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.     

Influence of solute-solvent coordination on the orientational relaxation of ion assemblies in polar solvents

We have investigated the rotational dynamics of lithium thiocyanate (LiNCS) dissolved in various polar solvents with time and polarization resolved vibrational spectroscopy. LiNCS forms multiple distinct ionic structures in solution that can be distinguished with the CN stretch vibrational frequency of the different ionic assemblies. By varying the solvent and the LiNCS concentration, the number and type of ionic structures present in solution can be controlled. Control of the ionic structure provides control over the volume, shape, and dipole moment of the solute, critical parameters for hydrodynamic and dielectric continuum models of friction. The use of solutes with sizes comparable to or smaller than the solvent molecules also helps amplify the sensitivity of the measurement to the short-ranged solute-solvent interaction. The measured orientational relaxation dynamics show many clear and distinct deviations from simple hydrodynamic behavior. All ionic structures in all solvents exhibit multi-exponential relaxation dynamics that do not scale with the solute volume. For Lewis base solvents such as benzonitrile, dimethyl carbonate, and ethyl acetate, the observed dynamics strongly show the effect of solute-solvent complex formation. For the weak Lewis base solvent nitromethane, we see no evidence for solute-solvent complex formation, but still see strong deviation from the predictions of simple hydrodynamic theory.     

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.     

From precipitation to vesicles: a study on self-organized assemblies by alkylammonium and its mixtures in polar solvents

The formation of self-organized assemblies by C_nH_2n+1NH₂·HCl and C₁₂H₂₅SO₄ Na in polar solvents was studied. By adding 1-propanol or 2-propanol, the precipitation formed in aqueous solution was resolved and a bilayer structure was discovered. Furthermore, multilamellar vesicle formation could be observed by methods of adjusting the molar ratio of C_nH_2n+1NH₂·HCl to C₁₂H₂₅SO₄Na, changing the length of the hydrocarbon chain of C_nH_2n+1NH₂·HCl, or adding 1-octanol to the mixed surfactant systems. Received: 18 January 2001 Accepted: 15 March 2001     

Inorganic polar blocks into controlled acentric assemblies

We show here a strategy to predict the crystal structure, formulate, and prepare new noncentrosymmetric (NCS) bismuth-phosphate based compounds. It is based on the cooperative-arrangement of polar building units (BUs) which can be created at particular stoichiometric conditions. The arrangement of such BUs into NCS compounds arise from the shortest-periodicity of repartition of the cationic charges in NCS structures than in the plausible, but never observed centrosymetric polytypes. This work validates the possibilities for the prediction of an extended series of novel compounds, tuning the size of BUs within a variety of controlled edifices. Despite their closed chemical composition, all the predicted terms appeared strikingly stable at precise stoichiometries.     

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.     

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.     

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.     

Hydrogen bonded supramolecular polymers in moderately polar solvents

Hydrogen bonded assemblies are usually decomposed by polar organic solvents. However, we have succeeded in preparing a strongly associated supramolecular polymer which forms viscous solutions in competitive solvents such as tetrahydrofuran.     

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.     

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.     

Spectroscopic properties of anisole in mixed polar solvents

Absorption and emission spectra, fluorescence quantum yields and lifetimes were studied for anisole in binary ethanol-water mixtures. The spectroscopic and photophysical properties show an exceptional behaviour in the aqueous solution. In water a strong nonradiative process, originating from the fluorescent state decreases the emission yield and lifetime. The results are discussed in terms of short range interactions between the solute and solvent.     

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.     

Carboxylate binding in polar solvents using pyridylguanidinium salts

A series of thiourea and guanidinium derivatives have been prepared and their ability to bind a carboxylate group has been investigated. Guanidinium 33, featuring two additional amides and a pyridine moiety, proved to be the most potent carboxylate binding site and was able to bind acetate in aqueous solvent systems (K(ass) = 480 M(-1) in 30% H(2)O-DMSO). The pyridine moiety is critical to obtaining strong binding, and comparison with the binding properties of analogous compounds in which the pyridine is replaced by a benzene ring provides a striking example of enthalpy-entropy compensation.     

Solubility of sulphur dioxide in polar organic solvents

The solubility of sulphur dioxide in N,N-dimethylformamide, dimethyl sulphoxide, N,N-dimethyl acetamide, sulpholane, tributyl phosphate and diethanolamine has been determined. A solubility model is proposed and the solubilities calculated by the model show good agreement with experimental data.     

Computer simulation of cellulose solvation in polar solvents

The solvation of cellulose molecules in water and N-methylmorpholine N-oxide has been investigated by the molecular-dynamics method. An analysis of simulation results yields the conformational characteristics of cellulose molecules in these solvents: the mean-square distance between polymer chain ends and the radial distribution function of monomer units. The radial distribution functions of oxygen atoms of solvents relative to protons of carbohydrate molecules are estimated. This step makes it possible to draw conclusions about the number and character of hydrogen bonds and the local structure of solvate shells.