Wettability of siloxane surfaces assessed by molecular dynamics, contact angle determination, and X-ray photoelectron spectroscopy

Abstract

Complexity of organic coatings on soil mineral particles hampers identification of the contribution of individual compounds to soil wettability. This study focused on the impact of different organic coatings on surface wettability by using dimethyl(DMS)-, aminopropyl(APS)-, and octadecyl(ODS)-siloxane coated glass slides prepared by reaction with organosilanes. Surfaces were examined for experimental contact angle (CAexp) and coating thickness (texp) determined by X-ray photoelectron spectroscopy. Molecular dynamics simulations (MDS) were performed on models with a water nanodroplet in contact with the (001) surface of tridymite coated by DMS, APS, and ODS monolayers. Chain length, polarity of terminal group, and coating density were found as the main factors determining surface wettability. Calculated CAMDS for DMS and APS coatings were distinctly < 90°, while CAMDS for ODS was > 90°. The main factor for small CAMDS was the polarity of the terminal amino group for APS, and the short alkyl chain for DMS. Measured CAexp was ≥ 90° with texp ≥ 1 nm for all coatings, except for the sample prepared with dimethyldiethoxysilane. This hinted a more complex surface coating with a multilayer arrangement. The results showed that combined MDS and experimental results from well-defined model systems will contribute to a better understanding of factors determining soil wettability.