Effect of nanometer-scale phase separation on wetting of binary self-assembled thiol monolayers on Au(111)

The advancing contact angle of water has been measured on three types of the binary self-assembled monolayers (SAMs) on Au(111) with known surface structure at nanometer scale: a homogeneously mixed SAM of undecanethiol (UDT) and 11-mercaptoundecanoic acid (MUDA) (1), a SAM of hexadecanethiol (HDT) and 3-mercaptopropionic acid (MPA) phase-separated into nanometer-scale domains with different heights (2), and a SAM of UDT and MUDA artificially phase-separated into nanometer-scale domains having approximately equal heights (3). The homogeneously mixed SAM, 1, shows a linear variation of the cosine of the contact angle, cos θ, with the surface mole fraction of COOH-terminated thiol, x_surf^COOH, while in the case of 2 the cos θ versus x_surf^COOH plot is strongly concave; the cos θ depends little on x_surf^COOH for x_surf^COOH < 0.5 and abruptly increases with x_surf^COOH for x_surf^COOH > 0.5. In contrast, the phase-separated SAM, 3, shows a linear variation of cos θ with x_surf^COOH, as is the case of 1, indicating that the contact angle does not reflect the phase separation in nanometer scale. The difference in the height of the domains of the two thiols is the primary factor for the nonlinear cos θ versus x_surf^COOH plot for 2. The Young-Laplace equation explains the nonwetting of the lower domains of 2.