Contact angle self-assembled monolayers

Section 4.1 briefly describes some of the commonly employed experimental tools and procedures. Chaudhury et al., Israelachvili et al. and Tirrell et al. employed contact mechanics based approach to estimate surface energies of different self-assembled monolayers and polymers. In these studies, the results of these measurements were compared to the results of contact angle measurements. These measurements are reviewed in Section 4.2. The JKR type measurements are discussed in Section 4.2.1, and the measurements done using the surface forces apparatus (SFA) are reviewed in Section 4.2.2. 

Self-assembled monolayers are formed spontaneously by the immersion of an appropriate substrate into a solution of active surfactant in an organic solvent. After the substrate is immersed for a time from minutes to hours, it is rinsed with ligroin, methanol, distilled water, and dried in a steam of nitrogen. An apparent effect of the monolayer coating is the drastic change in wettability of the surface so that the measurement of the contact angle can be considered as an effective way to detect the formation of the SAMs. 

Fig. 4 Water contact angle data (a) and surface energy data (b) from a surface energy library produced through the graded UV-ozonolysis of an ODS self-assembled monolayer on sihcon. (Reproduced with permission from [17])...

Lastly, Polystyrene (PS) brushes on silicate substrates were grafted via carbocationic polymerization from self-assembled monolayer (SAM) initiators as reported by Brittain et al The carbocationic initiators, 2-(4-(ll-triethoxysilylundecyl))phenyl-2-methoxypropane and 2-(4-trichlorosilyl-phenyl)-2-methoxy-d3-propane, and their corresponding SAMs were prepared on various substrates. The monolayers were characterized by FTIR-ATR, contact angles, and X-ray reflectometry. The growth of the PS brushes from... 

Arce etal. [140, 141] have studied the dynamics of 1-dodecanethiol and bu-tanethiol SAMs on Au(lll), applying ex situ and in situ STM. The potential of zero charge for the thiol-modified Au(lll) electrode was determined for self-assembled monolayers of octade-canethiolate (—0.52 V), undecanethiolate (—0.49 V), propanethiolate (—0.3 V), and H, //,2//,2 //-perfluorodecanethiolate (1.04 V) ]142]. The potentials (expressed versus Ag]AgCl]saturated KCl electrode) were determined from the measurements of the contact angle for a droplet of 0.1 M NaCl04 aqueous solution. 

Another well-represented category was that of self-assembled monolayers (SAMS) and other supramolecular systems. The experiments on the SAMS included studies of the surface pKa of one system (110), the kinetics and thermodynamics of the self-assembly process (111), and the characterization of the SAM surface by study of solution contact angles (112). The experiments on supramolecular systems included studies on chemical equilibria in such systems (113, 114, 115), the kinetics of inclusion phenomena (116), and the use of solvatochromic probes in studying inclusion phenomena (117). 

Contact Angles of Aqueous Solutions on Copper Surfaces Bearing Self-Assembled Monolayers 112... 

DePalma and Tillman investigated self-assembled monolayer films from three silanes, tridecafluorooctyltrichlorosilane, undecyltrichlorosilane, and octadecyl-trichlorosilane, on silicon, a popular model substrate for such studies with great relevance to potential semiconductor coating applications. They characterized the films by ellipsometry and contact angle measurements (data for trideca-fluorooctyltrichlorosilane are included in Table 1), but more usefully from an applicational viewpoint, they carried out friction and wear measurements with a pin-on-disk device where the silicon wafer substrate, coated with monolayer, is moved under a spherical glass slider. Optical microscopy was used to assess wear. Table 2 summarizes DePalma and Tillman s data and their comparison with the classical self-assembled monolayer friction studies of Levine and Zisman [18]. 

This section dealt with the novel functionality of a photoresponsive monolayer to carry out the displacement of liquids simply by photoirradiation as a result of the photocontrol of surface energy. Self-assembled monolayers of CRA-CM are designed and fabricated to ensure sufficient /Z photoisomerizability even in densely packed monomolecular layers, which are quite stable toward solvent treatments. The following critical condition should be met in order to realize the surface-mediated photomanipulation of liquid motion 0 ec < adv In this context, contact-angle hysteresis plays an essential role in this kind of dynamic behavior of liquid displacement. 

In their experiments the authors worked with gold-coated silica spheres, with self-assembled monolayers of mixtures of undecanethiols and co-hydroxy undecane thiols, to vaiy the contact angle between 20 and 100°. The results were compared with sessile drop measurements on the same system. A systematic difference up to 20° for the receding angles) between the two sets of results was observed, but since these differences depend on the nature of the solid surface it was not possible to pinpoint the origin definitely. A remarkable feature was that in the AFM method, hysteresis was very small, if not absent. Anyhow, this appears to be a promising technique. 

The structure of self assembled monolayers terminated with phenol and 2-chlorophenol moieties was studied by reflectance FUR, X-ray reflectometry, solid state C NMR spectroscopy and measurements of contact angle with water. The pH values at half dissociation (pHi/2) of the monolayers were >12.5 and >12, respectively, which is at least 2.5 pH units higher than those of half dissociation of the corresponding phenols in solution, as denoted by their pXa values. The pH at which a certain contact angle was achieved was lower for the 2-chlorophenol moieties, in accordance with their higher acidity. ... 

Recently Mahapatro, et al., demonstrated the surface modification of functional self assembled monolayers on 316L stainless steel via lipase (Novozyme-435) catalysis (Scheme 4) 86). SAMs of 16-mercaptohexadecanoic acid (-COOH SAM) and 11-mercapto-l-undecanol (-OHSAM) were formed on 316L SS, and lipase catalysis was used to attach therapeutic drugs, perphenazine and ibuprofen, respectively, on these SAMs. The reaction was carried out in toluene at 60 °C for 5 h using Novozyme-435 as the biocatalyst. The FTIR, XPS and contact angle measurements collectively confirmed the biocatalytic surface modification of SAMs. 

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