Formation of PDADMAC monolayers evaluated in situ by QCM and streaming potential measurements

• PDADMAC adsorption was quantitatively evaluated.
• The stability of PDADMAC monolayers was determined.
• An efficient method of determining binding energies was developed.

Kinetics of adsorption and monolayer stability of the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) were determined. Initially, the bulk characteristics of the polyelectrolyte were acquired using the DLS and microelectrophoretic measurements. These comprised the diffusion coefficient and electrophoretic mobility determined as a function of ionic strength at pH 5.8. From these measurements, the hydrodynamic diameter, zeta potential and the amounts of electrokinetic charge per molecule were calculated. Subsequently, the kinetics of PDADMAC adsorption was evaluated under in situ conditions using the quartz crystal microbalance with dissipation (QCM-D) and streaming potential measurements. The latter allowed one to derive the calibration dependencies of the zeta potential on the polyelectrolyte coverage for various ionic strength successfully interpreted in terms of the 3-dimensional (3D) electrokinetic model. Using these data, the PDADMAC desorption kinetics were quantitatively analyzed. In this way, the desorption constants, the equilibrium adsorption constants, and the binding energies of PDADMAC were determined. The energy varied between −20.5 and −19.7 kT, for ionic strength of 10−3 and 0.15 M, respectively. This agree with the proposed model of discrete electrostatic interactions among ion pairs present at the polyelectrolyte chain and the substrate surface. The mean-field electrostatic interactions approach proved inadequate.

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