GRAVIMETRIC MONITORING OF NONIONIC SURFACTANT ADSORPTION FROM NONAQUEOUS MEDIA ONTO QUARTZ-CRYSTAL MICROBALANCE ELECTRODES AND COLLOIDAL SILICA

The adsorption from benzene of a series of monodisperse poly(ethylene glycol) monododecyl ethers (C(12)E(n), n = 3, 5, and 8) and a polydisp erse nonyl phenol ethoxylate with an average ethylene oxide chain leng th of 5 (N-5) Onto gold and silicon dioxide surfaces has been investig ated in situ using quartz crystal microgravimetry (QCM). The frequency shifts caused by surfactant adsorption onto the QCM electrodes, as a function of the equilibrium surfactant concentration, cannot be descri bed by Langmuir adsorption. The introduction of a concentration-depend ent term to the frequency shift revealed a frequency dependence on sur factant concentration. This suggests contributions from the bulk solut ion properties. Hence, the linear relationship between the mass of sur factant adsorbed and frequency shift, implied the Sauerbrey equation f requently used in QCM experiments, could not be assumed. Complementary adsorption experiments onto colloidal silica enabled evaluation of th e bulk solution contributions to the measured QCM frequency shifts. Co rrected frequency shifts for C(12)E(3), C(12)E(5), and C(12)E(8) adsor bed onto SiO2 yielded a 1.15 Hz/ng calibration factor for the QCM, sug gesting that the resonance frequency shifts in benzene are as predicte d by the Sauerbrey equation for in-air measurements. For N-5 adsorbed onto SiO2, however, a calibration factor of 5.5 Hz/ng was obtained. It seems this increased sensitivity is due to interactions between the p henyl ring in N-5 and the benzene solvent. These calibration factors w ere used to calculate the surface coverage for each surfactant(and hen ce area per surfactant molecule) at the solid-benzene interface.