Effects of divalent salt on adsorption kinetics of a hydrophobically modified polyelectrolyte at the neutral surface-aqueous solution interface

Adsorption of a hydrophobically modified polyelectrolyte on hydrophobized silica surfaces in aqueous divalent salt solutions was studied using an ellipsometric technique. The results indicate three distinct stages in adsorption: a relatively short induction period, a surface accumulation region, and a plateau (quasi equilibrium) region. The induction period remains unchanged with increasing divalent salt concentration. The surface accumulation rate decreases while the equilibrium (quasi) adsorbed amount increases with increasing divalent ionic strength. Divalent salt (CaCl₂) produces much higher adsorbed amounts compared to a monovalent salt (NaCl). The analysis of kinetic data (surface accumulation rate) with a transport-limited regime model suggests the reduction of diffusion coefficient (i.e. the increase of size of the adsorbent) with increasing divalent salt concentration. At low concentrations of divalent salt (namely 0.01 M CaCl₂), it is possible that the substantially screened individual chains are predominantly adsorbed on the surface. On the contrary, at moderate and high concentrations of divalent salt (namely 0.075 M onwards), polyelectrolyte aggregates or micelles are predominantly adsorbed on the surface.