Structure of Aqueous Water Films on Textured −OH-Terminated Self-Assembled Monolayers

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 D2O are significantly different from those of bulk D2O. 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 D2O character near 9-MNO surfaces, whereas 4-MDU, 6-MHE, and 11-MUD surfaces exhibit increasingly symmetric hydrogen-bonded D2O character. From this, we propose a model for film structure.

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“…Static contact angle measurements of the resulting SAMs, hereafter identified by the terminal function they present (CH 3 , COOH, NH 2 and OH SAMs) were made using a contact angle goniometer (Biolin Scientific, Manchester, UK). Values of 101 ± 1°, 50 ± 1°, 35 ± 1° and 11 ± 1° (n = 3, mean ± SD) found for the CH 3 , NH 2 , OH and COOH, terminated SAMs respectively are in agreement with previous reports 40 – 43 .…”

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“…53 A near-infrared uorescent dye, 3,3 0 -diethylthiatricarbocyanine iodide (DBT), easily enters the brain and selectively binds to myelin bers. 54 As shown in Fig. 6E, administration of topotecan effectively reverses uorescence in EAE mice.…”

“…Myelination is imaged using 3,3-diethylthiatricarbocyanine iodide (DBT), a near-infrared dye that readily enters the brain and specically binds to myelinated bers. 54 Briey, mouse is intravenously injected with DBT (0.5 mg kg À1 , 100 mL PBS) through the tail vein and image acquisition is performed at 4 min post-injection. 53 The settings are Epi-FI, Ex740/Em790, binning 8, FStop 2, FOV D, height 1.50.…”

Spectroscopic analysis of molecular fluids at the solid-liquid interface

“…These parameters for the chlorobenzene fluids are shown in We have previously shown PM-IRRAS to be sensitive to single monolayers of fluids. 51 However, spectroscopic selection rules dictate that PM-IRRAS is only sensitive to molecular vibrations with a component of the changing dipole that is perpendicular to the interface. Hence, these results could illustrate one of two scenarios: 1) a fluid film is not being formed, or 2) the fluid molecules' molecular vibrations are oscillating in a plane that is perfectly parallel to the Ag substrate's interface.…”

“…The film is spectroscopically probed at the apex of the surface's rotation. 61,63 Lab temperatures are monitored and range between 20 -21°C.…”