Structure and Wettability of Methoxy-Terminated Self-Assembled Monolayers on Gold

Abstract

This manuscript describes the structure and wettability of self-assembled monolayers (SAMs) derived from the adsorption of a series of ω-methoxyalkanethiols (CH3O(CH2) n SH, where n = 9−14) onto the surface of gold. Using ellipsometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and contact angle measurements, the interfacial properties were examined as a function of chain length. Analysis by ellipsometry revealed a progressive increase in the thickness of the films as the chain length of the adsorbate was increased. Similarly, analysis by PM-IRRAS revealed a progressive increase in the conformational order of the methylene chains of the films. Moreover, the frequency and intensity of two characteristic C−H stretching bands of the methoxy groups were observed to vary systematically as a function of odd- versus even-numbered chain length, reflecting an alternating structural change in the films (i.e., an “odd−even” effect). Studies of contact angle wettability revealed that the methoxy-terminated SAMs were more wettable than SAMs derived from the corresponding methyl-terminated alkanethiols (CH3(CH2) n +1SH), consistent with a substantial polar influence of the terminal ether moiety. In addition, the contact angles were observed to increase with increasing chain length of the methoxy-terminated SAMs. Moreover, small but systematic variations in wettability as a function of odd- versus even-numbered chain length were also observed; their origin was attributed to the influence of surface dipoles.

Molecular Origin of the Odd–Even Effect of Macroscopic Properties of n-Alkanethiolate Self-Assembled Monolayers: Bulk or Interface?

“…The parity effect in the ν s (CH 3 ) and ν a (CH 3 ) peak intensities, also observed in previous studies, ,,, is significant because it signals the presence of orientational order at the interfaces of the SAMs prepared herein. Additionally, both the symmetric and antisymmetric CH 3 stretching modes exhibit an odd–even oscillation of their peak positions (Figures b and S12d), a characteristic hereto reported only for CH 3 O­(CH 2 ) n SAu SAMs . There is a 0.7 ± 0.2 cm –1 difference in the ν s (CH 3 ) peak positions of n even and n odd and a 1.1 ± 0.2 cm –1 difference for ν a (CH 3 ).…”

“…Additionally, both the symmetric and antisymmetric CH 3 stretching modes exhibit an odd−even oscillation of their peak positions (Figures 4b and S12d), a characteristic hereto reported only for CH 3 O(CH 2 ) n SAu SAMs. 73 There is a 0.7 ± 0.2 cm −1 difference in the ν s (CH 3 ) peak positions of n even and n odd and a 1.1 ± 0.2 cm −1 difference for ν a (CH 3 ). This odd− even effect could originate from (i) the different interactions/ chemical environments of the CH 3 groups in SAM odd and SAM even and associated intermolecular inductive effect (CH 3 ••• CH 3 in n odd and CH 3 •••CH 2 in n even ), 73,74 (ii) a difference in the conformational disorder near the chain termini, 10,29 or (iii) the different interaction potentials of molecules (e.g., water) physisorbed to SAMs of n odd versus n even 75 .…”

“…There is a 0.7 ± 0.2 cm –1 difference in the ν s (CH 3 ) peak positions of n even and n odd and a 1.1 ± 0.2 cm –1 difference for ν a (CH 3 ). This odd–even effect could originate from (i) the different interactions/chemical environments of the CH 3 groups in SAM odd and SAM even and associated intermolecular inductive effect (CH 3 ···CH 3 in n odd and CH 3 ···CH 2 in n even ), , (ii) a difference in the conformational disorder near the chain termini, , or (iii) the different interaction potentials of molecules (e.g., water) physisorbed to SAMs of n odd versus n even . We cannot at present say which of these three possibilities is the operative mechanism.…”

Electrochemistry of redox-active self-assembled monolayers

“…The motivation for this study was to address experimental work that showed that the length of a chain (odd or even number of carbon atoms) of long-chain carboxylic acid monolayers determined whether oriented growth is observed [324,331,445,446]. MD simulations were used to investigate the confirmation of head groups of both odd and even number of carbon atoms of the chains.…”

Functionalization and patterning of monolayers on silicon(111) and polydicyclopentadiene

“…From the literature of monolayers on gold we know several important characteristics about how molecules with these functional groups assemble into monolayers. [442][443][444] Ether bonds promote disorder in monolayers as they favor gauche over trans conformations by approximately 0.1 to 0.2 kcal mol -1 . 445 Whitesides et al…”