Jbfn Engberts

LIQUID-CRYSTALLINE AND THERMOCHROMIC BEHAVIOR OF 4-SUBSTITUTED 1-METHYLPYRIDINIUM IODIDE SURFACTANTS

Abstract The mesogenic behaviour of a series of thirty-one 1-alkyl-4-(or 2-)alkylpyridinium salts and of a homologous series of four 1-methyl-4-n-alkoxycarbonylpyridinium iodides is described. The occurrence and stability range of the thermotropic phases depend dramatically on the structure of the surfactants. Mesophases are only observed if the 1-alkyl group is methyl. Furthermore, an unusually strong dependence of the mesogenic behaviour on the number of carbon atoms in an unbranched alkyl chain is found. Alkyl chain branching, especially close to the headgroup, in 1-methy1-4-(C12-alky1)Pyridinium iodides lowers the clearing point and, to a lesser extent, the melting point. Apparently, the mesophase is destabilized due to the lack of space to accommodate a protruding side chain. The mesophases of the unbranched as well as the branched compounds are identical to the smectic A phase found for carbohydrate mesogens with one alkyl chain (smectic Ad). The mesophase of the peg-shaped compound 1-methyl-4-(17-t...

Exchangeability of phospholipids between anionic, zwitterionic and cationic membranes

Abstract The spontaneous transport of phospholipid molecules between artificial vesicles has been monitored by free-flow electrophoresis. First, the transfer of traces of the negatively charged [ 3 H] dimyristoylphosphatidylglycerol (DMPG) between anionic and zwitterionic or cationic and zwitterionic vesicles is followed. Anionic vesicles were generated from dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylglycerol (molar ratio 9/1), neutral ones from DMPC and cationic ones from DMPC and 4-(17-tritriacontyl)- N -methylpyridiniumchloride mixed in a molar ratio of 9/1. A halftime of 50 min was calculated for the transfer of DMPG from the anionic donors to the neutral acceptors. In the opposite direction transfer proceeded much slower ( t 1/2 = 900 min). On the other hand, DMPG moved with a t 1/2 of 520 min from the neutral towards the cationic vesicle population, whereas no transfer at all occurred in the reverse direction. In contrast, with the zwitterionic [ 3 H] DMPC, similar t 1/2 values (≈100 min) were found in the above-mentioned donor—acceptor systems. These results demonstrate that membrane charges can strongly modulate the transfer capacity of anionic phospholipids.

Thermotropic and lyotropic liquid crystalline behaviour of 4-alkoxyphenyl beta-D-glucopyranosides

The liquid crystalline properties of a series of 4-alkoxyphenyl beta-D-glucopyranosides (methoxy to decyloxy and dodecyloxy) were studied using polarized light microscopy and differential scanning calorimetry. The compounds with the shortest alkoxy substituents are not liquid crystalline. The butoxy derivative displays a monotropic smectic A phase and the higher homologues display enantiotropic smectic A phases. The lyotropic behaviour was studied as a function of concentration and temperature. Hexagonal, cubic and lamellar phases were observed for compounds with alkoxy chains longer than butoxy. The nonyloxy derivative forms long ribbons in dilute solution as revealed by electron microscopy.

A MOLECULAR-DYNAMICS COMPUTER-SIMULATION STUDY OF THE TEMPERATURE-DEPENDENCE OF HYDRATION OF 1,4-DIOXANE AND 1,3-DIOXANE

Abstract This paper describes a study of the hydration of 1,3-dioxane and 1,4-dioxane at two different temperatures using different molecular dynamics (MD) computer simulation techniques. Three major conclusions have been drawn. Firstly, the simulations of 1,4-dioxane—water and 1,3-dioxane—water at constant pressure lead essentially to the same conclusions as earlir MD studies at constant volume. Secondly, the numerical values of dynamic properties depend critically on the density of the system. Simulations at constant pressure provide densities which are dependent on the periodicity requirement imposed on the system by the periodic boundary conditions. The smaller the periodic box, the stronger this effect is. Thirdly, in 1,4-dioxane—water an increase in temperature results in an enhanced mobility of water molecules in the solvation shell, whereas in the case of 1,3-dioxane—water these water molecules become more strongly bound by the solute. This effect is entirely due to a reduction of the mobility of water molecules in the 1,3-dioxane oxygen hydration subshells. The contrasting behavior is explained in terms of a situation where solvent—solvent interactions dominate solute—solvent interactions in 1,4-dioxane—water at both temperatures and in 1,3-dioxane—water at the lower temperature, while the opposite situation holds for 1,3-dioxane—water at the higher temperature.

MOLECULAR-DYNAMICS COMPUTER-SIMULATION OF THE HYDRATION OF 2 SIMPLE ORGANIC SOLUTES - COMPARISON WITH THE SIMULATION OF AN EMPTY CAVITY

The hydration of two simple organic solutes has been studied using the molecular dynamics (MD) computer simulation method. Results of the simulations of a single 1,4-dioxane or 1,3-dioxane molecule dissolved in 122 water molecules are compared with those of a MD simulation of an empty cavity of corresponding size in 216 water molecules. This yields the opportunity to trace the specific effects of the polar and dispersion solute-solvent interactions on the properties of the water molecules in the hydration shell of the solute. The hydration shell properties of 1,4-dioxane (μ) = 0·14 D) are very similar to those of the corresponding cavity, whereas those of 1,3-dioxane (μ) = 1·91 D) show significant deviations. Earlier conclusions that water structure-making and water structure-breaking properties of 1,4-dioxane are about equally balanced, while 1,3-dioxane is definitely structure-breaking, are confirmed. Moreover, it is shown that a slower self-diffusion and reorientation of water molecules upon addition o...

A molecular dynamics computer simulation study of the hydration of bis(methylsulphonyl)methane in water

The molecular dynamics computer simulation technique has been applied to study the hydration of bis(methylsulphonyl)methane (1) in water. This 1,3-disulphone has water-structure breaking properties as is deduced from both simulated time-averaged and time-dependent properties. The time-averaged properties of water molecules in the various atomic hydration shells can be directly related to the solute atom under consideration. Time-dependent properties show a mutual influencing of the hydration shells of neighbouring atoms. Moderate sulphonyl oxygen-water hydrogen bonding competes with water-water hydrogen bonding in the same hydration shell, while methylene hydrogen-water hydrogen bonding is stronger than water-water hydrogen bonding. These results are in accord with previous interpretations of 1H-N.M.R. chemical shift data for the central methylene moiety of (1) in mixtures of water with 1,4-dioxane, 1,3-dioxane, and 1,2-dimethoxyethane.

Partitioning of organic solutes between the micellar and the aqueous phase as studied with an artificial kidney

Abstract The partitioning of p -nitrophenyl acetae and of p -methylphenylsulfonylmethyl methanesulfonate into CTAB micelles in aqueous solution has been quantitatively investigated by means of a Cordis Dow C-Dak 1.30 artificial kidney.