Solution behavior of ionomers. I. Metal sulfonate ionomers in mixed solvents

The solution behavior of metal sulfonate-containing ionomers has been investigated in various mixed solvent systems. Ionomers, such as lightly sulfonated polystyrene (sodium salt) and sulfonated ethylene-propylene-diene terpolymer (metal salts) are generally insoluble in typical hydrocarbon solvents, but readily dissolve when small amounts of alcohols or other polar cosolvents are present. At relatively low polymer concentration these ionomers display unusually high thickening behavior in nonpolar solvents when compared with nonionic polymers because of association of the metal sulfonate groups. The addition of modest levels of polar cosolvent markedly decreases the solution viscosity and gives rise to viscosity-temperature relationships different from those of conventional polymer solutions. For example, such solutions can display vicosities which increase, are relatively constant, or display maxima or minima over broad temperature ranges. These observations are interpreted as arising from a temperature-dependent preferential interaction of the cosolvent with the sulfonate groups. While these ionomers can be regarded as polyelectrolytes of low charge density, they do not display the typical “polyelectrolyte” behavior often observed in aqueous solutions. This anomalous behavior is attributed to the fact that the metal sulfonate groups are largely un-ionized in solvents of low dielectric constant. Therefore, the solution behavior is dominated by ion pair interactions rather than free ions.     

Solute-solvent and solvent-solvent interactions in the preferential solvation of 4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide in 24 binary solvent mixtures

The molar transition energy (E(T)) polarity values for the dye 4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide were collected in binary mixtures comprising a hydrogen-bond accepting (HBA) solvent (acetone, acetonitrile, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF)) and a hydrogen-bond donating (HBD) solvent (water, methanol, ethanol, propan-2-ol, and butan-1-ol). Data referring to mixtures of water with alcohols were also analyzed. These data were used in the study of the preferential solvation of the probe, in terms of both solute-solvent and solvent-solvent interactions. These latter interactions are of importance in explaining the synergistic behavior observed for many mixed solvent systems. All data were successfully fitted to a model based on solvent-exchange equilibria. The E(T) values of the dye dissolved in the solvents show that the position of the solvatochromic absorption band of the dye is dependent on the medium polarity. The solvation of the dye in HBA solvents occurs with a very important contribution from ion-dipole interactions. In HBD solvents, the hydrogen bonding between the dimethylamino group in the dye and the OH group in the solvent plays an important role in the solvation of the dye. The interaction of the hydroxylic solvent with the other component in the mixture can lead to the formation of hydrogen-bonded complexes, which solvate the dye using a lower polar moiety, i.e. alkyl groups in the solvents. The dye has a hydrophobic nature and a dimethylamino group with a minor capability for hydrogen bonding with the medium in comparison with the phenolate group present in Reichardt's pyridiniophenolate. Thus, the probe is able to detect solvent-solvent interactions, which are implicit to the observed synergistic behavior.     

Associating polymers in a well-defined extensional flow field: Influence of solvent quality and polymer concentration

The conformational response of an associating-type random coil macromolecule in solution was investigated utilizing an opposing jet device. This device, capable of generating a well-defined elongational flow field, is quite useful for probing intra- and intermolecular interactions of lightly sulfonated polystyrene ionomers in both nonpolar and polar solvent systems. Below a critical concentration in nonpolar media, such ionomers qualitatively follow trends predicted by dilute solution theory, although intramolecular ionic associations markedly increase the critical elongational shear rate. With further increases in concentration, the extensional behavior is determined by the initial formation of relatively strong intermolecular associations. At even higher polymer concentrations, a third regime is observed where the conformational relaxation process becomes even more facile. On the contrary, in a polar solvent, the conformational relaxation process is markedly enhanced (i.e., critical elongational shear rate is reduced) due to the polyelectrolyte effect, i.e., dissociation of a significant level of the counterions. The effect of this dissociation process influences the relaxation process over the entire concentration region examined. These findings are compared directly with solution rheology, where in low polarity solvents the reduced viscosity is markedly diminished by ion pair-type interactions, and in more polar environments the reduced viscosity is enhanced due to the dissociation of the counterions from the vicinity of the chain backbone.     

Solution viscosity behavior of polystyrene-based cationic ionomers. Effects of ion content and solvent

Dilute-solution viscosities of polystyrene-based cationic ionomers containing ammonio or phosphonio groups were measured in several solvents. In polar solvents with dielectric constant (ε_r) beyond 10, the ionomers showed a typical polyelectrolyte behavior, indicating that a large part of ionic groups were dissociated into ions. In nonpolar solvents with low ε_r, the reduced viscosity of the ionomers linearly decreased with a decreasing ionomer concentration. At low polymer concentrations, every ionomer gave a reduced viscosity lower than that of the corresponding chloromethylated polystyrene. With an increasing ion content, the intrinsic viscosity progressively decreased if the nonpolar solvents had a low acceptor number (A_N), such as toluene or tetrahydrofran (THF). In the halogenated solvents with high A_N value, such as chloroform, however, the intrinsic viscosity was hardly dependent on the ion content. This indicates that the intramolecular aggregation among the ionic groups is inhibited in the halogenated solvents due to a strong anion solvation. An addition of a protic solvent to a nonpolar solvent eliminates the aggregation between ionic groups and leads to polyelectrolyte behavior. © 1994 John Wiley & Sons, Inc.     

Ion Pairing in Protic Ionic Liquids Probed by Far‐Infrared Spectroscopy: Effects of Solvent Polarity and Temperature

The cation–anion and cation–solvent interactions in solutions of the protic ionic liquid (PIL) [Et₃NH][I] dissolved in solvents of different polarities are studied by means of far infrared vibrational (FIR) spectroscopy and density functional theory (DFT) calculations. The dissociation of contact ion pairs (CIPs) and the resulting formation of solvent‐separated ion pairs (SIPs) can be observed and analyzed as a function of solvent concentration, solvent polarity, and temperature. In apolar environments, the CIPs dominate for all solvent concentrations and temperatures. At high concentrations of polar solvents, SIPs are favored over CIPs. For these PIL/solvent mixtures, CIPs are reformed by increasing the temperature due to the reduced polarity of the solvent. Overall, this approach provides equilibrium constants, free energies, enthalpies, and entropies for ion‐pair formation in trialkylammonium‐containing PILs. These results have important implications for the understanding of solvation chemistry and the reactivity of ionic liquids.     

What can you learn from a molecular probe? New insights on the behavior of C343 in homogeneous solutions and AOT reverse micelles

The behavior of C343, a common molecular probe utilized in solvation dynamics experiments, was studied in homogeneous media and in aqueous and nonaqueous reverse micelles (RMs). In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission bands showing that the solvatochromic behavior of the dye depends not only on the polarity of the medium but also on the hydrogen-bonding properties of the solvent. Specifically, in the ground state the molecule displays a bathochromic shift with the polarity polarizability (pi) and the H-bond acceptor (beta) ability of the solvents and a hypsochromic shift with the hydrogen donor ability (alpha) of the media. The carboxylic acid group causes C343 to display greater sensitivity to the beta than to the pi polarity parameter; this sensitivity increases in the excited state, while the dependence on alpha vanishes. This demonstrates that C343 forms a stable H-bond complex with solvents with high H-bond acceptor ability (high beta) and low H-bond donor character (low alpha). Spectroscopy in nonpolar solvents reveals J-aggregate formation. With information from the Kamlet-Taft analysis, C343 was used to explore RMs composed of water or polar solvents/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/isooctane using absorption, emission, and time-resolved spectroscopies. Sequestered polar solvents included ethylene glycol (EG), formamide (FA), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA). Dissolved in the AOT RM systems at low concentration, C343 exists as a monomer, and when introduced to the RM samples in its protonated form, C343 remains protonated driving it to reside in the interface rather than the water pool. The solvathochromic behavior of the dye depends the specific polar solvent encapsulated in the RMs, revealing different types of interactions between the solvents and the surfactant. EG and water H-bond with the AOT sulfonate group destroying their bulk H-bonded structures. While water remains well segregated from the nonpolar regions, EG appears to penetrate into the oil side of the interface. In aqueous AOT RMs, C343 interacts with neither the sulfonate group nor the water, perhaps because of intramolecular H-bonding in the dye. DMF and DMA interact primarily through dipole-dipole forces, and the strong interactions with AOT sodium counterions destroy their bulk structure. FA also interacts with the Na+ counterions but retains its H-bond network present in bulk solvent. Surprisingly, FA appears to be the only polar solvent other than water forming a "polar-solvent pool" with macroscopic properties similar to the bulk.     

Interactions of a sulfonated ionomer with surfactant in nonaqueous media

The viscometric behavior of dilute solutions of the sodium salt of sulfonated polystyrene (0–6 mol % sulfonation level), with and without surfactant, is investigated to determine the extent of interaction as the structure of the solvent surfactant, and polymer concentration is varied. Reduced viscosity measurements confirm that formation of a polymer–surfactant complex in a relatively polar solvent is controlled to a large extent by charge–charge and hydrophobic forces. The magnitude of these specific interactions is dependent upon the relative polarity of the solvent medium. In a polar solvent, such as dimethylsulfoxide, the hydrophobic forces are strong enough to prevent expansion of the polymer chain at all surfactant concentrations studied. However, in a less polar medium (as in dimethylformamide) the hydrophobic forces are weaker and cannot prevent some chain expansion. It is interesting to note that in this solvent the polystyrene–cationic surfactant complex exhibits a polyelectrolyte effect. Finally, in a lower-polarity medium (cyclohexanone) where the hydrophobic forces are weak, solution behavior is dominated by the interaction of the surfactant with the intramolecular sulfonate ion-pair aggregates.     

Chromatographic and spectroscopic methods for the determination of solvent properties of room temperature ionic liquids

Room temperature ionic liquids are novel solvents with favorable environmental and technical features. Synthetic routes to over 200 room temperature ionic liquids are known but for most ionic liquids physicochemical data are generally lacking or incomplete. Chromatographic and spectroscopic methods afford suitable tools for the study of solvation properties under conditions that approximate infinite dilution. Gas-liquid chromatography is suitable for the determination of gas-liquid partition coefficients and activity coefficients as well as thermodynamic constants derived from either of these parameters and their variation with temperature. The solvation parameter model can be used to define the contribution from individual intermolecular interactions to the gas-liquid partition coefficient. Application of chemometric procedures to a large database of system constants for ionic liquids indicates their unique solvent properties: low cohesion for ionic liquids with weakly associated ions compared with non-ionic liquids of similar polarity; greater hydrogen-bond basicity than typical polar non-ionic solvents; and a range of dipolarity/polarizability that encompasses the same range as occupied by the most polar non-ionic liquids. These properties can be crudely related to ion structures but further work is required to develop a comprehensive approach for the design of ionic liquids for specific applications. Data for liquid-liquid partition coefficients is scarce by comparison with gas-liquid partition coefficients. Preliminary studies indicate the possibility of using the solvation parameter model for interpretation of liquid-liquid partition coefficients determined by shake-flask procedures as well as the feasibility of using liquid-liquid chromatography for the convenient and rapid determination of liquid-liquid partition coefficients. Spectroscopic measurements of solvatochromic and fluorescent probe molecules in room temperature ionic liquids provide insights into solvent intermolecular interactions although interpretation of the different and generally uncorrelated "polarity" scales is sometimes ambiguous. All evidence points to the ionic liquids as a unique class of polar solvents suitable for technical development. In terms of designer solvents, however, further work is needed to fill the gaps in our knowledge of the relationship between ion structures and physicochemical properties.     

Preferential solvation and solvation shell composition of free base and protonated 5, 10, 15, 20-tetrakis(4-sulfonatophenyl)porphyrin in aqueous organic mixed solvents

The solvatochromic properties of the free base and the protonated 5, 10, 15, 20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) were studied in pure water, methanol, ethanol (protic solvents), dimethylsulfoxide, DMSO, (non-protic solvent), and their corresponding aqueous-organic binary mixed solvents. The correlation of the empirical solvent polarity scale (E(T)) values of TPPS with composition of the solvents was analyzed by the solvent exchange model of Bosch and Roses to clarify the preferential solvation of the probe dyes in the binary mixed solvents. The solvation shell composition and the synergistic effects in preferential solvation of the solute dyes were investigated in terms of both solvent-solvent and solute-solvent interactions and also, the local mole fraction of each solvent composition was calculated in cybotactic region of the probe. The effective mole fraction variation may provide significant physico-chemical insights in the microscopic and molecular level of interactions between TPPS species and the solvent components and therefore, can be used to interpret the solvent effect on kinetics and thermodynamics of TPPS. The obtained results from the preferential solvation and solvent-solvent interactions have been successfully applied to explain the variation of equilibrium behavior of protonation of TPPS occurring in aqueous organic mixed solvents of methanol, ethanol and DMSO.     

Studies of polymers from cyclic dienes. V. Cationic polymerization of cyclopentadiene,influence of polymerization conditions on the polymer structure

Cationic polymerization of cyclopentadiene was studied with several Friedel-Crafts catalysts, and the influence of polymerization conditions on the polymer structure was investigated. Polycyclopentadiene contained higher amounts of the 1,2 structure when a stronger catalyst and a more polar solvent were used. This fact is discussed in terms of the tightness of the growing ion pair. The polymer structure did not vary with polymerization temperature in toluene solvent. In methylene chloride at around 0°C. the structural variation with catalysts was much smaller, suggesting a freer nature of the growing ion pair. The viscosity data also support the change in the structure of the ion pair under similar conditions. The use of aliphatic hydrocarbon solvents gave the highest contents of the 1,2 structure. This results was ascribed to the lack of solvation, considering the dependence of the polymer structure on the monomer concentration, which was found only in this solvent. Furthermore, isomerization during propagation was observed in polar solvents at higher temperature.     

Stabilities of ion/radical adducts in the liquid phase as derived from the dependence of electrochemical cleavage reactivities upon solvent.

The idea that significant ion/radical interactions should vary with solvent if they do exist in the liquid phase was pursued by an investigation of the dissociative electron-transfer reactivity of carbon tetrachloride and 4-cyanobenzyl chloride in four different solvents, 1,2-dichloroethane, N,N-dimethylformamide, ethanol, and formamide, by means of their cyclic voltammetric responses. Modification of the conventional dissociative electron transfer theory to take account of an interaction between fragments in the ion/radical pair resulting from the dissociative electron reaction allows a satisfactory fitting of the experimental data leading to the determination of the interaction energy. There is an approximate correlation between the interaction energies in the ion/radical pair and the solvation free energies of the leaving anion, Cl(-). The interaction is maximal in 1,2-dichloroethane, which is both the least polar and the least able to solvate Cl(-). The interaction is smaller in the polar solvents, albeit distinctly measurable. The two protic solvents, ethanol and formamide, which are the most able to solvate Cl(-), give rise to similar interaction energies. The interaction is definitely stronger in N,N-dimethylformamide, which has a lesser ability to solvate Cl(-) than the two other polar solvents. The existence of significant ion/radical interactions in polar media is thus confirmed and a route to their determination opened.     

Solvent effects on the photochemistry of dimethyl sulfoxide–Cl complexes studied by combined pulse radiolysis and laser flash photolysis

Photolysis of complexes of dimethyl sulfoxide (DMSO) with chlorine atoms results in rapid and permanent photobleaching which may be due to intramolecular hydrogen abstraction. The effects of solvent polarity were examined in a wide variety of DMSO–carbon tetrachloride mixed solvents. The quantum yields of photobleaching decreased from 0.27 to 0.08 as the solvent polarity increased, while significant changes were observed in the low DMSO concentration range (<0.2 mol dm⁻³). This cannot be accounted for by simple solvent polarity effects. The effects of polar and nonpolar additives were also examined and it is concluded that the specific solvation effect of DMSO was the main cause of the significant change in quantum yields in the low concentration range of DMSO.     

Dynamic stokes shift and excitation wavelength dependent fluorescence of dipolar molecules in room temperature ionic liquids

Room temperature ionic liquids (RTILs) are viscous media consisting entirely of ions. Because of the complex nature of various interactions in these media, the solvent properties of the RTILs are very little understood. Since the fluorescence response of molecules comprising conjugated electron donor and acceptor groups, referred to as dipolar molecules, is one of the most frequently exploited sources of information on complex media, whose properties are largely unknown, it is possible to obtain insight into the structure and dynamics of the RTILs by studying the fluorescence behavior of dipolar solutes in these complex media. The most commonly exploited utility of a fluorescent dipolar system is in the estimation of the polarity of the media from its steady state fluorescence response. While several dipolar systems do provide estimates of the polarity of various RTILs, there can be circumstances when the steady state emission frequency of a dipolar system may not truly reflect the equilibrium solvation energy and, hence, the polarity of the medium. The fluorescence response of a dipolar system can be dependent on the excitation wavelength, an observation not commonly encountered in conventional solvents of similar polarities. On the other hand, the time-resolved fluorescence behavior of a dipolar solute in polar medium is one of the primary sources of information on the time-scale of reorganization of the solvent molecules around the photoexcited species. As the RTILs are sufficiently polar media, the time-dependent fluorescence data of the dipolar systems provide insight into the dynamics and mechanism of solvation in these media, which differ considerably from the conventional solvents. These aspects have been discussed taking into consideration the inherent absorption and fluorescence behavior of the imidazolium ionic liquids.     

Molecular interpretation for the solvation of poly(acrylamide)s. I. Solvent-dependent changes in the C=O stretching band region of poly(N,N-dialkylacrylamide)s

The solvation of poly(N,N-dimethylacrylamide) (PNdMA) and poly(N,N-diethylacrylamide) (PNdEA) in various protic and aprotic solvents has been studied by using infrared (IR) spectroscopy. Because PNdMA and PNdEA have the same polar functional group, their IR spectra show quite similar solvent effects. Unexpectedly, the solvent-dependent changes of the C=O stretching vibration (nu(C=O)) bands of the two polymers cannot be explained only by dielectric constants of the solvents. Then, infrared spectra of N,N-dimethylacetamide (NdMA) and N,N-diethylacetamide (NdEA), monomer models for PNdMA and PNdEA, respectively, in the same solvents as the polymer solutions have also been examined. Interestingly, the solvent-dependent spectra in the nu(C=O) band region of NdMA and NdEA are correlated with those of PNdMA and PNdEA, respectively, except for slight deviations, which may be ascribed to molecular mobility and/or exclusive volume. These correlations permit one to regard the solvation of the polymers as that of the corresponding monomers. As a result, we have proposed the assignments of nu(C=O) bands for the PNdMA and PNdEA solutions regarding the interactions between solvents and NdMA and NdEA as hydrogen bondings. In the IR spectra of PNdMA and PNdEA in the protic solvents, two C=O bands are mainly observed; one appears at a similar frequency to that of a C=O band observed for the monomer solution, and the other is characteristic of the polymer systems. The former band is likely to reflect the solvation behavior of PNdMA and PNdEA. The results clearly show that the solvation of a polymer can be interpreted at the molecular level using infrared spectroscopy sensitive to solvent effects.     

Preferential Solvation and Solvatochromic Parameters in Mixtures of Poly(ethylene glycol)-400 with Some Molecular Organic Solvents

Polyethylene glycols have become more popular alternate reaction media due to interesting properties like non-toxicity, bio-degradability, and full miscibility with water and organic solvents. Binary mixtures of polyethylene glycols with common solvents can be useful to tune their physical and chemical properties and to facilitate chemical and physical processes. In this study, solvatochromic parameters were spectrophotometrically determined for binary solvent mixtures of poly(ethylene glycol)-400 (PEG-400) with methanol, 2-propanol, 1-butanol, dimethyl sulfoxide, N,N-Dimethylformamide, and dichloromethane under ambient conditions, over the whole range of mole fractions. The solvatochromic parameters showed different trends in protic and aprotic solvents mixed with PEG-400. Methanol/PEG-400 mixtures showed special properties in polarity and polarizability so that the mixtures are more dipolar/polarizable than their pure components. Positive or negative deviations from ideal behavior confirmed that the indicators were involved in a preferential solvation process in the solvent mixtures. These deviations from ideality can be attributed to strong solvent–solvent interactions in the binary mixtures.     

磺化聚苯乙烯镧离子型聚合物的稀溶液性质

聚电解质行为;比浓粘度;电解质;磺化聚苯乙烯镧离子型聚合物的稀溶液性质     

Solvent extraction of tetraphenylarsonium ion pairs from mixed aqueous-organic solutions

Ion pairs of tetraphenylarsonium cation with iodide, bromide, and perrhenate anions were extracted into chloroform from mixed aqueous-organic solutions. The extraction of these ion pairs was found to increase in the presence of polar aprotic solvents in the mixed aqueous-organic phase. This effect was correlated with literature data on free energy of transfer of the Ph₄As⁺ ion, and ascribed to ion pair/solvent and/or ion/solvent interactions in the organic phase.     

Extraction of organic compounds with room temperature ionic liquids

Room temperature ionic liquids are novel solvents with a rather specific blend of physical and solution properties that makes them of interest for applications in separation science. They are good solvents for a wide range of compounds in which they behave as polar solvents. Their physical properties of note that distinguish them from conventional organic solvents are a negligible vapor pressure, high thermal stability, and relatively high viscosity. They can form biphasic systems with water or low polarity organic solvents and gases suitable for use in liquid–liquid and gas–liquid partition systems. An analysis of partition coefficients for varied compounds in these systems allows characterization of solvent selectivity using the solvation parameter model, which together with spectroscopic studies of solvent effects on probe substances, results in a detailed picture of solvent behavior. These studies indicate that the solution properties of ionic liquids are similar to those of polar organic solvents. Practical applications of ionic liquids in sample preparation include extractive distillation, aqueous biphasic systems, liquid–liquid extraction, liquid-phase microextraction, supported liquid membrane extraction, matrix solvents for headspace analysis, and micellar extraction. The specific advantages and limitations of ionic liquids in these studies is discussed with a view to defining future uses and the need not to neglect the identification of new room temperature ionic liquids with physical and solution properties tailored to the needs of specific sample preparation techniques. The defining feature of the special nature of ionic liquids is not their solution or physical properties viewed separately but their unique combinations when taken together compared with traditional organic solvents.