Oriented Crystalline Thin Films of Tetracosanedioic Acid and Its Metal Salts at the Air-Aqueous Solution Interface

Abstract

Langmuir monolayers are conventionally prepared by spreading, on a water surface, amphiphilic molecules containing hydrophilic head groups, which interact with the water molecules, and hydrophobic chains, which favor lateral aggregation in air. The crystalline packing of the molecules in the monolayer has been determined in situ, with almost atomic resolution by means of synchrotron X-ray diffraction using a monochromatic beam at grazing incidence. With the use of this technique, the two-dimensional crystalline packing arrangement of monolayers of long-chain alcohols, carboxylic acids, amides, aamino acids, and phospholipids could be determined. Recently, the formation at the air±water interface of crystalline monolayer and multilayer films obtained from symmetric molecules with end groups equally hydrophobic or hydrophilic, such as long-chain linear fluorocarbons and hydrocarbons, oligothiophenes (containing four, five, and six rings), and a,w-alkanediols, has been reported. In these films at low surface pressure the long molecular axis is aligned either normal to the water surface or tilted by up to 40 from the normal. Moreover, the formation of crystalline interdigitated trilayers from water-insoluble long-chain mandelic acid and phenylethylamine molecules contained in the subphase was proven by grazing incidence X-ray diffraction (GID). The changes in packing arrangement of monolayers composed of long-chain carboxylic acids, as induced by the presence in the subphase of Cd, Ca, or Pb ions, have also been determined by GID. Over pure water and in the uncompressed state, the chains are tilted by approximately 30 from the normal to the water surface, whereas over solutions containing the metal ions, the chains are vertically aligned. This report describes the self-assembly process of a,wtetracosanedioic acid, HOOC±(CH2)22±COOH, on aqueous subphases containing divalent Cd and Pb ions. These positive ions have a dramatic influence on the packing of the diacid molecules: they bind to both carboxylate ends of the molecule and force the long hydrocarbon chain to lie parallel to the water surface and to self-assemble as multilayer domains about 50 Š thick, but with the molecules and the bound ions perfectly ordered laterally over distances up to 1000 Š. GID and specular X-ray reflectivity (XR) were used to characterize the oriented crystalline multilayers of these cadmium and lead salts. Such multilayer films preserve their orientation during the transfer from the water surface onto a mica support, as imaged by scanning force microscopy (SFM), and can be used in the preparation of quantum dots (see following communication). The GID experiments were carried out at the synchrotron beamline BW1, HASYLAB, on the liquid surface diffractometer, as described elsewhere. The samples were prepared by spreading solutions of the diacid onto pure water and onto aqueous subphases containing the metal ions, at 15 C, to give a nominal molecular area of 36 Š (trough area divided by the number of spread molecules). Spreading was followed by reducing the surface area of the film to a nominal molecular area of 18 Š, with a maximum surface pressure increase of 1 mN/m and cooling down to 5 C. Preliminary evidence for the interaction between the diacid molecules and the metal ions present in the subphase leading to salt formation was obtained from the surface pressure±area isotherms (Fig. 1a). While the diacid spread on water leads to a small increase in surface pressure only at relatively small nominal molecular areas, the isotherm performed on the subphases containing ions resulted in a pressure increase of about 20 Š/molecule. On the basis of the isotherm alone, one might assume conventional monolayer formation for the latter system. However, as we shall see, the GID results provide unambiguous evidence for the formation of multilayers with the diacid hydrocarbon chain aligned parallel to the liquid surface.