Influence of the Position of the Hydrophilic Group on the Monolayer Properties of Rigid Amphiphiles with Laterally Attached Headgroups

Abstract

The π/A isotherms of rodlike 4,4-didecyloxy-p-terphenyl derivatives incorporating hydrophilic 1,2-diol groups in different lateral positions were recorded with dependence on the temperature, and the thin films were investigated by Brewster angle microscopy. Depending on the position of the hydrophilic groups and on the type of spacer between the hydrophilic group and the rigid core, different types of isotherms with different temperature dependences have been found. Two sharp breaks separated by a large plateau occur along the π/A isotherms of the 2-substituted and 2'-substituted compounds. In the plateau region a defined formation of a triple layer is proposed. For the 3-substituted compounds with a peripheral position of the hydrophilic groups, the type of isotherm largely depends on the type and the length of the spacer unit between the hydrophilic group and the rigid core. Two transition regions can be found in the slope of their isotherms. A tentative model of the organization of these molecules with dependence on the lateral pressure is proposed. Accordingly, the first transition is attributed to a transition of the rigid cores from a flat arrangement on the surface to a tilted arrangement. In the second transition region either formation of a defined triple layer (short spacer) or transition to a more densely packed monomolecular film with the rigid cores arranged parallel or tilted to the surface normal (long polyether chains) should take place. If the hydrophilic groups are coupled via lipophilic alkyl chains, the molecules are organized immediately after spreading and π/A isotherms similar to those of amphiphiles with terminal hydrophilic groups have been found. Thus, by slight changes of the molecular structure completely different supermolecular organizations at the water surface can be realized.