PREPARATION AND CHARACTERIZATION OF MIXED MONOLAYER ASSEMBLIES COMPOSED OF THIOL ANALOGS OF CHOLESTEROL AND FATTY-ACID

Mixed self-assembled monolayers provide an attractive model system for investigating the role of different molecules in biological membranes . This paper describes the preparation and characterization of a novel type of mixed monolayer assemblies composed of thiol analogues of cho lesterol and fatty acid. The mixed: monolayers are prepared by coadsor bing 11-mercaptoundecanoic acid (MUA) and thiocholesterol(cholest-5-en e-3 beta-thiol, TC) from solution directly onto evaporated gold surfac es. The influence of TC on the molecular composition and conformation in the mixed monolayer is analyzed by using a combination of infrared reflection-absorption spectroscopy (IRAS), X-ray photoelectron spectro scopy (XPS), ellipsometry, contact angle measurement, and cyclic volta mmetry. The results indicate that the TC molecules maintain their conf ormation in the mixed monolayers, whereas the MUA molecules display a significantly more disordered conformation as compared to the MUA mole cules in the pure monolayer. Cyclic voltammetry shows that the mixed m onolayers are more densely packed and less permeable than the pure TC and MUA monolayers. The kinetics of the coadsorption of TC and MUA fro m ethanol indicates that adsorption of TC initially is strongly prefer red over MUA but that MUA dominates over TC at long coadsorption times . This is because there is a larger energy gain per unit area in formi ng monolayers with MUA, Further, it is also seen that the number of mo lecules per unit area changes with the molecular composition, as a con sequence of the different sizes of TC and MUA. We present herein a met hod for calculating the mole fraction of TC on the gold surface, (chi TC), which accounts for this variation. As a consequence of the dissim ilar size and shape of the two molecules, the wetting properties of th e mixed monolayer are found to be mainly governed by the fractional ar ea of TC, rather than by the molecular composition of TC, (chi TC).