Assess the Intramolecular Cavity of a PAMAM Dendrimer in Aqueous Solution by Small-Angle Neutron Scattering

We present a contrast variation small-angle neutron scattering (SANS) study of a series of neutral PAMAM dendrimers in aqueous solutions using three different generations (G4-6) at a concentration of about 10 mg/mL. Varying the solvent hydrogen-deuterium ratio, the scattering contributions from the water molecules and the constituent components of PAMAM dendrimer can be determined. Using an analytical model of the scattering cross section I(Q) incorporating the effect of water penetration, we have quantified the intramolecular space of PAMAM dendrimer by evaluating the number of guest water molecules, and we draw a direct comparison to computational predictions. As expected, the overall available internal cavity was seen to increase as a function of increasing dendrimer generation. However, the fraction of water accessible volume of a dendrimer was found to remain invariant for the three generation PAMAM dendrimers studied in this report. We have also estimated the average water density inside a dendrimer, which is found to be higher than that of bulk water.