Photophysical investigation of starburst dendrimers and their interactions with anionic and cationic surfactants

Abstract

Fluorescence spectroscopy has been used to characterize the structure of a unique class of anionic macromolecules: starburst dendrimers (SBDs) possessing an external anionic surface. Pyrene was used as a photoluminescence probe to sense various hydrophobic sites in the microheterogeneous architecture offered by poly(amidoamine) starburst dendrimers possessing sodium carboxylated surfaces. A series of 10 different "half-generations, n.5" starburst dendrimers, which differ systematically in molecular weight, size, and surface charge density, have been studied. The probe method provides experimental evidence for a structural surface transition between generations 3.5 and 4.5. The same probe method was applied to study how the structural properties of the starburst dendrimers determine their interactions with small ionic amphiphilic molecules. Whereas starburst dendrimers do not noticeably affect the micellization process of an anionic surfactant (sodium dodecyl sulfate, SDS), the association process with a cationic surfactant (dodecyltetrammonium bromide, DTAB) leads to the formation of two different types of SBD-templated surfactant aggregates. Addition of DTAB to aqueous solutions containing the earlier generation (0.5-3.5) SBD leads to the formation of SBD-templated surfactant aggregates that result from noncooperative, random condensation of surfactant molecules on the anionic dendrimer surface. Addition of DTAB to aqueous solutions containing later generation (4.5-9.5) SBDs leads to the formation of SBD-templated surfactant aggregates resulting from initial, noncooperative, random condensation, followed by cooperative condensation of surfactant molecules on the anionic dendrimer surface. The results are shown to be consistent with a change in the morphology of SBDs from an open, branched structure for generations 0.5-3.5 to a closed, increasingly compact surface for generations 4.5-9.5.the early generations were found to possess average structures that were very open, whereas the later generations were more closed,

Characterization of Starburst Dendrimers by EPR. 3. Aggregational Processes of a Positively Charged Nitroxide Surfactant

“…In the presence of SBD below the cmc, we postulate that the pertinent structures 8,17 are two supramolecular complexes 3, one representative of earlier generations (e.g., 1.5-SBD) and a second representative of later generations (e.g., 5.5-SBD).…”

“…These EPR results complement those reported earlier 8 for SBD and spin-labeled surfactants below the cmc and are consistent with studies of SBD and surfactants investigated by the fluorescence method. 17 The structures deduced involving the noncovalent binding characteristic of supramolecular systems suggest a rich array of structural possibilities for producing novel aggregates between polyelectrolytes and SBD employing the principles of supramolecular chemistry. We hope the results will stimulate further experimentation in the design of supramolecular structures involving dendrimers and noncovalently associating species such as polyelectrolytes and water soluble polymers.…”

“…These interactions and the complexes formed between polyelectrolytes and surfactant aggregates have been the subject of a range of investigations in recent years. [17][18][19][20][21][22] In particular, the aggregation of dodecyl trimethylammonium bromide (DTAB) in n.5-SBD solutions has been investigated using the fluorescence probe method. 17 The latter investigation describes the formation of both primary (noncooperative) and secondary (cooperative) binding of DTAB at the SBD/water interface as a function of DTAB concentration and SBD generation.…”

Model for the Interactions between Anionic Dendrimers and Cationic Surfactants by Means of the Spin Probe Method

“…For DTAB we referred to the same cmc (14 mM) as used by Caminati and co-workers in a study on the aggregational behavior of DTAB in the presence of n.5SBDs. 25 The starting solutions of the surfactants were prepared well above their cmc, that is, [CTAB] ) 8 mM and [DTAB] ) 80 mM. These concentrated solutions were warmed at 313 K to provide clear solutions.…”

The symmetry of functionalized poly(propylene imine) dendrimers probed with hyper-Rayleigh scattering

“…PII S0009-261 4(96)00840-8 molecules, other than ESR studies on modified PAMAMs [10,11]. Here, we report our results of probing appropriately modified poly(propylene imine) dendrimers [12] with hyper-Rayleigh scattering (HRS) [13-151.…”