CHAIN-LENGTH DEPENDENCE OF THE STRUCTURE AND WETTING PROPERTIES IN BINARY COMPOSITION MONOLAYERS OF OH-TERMINATED AND CH3-TERMINATED ALKANETHIOLATES ON GOLD

Equimolar, binary-component, self-assembled monolayers (SAMs) of -S(CH2)(15)CH2OH and -S(CH2)((15+m))-CH3, with m varied systematically from -6 to +6, were prepared on evaporated gold films by self-assembly from ethanol solutions of the corresponding thiols. Single-wavelength ellipsometry, X-ray photoelectron spectroscopy, and infrared reflection spectroscopy (IRS) were used to monitor the coverage, composition, and chain conformational order, respectively, in the air/SAM structures, while liquid drop contact angle measurements with hydrogen-bonding and hydrocarbon liquids were used to probe the structural and chemical characteristics at the liquid/SAM interface. Both the IRS C-H stretching mode characteristics and the hydrocarbon liquid contact angles show sharp changes as m increases through the region of -4 to 0, while the hydrogen-bonding liquid contact angles remain essentially constant across the range of m values. The combined data lead to a general structural model in which monolayer structures shift from randomly placed, protruding HO-terminated chain segments which screen the underlying CH3 surface at low m to contiguously placed, protruding CH3-terminated chains, organized in clusters and extending away from the underlying CH2OH surface at high m. These structures require that the monolayers approach random mixing at low m but be phase-segregated at high m.