Molecular Dynamics Simulation, Contact Angle determination, and X-Ray Photoelectron Spectroscopy explain the wetting properties of siloxane model surfaces

The organic coatings of soil particles determine soil wettability, a crucial property for soil functioning. However, the complexity of natural coatings hampers identification of the contribution of individual compounds to soil wetting properties, usually quantified in terms of contact angle (CA). Here, we investigated the impact of chain length and polarity of the head group on CA by using soda-lime glass slides, serving as a model for quartz or feldspar surfaces, coated by siloxane units, serving as a model for organic coating compounds. Siloxane surfaces were prepared by treatment with dichlorodimethyl-(DCDMS) and dimethyldiethoxysilane (DMDES; chain length both 1C; DMS unit), aminopropyltriethoxysilane (APTES; chain length 3C; APS unit), and octadecyltrichlorosilane (OTS; chain length 18C; ODS unit). The experimentally determined sessile drop CA (CA_exp) and the coating thickness (t_exp) derived from XPS spectra were compared to CA_MDS and t_MDS resulting from molecular dynamics simulation (MDS) for the models of the (001) tridymite surface coated by DMS/APS/ODS monolayers, assuming monodentate binding. MDS revealed that chain length, polarity of the terminal group, and coating density are the main factors determining surface wettability. Calculated CA_MDS for DMS and APS coatings were distinctly < 90◦, while CA_MDS for ODS was > 90◦. The main factor for small CA_MDS was the short alkyl chain for DMS and the polarity of the terminal amino group for APS. Accordingly, t_MDS was <1 nm for DMS and APS and >1 nm for ODS, as CA analysis depth is about 1 nm [1]. With exception of the DMDES-treated slide (CA_exp <90°, t_exp about 1 nm), CA_exp, in agreement with t_exp > 1 nm, was ≥ 90°. This hinted on multilayer formation during preparation and an arrangement of the APS units that exposed a high amount of the hydrophobic backbone that outweighed the impact of the polar amino groups. Similar CA_exp and CA_MDS in case of DMDES-treatment probably resulted from the low reactivity of DMDES that left uncoated areas, which allowed water molecules to access polar sites of the glass surface. Both, multilayer formation and incomplete coating as well as masking of polar terminal groups by inclined arrangement must be assumed to occur for natural soil particle coatings. To summarize, MDS could explain the differences between MDS and experimental results with respect to experimental and modelled CA and coating thickness. Combination of an ideal und defined system (MDS) and an idealized and defined system (exp) thus proves to be a promising approach to get a better understanding of the factors determining soil wetting properties.

[1] Daniel Tunega, Roland Šolc, Peter Grančič, Martin H. Gerzabek, Marc-Oliver Goebel, Georg Guggenberger, Joerg Bachmann, Susanne K. Woche. 2023. Wettability of siloxane surfaces assessed by molecular dynamics, contact angle determination, and X-ray photoelectron spectroscopy, Applied Surface Science, Volume 611, Part B, 155680. DOI: doi.org/10.1016/j.apsusc.2022.155680