Effect of Intermolecular and Interfacial Interactions on the Three- and Two-Dimensional Structure and Phase Behavior of Three-Block Liquid-Crystalline Siloxane Derivatives

Using several experimental techniques, we have investigated the thermodynamics and the structure of the bulk, monolayers, and multilayers of new three-block organosiloxanes based on the pentyloxy-4‘-cyanobiphenyl mesogenic unit. Compared to conventional amphiphiles or smectogens, these molecules contain an additional siloxane moiety whose bulkiness was gradually increased. This enables fine tuning of the steric effect imposed by the siloxane parts and allows determination of the balance between molecule−substrate and molecule−molecule interactions as well as of the effect of the siloxane end group on the microscopic structure, order, and film homogeneity. We show that the three- and two-dimensional behaviors are highly correlated and are determined by the subtle balance between these interactions. We particularly showed that surface effects induce a three-layer smectic film whatever the bulk state. Various 2D phenomena have been observed such as wetting, mono- and bilayer melting, liquid droplet freezing, several 2D phase transitions, and a tricritical point. These are reversible phenomena and easily controllable via lateral density change, making the present organosiloxane molecules a good model system to investigate in much more detail some challenging physical aspects in two dimensions.