Pawilai Chinwangso

SAMs on gold derived from the direct adsorption of alkanethioacetates are inferior to those derived from the direct adsorption of alkanethiols.

Self-assembled monolayers (SAMs) on gold derived from the direct adsorption of thioacetic acid S-decyl ester (C10SAc) and thioacetic acid S-octadecyl ester (C18SAc) were compared to the corresponding SAMs derived from the analogous adsorption n-decanethiol (C10SH) and n-octadecanethiol (C18SH). All SAMs were characterized using ellipsometry, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). The comparison revealed that the SAMs generated from the thioacetates are not as densely packed and well ordered as the SAMs generated from the thiols. Furthermore, studies of the kinetics of adsorption found that the thioacetates adsorb more slowly than the corresponding thiols.

Self-Assembled Monolayers Generated from Unsymmetrical Partially Fluorinated Spiroalkanedithiols.

Self-assembled monolayers (SAMs) were prepared on gold substrates from an unsymmetrical partially fluorinated spiroalkanedithiol adsorbate with the specific structure of [CH3(CH2)7][CF3(CF2)7(CH2)8]C[CH2SH]2 (SADT) and compared to SAMs formed from the semifluorinated monothiol F8H10SH [CF3(CF2)7(CH2)10SH] of analogous chain length and n-octadecanethiol. The adsorbate with two alkyl chains, one terminally fluorinated and the other nonfluorinated, was designed to form monolayers in which the bulky helical fluorocarbon segments assemble on top of an underlying layer of well-packed trans-extended alkyl chains. Different combinations of deposition solvents and temperatures were used to produce the bidentate SAMs. Characterization of the resulting monolayers revealed that SAMs formed in DMF at room temperature allow complete binding of the sulfur headgroups to the surface and exhibit higher conformational order than those produced using alternative solvent/temperature combinations. The reduced film thicknesses and enhanced wettability of the SADT SAMs, as compared to the SAMs generated from F8H10SH, suggest loose packing and an increase in the tilt of the terminal fluorocarbon chain segments. Nevertheless, the density of the underlying hydrocarbon chains of the SADT SAMs was higher than that of the F8H10SH SAMs, owing to the double-chained structure of the new adsorbate. The conformational orders of the SAM systems were observed to decrease as follows: C18SH > F8H10SH > SADT. However, the SAMs formed from this new double-chained bidentate adsorbate in DMF expose a fluorinated interface with a relatively low surface roughness, as determined by contact-angle hysteresis.

Structure, Wettability, and Thermal Stability of Organic Thin-Films on Gold Generated from the Molecular Self-Assembly of Unsymmetrical Oligo(ethylene glycol) Spiroalkanedithiols.

Organic thin-films on gold were prepared from a set of new, custom-designed bidentate alkanethiols possessing a mixture of normal alkane and methoxy-terminated tri(ethylene glycol) chains. The new unsymmetrical spiroalkanedithiol adsorbates were of the form [CH3O(CH2CH2O)3(CH2)5]-[CH3(CH2)n+1]C[CH2SH]2 where n = 3 and 14; designated EG3C7-C7 and EG3C7-C18, respectively. Their corresponding self-assembled monolayers (SAMs) on gold were characterized and compared with monothiol SAMs derived from an analogous normal alkanethiol (C18SH) and an alkanethiol terminated with an oligo(ethylene glycol) (OEG) moiety (i.e., EG3C7SH). Ellipsometric data revealed reduced film thicknesses for the double-chained dithiolate SAMs, which perhaps arose from the phase-incompatible merger of a hydrocarbon chain with an OEG moiety, contributing to disorder in the films and/or an increase in chain tilt. The comparable wettabilities of the SAMs derived from EG3C7SH and EG3C7-C7, using water as the contacting liquid, are consistent with exposure of the OEG moieties at both interfaces, whereas the lower wettability of the SAM derived from EG3C7-C18 is consistent with exposure of hydrocarbon chains at the interface. The data collected by X-ray photoelectron spectroscopy confirmed the formation of the new OEG-terminated dithiolate SAMs, and also revealed them as less densely packed monolayers due in part to the large molecular cross section of the OEG moieties and to their double-chained structure with dual surface bonds. Mixed SAMs formed from pairs of monothiols having chain compositions analogous to those of the chains of the new dithiols showed that an EG3C7SH/heptanethiol-mixed SAM and the EG3C7-C7 SAM produced almost identical characterization data, revealing the favorable film formation dynamics for adsorbate structures where the alkyl chains can assemble beneath the phase-incompatible OEG termini. For the mixed SAM formed from EG3C7SH/C18SH, the data indicate that the EG3C7SH component failed to incorporate in the film, demonstrating that the blending of phase-incompatible chains is sometimes best accomplished when both chains exist on a single adsorbate structure. Furthermore, the results of solution-phase thermal desorption tests revealed that the OEG-terminated films generated from the bidentate EG3C7-C7 and EG3C7-C18 adsorbates exhibit enhanced thermal stability when compared to the film generated from monodentate EG3C7SH. In a brief study of protein adsorption, the multicomponent SAMs showed a greater ability to resist the adsorption of fibrinogen on their surfaces when compared to the SAM derived from C18SH, but not better than the monolayer derived from EG3C7SH.

Unsymmetrical Spiroalkanedithiols Having Mixed Fluorinated and Alkyl Tailgroups of Varying Length: Film Structure and Interfacial Properties

A custom-designed series of unsymmetrical spiroalkanedithiols having tailgroups comprised of a terminally fluorinated chain and a hydrocarbon chain of varying lengths were synthesized and used to prepare self-assembled monolayers (SAMs) on gold substrates. The specific structure of the adsorbates was of the form [CH3(CH2)n][CF3(CF2)7(CH2)8]C[CH2SH]2, where n = 7, 9, and 15 (designated as F8H10-C10, F8H10-C12, and F8H10-C18, respectively). The influence of the length of the hydrocarbon chain in the bidentate dithiol on the structure and interfacial properties of the monolayer was explored. A structurally analogous partially fluorinated monodentate alkanethiol and the corresponding normal alkanethiols were used to generate appropriate SAMs as reference systems. Measurements of ellipsometric thickness showed an unexpectedly low film thickness for the SAMs derived from the bidentate adsorbates, possibly due to disruptions in interchain packing caused by the fluorocarbon chains (i.e., phase-incompatible fluorocarbon-hydrocarbon interactions), ultimately giving rise to loosely packed and disordered films. Analysis by X-ray photoelectron spectroscopy (XPS) were also consistent with a model in which the films were loosely packed; additionally, the XPS spectra confirmed the attachment of the sulfur headgroups of the bidentate adsorbates onto the gold substrates. Studies of the SAMs by polarization modulation-infrared reflection-adsorption spectroscopy (PM-IRRAS) suggested that as the length of the hydrocarbon chain in the adsorbates was extended, a more ordered surface was achieved by reducing the tilt of the fluorocarbon segment. The wettability data indicated that the adsorbates with longer alkyl chains were less wettable than those with shorter alkyl chains, likely due to an increase in interchain van der Waals forces in the former.