Dewetting of liquids on ceramic surfaces at high temperatures.

The influence of surface structure and chemistry on high-temperature dewetting of silicate liquids on ceramic surfaces has been investigated. Model systems based on well-defined crystallography and known chemistry have been used to illustrate the effect of surface roughness and chemistry on the dewetting process. Reconstructed ceramic surfaces provide ideal substrates to study effects of surface roughness. It has been shown that the morphology of dewet droplets depend on the length scale and the crystallography of the facets on the surface. Complex pattern formation due to solute redistribution during dewetting is illustrated in the case of SiO2 dewetting on (001) rutile substrates. The role of kinetics on the dewetting process has also been clarified.     

Ionic radii for Group 1 halide crystals and ion-pairs

Internuclear separations calculated from empirical soft-sphere radii for seventeen crystalline Group 1 halides with rock-salt structures and from empirical hovering-sphere radii for fourteen gaseous Group 1 halide ion-pairs agree well with experimental measurements. Two lithium and four fluoride ion-pairs appear to be anomalous. Soft-sphere ionic radii are compared with atomic radii of noble gases.     

Hexacyanoferrate(III) as a mediator in the determination of total iron in potable waters as iron(II)-1,10-phenanthroline at a single-use screen-printed carbon sensor device

Hexacyanoferrate(III) was used as a mediator in the determination of total iron, as iron(II)-1,10-phenanthroline, at a screen-printed carbon sensor device. Pre-reduction of iron(III) at −0.2 V versus Ag/AgCl (1 M KCl) in the presence of hexacyanoferrate(II) and 1,10-phenanthroline (pH 3.5-4.5), to iron(II)-1,10-phenanthroline, was complete at the unmodified carbon electrode surface. Total iron was then determined voltammetrically by oxidation of the iron(II)-1,10-phenanthroline at +0.82 V, with a detection limit of 10 μg l⁻¹.In potable waters, iron is present in hydrolysed form, and it was found necessary to change the pH to 2.5-2.7 in order to reduce the iron(III) within 30 s. A voltammetric response was not found at lower pH values owing to the non-formation of the iron(II)-1,10-phenanthroline complex below pH 2.5.Attempts to incorporate all the relevant reagents (1,10-phenanthroline, potassium hexacyanoferrate(III), potassium hydrogen sulphate, sodium acetate, and potassium chloride) into a modifying coated PVA film were partially successful. The coated electrode behaved very satisfactorily with freshly-prepared iron(II) and iron(III) solutions but with hydrolysed iron, the iron(III) signal was only 85% that of iron(II).     

Some jacobi matrices with decaying potential and dense point spectrum

We discuss doubly infinite matrices of the formM _ij= _i,j+1+ _i,j–1+V _{i ij} as operators on ². We present for each >0, examples of potentialsV _n with |V _n|=O(n ^–1/2+) and whereM has only point spectrum. Our discussion should be viewed as a remark on the recent work of Delyon, Kunz, and Souillard.Research partially supported by USNSF under grant MCS 81-20833     

Gas chromatographic determination of organolead compounds with an alternating current plasma detector

The performance of the gas chromatography/alternating current plasma detector as a selective detector for organolead compounds is investigated. The helium make-up flow rate and the spatial position from which the lead emission is viewed, have an effect on the detector response. The detection limit for tetrabutyl lead was established as 130 pg/s and the lead selectivity ratio was found to exceed 13,800. Some applications of organolead determination in complex matrices were also studied in order to demonstrate the selectivity and sensitivity of the alternating current plasma detector.     

Transference numbers for ultra concentrated solutions of aqueous hydrochloric acid at 25°C

Transference numbers of HCl(aq) solutions at 25°C, from 0.01 to 13.6 mol-kg^–1(m) have been obtained by measuring the emf of cells with transference using hydrogen gas/platinum electrodes. Good agreement is obtained at concentrations up to 1 m with all previous data, and our results strongly corroborate those of King and Spiro over the 2–8m concentration range. The transference numbers of the hydronium ion fit the empirical equation, _H ^HCl = 0.821 + 0.0457m ^1/2 – 2.476×10^–2m – 1.90×10^–4 m ² – 1.45×10^–5 m ³ the maximum deviation in T _H ^HCl being 0.003.     

Effect of low amounts of cholesterol on the swelling behavior of floating bilayers

The effect of the addition of 1, 2, 4, and 6 mol % cholesterol to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) floating bilayers has been investigated by neutron reflectivity. All samples exhibited fully stable and reversible gel and fluid phases. Around the main lipid phase transition temperature, DPPC double bilayers exhibit large increases in the water layer separating the bilayers and the upper bilayer roughness. The inclusion of low amounts of cholesterol reduced the swelling of the water layer between the bilayers and the upper bilayer roughness and progressively widened the temperature range over which swelling occurs. Results from asymmetric bilayers are also reported. A higher amount of cholesterol in the lower bilayer induces a smaller swelling of the water layer between the bilayers than in the symmetric case. Finally, the effect of the inclusion of a leaflet of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) was investigated. The presence of a leaflet with a higher gel-transition temperature (T(m)) modifies the phase behavior of the lower T(m) leaflet.     

Flow-injection technique for the determination of low levels of phosphorus in natural waters

An inexpensive flow-injection instrument for determining low concentrations of dissolved reactive phosphorus in natural waters is reported. The unique feature is the use of an inexpensive detector consisting of a flow cell and a simple photometer that incorporates a super-bright light-emitting diode as the source and a photodiode as the detector. The tin(II) chloride-molybdate method was optimized using a modified simplex optimization method. Silicate interference up to 5 mg Si l^?1 was removed by addition of 0.10% (w/v) tartaric acid. Using the tartaric acid-modified optimized reagents, a detection limit of 0.6 μg P l^?1 was achieved. The method was linear over the range 0–100 μg P l^?1 with an excellent precision (r.s.d. 2.9% at 2.0 and 0.5% at 50 μg P l^?1). An in-line pre-concentration anion-exchange column was used to obtain an even lower detection limit of 0.1 μg P l^?1 and applied to the analysis of real samples.     

Anomalous NMR behavior of meso compounds with remote stereogenic centers on addition of chiral shift reagent or chiral solvating agent

A problem has arisen in using chiral shift reagents (CSR) and chiral solvating agents (CSA) to determine meso and racemic forms of diastereoisomers in which the stereogenic centers of the molecules are separated by achiral spacers. It is found that NMR signals of both meso and racemic forms of diastereoisomers may exhibit doubling on addition of CSR/CSA, which means that unequivocal assignments cannot be made without characterizing the effects for separate meso and racemic forms; this is particularly important for additions of CSR/CSA at relatively low concentrations, which always result in the splitting of some NMR signals of diastereoisomers. The phenomenon is demonstrated in the (31)P NMR spectra of meso and racemic forms of three spermine-bridged gem-disubstituted cyclotriphosphazatrienes, 1a-c, and compared with analogous achiral molecules, the per-substituted spermine-bridged cyclotriphosphazatrienes 2a-d. As expected, only one set of (31)P NMR signals was observed for the achiral compounds 2a-d, even on addition of CSA. Two sets of (31)P NMR ABX multiplets corresponding to meso and racemic diastereoisomers were observed for compounds 1a-c; on addition of CSA, the signals of at least one of the multiplets for each compound separated into more than the expected groups of three lines with an intensity distribution of 2:1:1. To understand this phenomenon, the meso and racemic forms of 1a and 1b and the meso form of 1c have been separated and characterized by X-ray crystallography. On addition of CSA to the racemic forms of 1a and 1b, the (31)P NMR spectrum shows the expected doubling of signals, but, unexpectedly, the same is observed for each of the meso forms of 1a-c. Analogous results using both CSA and CSR have been obtained for the meso and racemic forms of the diastereoisomeric piperazine-bridged macrocyclic-phosphazene compound, 3, whereas no effect was observed for the two meso forms of the doubly bridged macrocyclic-phosphazene compound 4. The phenomenon of doubling of the (31)P NMR signals of the meso form of singly bridged cyclotriphosphazatrienes, 1a-c and 3, is explained by consideration of the equilibrium in solution of independent complexation of a chiral ligand with molecules that have two chiral cyclophosphazene moieties separated by an achiral spacer group. The results show that the stereogenicity of such diastereoisomeric molecules in solution cannot be characterized unequivocally by NMR measurements on addition of either CSR or CSA.     

Absence of continuous symmetry breaking in a one-dimensional n−2 model

For a one-dimensional array ofS ^N–1 spins (N 2) with isotropic pair interactions (and more general systems) with J(j–i) obeying sup_n[n^–1 ₁ ⁿ j ²|J(j)|]<, we prove that every equilibrium state is invariant under the natural action ofSO(N). In particular, there is no long-range order of the conventional type. Included is the caseJ(n)=n ^–2.Research partially supported by U.S.N.S.F. Grant No. MCS-78-01885.S. Fairchild Scholar at Caltech. On leave from Departments of Mathematics and Physics, Princeton University, Princeton, New Jersey 08544.