Rheological investigations of viscoelastic surfactant solutions, both theoretical and experimental, have primarily focused on anionic or cationic surfactant molecules and charged counterion pairs. In this article, we present an experimental investigation on the linear flow properties (stress relaxation) of uncharged aqueous dimer acid betaine solutions as a function of surfactant and salt concentration. In the zwitterionic dimer acid betaine solutions, the electrostatic interactions among the surfactant molecules are primarily not screened by counterions. As a consequence, the rheological properties are different from charged monomeric surfactant solutions. The analysis of the rheological properties of dimer acid betaine reveals that this surfactant system behaves differently from anionic or cationic compounds. Dimer acid betaine solutions with and without salt do not show monoexponential relaxation phenomena that are typical for kinetically controlled relaxation mechanisms. Introducing NaCl also leads to gel forming and an even higher stability of the aggregates. As a conclusion, the rheological properties such as the zero shear viscosity, relaxation times, or relaxation moduli are mainly controlled by diffusion (reptation) processes. A model for the linear stress decay developed for ionic surfactants was used to characterize basic structural parameters. The aggregation of the surfactant and the structure of the supramolecular aggregates are, however, not completely decoded by the used model showing the complex nature of zwitterionic dimeric surfactant systems.
Dedicated to Prof. AIberf Eschenmoser on the occasion of his 60th birthday (28.5.85) The yield of the main product from the reaction of heptamethyl dicyanocobyrinate with 0, in the presence of ascorbic acid has been optimised by systematic variation of the reaction conditions, and the structure of the obtained 'stable yellow corrinoid' has been established by X-ray analysis. From the relationship between the structures of analogous xanthocorrinoids obtained likewise from a series of dicyanocobyrinic-acid derivatives, which has been prepared by systematic modifications of the c-acetic-acid chain, and the functionality of the substituent at C(7). a reaction mechanism is suggested.Stable yellow corrinoids, also termed 'xanthocorrinoids' [2], are formed as by-products of most reactions of cyanocobalamin (vitamin B,,) and its derivatives, which proceed with change of the oxidation state of the complex-bonded Co ion [3b]. They are stable to air and to light, and show, as a common feature, absorption bands at about 460480 nm, a spectral range which is also characteristic for some yellow compounds formed on degradation of vitamin B,, by bacteria (e.g. Aerohucter uerogenes [4] and Pseudomonus rubescens . Moreover, metal-free yellow corrinoids have been obtained from the culture fiitrates of phototrophic bacteria as Rhodopsezidomonus sphueroides or Rh. cupsulufa, when grown in a medium deficient in Co [lo]. However, as the structures of the above mentioned xanthocorrinoids are, with one exception, unknown, it is questionable whether all of them belong to the same class of compounds.Among the different reactions, which lead to the formation of stable yellow corrinoids from vitamin-B,, derivatives (cf. Fig. I ) , we investigated first the formation of such pigments under the conditions of Udenfriend's reaction using heptamethyl Cox, Copdicyanocobyrinate (1) as a substrate. Now, after a series of experiments directed towards the elucidation of the mechanism of the formation of xanthocorrinoids using different dicyanocobyrinic-acid derivatives as substrates has been completed [ 1 11, the experimental ') Part I [I]. 2, ') 4,
Most reactions of cyanocobalamin (vitamin Biz) which proceed with alteration of the oxidation state of the complexbonded cobalt ion are accompanied by the formation of by-products which are called stable yellow corrinoids['! Despite numerous attempts, it has not previously proved possible to elucidate the structure of these compounds. Owing to the biological (metabolism of the vitamin B1 2-coenzyme in the living cell) and pharmacological interest (incompatibility of cyanocobalamin and vitamin C in mixtures of vitamins), we have investigated the formation of such pigments by treating dicyanocobyrinic heptamethyl ester ( I )[' I with ascorbic acid. As the former is readily soluble in aprotic solvents, its use as substrate is more advantageous from the preparative and analytical point of view.Thus, reaction of a solution of ( 1 ) in methanol-water with ascorbic acid in the presence of oxygen yields a yellow cor-
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.