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, xsurfCOOH, while in the case of 2 the cos θ versus xsurfCOOH plot is strongly concave; the cos θ depends little on xsurfCOOH for xsurfCOOH < 0.5 and abruptly increases with xsurfCOOH for xsurfCOOH > 0.5. In contrast, the phase-separated SAM, 3, shows a linear variation of cos θ with xsurfCOOH, 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 xsurfCOOH plot for 2. The Young-Laplace equation explains the nonwetting of the lower domains of 2.
|出版ステータス||Published - 1998 4月 28|
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