Molecular recognition and adsorption equilibria in starburst dendrimers: gas structure and sensing via molecular theory

Abstract

An idealized model for the dendrimer polyamidoamine is examined as a gas/chemical sensor. The system considered is a solution of this dendrimer in a binary mixture of two solvents: one being the analyte molecules and the other the placebo molecules. The analyte species possesses special affinity to the corona (the surface) of the dendrimer, or to the exo-receptors; while the other fluid being neutral. Both Monte Carlo simulation and integral equation studies have been carried out to determine the excess adsorption of analyte population on the surfaces of dendrimers.In the simulation studies, we explicitly account for the presence of the solvent molecules (solvent-explicit). As a consequence, we find that at low gas permeation, the dendrimers exhibit dense core behavior. However, at high gas contents, the dendrimers transit to the dense shell configuration. This behavior is clearly shown in the values of Rg (radius of gyration) at difference gas densities. By functionalizing the end groups, we observe pronounced analyte aggregation around the corona. Although there is no unusual behavior in these observations, we put the interrelations on a quantitative basis by showing the amounts or variations of the “molecular recognition” as function of the temperature, affinity strength, gas density and the composition.To decipher the behavior on a theoretical basis, we apply a self-consistent closure to the Ornstein–Zernike equations for calculating the structures of the dendrimer–gas A–gas B mixture. We are able to reproduce accurately the structural information as well as the thermodynamic properties for such mixtures, notwithstanding the large size disparity between the dendrimer and fluid molecules (up to 10:1 ratio).