Remarkable electrokinetic features of charge-stratified soft nanoparticles: mobility reversal in monovalent aqueous electrolyte.

Abstract

The electrokinetic behavior of G6.5 carboxylate-terminated poly(amidoamine) (PAMAM) starburst dendrimers (8 ± 1 nm diameter) is investigated over a broad range of pH values (3-9) and NaNO3 concentrations (c(∞)= 2-200 mM). The dependence of nanodendrimer electrophoretic mobility μ on pH and c(∞) is marked by an unconventional decrease of the point of zero mobility (PZM) from 5.4 to 5.5 to 3.8 upon increase in salt concentration, with PZM defined as the pH value at which a reversal of the mobility sign is reached. The existence of a common intersection point is further evidenced for series of mobility versus pH curves measured at different NaNO3 concentrations. Using soft particle electrokinetic theory, this remarkable behavior is shown to originate from the zwitterionic functionality of the PAMAM-COOH particles. The dependence of PZM on c(∞) results from the coupling between electroosmotic flow and dendrimeric interphase defined by a nonuniform distribution of amine and carboxylic functional groups. In turn, μ reflects the sign and distribution of particle charges located within an electrokinetically active region, the dimension of which is determined by the Debye length, varied here in the range 0.7-6.8 nm. In agreement with theory, the electrokinetics of smaller G4.5 PAMAM-COOH nanoparticles (5 ± 0.5 nm diameter) further confirms that the PZM is shifted to higher pH with decreasing dendrimer size. Depending on pH, a mobility extremum is obtained under conditions where the Debye length and the particle radius are comparable. This results from changes in particle structure compactness following salt- and pH-mediated modulations of intraparticle Coulombic interactions. The findings solidly evidence the possible occurrence of particle mobility reversal in monovalent salt solution suggested by recent molecular dynamic simulations and anticipated from earlier mean-field electrokinetic theory.