Compositionally mixed, self-assembled monolayers (SAMs) derived from 16,16,16-trifluorohexadecanethiol and a normal alkanethiol, either hexadecanethiol or pentadecanethiol, were formed on Au(111) substrates. The relative composition of the films was determined using X-ray photoelectron spectroscopy and was found to approximately equal the equimolar composition of the isooctane solution from which they were formed. The frictional properties of the mixed films were measured on the nanometer scale using atomic force microscopy and were observed to decrease when the chain length of the CH(3)-terminated component was shortened by one methylene unit (i.e., when hexadecanethiol was replaced by pentadecanethiol). For comparison, the frictional properties of a mixed-chain-length CH(3)-terminated SAM derived from hexadecanethiol and pentadecanethiol in a 1:1 ratio was also examined. In contrast to the mixed CF(3)/CH(3) system, the latter mixed-chain-length system exhibited relatively higher friction when compared to single-component SAMs derived solely from either hexadecanethiol or pentadecanethiol. For both types of mixed films, the change in frictional properties that occurs as a result of modifying the position of neighboring terminal groups with respect to the surface plane is discussed in terms of the influence of local packing environments on interfacial energy dissipation (friction).
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.
Self-assembled monolayers (SAMs) have received considerable recent attention as molecular-level lubricants in, for example, micro-electro-mechanical systems (MEMS).[1] Of particular interest as tribological films have been SAMs terminated by fluorocarbon groups, because of their inert nature and enhanced thermal stability. Surprisingly however, fluorocarbon films were shown to actually produce higher coefficients of friction (relative to CH3-terminated films) in atomic force microscopy (AFM) studies. Subsequent work has concluded that the increased van der Waals radius of the fluorine groups (∼45%) causes a steric disruption of the order of the molecular surface giving rise to an increased friction.[2] We present results from a direct comparison of the adhesive, mechanical and frictional properties of SAMs terminated by CF3 and CH3 groups using both Interfacial Force Microscopy[3] (IFM) and the AFM. IFM results are shown for a two micron tungsten tip interacting with C16 alkylthiol molecules assembled on Au(111) single-crystal surfaces. AFM results involve a ∼20 nm tip interacting with the same two molecules assembled on Au films deposited on mica surfaces. A direct AFM comparison is accomplished by using a “nanografting technique”.[4]
Glycosylation of monosaccharides 4, 8, and 10 containing an intramolecular disul®de bridge by applying Helferich-conditions and an excess of acetobromohexose 5 or 11 as donor substrate yielded 6-thiocyanato--(134)-thiodisaccharides 6, 9, and 12 regio-and stereospeci®cally in moderate to good yields. In addition, the formation of trisaccharides 7 and 13 could be observed, albeit in low yield. This novel route for thioglycoside formation may open access to a variety of bioactive disaccharide analogues.
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