  We investigated the effect of surfactants on an interface between two kinds
of liquid by molecular dynamics simulation. We adopted the simple bead-spring
model with two atoms as the surfactants. We controlled the interfacial tension
of the surfactant adsorbed on the interface by changing the bond length.
Although the interface's structure changed depending on the magnitude of the
interfacial tension, the interface was stable even under conditions where the
interfacial tension was virtually zero. The Fourier spectrum of the
fluctuations of the surface structure showed a crossover from $q^2$ to $q^4$
when the interfacial tension was almost zero, where $q$ is the wavenumber. This
crossover means that the bending rigidity is dominant for the restoring force
when the surfactant molecules are sufficiently absorbed on the interface and
the interfacial tension is almost zero, whereas the interfacial tension is
dominant when the interfacial tension is a finite value.
