Water Structure Next to Ordered and Disordered Hydrophobic Silane Monolayers: A Vibrational Sum Frequency Spectroscopy Study
2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 4, 1780-1790 p.Article in journal (Refereed) Published
Vibrational sum frequency spectroscopy (VSFS) has been used to study the structure of water incontact to fused silica surfaces, hydrophobically modified with series of alkyl trichlorosilanemonolayers of varying degrees of order. The interfacial molecular structural information wascomplemented using total internal reflection (TIR) Raman spectroscopy. The silane moleculesconsisted of octadecyltrichlorosilane (OTS) and its shorter chain analogue with eight carbonatoms. The VSFS spectra show a direct correlation between monolayer order and the intensity ofthe free OH mode, characteristic of straddling water molecules vibrating in direct contact to thehydrophobic layer, with a concurrent reduction of the bands associated with hydrogen bondedwater molecules. The results imply that the structure of water in the most ordered monolayers isnot much affected beyond the first layer of water molecules, with bulk isotropic propertiesbecoming apparent already at sub-nanometer distances from the surface. Contact anglemeasurements with both water and hexadecane were also performed in an effort to relatemacroscopically measureable parameters to the molecular surface structure provided by VSFSand TIR Raman. Only the receding contact angles, and in particular those with hexadecane, werefound to correlate with the monolayer order. Finally, to obtain an additional insight into the waterstructure in direct contact to an ordered hydrophobic surface, isotopic dilution experiments withD2O were also performed. They indicate that the vibrational coupling mechanism of watermolecules at the hydrophobized solid/water interface is different from what has been observed atthe liquid/vapor interface.
Place, publisher, year, edition, pages
2013. Vol. 117, no 4, 1780-1790 p.
TIR Raman, silica interface, contact angles, surface molecular structure, octadecyltrichlorosilane
IdentifiersURN: urn:nbn:se:kth:diva-101080DOI: 10.1021/jp310732fISI: 000314492400030ScopusID: 2-s2.0-84874413657OAI: oai:DiVA.org:kth-101080DiVA: diva2:546304
FunderSwedish Research Council
Uppdaterad från Manuskript till Artikel (20130308) QC 201303082012-08-232012-08-232013-03-08Bibliographically approved