Interaction of a polycation with small oppositely charged dendrimers

Complex formation between small carboxyl-terminated dendritic polymers and oppositely charged poly(dimethyldiallylammonium chloride) was monitored by turbidimetry, dynamic light scattering, viscometry and potentiometric titration. All techniques reveal a discontinuity at a well-defined pH, corresponding to the point of incipient complex formation. Conversion of this critical pH to a surface charge density sigma(crit) leads to the observation that sigma(crit), to a first approximation, varies linearly with the Debye-Huckel parameter kappa and that binding occurs more easily (i.e., at lower sigma(crit)) for the larger dendrimer. A model is presented in which the distortion of the polyelectrolyte backbone in the complex is described in terms of the elastic resistance to bending around the contour of the spherical macroion, acting in opposition to attractive Coulombic forces. The elastic resistance is treated as a Hookean effect obtained from the measured persistence length. This simple theory produces the principal features of the experimental system.