Dual Characterization of Hydrophobically Modified Polyamidoamine Dendrimers and their Surfactant Aggregate Hosts by Pyrene Excimer Fluorescence

Abstract

This thesis explores why the conformational response of generation-0 polyamidoamine dendrimers end-labeled with four identical 1-pyrenealkanoyl groups (PyCX-PAMAM-G0 with X = 4, 6, 8, 10, and 12 for a butyroyl, hexanoyl, octanoyl, decanoyl, and dodecanoyl linker, respectively) to their local environment makes them excellent molecular probes to investigate surfactant aggregates. The conformation of the dendrimers was studied in polar organic solvents, spherical micelles, and non-spherical surfactant aggregates (NSSA) using pyrene excimer formation (PEF) and the model-free analysis (MFA) of the fluorescence decays. In N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) and in micelles of sodium dodecyl sulfate (SDS) or dodecyltrimethylammonium bromide (DTAB), the dendrimers with shorter (X = 4, 6, 8) alkanoyl linkers adopted an ideal conformation. In contrast, the PyC10- and PyC12-PAMAM-G0 dendrimers experienced a conformational inversion in pure surfactant micelles driven by the hydrophobicity gradient (HG) generated between the polar surface and the hydrophobic interior of the micelles. The conformational inversion of the PyC10- and PyC12-PAMAM-G0 dendrimers was further investigated with mixed micelles prepared from SDS and DTAB mixtures. The decrease in conformational inversion as the micellar shape evolved from a sphere to an elongated ellipsoid with increasing DTAB content led to the idea of the spatial partitioning theory (SPT). The SPT attributes changes in the average conformation of the dendrimers to the change in the volume fractions of the two regions found inside the mixed micelles, between which the dendrimers partition themselves. These two regions were the polar edge region, which was made of ~ 70 charged SDS molecules, and had a curved surface and a high HG, and a more hydrophobic middle region with a lower surface curvature and a low HG formed by the remaining neutralized surfactants. The SPT provided a robust fundamental framework to predict how the average rate constant (<k>) for PEF, obtained from the MFA of the fluorescence decays acquired with the PyCX-PAMAM-G0 samples, was affected by the composition of the NSSA the dendrimers interacted with. The sensitivity of the conformational inversion of the PyC10- and PyC12-PAMAM-G0 dendrimers to their local environment shows the potential of these dendrimers as molecular probes for NSSA formed upon the addition of NaCl or DTAB to aqueous solutions of SDS micelles. Partitioning of the dendrimers with longer C10 and C12 linkers between the edge and middle regions rationalized the changes in <k> observed as a function of salt concentration, DTAB content, or both. The generality of the SPT, which applied to all surfactant systems investigated in this thesis, provided strong support for the two regions coexisting in the NSSA, with the edge region being constituted of the same number of charged surfactants as that found in a pure SDS micelle. This insight led to a proposal for the mechanism leading to the formation of NSSA, when salt or oppositely charged surfactants are added to an SDS aqueous solution. Together, the results presented in this thesis suggest that the PyCX-PAMAM-G0 dendrimers constitute outstanding molecular probes to study NSSA in solution.