The anchoring structures of binary mixtures of 4'-n-octyl-4-cyanobiphe
nyl (8CB) and 4'-n-dodecyl-4-cyanobiphenyl (12CB) on molybdenum disulf
ide (MoS2) are directly observed by scanning tunneling microscopy (STM
) in order to investigate the boundary condition of liquid crystal mix
tures at the molecule substrate interface. On the basis of the highly
reproducible STM images, the anchoring structures of binary mixtures a
re clearly divided into three categories: a homogeneous single-row typ
e consisting of only 8CB (in the case of 8CB:12CB = 90:10, 80:20, and
70:30 (mol %)), an inhomogeneous (mixed) double-row type consisting of
8CB and 12CB (60:40, 50:50, 40:60, and 30:70), and a homogeneous doub
le-row type consisting of only 12CB (20:80 and 10:90). The ratio of 8C
B and 12CB adsorbed on MoS2 substrate in all mixtures is different fro
m that in the bulk. Furthermore, a new phase transition only available
at the boundary is confirmed for the first time. The correlation betw
een those anchoring structures of liquid crystal mixtures and the bulk
phase diagram is discussed from the viewpoint of anchoring phase form
ation and selective adsorption of mixtures at a molecular level.