Preparation and characterization of well-defined sterically stabilized latex particles with narrow size distribution

 The synthesis and comprehensive characterization of a purely sterically stabilized latex with narrow size distribution is reported. By use of non-ionic initiators no chemically bound surface charges are generated. Stabilization of the particles is achieved through use of a non-ionic surfactant having a double bond in the hydrophobic part which is chemically bound to the surface. Analysis of the latex particles thus generated by transmission electron microscopy, disc centrifugation, and small-angle X-ray scattering (SAXS) reveals that the size distribution is narrow (standard deviation between 6 and 10%). SAXS furthermore demonstrates that the surfactant is located in a thin layer on the surface. The interaction of the particles is purely repulsive as shown by the analysis of the turbidity of the latex. The systems obtained herein may serve as model systems of water-borne purely sterically stabilized colloid particles. Received: 23 December 1997 Accepted: 18 May 1998     

Sound propagation in metallic glasses—a new look at the tunneling model

A variety of amorphous metals exhibit a characteristic behavior in their acoustic properties at low frequencies which differs from the predictions of the standard tunneling model. We point out that a lower cut-off _min for the tunnel matrix element, which is needed for consistency of the tunneling model, leads to an upper bound on relaxation times induced by the conduction electrons. We derive explicit expressions for the velocity shift and internal friction for the normalconducting and superconducting case. It is shown that a maximum relaxation time plays an essential role at audio frequencies. The corresponding change of the acoustic properties is in qualitative agreement with vibrating reed experiments.     

Orientation of nonionic surfactants on solid surfaces: n-alkyl polyglycol ethers on montmorillonite

Summary Complexes of nonionic surfactantsR-(OCH₂CH₂)x OH with montmorillonite have been studied (R =n-hexadecyl,n-octadecyl and oleyl ;x=2, 10 and 20).On internal surfaces the surfactant molecules are arranged in bilayers. Withx=2 the alkyl chain and about one half of the polar group, (-R OCH₂CH₂O-) stand perpendicularly whereas the terminal —CH₂CH₂OH group is attached to the silicate surface (₁-phase). The bilayer thickness decreases stepwise with rising temperature due to the formation of kinks (i-phases). At higher temperature (52 °C withR=C₁₈H₃₇-, 43°C withR=n-C₁₆H₃₃-, and 12 °C withR = oleyl-) surfactant molecules are squeezed out of the interlayer space reversibly, the packing density decreases, the whole polar head group gets attached to the silicate surface and the alkyl chains rearrange into a gauche-block structure. This structure undergoes further structural changes at still higher temperatures ( _i -phases).The complexes withx =10 and 20 form -phases even at room temperature. These -phases take up long chain alkanol molecules under formation of -structures which rearrange at higher temperatures into -phases. Long-chain impurities of the surfactants can be intercalated in a similar way.Previous data indicating mono- or bilayers of flatly lying surfactant molecules refer to metastable phases due to steric hindrances of lattice expansion.It is proposed that the surfactant molecules build up similar films on the external surfaces, which can adopt - or -structures depending upon number of ethylene oxide groups and temperature. The films of hexadecyl polyglycol ethers for instance are about 27 Å thick in the -phases and about 17 Å in the a-phases.

---

Zusammenfassung Es wurden Montmorillonitkomplexe mit nichtionogenen TensidenR(-OCH₂CH₂) _x ,OH (R =n-hexadecyl-,n-octadecyl- und oleyl-;x = 2, 10 and 20) hergestellt. Die Tensidmoleküle bilden zwischen den Silicatschichten bimolekulare Filme.Mitx=2 sind die hydrophoben Reste und die anschließende -OCH₂CH₂-Äthergruppe bei niedriger Temperatur gestreckt und senkrecht zu den Silicatschichten orientiert ( _i -Phase); nur die endständige HOCH₂CH₂-Gruppe sitzt direkt auf der Silicatschicht auf. Beim Erwärmen erniedrigt sich die Dicke der bimolekularen Tensidschicht stufenweise durch den Einbau von Kinken ( _i -Phasen). Bei höheren Temperaturen (52°C mitR =C₁₈H₃₇-, 43 °C mitR =C₁₆H_33- und 12 °C mitR = oleyl) werden Tensidmoleküle reversibel aus den Schichtzwischenräumen verdrängt, die Packungsdichte erniedrigt sich, die gesamte polare Gruppe kommt in direkten Kontakt mit der Silicatschicht und die Alkylketten ordnen sich in eine Gaucheblockstruktur ( _i -Phase). Diese kann bei noch höheren Temperaturen weitere Phasenumwandlungen erleiden.Mitx =10 undx = 20 werden auch bei Zimmertemperatur nur -Phasen gebildet. Diese -Phasen können zusätzlich langkettige Alkanolmoleküle aufnehmen und ternäre Komplexe mit -Struktur bilden, die sich beim Erhitzen reversibel in -Formen umwandeln. Langkettige polare Verunreinigungen in den Tensiden wirken ähnlich wie die Alkanolmoleküle.An den äußeren Oberflächen werden die Tensidmoleküle gleichartige Filme mit - oder -Struktur bilden, je nach der Zahl der -CH₂CH₂O-Gruppen und der Temperatur. Ein Film aus Hexadecylpolyglykoläthern wird etwa 27 Å dick sein in der -Phase und etwa 17 Å in der -Form.

     

Screening an In-House Isoquinoline Alkaloids Library for New Blockers of Voltage-Gated Na+ Channels Using Voltage Sensor Fluorescent Probes: Hits and Biases

Voltage-gated Na⁺ (Na_V) channels are significant therapeutic targets for the treatment of cardiac and neurological disorders, thus promoting the search for novel Na_V channel ligands. With the objective of discovering new blockers of Na_V channel ligands, we screened an In-House vegetal alkaloid library using fluorescence cell-based assays. We screened 62 isoquinoline alkaloids (IA) for their ability to decrease the FRET signal of voltage sensor probes (VSP), which were induced by the activation of Na_V channels with batrachotoxin (BTX) in GH3b6 cells. This led to the selection of five IA: liriodenine, oxostephanine, thalmiculine, protopine, and bebeerine, inhibiting the BTX-induced VSP signal with micromolar IC₅₀. These five alkaloids were then assayed using the Na⁺ fluorescent probe ANG-2 and the patch-clamp technique. Only oxostephanine and liriodenine were able to inhibit the BTX-induced ANG-2 signal in HEK293-hNa_V1.3 cells. Indeed, liriodenine and oxostephanine decreased the effects of BTX on Na⁺ currents elicited by the hNa_V1.3 channel, suggesting that conformation change induced by BTX binding could induce a bias in fluorescent assays. However, among the five IA selected in the VSP assay, only bebeerine exhibited strong inhibitory effects against Na⁺ currents elicited by the hNav1.2 and hNav1.6 channels, with IC₅₀ values below 10 µM. So far, bebeerine is the first BBIQ to have been reported to block Na_V channels, with promising therapeutical applications.     

Bifurcations in a model of magnetoconvection

Bifurcations from oscillatory solutions are studied in a truncated model of two-dimensional Boussinesq magnetoconvection. The fifth order system of nonlinear differential equations is integrated numerically and in certain parameter regimes there is a bifurcation from symmetrical to asymmetrical oscillations followed by a period-doubling cascade. After the accumulation point there is a semiperiodic cascade leading to chaotic behaviour. Then the semiperiodic cascade is repeated in reverse, followed by a period-halving cascade and a bifurcation back to symmetry. Finally, the branch of oscillatory solutions terminates with a symmetrical heteroclinic orbit that connects two saddle-foci. The interval with aperiodic solutions contains many pairs of narrow windows with asymmetrical or symmetrical periodic solutions, each with its own cascade. This pattern of behaviour is likely to occur when a periodic orbit approaches a symmetrical pair of saddle-foci with eigenvalues that satisfy Shil'nikov's inequality.     

The transmission properties of optical waveguides in SIMOX structures

The numerical aperture of step index planar waveguides in SIMOX structures is 3.16. Analysis suggests that some of the higher-order modes that are supported by the waveguide will be reflected at the output face, due to total internal reflection. This situation therefore suggests that the propagation losses of such waveguide structures may be lower than those measured by conventional methods.     

Book reviews

MODERN CRYSTALLOGRAPHY I, by B. K. Vainshtein, Springer Series in Solid State Sciences 15. Springer-Verlag, New York, 1981. 399 pp., USf47.50.

SPECTROSCOPY IN CHEMISTRY AND PHYSICS: Modern Trends, Edited by F. J. Comes, A. Muller and W. J. Orville-Thomas (Elsevier Scientific Publishing Company). Price US $87.75, Dfl 180.00.

ELECTRONIC STATES OF MOLECULES AND ATOM CLUSTERS-Foundations and Prospects of Semiempirical Methods, by G. Del Re, G. Berthier and J. Serre. Springer Lecture Notes in Chemistry, Vol. 13. Price: US817.50 (Springer-Verlag, Berlin, 1980).

MATERIALS FOR THE FUTURE, Edited by P. M. Kelly, Australian Academy of Science, 1981.     

Lead Fractionation Analysis in Lipstick Samples by Inductively Coupled Plasma-Mass Spectrometry

A new analytical approach based on fractionation was introduced for lead in lipstick samples. Different separation techniques including n-hexane, glycerol extraction, and activated carbon adsorption were used to characterize the lipid fraction, polar and aromatic components of the samples. Additionally, artificial saliva and food stimulant extractions were used for the risk assessment studies. Trace metal levels in fractions were determined by inductively coupled plasma-mass spectrometry. Method validation parameters in the total element determinations were defined in terms of detection limits, accuracy, and precision. The limits of detection and quantification were 0.02 and 0.07 mg kg^?1 for Pb; whereas the repeatability and reproducibility of the results based on percent relative standard deviation were 3.0% and 7.2% for lead, respectively.