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 titra tion. All techniques reveal a discontinuity at a well-defined pH, correspon ding to the point of incipient complex formation. Conversion of this critic al pH to a surface charge density sigma(crit) leads to the observation that sigma(crit), to a first approximation, varies linearly with the Debye-Huck el parameter kappa and that binding occurs more easily (i.e., at lower sigm a(crit)) for the larger dendrimer. A model is presented in which the distor tion of the polyelectrolyte backbone in the complex is described in terms o f the elastic resistance to bending around the contour of the spherical mac roion, acting in opposition to attractive Coulombic forces. The elastic res istance is treated as a Hookean effect obtained from the measured persisten ce length. This simple theory produces the principal features of the experi mental system.