Mapping single macromolecule chains using the colloid deposition method: PDADMAC on mica

Abstract

Monolayers of the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) on mica were thoroughly characterized using the streaming potential and the colloid deposition methods. Initially, the stability of the monolayers was determined by performing desorption experiments carried out under diffusion-controlled regime. It was shown that the desorption of the polyelectrolyte at the ionic strength range 0.01–0.15M is negligible over the time of 20h. The structure of PDADMAC monolayers and orientation of molecules were evaluated using the colloid deposition measurements involving negatively charged polystyrene latex microspheres, 820nm in diameter. The functional relationships between the polyelectrolyte coverage and latex coverage deposited within 20h were acquired by direct optical microscope. In this way the influence of ionic strength varied in the range 0.15–0.01M on the molecule orientation in monolayers was determined. It was shown that for ionic strength of 0.15M nearly one to one mapping of polyelectrolyte chains by colloid particles can be achieved for PDADMAC coverage below 0.1%. In this way, because of a considerable surface area ratio between the macromolecule and the colloid particle, an enhancement factor of 103 can be attained. This behavior was quantitatively interpreted in terms of the random site adsorption model whereas the classical mean-field theory proved inadequate. On the other hand, for lower ionic strength, it was confirmed that an irreversible immobilization of latex particles can only occur at a few closely spaced PDADMAC chains. It was shown that these experimental results were consistent with the side-on adsorption mechanisms of PDADMAC at mica for the above ionic strength.