Sorption of hydrophobic, negatively charged microspheres onto a stagnant air/water interface

Experiments to study the sorption of surfactant-free, hydrophobic, negatively charged, polystyrene latex microspheres onto a stagnant air/water interface, in the absence of convection, as a function of ionic strength of NaCl electrolyte solution (from 0 to 1.0 M) were performed. Sorption of colloidal particles onto air/water interface was irreversible, due to a net negative energy change associated with transferring particles from the bulk solution to the interface. The sorption parameters, the asymptotic surface coverage, theta(mx) and the characteristic time of sorption, t(ch), were strong functions of ionic strength. The values of theta(mx) ranged from a minimum of 1.2% monolayer at 1.0 M NaCl to a maximum of 8% monolayer at 0.05 M NaCl. Sorption of colloidal particles onto the air/water interface followed an exponential relationship with time and was fully described by theta(mx) and t(ch). Below 0.05 M NaCl, theta(mx) increased, but t(ch) decreased with increasing ionic strength, reaching a maximum and a minimum, respectively, at 0.05 M NaCl. Beyond 0.05 M NaCl, theta(mx) decreased, but t(ch) increased exponentially with increasing ionic strength. The measured change of theta(mx) and t(ch) with ionic strength was attributed to the accessibility of the interface to sorbing particles determined by the combined effect of the energy barrier at the interface and that between particles in the bulk solution. (C) 1998 Academic Press.