Dynamic percolation in water in oil microemulsions revisited

Microemulsions (dispersions of water droplets, typical radius about 10 nm, in oil) show a particular percolation pattern, a so-called dynamical percolation. Predictions of scaling theory and Monte Carlo simulations were compared with experimental static and frequency dependent conductivity data. The latter gives evidence of two different time scales of charge transport.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Rheological analysis of triblock copolymer-microemulsion networks

The rheological properties of microemulsion-triblock copolymer [poly(oxyethylene)-b-polyisoprene-b-poly(oxyethylene)] networks were studied by oscillatory shear measurements. The principal viscoelastic pattern of our systems stays the same whether the surfactant is ionic or nonionic. A closer examination shows, however, the enormous influence of the respective phase behavior of the underlying microemulsions. Also, specific interactions between the surfactants and the hydrophilic poly(oxyethylene) blocks lead to a significant effect on the formation of network structures.Characteristic rheological data such as plateau moduli and relaxation times for a series of different samples were derived differing in polymer or droplet concentrations, apart from the molecular weights of the poly-(oxyethylene) and the polyisoprene blocks. Finally, we could construct polymer and droplet concentration invariant master — mastercurves using the temperature invariant mastercurves.     

Fourier transform ion cyclotron resonance study of ion-molecule reactions of [M – OCH3]+ ions of methyl 2,3,4,6-tetra-O-methyl-d-hexopyranosides with ammonia

Glycosidic oxocarbenium ions A₁⁺ were formed by isobutane chemical ionization from methyl 2,3,4,6-tetra-O-methyl-β-D -mannopyranoside, methyl 2,3,4,6-tetra-0-methyl-β-D -galactopyranoside and methyl 2,3,4,6-tetra-O-methyl-β-D -glucopyranoside (the ring - O-being converted into ? O ? ), and then- reaction with ammonia was studied by Fourier transform ion cyclotron resonance Spectrometry. Very slow formation (reaction efficiency 0.6-1.4%) of the adduct ion [A₁ + NH₃]⁴ was observed as the main process for carefully thermalized ions A₁⁺. Interestingly, the efficiency of the adduct ion formation depends on the sterochemistry of ions A₁⁺.     

Kinetic energy release and position of transition state during the intramolecular substitution of ionized 2-benzoyl pyridines

The loss of substituents X from molecular ions of ortho substituted 2-benzoyl pyridines has been investigated as a function of the dissociation energy of the C? X bond. Comparison of unimolecular and collisional induced decompositions of the resulting [M ? X]⁺ ions and reference ions arising from 3-hydroxypyrido[1,2-α]indole shows that cyclic fragment ions are formed in every case by an intramolecular substitution reaction with the exception of the parent compound (X = H), which gives rise to a mixture of [M ? H]⁺ ions with different structures. The heat of formation of the cyclic ion has been estimated experimentally and by calculation using thermochemical data, and from this value and the appearance energies, the activation energies of the reverse reactions have been evaluated for the different reaction systems. Measurement of the kinetic energy release during the substitution reactions shows that only part of the reverse activation energy is released as kinetic energy. The energy partitioning quotient varies from 0.37 to 0.08 depending on the dissociation energy of the C? X bond or the reaction enthalpy. A sudden change in the energy partitioning quotient is observed with increasing exothermicity of the reaction, paralleling the behaviour of similar reaction systems. These results are interpreted as a demonstration of the influence of the variation of transition state position on the energy partitioning quotient.     

Zum Mechanismus massenspektrometrischer Fragmentierungsreaktionen.—I Methoxygruppenwanderungen in den Massenspektren der Methyläther von linearen und verzweigten aliphatischen Polyalkoholen

The formation of 1,1-dimethoxy-alkyl rearrangement ions in the mass spectra of methyl ethers of linear diols, 1,2,3-triols and of derivatives of pentaglycerol and pentaerythritol has been investigated by deuterium labelling and mass measurements. Methoxy group migrations do not occur, or at least only to a small amount, in the mass spectra of the diol-dimethyl ethers. The mass spectra of methyl ethers of 1,2,3-triols exhibit characteristic peaks of the rearrangement ions +CH(OCH₃)₂ and +CR(OCH₃)₂. These ions arise by a 1,3-migration of a methoxy group, probably during a one step degradation of the molecular ion to give a molecule methyl alkenyl ether and a H-atom or alkyl radical as neutral fragments. Large peaks of the rearrangement ion +CH(OCH₃)₂ are observed in the mass spectra of compounds of the following type: A radical ion, formed by loss of HY from the molecular ion, rearranges by methoxy group migration in the mass spectra of these substances. Rearrangement by migration of hydroxy and acetoxy groups are also observed, but no migration of a Cl-atom.     

On the Formation of Water/Oil-Microemulsions

The formation of a so-called water/oil (W/O)-microemulsion (AOT/isooctane/water) was followed by light scattering and ultracentrifuge measurements. With increasing weighed-in water concentrations the microemulsion is stabilized by repeated aggregational processes of micelles containing water due to a decrease of the free interfacial enthalpy. This process conforms very satisfactorily to a model describing an adsorption of surfactants at the water/hydrocarbon interface resulting from dipole-image dipole interactions.     

Proton bound homodimers and heterodimers of amides and amines in the gas phase. Equilibrium studies by Fourier transform ion cyclotron resonance spectrometry

By injection of the proton bound homodimer [DMF.H+.DMF] of N,N-dimethylformamide (DMF) generated in an external ion source into a mixture of DMF and a second base within the cell of a Fourier transform ion cyclotron resonance (FT-ICR) spectrometer the equilibria between [DMF.H+.DMF] and the other possible proton bound dimers [DMF.H+.base] and [base.H+.base] have been studied for 13 different bases. Strongly polar bases like aliphatic amides and dimethyl sulfoxide (DMSO) exchange both DMF in [DMF.H+.DMF] by a two step process, while the almost non-polar amines exchange only one DMF. If the base is a primary or secondary amine, the proton bound heterodimer [DMF.H+.amine] reacts further by the addition of one DMF to create a proton bound trimer [(DMF)2.H+.amine]. The affinity deltaG(DMFH+) of the bases towards protonated DMF relative to neutral DMF depends linearly on the difference deltaGB of the gas phase basicity of DMF and the other base, but different correlation lines are obtained for polar and non-polar ligands (deltaGDMFH+ = 0.44GB(base)-375 [kJ/mol] (r = 0.97) and deltaGDMFH+ = 0.46GB(base)-397 [kJ/mol] (r = 0.99), respectively). This different behavior is explained by a different character of the proton bridge in the heterodimers containing only polar ligands and those incorporating a non-polar ligand besides DMF. The former dimers contain a more or less symmetric proton bridge while the latter can be viewed as a protonated base solvated by DMF. The available data have been used to calculate the molecular pair gas phase basicity of DMF and the 13 bases used and to estimate the dissociation energies of the bonds of the proton bridge in various proton bound heterodimers.     

Effects of internal hydrogen bonds between amide groups: protonation of alicyclic diamides

The proton affinity (PA) of cyclopentane carboxamide 1, cyclohexane carboxamide 2 and their secondary and tertiary amide derivatives S1, S2, T1 and T2, was determined by the thermokinetic method and the kinetic method [PA(1) = 888 +/- 5 kJ mol(1); PA(2) = 892 +/- 5 kJ mol(1); PA(S1) = 920 +/- 6 kJ mol(1); PA(S2) = 920 +/- 6 kJ mol(1); PA(T1) = 938 +/- 6 kJ mol(1); PA(T2) = 938 +/- 6 kJ mol(1)]. Special entropy effects are not observed. Additionally, the effects of protonation have been studied using an advanced kinetic method for all isomers 37 of cyclopentane dicarboxamides and cyclohexane dicarboxamides (with the exception of cis-cyclopentane-1,2-dicarboxamide) and their bis-tertiary derivatives T3T7 by estimating the PA and the apparent entropy of protonation Delta(DeltaS(app)). Finally, the study was extended to bicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxamide 8 and its bis-tertiary derivative T8, to all stereoisomers of bicyclo[2.2.1]heptane-2,3-dicarboxamide 9, their secondary and tertiary amide derivatives S9 and T9, and to endoendobicyclo[2.2.1]heptane-2,5-dicarboxamide 10 and the corresponding secondary and tertiary derivatives S10 and T10. Compared with 1 and 2, all alicyclic diamides exhibit a significant increase of the PA (DeltaPA) and special entropy effects on protonation. For alicyclic diamides, which can not accommodate a conformation appropriate for building a proton bridge, the values of DeltaPA and Delta(DeltaS(app)) are small to moderate. This is explained by ion / dipole interactions between the protonated and neutral amide group which stabilize the protonated species but hinder the free rotation of the amide groups. If any of the conformations of the alicyclic diamide allows formation of a proton bridge, DeltaPA and Delta(DeltaS(app)) increase considerably. A spectacular case is cis-cyclohexane-1,4-dicarboxamide 7c which is the most basic monocyclic diamide, although generation of the proton bridge requires the unfavorable boat conformation with both amide substituents at a flagpole position. A pre-orientation of the two amide groups in such a 1,4-position in 10 results in a particularly large PA of < 1000 kJ mol(1). The observation of comparable values for Delta(DeltaS(app)) for linear and monocyclic diamides indicates that a major part of the entropy effects originates from freezing the free rotation of the amide groups by formation of the proton bridge. This is corroborated by observing corresponding effects during the protonation of dicarboxamides containing the rigid bicyclo[2.2.1]heptane carbon skeleton, where the only internal movements of the molecules corresponds to rotation of the amide substituents.     

Zum Mechanismus massenspektrometrischer Fragmentierungsreaktionen—IXSterische Effekte in den Massenspekten der Stereoisomeren von Decalin-2,3-diolen,Decalin-2,4-Diolen und Ihen Dimethyläthern

The mass spectra of all stereoisomers of decalin-2,3-diol, the corresponding dimethyl ethers and of some deuterated derivatives are discussed. The mass spectra of isomeric decalin-2,3-diols differ only slightly in ion intensities. The mass spectra of the stereoisomeric 2,3-dimethoxy-decalins are nearly identical within the series of transand cisderivatives. A mass spectrometric identification of the stereoisomers of these compounds is therefore diffucult. Stereoselective eliminations from the molecular ion are not observed. The mass spectra -of stereoisomeric decalin-1,4-diols show characteristic differences in the intensities of the[M ? H₂O]⁺˙-ions, which can be related to the geometry of the molecules in a similiar mode as was the case with cyclohexane-1,4-diols, The sterechemical control of the elimination of H₂O from the molecular ions has been confirmed by deuterium labelling. The mass spectra of stereoismeric 1,4-dimethoxy-decalins also differ characteristically in the intensities of the [M ? CH₃OH]⁺˙ ions. Furthermore peak due to the [M ? CH₂O]⁺˙ ions are only observed in the mass spectra of those stereoisomers, which have at least one conformation with a short distance between the two methoxy. The stereospecifity of the CH₃OH- and CH₂O-eliminationjs has also been determined by deuterium labelling.