Odd-even effect in the smectic A-cholesteric tricritical pressure of the cholesteryl n-alkanoates. High pressure studies on nine homologues

Abstract

The tricritical pressure, Ptcp, for the smectic A-cholesteric phase transition was determined by optical measurements for nine members of the homologous series of the cholesteryl n-alkanoates. Ptcp shows a clear odd-even effect with the length of the n-alkyl chain. The dependence of Ptcp on the length of the chain is not the same as that of the total smectic A-cholesteric transitional enthalpy, ΔH, at 1 bar. A comparison of both dependences gives hints to a different influence of the chain length on the pretransitional and discontinuous part of ΔH.     

X-ray absorption and X-ray photoelectron spectroscopy of a rhodium colloid

Transition metal colloids are potential precursors of heterogeneous catalysts with application to selective chemical reactions. Sample preparation techniques are described. Experimental details are given of the characterization of these often air-sensitive particles by X-ray photoelectron and X-ray absorption spectroscopy. First results obtained with both techniques for a Rh-colloid show that the metal is mainly present in the zerovalent chemical state. But the spectra indicate further chemical states of Rh which can be assigned to the outermost metal atoms of the colloid interacting with organic ligands or to the educt Rh-halides.     

Defects at surfaces and interfaces — A scattering theoretical approach

A theory for isolated defects at surfaces and interfaces of semiinfinite solids is presented and applied to vacancies at or near surfaces. Vacancy-induced changes in the surface electronic structure of a simple cubic s-band model are discussed in detail and are found to converge very slowly to corresponding bulk vacancy results when the defect is moved into the solid. Results for Ga and As vacancies at GaAs surfaces are discussed in connection with recent experimental data.     

Der Druckeinflu? auf die Bildung von Hetero-Excimeren. 2. Mitt. Verschiedene Donor-Akzentor-Systeme in n-Hexan

Ohne Zusammenfassung     

An improved HPTLC method for the rapid identification and quantification of sulfonamides

Summary R_f values of twenty therapeutic sulfonamides have been estimated in seven solvent systems. Based on these data a simple identification scheme was developed demanding only three solvent systems for complete separation.Quantification of corresponding fluorescamine derivatives at nanogram level was performed by the use of an TLC-scanner configured online with a microcomputer. Mathematical treatment of standard curves using the computer program Curve Fitter are discussed. Validity and reproducibility of results are compared with data obtained by HPLC and show good correlation.