Structure of Aqueous Water Films on Textured −OH-Terminated
Self-Assembled Monolayers
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Abstract
We report the thickness and interfacial
molecular structure of
thin (1–3 nm) aqueous films supported on hydroxyl-terminated
self-assembled monolayers over a silver substrate. The water film
structure is studied as a function of varying the monolayer’s
methylene chain lengths. Analysis techniques include ellipsometry,
contact angle, and polarization modulation reflection adsorption infrared
spectroscopy. The aqueous film thicknesses follow 4-mercaptobutanol
(4-MBU) > 11-mercaptoundecanol (11-MUD) > 6-mercaptohexanol
(6-MHE)
> 9-mercaptononanol (9-MNO). Water contact angle measurements across
the same surfaces are very similar; however, vibrational spectroscopic
analysis of the films shows that intermolecular bonding patterns of
D<sub>2</sub>O are significantly different from those of bulk D<sub>2</sub>O. This evokes unique interfacial molecular architectures
for each of these films. The structural differences depend on the
nature of the SAM structure and resulting water–SAM interactions,
which are evident from PM-IRRAS data. Spectroscopic peak intensity
ratios of ν(O–D) modes suggest more asymmetric hydrogen-bonded
D<sub>2</sub>O character near 9-MNO surfaces, whereas 4-MDU, 6-MHE,
and 11-MUD surfaces exhibit increasingly symmetric hydrogen-bonded
D<sub>2</sub>O character. From this, we propose a model for film structure