The formation of self-assembled monolayers (
SAMs) of adsorbed cationic or anionic
surfactant molecules on atomically flat H-terminated Si(111) surfaces in aqueous solutions was investigated by in situ AFM measurements, using octyl trimethylammonium
chloride (C8TAC),
dodecyl trimethylammonium chloride (
C12TAC), octadecyl trimethylammonium
chloride (C18TAC))
sodium dodecyl sulfate (STS), and
sodium tetradecyl sulfate (SDS). The adsorbed
surfactant layer with well-ordered molecular arrangement was formed when the Si(111) surface was in contact with 1.0x10(-4) M C18TAC, whereas a slightly roughened layer was formed for 1.0x10(-4) M C8TAC and
C12TAC. On the other hand, the addition of
alcohols to solutions of 1.0x10(-4) M C8TAC,
C12TAC, or SDS improved the molecular arrangement in the adsorbed
surfactant layer. Similarly, the addition of a
salt, KCl, also improved the molecular arrangement for both the cationic and anionic
surfactant layers. Moreover, the adsorbed
surfactant layer with a well-ordered structure was formed in a
solution of mixed cationic (
C12TAC) and anionic (SDS)
surfactants, though each
surfactant alone did not form the well-ordered layer. These results were all explained by taking into account electrostatic repulsion between ionic head groups of adsorbed
surfactant molecules as well as hydrophobic interaction between their alkyl chains, which increases with the increasing chain length, together with the increase in the hydrophobic interaction or the decrease in the electrostatic repulsion by incorporating alcohol molecules into the adsorbed
surfactant layer, the decrease in the electrostatic repulsion by increasing the concentration of counterions, and the decrease in the electrostatic repulsion by alternate arrangement of cationic and anionic
surfactant molecules. The present results have revealed various factors to form the well-ordered adsorbed
surfactant layers on the H-Si(111) surface, which have a possibility of realizing the third generation surfaces with flexible structures and functions easily adaptable to circumstances.