EPR Investigation of the Adsorption of Dendrimers on Porous Surfaces

Guest−host interactions of poly(amidoamine) dendrimers and porous surfaces, such as homoporous silica of different pore sizes, activated alumina (acidic, neutral, basic), and MCM-41 zeolite (Si/Al=30), were investigated by electron paramagnetic resonance (EPR) spectroscopy. Two generations of dendrimers were selected, G2 and G6, representing “early” and “late” generations, which were labeled with a nitroxide radical. It was found that the main parameters regulating the adsorption and interaction of the dendrimers and porous surfaces are (I) the size of the pores of the porous materials (host) relative to the size of the dendrimer (guest), (II) the total surface area of the host that is accessible to the guest dendrimers, and (III) the chemical composition of the host and guest surfaces that come into contact upon adsorption. If the pore size of the host surface is smaller than the guest dendrimer diameter, then adsorption of the dendrimer occurs exclusively on the external host surface. If the pore size of the host surface is bigger than the dendrimer diameter, then the internal surface area is accessible to the dendrimers and large amounts of dendrimers are adsorbed. The chemical composition of the host surface and guest surface determines the strength of the host−guest interaction. An increase in alumina content (silica < zeolite MCM-41 < alumina) of the host increased the electrostatic interaction of the dendrimer with the host surface. Increased protonation of the external dendrimer amino groups also increased the electrostatic interaction. For the large dendrimers (G6), which possess a closed surface structure, electrostatic interactions are the main forces of host−guest interaction. Therefore, the strongest interaction of G6 was observed with basic alumina. For small dendrimers (G2), which possess an open structure, hydrophobic interactions between the low polarity internal dendrimer structure and the low polarity Si−O−Si groups of the host surface provide an additional host−guest interaction. High protonation of the dendrimer surface and high alumina content of the host surface inhibits this hydrophobic type of interactions. The combination of low polar Si−O−Si groups and highly polar AlO - groups in the zeolite MCM-41 probably cause the strongest interaction of the guest G2 dendrimer with MCM-41 compared to the other investigated host surfaces.