QUARTZ-CRYSTAL MICROBALANCE AND SURFACE-PLASMON RESONANCE STUDY OF SURFACTANT ADSORPTION ONTO GOLD AND CHROMIUM-OXIDE SURFACES

The adsorption of two nonionic surfactants on hydrophobic gold and hydrophilic chromium oxide surfaces from aqueous solution has been investigated using a quartz crystal microbalance (QCM) and surface plasmon resonance (SPR). Adsorption isotherms for a nonyl phenol ethoxylate with average ethylene oxide chain length of 9 (N9) on gold and for octaethylene glycol monododecyl ether (C(12)E(8)) on both gold and chromium oxide surfaces have been measured by QCM. Isotherms for N9 and C(12)E(8) on the gold surface can be described by Langmuir adsorption, while that of C(12)E(8) on the chromium oxide surface was S-type. Complementary SPR experiments of C(12)E(8) adsorption onto gold suggest that the resonance frequency change of the QCM crystal in solution with adsorbed C(12)E(8) (at saturation coverage) is 80% greater than that predicted by the Sauerbrey equation for air measurements. This implies a 2.08 Hz/ng change for the QCM in contact with aqueous solution. This calibration factor was used to calculate the saturated surface coverage for the surfactants (and hence area per surfactant) at the solid/liquid interface. These data are in good agreement with those reported in the Literature for N9 and C(12)E(8) adsorbed onto various hydrophobic and hydrophilic surfaces.