FT-IR/ATR and SEM study of colloidal particle deposition

Abstract

Fourier transform infrared attenuated total reflectance (FT-IR/ATR) spectroscopy and scanning electron microscopy (SEM) have been used to study the deposition of 406 nm diameter amidine functionalized latex particles at silica surfaces at pH 6.6 and 22.0. SEM micrographs were used to determine the area fraction of the surface covered by latex particles as a function of solution concentration and time. At pH 10.6 no deposition was observed. At pH 6.6 the maximum coverage reached at all concentrations was approximately 48% and the time necessary to reach the maximum coverage increased with decreasing concentration of latex particles. The deposited latex particles were not removed from the surface by removing the latex particles from solution and increasing the pH from 6.6 to 10.6. These results are in agreement with the expected deposition mechanism. For the FT-IR/ATR results fused silica ATR elements were used. These crystals exhibit a window in the mid-IR between 3100 and 2800 wavenumbers. The latex particles have several bonds which absorb IR light in this frequency range. The absorbance of the asymmetric –CH 2 stretching band of the polystyrene latex was chosen to follow the deposition kinetics. The kinetics of deposition followed in this manner were similar to the deposition studied by SEM. In particular, the final surface coverage was equivalent in both cases. However, the time taken to reach the maximum was slightly greater for the FT-IR/ATR experiments. Finally, the maximum coverage was used to determine the interparticle spacing and hence the interaction distance of the overlapping double layers between particles. The maximum coverage expected for noninteracting particles is 54.7% if the random sequential adsorption (RSA) model is used or 61.1% if ballistic deposition is active. The average distance between particles was found to be 2.6 times the Debye length assuming RSA and 5.2 times the Debye length assuming ballistic deposition. This seems to indicate that ballistic deposition is occurring in the system studied.