrolyte with nanometer size 14) . Nevertheless, the expected pH-dependence is not observed for the sulfate latex.…”

“…In the adsorption of dendritic polyelectrolytes, however, the charge of substrate/collector affects the maximum surface coverage 13), 14) . To confirm the effect of collector charge on θmax in colloid transport in porous media, we need experiments as carried out in this study.…”

“…As shown in the figure, the calculated values are much higher than the experimental data. The RSA model with the lateral repulsive force successfully describes the θmax for the deposition of nanoparticles 12),13), 14) . The success of the RSA is because the transport of nanoparticles is governed by Brownian diffusion; the diffusion controlled process matches the assumption in the RSA model.…”

“…In the case of colloidal particles, the lateral double layer repulsion further reduces the maximum surface coverage due to the increase of the effective radius. Thus, the maximum surface coverage decreases with decreasing salt concentration due to the development of diffuse double layer 7),12),13), 14) . The RSA model with electric double layer potential successfully describes the maximum surface coverage of diffusional deposition of nanoparticles and dendritic polyelectrolytes on planar surfaces 12), 13) .…”

Metal ion remediation by polyamidoamine dendrimers: a comparison of metal ion, oxidation state, and titania immobilization

“…Adsorption of metal laden dendrimers is commonly examined for loading of nanoparticles onto metal oxide supports (e.g., silica (Castillo and Kuhn 2012;Huang et al 2008), alumina (Alexeev et al 2006;Deutsch et al 2007;Lopez-De Jesus et al 2008), titania (Crump et al 2008;Scott et al 2005), and zirconia (Deutsch et al 2007)) for the synthesis of heterogeneous catalysts. More in-depth studies also exist (Cahill et al 2008). To confirm the immobilization, FTIR spectroscopy was performed onto the dendrimer/titania composite (Fig.…”