Self-Assembled Monolayers of Branched Thiols and Disulfides on Gold: Surface Coverage, Order and Chain Orientation

Self-assembly of several branched thiols possessing two long alkane chains and corresponding disulfides on gold surfaces are described. The self-assembled monolayers (SAMs) obtained were investigated by contact angle measurements, Fourier transform infrared spectroscopy (FT-IR), surface plasmon resonance (SPR), and atomic force microscopy (AFM). Monolayers formed by the disulfides were shown to be significantly thinner (SPR) and much more disordered (FT-IR, contact angles) than SAMs of the thiol counterparts. The presence of polar functional groups and complementary H-bond donors/acceptors in the alkane chains of branched disulfides was shown to assist the formation of better packed monolayers. Compared to SAMs of octadecanethiol, the branched thiols investigated in this study gave SAMs with a significantly reduced tilt angle, as seen in the FT-IR spectra. AFM revealed the lattice of one of the thiols on Au(111) with molecular (lattice) resolution showing a reduced area per molecule (as compared to octadecanethiol) which is consistent with a reduced tilt angle.