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Abstract:
Mixtures of two long-chain alkanethiols, HS(CH2)nX and HS(CH2)mY (X, Y = CH3, OH; n > m), in which the alkyl chains have different lengths, adsorb from solution onto gold and form monolayers comprising a densely packed inner region adjacent to the gold surface and a disordered outer region in contact with the solution. When X = Y = CH3 (n = m), this disordered phase makes the mixed monolayer more oleophilic than the ordered, pure (i.e., single-component) monolayers. When X = Y = OH, the pure monolayers are wet by water, but the mixed monolayers are less hydrophilic because nonpolar polymethylene chains are exposed at the surface. When X = Ch3, Y = OH (n = 21, m = 11), a very sharp transition occurs from a monolayer composed largely of the longer, methyl- terminated component to the shorter, hydroxyl-terminated component as the mole fraction of HS(CH2)11OH in the adsorption solution is increased. From solutions containing two thiols, adsorption of the thiol with the longer chain is preferred. This preference is greater when the monolayers are adsorbed from ethanol than from isooctane. The mixed monolayers do not act as ideal two- dimensional solutions. The adsorption isotherms suggest a positive excess free energy of mixing of the two components in the monolayer. The compositions of the monolayers appear to be determined largely by thermodynamics, although in some cases there is also a kinetic contribution to the composition. (AW)
| Limitations: |
 APPROVED FOR PUBLIC RELEASE |
| Description: |
Interim technical rept. |
| Pages: |
81 |
| Report Date: |
MAY 89 |
| Contract Number: |
N00014-85-K-0898, N00014-86-K- |
| Report Number: |
A683802 |
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