Several rodlike 4,4‘‘-bis(decyloxy)-p-terphenyl derivatives incorporating nonionic hydrophilic groups in the lateral 2‘-position (2-oxa-4,5-dihydroxypentyl, 2,5-dioxa-7,8-dihydroxyoctyl, 2,5,8-trioxa-10,11-dihydroxyundecyl, and 2,5,8,11-tetraoxa-13,14-tetradecyl groups) and 2‘-(2-oxa-4,5-dihydroxypentyl)-4,4‘‘-diundecyl-p-terphenyl form well-ordered thin films when spread at the air−water interface. One observes two sharp breaks in the pressure/area isotherms separated by a large plateau. The first break occurs at an area of ca. 0.90 nm2/molecule, an area which corresponds to a side-on arrangement of the terphenyl units at the interface. The plateau corresponds to a first-order phase transition. The surface pressure related to this transition significantly rises with an increasing number of oxyethylene units in the hydrophilic lateral groups. Brewster angle microscopic investigations indicate the formation of fluid domains in this region. In some cases these domains coalesce to a homogenous layer. The surface potential is nearly constant in the region of the plateau, which can be explained by a defined collapse due to the formation of a triple layer consisting of a bilayer on top of the monolayer.

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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.

Three 4,4′′-didecyloxy-p-terphenyl derivatives with laterally attached crown ether units of different ring size [(1,4,7,10-tetraoxacyclododecyl)-, (1,4,7,10,13-pentaoxacyclopentadecyl)-, and (1,4,7,10,13,16-hexaoxacyclooctadecyl)-2-methyl 2,5-bis(4-decyloxyphenyl)benzoate] have been synthesized, and their Langmuir films were investigated by means of a film balance. They form well-ordered thin films when spread at the air-water interface which are characterized by two sharp breaks in their surface pressure-area isotherms separated by a large plateau. The lateral pressure in the plateau regionsin which the molecules undergo a first-order phase transition with formation of a defined triple layerssignificantly increases on enhancing the number of oxyethylene units in the crown ether units. Additionally, it depends on the type and concentration of alkali metal ions (Li + , Na + , K + , Rb + , Cs + ) in the subphase. Cations which fit the cavity of the appended crown ethers most significantly stabilize the films. It seems that the surface pressure in the plateau region is reminiscent of the binding constants of alkali metal ions to the crown ether units. This shows that facial amphiphiles could represent an interesting new platform for the investigation of guest-host interactions and molecular recognition processes at the air-water interface.