Phase Behavior, Structure, and Physical Properties of the Quaternary System Tetradecyldimethylamine Oxide, HCl, 1-Hexanol, and Water

The phase diagram of the ternary surfactant system tetradecyldimethylamine oxide (TDMAO)/HCl/1-hexanol/water shows with increasing cosurfactant concentration an L(1) phase, two L(alpha) phases (a vesicle phase L(alpha1) and a stacked bilayer phase L(alphah)), and an L(3) phase, which are separated by the corresponding two-phase regions L(1)/L(alpha) and L(alpha)/L(3). In this investigation, the system was studied where some of the TDMAO was substituted by the protonated TDMAO. Under these conditions, one finds for constant surfactant concentration of 100 mM TDMAO a micellar L(1) phase, an L(alpha1) phase (consisting of multilamellar vesicles), and an interesting isotropic L(1)(*) phase in the middle of the L(1)/L(alpha) two-phase region. The L(1)(*) phase exists at intermediate degrees of charging of 30-60% and for 40-120 mM TDMAO and 70-140 mM hexanol concentration. At surfactant concentrations less than 80 mM the L(1)(*)-phase borders directly on the L(1) phase. The phase transition between the L(1) phase and the L(1)(*) phase was detected by electric conductivity and rheological measurements. The conductivity values show a sharp drop at the L(1)/L(1)(*) transition, and the zero shear viscosity of the L(1)(*) phase is much lower than in L(1) phase. The form and size of the aggregates in L(1)(*) were detected with FF-TEM and SANS. This phase contains small unilamellar vesicles (SUV) of about 10 nm and some large multilamellar vesicles with diameters up to 500 nm. The system exhibits another peculiarity. For 100 mM surfactant, the clear L(alpha1)-phase exists only at chargings below 30%. With oscillating rheological measurements a parallel development of the storage modulus G' and the loss modulus G" was observed. Both moduli are frequency independent and the system possesses a yield stress. The storage modulus is a magnitude larger than the loss modulus. Copyright 2000 Academic Press.     

Phase behaviour and physical properties of the cationic quaternary system tetradecyldimethylamine oxide hydrochloric acid 1-hexanol water

 The phase behaviour and properties of the tetradecyldimethylamine oxide/HCl/hexanol/water quaternary surfactant system have been studied by means of electric conductivity, rheology, freeze-fracture transmission electron microscopy (FF-TEM) and small-angle neutron scattering (SANS). In this system the originally zwitterionic surfactant can become increasingly charged by protonation through the addition of HCl, i.e. the degree of charging can be changed continuously. An interesting, isotropic phase (L₁ ^* phase) of low viscosity was observed for intermediate degrees of charging. From viscosity and conductivity measurements this phase can clearly be distinguished from the conventional L₁ phase that is composed of micelles. Investigation of the structures present by means of FF-TEM and SANS showed that the L₁ ^* phase is made up of unilamellar vesicles of extremely small diameter of 8–10 nm. Evidently such highly curved structures are stabilized by the electrostatic conditions in this system. Received: 11 July 1999/Accepted: 25 August 1999     

Untersuchung von Kartoffeln, Gerste und Malz

Ohne Zusammenfassung     

Conformation of poly(L-lysine)-graft-poly(ethylene glycol) molecular brushes in aqueous solution studied by small-angle neutron scattering

Small-angle neutron scattering (SANS) has been employed for the analysis of conformations of poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g -PEG) molecular bottle brushes in aqueous solutions. The degree of polymerisation of the PEG chains was systematically varied in order to unravel dependence of the conformational properties of the bottle brushes on the molecular weight of the grafted chains. The grafting density was kept constant and high enough to ensure strong overlap of the PEG chains. The scattering spectra were fitted on the basis of the model of an effective worm-like chain with the account of average radial distribution and local fluctuations of the PEG density in the bottle brush. The results of the fits indicate that molecular brushes retain weakly bent configuration on the length scale of the order of (or larger than) the brush thickness. This finding is in agreement with earlier simulation and recent theoretical results.     

DNA molecular motor driven micromechanical cantilever arrays

The unique ability of living systems to translate biochemical reactions into mechanical work has inspired the design of synthetic DNA motors which generate nanoscale motion via controlled conformational change. However, while Nature has evolved intricate mechanisms to convert molecular shape change into specific micrometer-scale mechanical cellular responses, the integration of artificial DNA motors with mechanical devices presents a major challenge. Here we report the direct integration between an ensemble of DNA motors and an array of microfabricated silicon cantilevers. The forces exerted by the precise duplex to nonclassical i-motif conformational change were probed via differential measurements using an in-situ reference cantilever coated with a nonspecific sequence of DNA. Fueled by the addition of protons, the open to close stroke of the motor induced 32 +/- 3 mN/m compressive surface stress, which corresponds to a single motor force of approximately 11 pN/m, an order of magnitude larger than previous classical hybridization studies. Furthermore, the surface-tethered conformational change was found to be highly reversible, in contrast to classical DNA motors which typically suffer rapid system poisoning. The direction and amplitude of motor-induced cantilever motion was tuneable via control of buffer pH and ionic strength, indicating that electrostatic forces play an important role in stress generation. Hybrid devices which directly harness the multiple accessible conformational states of dynamic oligonucleotides and aptamers, translating biochemical energy into micromechanical work, present a radical new approach to the construction of "smart" nanoscale machinery and mechano-biosensors.     

教师教学水平的时齐马尔可夫链评估方法

采用时齐马尔可夫链方法给出了一种评估教师教学水平的方法,对文［１］所给方法做了适当的改进,拓宽了该方法的应用范围,在多民族院校,使用本法可以克服由于学生基础不同等多方面的原因造成难以比较教师教学水平的难题．