Capsule-like assemblies in polar solvents

Calix[4]arene derivatives with four anionic groups at their upper rim form discrete 1:1 complexes with complementary calix[4]arene derivatives bearing four cationic groups at their upper rim. Each cation is bound by two anions, and vice versa, in a mutual chelate arrangement, reinforced by a network of ionic hydrogen bonds. These multiple electrostatic interactions lead to the formation of highly stable capsule-like assemblies even in polar protic solvents such as methanol and water. In the capsule interior a cavity is formed that is in principle large enough for the encapsulation of small aliphatic and aromatic guests (170-230 A(3)). Monte Carlo simulations in water reproducibly lead to the same regular opimized structures. These differ mainly by their inner volume and flexibility, as demonstrated by molecular dynamics calculations. Most half-spheres can be synthesized by way of the tetrakis(chloromethyl) or the tetrabromocalix[4]arene intermediate. Oppositely charged calix[6]arenes also form strong complexes, but no indication was found for a lock in the cone conformation. The formation of the ball-shaped complexes from calix[4]arene building blocks was studied with Job plots, NMR titrations, NOESY, and variable-temperature experiments, as well as ESI-MS measurements. Investigations aimed at the inclusion of various guest molecules were carried out with alcohols, sulfoxides, benzene derivatives, and ammonium, as well as pyrazinium guests. Although binding isotherms were generated with cationic guests, these must be considered to be loosely associated around the seam rather than included inside the capsule.     

Dispersion polymerization in polar solvents

The effect of the nitrogen purge, monomer purification, type of agitation, and presence of costabilizer on the particle size distribution (PSD) was investigated in the dispersion po-lymerization of styrene in ethanol and in the dispersion copolymerization of styrene and butyl acrylate in a water–ethanol mixture. Purging with nitrogen and, to a lesser extent, monomer purification, were of paramount importance to achieve monodispersity. The type of agitation had a week effect on the PSD, whereas the presence of costabilizer had no effect on the PSD. © 1995 John Wiley & Sons, Inc.     

Confinement of polar solvents within beta-cyclodextrins

Using molecular dynamics techniques, we examined equilibrium and dynamical characteristics pertaining to the solvation of a single beta-cyclodextrin (CD) in water and in dimethylsulfoxide (DMSO). Compared to its global minimum structure, the overall shape of the solute in solution is reasonably well preserved. While in aqueous solutions, the average number of solvent molecules retained within the central cavity of the oligosaccharide is close to 5, for DMSO, that number reduces to approximately 1. No evidence of significant orientational correlations of the trapped molecules were found in either solvent. The main contributions to the hydrogen-bond (HB) connectivity between the solute and the bulk phases are due to the more distal HO6-O6 hydroxyl groups, acting as HB donors and acceptors. The average residence time for retained DMSO was found to be in the nanosecond range, and it is, at least, 1 order of magnitude longer that the one observed for water. We also analyzed the characteristics of the solvation of the beta-CD in an equimolar water-DMSO mixture. In this environment, we found a preferential localization of a single DMSO molecule in the interior of the CD and a very minor retention of water. In the mixture, the characteristic time of residence of the trapped DMSO molecule increases by a factor of approximately 2. The observed difference was rationalized in terms of the fluctuations of the local concentrations of the two species in the vicinity of the CD top and bottom rims.     

Optical and fibre-optic sensors for vapours of polar solvents

An optical sensor is described which continuously measures the concentration of vapours of polar organic solvents such as alcohols, ethers, esters and ketones, but does not respond to hydrocarbons and chlorinated hydrocarbons. The detection is based on reversible decolorization of the blue thermal printer paper used in graphic plotters. Two sensing principles are exploited. In the first, a sensor sheet is placed in a flow-through cell. An LED acts as a source of yellow light and a phototransistor measures the light transmitted. In the second, the sensing membrane is placed at the end of a bifurcated fibre-optic and its diffuse reflectance is measured. The sensitivity of the devices towards various kinds of vapours has been studied and the detection limits vary from 10 to 1000 ppm for some of the most common technical solvents.     

New protic salts of aprotic polar solvents

Aprotic polar solvents such as DMF, acetonitrile or DMSO can be protonated to form stable triflate salts when treated with triflic acid. The reaction of the same solvents with N,N-bis(trifluoromethanesulfonyl)imide led to the isolation of the corresponding N,N-bis(trifluoromethanesulfonyl)imide salts.     

Self-assembly of molecular capsules in polar solvents

[structure: see text] We present a novel type of molecular capsule formed by self-organization of calix[4]arenes with several oppositely charged functional groups located at their upper rims. In highly polar solvents, the complementary half-spheres form stable 1:1 complexes with association constants of up to 7 x 10(5) M(-)(1) in methanol. The cavity inside the capsules is large enough for the inclusion of small aliphatic or (hetero)aromatic guest molecules.     

Hydrogen-bonded capsules in polar,protic solvents

A kinetically stable, dimeric capsule is formed by tetrahydroxyresorcinarene in methanol; it encapsulates tropylium and tetramethylammonium cations.     

Energetics of electron transfer reactions in polar solvents

The free energy change of an electron transfer reaction in a polar solvent is rigorously analyzed within the framework of the dielectric continuum model. An appropriate expression for the electrostatic energy between the two product ions separated by R is derived. The present result does not support a recent claim by Suppan that, if R is close to the contact distance, the electrostatic energy should be much larger in magnitude than estimated from the usual expression −e²/ε_sR.     

Complexes of acid amides with polar aprotic solvents

Solid complexes of bis(N-phenyl)-pyromellitic acid amide with equimolar binary mixtures of dimethylformamide, dimethylacetamide, dimethylsulfoxide and N-methylpyrrolidone were prepared and studied by evolved gas analysis using mass spectrometry. The activity sequence of these compounds in forming complexes with bis(N-phenyl)-pyromellitic acid amide was established.     

Complexes of acid amides with polar aprotic solvents

A study was made of the effects of organic carbonyl-containing compounds and inorganic acids on the character of complex formation of the solvent with polyacid amides, and in particular on the frequency of three-dimensional networks formed by hydrogen-bonds in solid polyacid amide films. It was found that the characteristics of the solid-phase cyclodehydration process can be varied within a wide range by means of active additives.

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Zusammenfassung Die Wirkung von organichen carbonyl-enthaltenden Verbindungen und anorganischen Säuren auf den Charakter der Komplexbildung von Lösungsmitteln mit Polysäureamiden wurde untersucht, inbesondere was die Häufigkeit von dreidimensionalen, durch Wasserstoffbindungen in den festen Polysäureamidfilmen zustande kommenden Vernetzungen betrifft. Es wurde festgestellt, daß die Kennwerte des Cyclodehydratisierungsprozesses in fester Phase innerhalb weiter Grenzen durch aktive Additive variiert werden können.

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Self-organization of spheroidal molecular assemblies in polar solvents

[reaction: see text] We report a number of 1:1 noncovalent complexes composed of a symmetrical trisphosphonate and various symmetrical trisammonium or trisamidinium compounds. The spheroidal complexes show high thermodynamic stability, with association constants Ka reaching 10(6) M(-1) in methanol and in some cases even exceeding 10(3) M(-1) in water. The observed Ka values correlate well with the different degree of preorganization of the complexation partners.     

Theory of migration-controlled electron-transfer reactions in polar solvents

A general solution of the problem of finding the rate constant of electron-transfer reactions in polar solvents without the restrictions of the diffusional approximation has been obtained. Expressions for the reaction rate constant at the limit of the random-jump mechanism, as well as a convenient equation describing the transition between the random-jump and diffusional reaction mechanisms, have been found. A test for identifying a random-jump electron-transfer mechanism has been proposed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 1, pp. 27–33, January–February, 1985.     

A method for the generation of solvated electrons in polar solvents

A simple method for generating solvated electrons in a polar liquid by a rapid discharge of a capacitor bank is suggested. New theoretical concepts of excess electrons in polar liquids are used.     

Dispersion of cellulose nanocrystals in polar organic solvents

In an attempt to prepare stable dispersions of cellulose nanocrystals in dipolar aprotic solvents, dilute aqueous suspensions of cellulose nanocrystals were prepared by sulfuric acid hydrolysis of cotton. The aqueous suspensions were freeze-dried, and then sonicated in the solvent of interest. Dispersions of 1 and 3% w/v concentration were prepared in polar organic solvents DMSO and DMF. The dispersions showed flow birefringence. The redispersion was incomplete, and there was some evidence for aggregation in the suspensions. A small amount of water appeared to be critical to suspension stability. Birefringent cellulose films were prepared from the dispersions by drying under vacuum and at ambient conditions.     

Complexes of amic acids with polar aprotic solvents

The polymer-polymer interactions of a polyamic acid (PAA) with polybases (polyvinylpyrrolidone, polyacrylonitrile and polyvinylpyridine) were studied by means of massspectrometric thermal analysis (MTA), TG, DTA and IR-spectroscopic methods in solution and in the solid state. The polybases were shown to substitute the solvent from its complex with PAA, forming H-bonds or salt-type polycomplexes with PAA. The MTA method proved to be very informative for the investigation of interpolymer interactions in the solid state.

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Zusammenfassung Mittels MS-Thermoanalyse (MTA), TG, DTA und IR-Spektroskopie wurden sowohl in Lösung als auch im festen Aggregatzustand die Polymer-Polymer-Wechselwirkungen zwischen Polyamicsäure (PAA) und Polybasen (Polyvinylpyrrolidon, Polyacrylnitril und Polyvinyl-pyridin) untersucht. Polybasen ersetzen das Lösungsmittel in seinem Komplex mit PAA und bilden dabei mit PAA Wasserstoffbindungen oder salzähnliche Polykomplexe. Die MTA-Methode erwies sich bei der Untersuchung von interpolymeren Wechselwirkungen im festen Aggregatzustand als sehr informativ.

     

Complexes of amic acids with polar aprotic solvents

The nonisothermal kinetics of solid-phase cyclodehydration of poly(4,4'-oxydiphenylene) pyromellitamic acid has been investigated with the aid of mass-spectrometric thermal analysis. It was established that depending on the method of preparation of the samples, the kinetics of cyclization vary from the polychromatic (film) to the monochromatic (fiber) kinetics. The possibility of the catalysis of the process as a result of the formation of complexes between the polymer and imidazole and its substituents is shown.

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Zusammenfassung Mittels massenspektrometrischer Thermoanalyse wurde die nichtisotherme Kinetik der Cyclodehydratation von Poly(4,4'-Oxydiphenylen)pyromellithsäure im festen Zustand untersucht. Es wurde festgestellt, daß die Cyclisationskinetik in Abhängigkeit vom Herstellungsverfahren der Proben von polychromatisch (Film) bis monochromatisch (Fiber) variiert. Die Möglichkeit der Katalyse des Vorganges infolge von Komplexbildung zwischen Polymer und Imidazol und seiner Substituenten wird gezeigt.

     

Solution properties of ion-containing polymers in polar solvents

The placement of ionic groups within the molecular structure of a polymer produces marked modification in physical properties. A large number of studies have been performed on these ion-containing polymers, but few have focused on the effects of anion–cation interactions (i.e., counterion binding or ionization) on hydrodynamic volume, especially as the molecular structure of the solvent and nature of counterion are varied. In this study changes in hydrodynamic volume are followed through reduced viscosity measurements as a function of the abovementioned molecular parameters. The dilute solution properties of various polyelectrolytes that contain sulfonate and carboxylate groups were investigated as a function of the counterion structure, charge density, molecular weight, and solvent structure. The polymeric materials were selected because of their specific chemical structure and physical properties. In the first instance a (2-acrylamide-2 methylpropanesulfonic acid)-acrylamide-sodium vinyl sulfonate terpolymer was synthesized and subsequently neutralized with a series of bases. Viscometric measurements on these materials indicate that the nature of the cation affects the ability of the polyelectrolyte to expand its hydrodynamic volume at low polymer levels. The magnitude of the molecular expansion is shown to be due in part to the ability of the counterion to dissociate from the backbone chain, which, in turn, is directly related to the solvent structure. The changes in solution behaviour of these inomers lend support for the existence of ion pairs (i.e., site binding) and ionized moieties on the polymer chains. Measurements performed in a variety of solvent systems further confirm this interpretation. In addition, and acrylamide-sodium vinyl sulfonate copolymer was partially hydrolyzed with sodium hydroxide to study the effect of varying the charge density at a constant degree of polymerization and counterion structure. The results show that the charge density has a significant effect on the magnitude of the reduced viscosity and dilute solution behaviour. These observations, made in aqueous and nonaqueous solvents, are related to the interrelation of hydrodynamic volume, counterion concentration, and site binding. Again the controlling factor is the degree of site binding of the counterion onto the polymer backbone. Finally, we observe that the increased hydrodynamic volume affects viscosity behavior beyond the polyelectrolyte effect regime. If the average charge density on the macromolecule is relative high and/or the molecular weight is large (≥ 10⁶) sufficient intermolecular interactions will occur to produce rapid changes in reduced viscosity.