We have used infrared spectroscopy, ellipsometry, and contact angle measurements to study self-assembled
monolayer (SAM) formation on aluminum native oxide from three alkanephosphonic acids: CF3(CF2)7(CH2)11PO3H2 (F8H11PA), and CH3(CH2)
n
PO3H2 (n = 15 (H16PA); n = 21 (H22PA)). These compounds show significant
differences in film structure and film formation kinetics. Strikingly, the methylene segment of the
semifluorinated F8H11PA SAM never reaches an ordered state even at long assembly times. This contrasts
with the ordered chains in equilibrium films from H16PA and H22PA. We attribute this behavior to steric
effects of the fluorocarbon segment and the phosphonic acid headgroup. F8H11PA represents an amphiphile
in which bulky head and tail groups prevent an interposed hydrocarbon segment from ordering. For all three
phosphonic acids, negative peaks attributed to loss of Al−OH groups in the infrared spectra of the monolayers
are consistent with a condensation reaction between the acids and surface hydroxyls to form bound
aluminophosphonate salts. With respect to kinetics, our results indicate that F8H11PA approaches its equilibrium
film structure considerably faster than the hydrocarbon phosphonic acids. We interpret the structural dependence
of film formation kinetics in terms of the T
c formalism advanced by Rondelez and co-workers (Langmuir
1994, 10, 4367−4373). We also suggest that the accelerated film formation exhibited by F8H11PA may be
due to chain entanglement and solubility effects, to the extent that this species may self-assemble as islands
of approximately vertically oriented chains which fill in as coverage increases. H22PA may also deposit as
islands, but in contrast, film formation for H16PA probably involves initially disordered chains with higher
tilt angles that order and reorient as film assembly proceeds.
equation,26 and HSAB theory20 have all been used in attempts to correlate reactivity with structural or thermodynamic properties. Success of these correlations is rather limited, as absolute and even relative rates of SN2 reactions have been found to be highly solvent (2) (a) Swain, C.
Dynamic contact angle studies on self-assembled thin films from C 7 F 15 CH 2 OCH 2 CH 2 CH 2 SiCl 3 reveal a range of interesting behavior. Solution-based processing conditions have been identified that allow preparation of essentially monolayer films on quartz exhibiting water adv/rec contact angles of 119/104 ( 2°and extremely low contact angle hysteresis (hexadecane adv/rec ) 74/70°, heptane adv/rec ) 56/55°) with hydrocarbon liquids. This compound provides an example of a fluorinated trichlorosilane that is able to deliver lowhysteresis films by deposition at room temperature. Adsorption of silane oligomers, formed by hydrolysis and condensation reactions in solution, was also found to occur, slower than but competitive with monolayer formation. This process became more significant as dip coating times increased. Ellipsometric data on silicon wafers confirmed that film thicknesses increased with dip time, while AFM imaging showed that the oligomeric material was deposited in the form of particulates. The effects of this process on water dynamic contact angles are discussed. We also compare contact angles on these films with those on films prepared from C n F 2n+1 CH 2 CH 2 SiCl 3 (n ) 6, 8, 10) and draw some conclusions regarding structure-property effects in these systems. Finally, we propose a mechanism that can account qualitatively for the bulk of the results observed here. Its central feature is reaction of the fluorinated alkyltrichlorosilane with surface-adsorbed water to yield a self-assembled monolayer consisting of silanetriol molecules hydrogen-bonded to the substrate. Effects of high humidity aging on dynamic contact angles of these films suggest that they are at most only lightly cross-linked when prepared under conditions utilized here.
Imidazolium
functionality has played a prominent role in research
on anion exchange membranes for use in alkaline electrochemical devices.
Base stability and degradation of these materials has been much studied,
but in many instances, product pathways have not been thoroughly delineated.
We report an NMR study of base-induced decomposition products from
three benzylimidazolium salts bearing varying extents of methyl substitution
on the imidazolium ring. The major products are consistent with a
hydrolytic ring fragmentation pathway as the principal mode of decomposition.
We observe several new products not previously reported in the literature
on imidazolium salt degradation, including benzilic acid rearrangement
products formally derived from intermediate 1,2-dicarbonyl compounds
or their equivalents. However, the overall reactions are complex,
the yields of observed products do not account for all consumed starting
materials, and mechanistic ambiguities remain.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.