The investigation of the thermal stability of self-assembled monolayers of symmetrical and mixed alkyl fluoroalkyl disulfides on gold is reported. The monolayers were annealed in air at temperatures ranging from 60 to 110°C for several hours and afterward characterized by contact angle measurements, polarized grazing incidence Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). In mixed alkyl perfluoroalkyl systems evidence was sought for heatinduced phase separation of the alkyl-and perfluoroalkyl-terminated molecules. The ester and amide linkages of the fluorocarbon chains are shown to be less stable than those of the hydrocarbon analogues. Fluoro substituents in R-position to the ester linkage make the ester bond susceptible to hydrolytic cleavage at temperatures between 60 and 70°C, whereas the corresponding amide and the γ-fluoro-substituted esters remain stable up to 80-100°C. After annealing for 10 h or longer at these temperatures contact angle measurements, XPS, and FTIR reveal partial loss of fluorocarbon chains in monolayers of symmetrical fluoroalkyl disulfides and mixed alkyl fluoroalkyl disulfides. In contrast, it is still possible to observe ordered domains with AFM in monolayers of mixed alkyl fluoroalkyl disulfides annealed at 100°C for 17 h. AFM scans in molecular resolution confirm that the lattice constant of the hexagonal lattice remains unchanged (6.2 Å). Since the monolayers of both corresponding symmetrical disulfides show significantly smaller lattice constants, heat-induced phase separation can be excluded. The results indicate nonexisting to very low mobility of the molecules within the layer at higher temperatures. Alternatively, the existence of disulfides rather than thiolates bound to the gold can explain the observations.

An evanescent wave ellipsometric technique is used to study the orientational wetting of 4Ј,4alkylcyanobiphenyls (nCB,nϭ5 -9) at the interface with a series of self-assembled monolayers. Brewster angle measurements are interpreted in terms of a global orientational wetting phase diagram (T,n,h 1 ), where T denotes temperature, n is the alkyl chain length, and h 1 is the cosine of the contact angle of water with the monolayer. At temperatures below the isotropic-nematic phase transition temperature T IN , an anchoring transition from planar to homeotropic alignment occurs as the surface of the monolayer is made more hydrophobic. Homeotropic anchoring of the nematic director at TϽT IN is associated with complete orientational wetting on approaching the isotropic-nematic phase boundary from temperatures above T IN . The anchoring-wetting transition shifts to less hydrophobic surfaces as n increases.

Self-assembled monolayers of alkanethiol derivatives have been used to control the anchoring of nematic films of alkylcyanobiphenyl (nCB) liquid crystals. Variation of the surface field, via the a-functional group presented at the liquid crystal interface, has allowed us to explore both orientational wetting and anchoring transitions. It is found that low-energy CF, surfaces give rise to homeotropic anchoring of 8CB whilst C 0 2 H and OH terminated surfaces yield planar anchoring. The anchoring transition lies close to 5CB on CF,-and 8CB on CH,-terminated surfaces. The molecular alignment in the interfacial region is studied using a modified evanescent wave ellipsometric approach in which the position of the Brewster angle represents our integrated order parameter. We find complete orientational wetting by homeotropic nematic films at the substrate/isotropic fluid interface, for sufficiently low energy surfaces. In contrast, in our systems, planar anchoring is not associated with orientational wetting.

The anchoring of the nematic liquid crystal (LC) 4′-n-octyl-4-cyanobiphenyl (8CB) at derivatized self-assembled monolayer (SAM) surfaces has been investigated using surface plasmon resonance (SPR) microscopy. Surfaces having distinct areas (patterns) covered by different ω-functionalized groups, one (-CF3) promoting homeotropic anchoring of the nematic director and the other (-OH) planar anchoring, were fabricated from photopatterned SAMs. Our results show that SPR microscopy, which is sensitive to the alignment on the mesoscale (i.e., within 300 nm of the interface) can be used to spatially resolve the anchoring of adsorbed LC films, with a lateral resolution on the order of a few micrometers, as a function of temperature. In addition, we observe surface melting of the crystalline phase at the SAM boundary by both homeotropic and planar aligned melts.