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"Ordered" structure in ionic dilute solutions: dendrimers with univalent and bivalent counterions
A Ohshima1, T Konishi, J Yamanaka
1Central Laboratory, Rengo Co., Ltd., 186-1-4, Ohhiraki, Fukushima-ku, Osaka 553-0007, Japan.
Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics
|December 12, 2001
Summary
Small-angle x-ray scattering reveals that poly(amido amine) dendrimers form two-state structures due to attractive forces, even with univalent counterions. This challenges assumptions about dendrimer interactions and counterion effects.
Area of Science:
- Supramolecular Chemistry
- Materials Science
- Physical Chemistry
Background:
- Poly(amido amine) dendrimers serve as intermediate ionic solutes between macroions and simple electrolyte ions.
- Understanding dendrimer solution behavior is crucial for their application in various fields.
Purpose of the Study:
- To investigate the structural properties and interparticle interactions of poly(amido amine) dendrimers in aqueous solutions using small-angle x-ray scattering (SAXS).
- To elucidate the role of counterions (univalent and bivalent) in mediating attractive forces between dendrimer molecules.
Main Methods:
- Small-angle x-ray scattering (SAXS) and ultra-small-angle x-ray scattering (USAXS) measurements were performed on aqueous solutions of G4, G7, and G10 dendrimers.
- Conductivity measurements were utilized to determine the effective charge density and charge number of the dendrimers.
- Analysis of scattering curves and structure factors to determine interparticle distances and structural inhomogeneities.
Main Results:
- SAXS curves of univalent acid solutions exhibited a single scattering peak, indicating macroion-like behavior, dependent on dendrimer concentration but not acid concentration or counterion species.
- Effective charge density was insensitive to acid concentration and counterion species, while bivalent counterions showed stronger association, reducing the dendrimer's effective charge number.
- Calculated nearest neighbor interparticle distances were smaller than average distances, suggesting counterion-mediated attractive forces leading to two-state structures in dendrimer solutions.
Conclusions:
- Counterion-mediated attraction exists in dendrimer solutions even with univalent counterions, challenging the notion that attraction only appears with bivalent counterions.
- The intensity of attraction is influenced by both effective charge density and effective charge number, unlike in macroionic solutions where only charge density is considered important.
- Dendrimer solutions exhibit structural inhomogeneities, likely due to weak attractions, indicating complex self-assembly behavior.