An interfacial rheometer for both stress- and strain-controlled measurements of shear rheological properties at liquid/liquid and gas/liquid interfaces is presented. The device is based on a rotating or oscillating biconical bob design in combination with a low friction electronically commutated motor system. The interfacial shear stress, viscosity, and dynamic moduli are obtained by solving the Stokes equations (low Reynolds number) along with the Boussinesq–Scriven interfacial stress tensor, which is used for the boundary conditions at the interface. An improved and simple numerical method for the calculation of the velocity distribution in the measuring cell is presented. The scope and limitations of the rheometer are discussed. Results from steady shear and oscillatory experiments as well as creep recovery and stress relaxation tests at both oil/water and air/water interfaces with adsorbed films of a globular protein (ovalbumin) and spread films of a surfactant (sorbitan tristearate) are presented.

The temperature-dependent self-assembly of the single-chain bolaamphiphile dotriacontan-1,1'-diyl-bis[2-(trimethylammonio)ethyl phosphate] (PC-C32-PC) was investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), X-ray scattering, rheological measurements, and dynamic light scattering (DLS). At room temperature this compound, in which two phosphocholine headgroups are connected by a C(32) alkyl chain, proved to be capable of gelling water very efficiently by forming a dense network of nanofibers (Kohler et al. Angew. Chem., Int. Ed. 2004, 43, 245). A specific feature of this self-assembly process is that it is not driven by hydrogen bonds but solely by hydrophobic interactions of the long alkyl chains. The nanofibers have a thickness of roughly the molecular length and show a helical superstructure. A model for the molecular structure of the fibrils which considers the extreme constitution of the bolaamphiphile is proposed. Upon heating the suspensions three different phase transitions can be detected. Above 49 degrees C, the temperature of the main transition where the alkyl chains become "fluid", a clear low-viscosity solution is obtained due to a breakdown of the fibrils into smaller aggregates. Through mechanical stress the gel structure can be destroyed as well, indicating a low stability of these fibers. The gel formation is reversible, but as a drastic rearrangement of the molecules takes place, metastable states occur.

Bipolar lipids with only one long alkyl chain and two polar headgroups at the ends are model compounds for the chemically more complicated bola lipids from Archea bacteria, for instance, the bis(diphytanyl) diglycerol tetraethers. [1] In Irlbachia alata and Anthocleista djalonensis, the bola lipid docosan-1,22-diyl-bis-[2-(trimethylammonio)ethylphosphate] (irlbacholine) has a C 22 alkyl chain that connects two polar phosphocholine groups and has been used in plant medicine. It was found to be effective against the fungus Trichophyton rubrum, which affects the skin, and other fungal infections. [2] In the course of studies on the possibility of using similar bola lipids with one long alkyl chain as membrane stabilizers, a bola phospholipid with a C 32 alkyl chain was synthesized, namely the lipid dotriacontan-1,32-diyl-bis-[2-(trimethylammonio)ethylphosphate] (PC-C32-PC, 1; Figure 1).[3] During the initial characterization of 1 we observed unusual aggregation behavior, which necessitated further physico-chemical investigations. To obtain sufficient quantities of 1 a more efficient synthetic route to long-chain bola compounds was necessary.We prepared lipid 1 in gram quantities by a newly developed simple and effective procedure. The construction of the chain with 32 carbon atoms is possible in a one-step synthesis by a copper-catalyzed coupling of undec-10-en-1-ylmagnesium bromide with 1,10-dibromodecane in a molar ratio of 2:1 (see Experimental Section). Dotriacontan-1,31-diene is then converted into dotriacontan-1,32-diol by hydroboration with disiamyl borane and subsequent oxidative hydrolysis. The importance of the new reaction is underlined by the possibility of functionalizing the 1-and 32-positions. The phosphocholine moieties were introduced by the reaction of dotriacontan-1,32-diol with 2-bromoethyl phosphoric acid dichloride in chloroform. This classic phosphorylation reagent was more effective than more modern variants. The trimethylammonium groups in 1 were introduced by established procedures. [4] Because of the long alkyl chain and the large polar headgroups at both chain ends of 1, it forms novel types of aggregate structures not observed previously. We investigated the self-assembly of the bipolar compound in aqueous solution using differential scanning calorimetry, electron microscopy, X-ray scattering, viscosity measurements, and FT-IR spectroscopy.In highly dilute aqueous solution (c < 0.5 wt %), a very viscous, almost transparent gel is observed. The 0.1-wt % sample shown in Figure 2 has a water/1 molar ratio of ca. 45 000:1; we see that an enormous amount of water can be "immobilized" by the gel structure. At temperatures above 50 8C or through mechanical stress the gel character disappears and a fluid solution is obtained; the viscosity drops by about five orders of magnitude. Preliminary viscosity measurements at room temperature show an indication of a yield stress (t 0 *) of roughly 7.2 Pa for a 0.8-wt % solution (Figure 3). Temperature-dependent viscosity measurements indicate a viscosity...

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