The role of salt in governing the adsorption mechanisms of micelle-forming polyelectrolyte/neutral diblock copolymers

The adsorption kinetics of amphiphilic poly(tert-butyl styrene)-block- poly(sodium styrenesulfonate) copolymers have been monitored as a function of a monovalent 1:1 salt (sodium chloride) to hydrophobic octadecylsilane (OTS) surfaces and compared to the adsorption kinetics of poly(sodium styrenesulfonate) homopolymer. The initial adsorption rate of the homopolymer shows a weak power-law dependence on the external salt concentration, φ _s ^1/10, at salt concentrations above 0.01 M, in agreement with diffusion-limited kinetics. Moreover, the adsorbed amount reaches a definitive plateau in less than 24 h at all salt concentrations. The initial adsorption rate of the copolymer, on the other hand, adsorbs 1-2 orders of magnitude more slowly than the homopolymer and does not follow diffusion-limited kinetics, which can be ascribed to the presence of micelles. Micelles adsorb to the surface, but the micelles must overcome a potential energy barrier in order to adsorb. That being said, the copolymer micelles eventually surpass the adsorbed amount of the homopolymer and show no signs of reaching saturation, even after 1 week of adsorption. The copolymer shows two types of adsorption kinetics at long times. If the external salt concentration φ _s is sufficiently higher than the counterion concentration associated with the adsorbed layer, the copolymers adsorb with φ _s ^2/5{log(t)} ^6/5 kinetics. In the other limit, where the internal ion concentration in the brush layer is higher than the external salt concentration, the adsorption kinetics show weak power-law behavior. Simple scaling relationships have been established to understand the qualitative differences between adsorption in the two regimes.